EP1360874A2

EP1360874A2 - Sound system and method of sound reproduction

Info

Publication number: EP1360874A2
Application number: EP02717406A
Authority: EP
Inventors: Lawrence R. Fincham
Original assignee: THX Ltd
Current assignee: THX Ltd
Priority date: 2001-02-09
Filing date: 2002-02-08
Publication date: 2003-11-12
Anticipated expiration: 2022-02-08
Also published as: JP2004521541A; WO2002065815A2; WO2002065815A3; DE60230386D1; ATE418255T1; EP1360874B1; CA2449410C; CA2449410A1

Abstract

A sound reproduction system comprises a left and right speakers located in close proximity, and a sound processor which provides audio signals to the pair of speakers. The sound processor preferably derives a cancellation signal from the difference between the left and right channels. The resulting difference signal is scaled, delayed (if necessary), and spectrally modified before being added to the left channel and, in opposite polarity, to the right channel. The spectral modification to the difference channel preferably takes the form of a low-frequency boost over a specified frequency range, in order to restore the correct timbral balance after the differencing process. Additional phase-compensating all-pass networks may be inserted in the difference channel to correct for any extra phase shift contributed by the spectral modifying circuit. The technique may be used in a surround sound system or an automobile sound system. In an automobile, the pair of speakers may be placed together in a common enclosure having a narrow slot near the base of the cone of the speakers. The speaker may be oriented perpendicularly with respect to the dashboard such that the sound emanates from the slot in the enclosure.

Description

SOUND SYSTEM AND METHOD OF SOUND REPRODUCTION

BACKGROUND OF THE INVENTION

1 ) Field of the Invention

[0001] The field of the present invention relates to sound reproduction and, more

specifically, to a speaker configuration and related sound processing for use in a sound

system.

2) Background

[0002] Attaining optimal sound quality in surround sound or multi-channel sound

systems, over the largest possible listening area, can be quite challenging. Some of the

difficulties in achieving optimal sound quality in such systems result from the fact that a

wide variety of different surround sound and multi-channel audio formats and speaker

configurations exist, so that a particular sound system may have reasonably acceptable

sound with respect to one or perhaps two audio formats yet sub-optimal sound with

respect to other audio formats. Therefore, where a consumer desires, for example, to

use a single sound system to play sound recordings in a variety of different formats,

different levels of sound quality, some of which are poor or impaired, are likely to be

experienced. While the user can adjust speaker positioning or relative balances to try to improve sound quality, such techniques may involve significant manual effort or

inconvenience, may be hard to reproduce consistently, and may benefit only one or

perhaps a few listeners in a relatively small portion of the listening area.

[0003] Existing surround sound recording formats include those referred to as

5.1 , 6.1 and 7.1. The 5.1 surround format comprises a compressed data stream

containing five channels, generally designated left, center, right, surround left, and

surround right, named for the speaker positions for which the channel information is

intended. A low frequency effects channel is formed by a combination of the five other

channels, and may be provided to a sub-woofer. The 6.1 surround format includes the

same five channels as the 5.1 surround format, but adds a surround back channel, which

may be fed to one or more back speakers in a surround sound system. The 7.1 surround

format is similar to the 6.1 surround format, but has two surround back channels

(surround back left and surround back right) rather than a single back channel, for a total

of seven channels. Thus, the 5.1 surround format has two surround channels (surround

left and right), the 6.1 surround format has three surround channels (surround left, right

and back), and the 7.1 surround format has four surround channels (surround left and

right, and surround back left and right).

[0004] Basic surround system speaker configurations generally include from six

to eight speakers placed at conventionally well-established locations, according to the

type of surround format they are intended to play. A six-speaker surround system

typically includes left, right and center speakers (with the right and left speakers spaced

widely apart), a sub-woofer, and surround left and right speakers (which may be

monopolar or dipolar in nature). A seven-speaker surround system typically includes the same speaker arrangement as the six-speaker surround system, but adds a back

surround speaker. An eight-speaker surround system typically includes the same

speaker arrangement as the six-speaker surround system, but adds a back left surround

speaker and a back right surround speaker.

[0005] The enjoyment experienced by a listener in a surround sound system can

be affected by a number of factors, including the listener's physical position relative to

the various speakers, as well as by the particular format of the audio track being played

on the system. For example, when a 5.1 surround format soundtrack is played on an

eight-speaker (7.1 ) surround system, certain anomalies may occur. An example is that, if

the 5.1 surround left and surround right audio signals are monaural, then the left and

right surround effects can disappear, being replaced by a single central "phantom"

sound image at the rear. Another phenomenon is that if the listener is positioned in the

middle of the surround left and surround right speakers, he or she may perceive the

surround left and right sound (if monaural) to be higher in volume that it otherwise

would be, primarily due to the additive effect of the sound waves intersecting at the

listener's position (known as a "lift" effect). If the sound pans from one side to the

other (e.g., from left to right), the sound volume may appear to increase as left/right

balance is achieved, and then appear to decrease as the sound continues to pan, even

though the audio output level remains constant, due to the same "lift" effect. The sound

quality may also seem to be "unstable," in the sense that if the listener moves from the

center position, the sound might seem to "flip" from one side to the other.

[0006] Some of these effects can be mitigated in 5.1 surround sound systems by

the use of adaptive de-correlation with respect to the surround left and right speakers, which derives two substantially de-correlated signals when the surround left and right

signals are monaural, in order to provide an improved enveloping surround effect.

[0007] When a 6.1 surround format soundtrack is played on an eight-speaker

(7.1 ) surround system, certain other anomalies may be experienced. Since the two rear

surround speakers (left and right) are each fed with an identical monaural signal (that is,

the same surround back signal), a centrally located "phantom" image may result when

the listener is positioned approximately equidistant from the speakers. Reported side

effects of this arrangement include "coloration" associated with the phantom image (for

example, the sound may seem "unnatural"), a narrow "sweet spot" due to lack of

sound image stability when the listener moves off center, and a comb filter effect (in

other words, nulls may be produced due to sound wave cancellation effects).

[0008] Besides surround systems, a variety of multi-channel recording and

playback systems also exist. Examples of some common multi-channel sound systems

are Dolby AC-3, DTS, and DVD-Audio, each of which has its own specific digital

encoding format. Unlike cinema sound, there is generally no single adopted standard of

either loudspeaker type (e.g., full range, satellite plus sub-woofer, dipole, monopole) or

speaker layout for most multi-channel audio formats. Any user therefore desiring to

listen to multi-channel soundtracks, and/or any of the surround formats (5.1., 6.1 and

7.1 ), is required either to accept one speaker layout optimized for a particular audio

format and experience a compromised performance for all others, or to reconnect

various speakers to suit the audio format a particular soundtrack.

[0009] Beyond the surround sound environment, other sound systems also face

similar challenges, such as attaining a suitably wide "sweet spot" in which the perceived area and stability of a stereo sound image is maximized. In most traditional

sound systems, the convention has been to place left and right speakers far apart

physically, under the theory that the human ear is thereby better able to perceive the

richness of the audio subject matter. However, under many left/right speaker

configurations, the sound at off-axis listening positions may be sub-optimal. The quality

of sound at a given off-axis listening position may be affected not only by the difference

between left and right volumes resulting from the different distances to the left and right

speakers, but also by the slight difference in time it takes the aural information to reach

the listener.

[0010] Accordingly, it would be advantageous to provide an improved sound

system which overcomes one or more of the foregoing problems or shortcomings.

SUMMARY OF THE INVENTION [0011] The present invention is generally directed to improved sound

reproduction systems and methods involving a speaker configuration and/or placement,

and related sound processing, for enlarging the perceived area and stability of a sound

image generated from right and left source signals.

[0012] In one aspect, a sound reproduction system comprises a pair of speakers

(left and right) located in close proximity, and a sound processor which provides audio

signals to the pair of speakers. According to a preferred embodiment, the sound

processor acts to "spread" the sound image produced by the two closely spaced

speakers by employing a cross-cancellation technique wherein a cancellation signal is

derived, for example, from the difference between the left and right channels. The resulting difference signal is scaled, delayed (if necessary) and spectrally modified before

being added to the left channel and, in opposite polarity, to the right channel. The

spectral modification to the difference channel preferably takes the form of a low-

frequency boost over a specified frequency range, in order to restore the correct timbral

balance after the differencing process which causes a loss of bass when the low-

frequency signals in each channel are similar. Additional phase-compensating all-pass

networks may be inserted in the difference channel to correct for any extra phase shift

contributed by the usually minimum-phase-shift spectral modifying circuit so that the

correct phase relationship between the canceling signal and the direct signal is

maintained over the desired frequency range.

[0013] Alternatively, a linear-phase network may be employed to provide the

spectral modification to the difference channel, in which case compensation can be

provided by application of an appropriate, and substantially identical, frequency-

independent delay to both left and right channels.

[0014] The various speaker configuration and sound processing embodiments as

described herein may be employed in connection with a surround sound system to

achieve improved sound reproduction. A sound reproduction system for a surround

sound stereophonic system may comprise a set of speakers (e.g., front, left, center,

surround left, and surround right), including a pair of surround back speakers located in

close proximity, and a sound processor. The sound processor receives left and right

surround channel signals (either side or rear surround signals), and generates a

difference signal therefrom. The resulting difference signal may be processed as

described above - i.e., scaled, delayed (if necessary) and spectrally modified before being added to the left channel and, in opposite polarity, to the right channel.

Additional phase-compensating all-pass networks may, as noted above, be inserted in

the difference channel to correct for any extra phase shift contributed by the usually

minimum-phase-shift spectral modifying circuit so that the correct phase relationship

between the canceling signal and the direct signal is maintained over the desired

frequency range.

[0015] In the automobile or vehicle context, the pair of central speakers may be

placed in a common enclosure with a central dividing partition that is inserted into or

else integral with the front console or dashboard of the automobile. In certain

embodiments, the center speakers may be placed with their diaphragms facing down

and in close proximity to a rigid reflecting surface such that substantially all of the sound

energy is directed forward, towards the listener, via an arrow slot in the enclosure. The

resultant radiating system provides the dual benefit of occupying less dashboard area,

where space is always at a premium, and possessing a very wide directional

characteristics due to the slot having dimensions that can be made very small with

respect to the wavelength the radiated sound.

[0016] Further embodiments, variations and enhancements are also disclosed

herein.

BRIEF DESCRIPTION OF THE DRAWINGS

[0017] FIG. 1 is a diagram illustrating playback of a soundtrack in a 5.1 surround

system. [0018] FIG. 2 is a diagram illustrating playback of a 5.1 surround format

soundtrack in a 7.1 surround sound system.

[0019] FIG. 3 is a diagram illustrating playback of a 6.1 surround format

soundtrack in a 7.1 surround sound system.

[0020] FIG. 4 is a diagram illustrating the concept of a "sweet spot" in the

context of 6.1 surround format playback in a 7.1 surround sound system.

[0021] FIG. 5 is a diagram illustrating movement of the phantom image in

conjunction with the listener's movement.

[0022] FIG. 6 is a diagram of a speaker configuration for a surround sound

system, in accordance with a preferred embodiment as described herein.

[0023] FIG. 7 is a diagram illustrating 6.1 surround format playback in the

surround sound system shown in FIG. 6.

[0024] FIG. 8 is a simplified block diagram of a sound processing system in

accordance with one or more embodiments as disclosed herein, as may be used, for

example, in connection with the speaker configuration illustrated in FIG. 6.

[0025] FIG. 9-1 is a more detailed diagram of a sound processing system as may

be used, for example, in connection with the system illustrated in FIG. 6

[0026] FIG. 9-2 is a diagram of a sound processing system in general accordance

with the layout illustrated in FIG. 9-1 , further showing examples of possible transfer

function characteristics for certain processing blocks.

[0027] FIG. 10 is a diagram of a sound processing system illustrating

representative transfer functions. [0028] FIG. 1 1 is a diagram of a sound system in accordance with the general

principles of the systems illustrated in FIGS. 8 and 9, as applied in the context of a

surround sound system.

[0029] FIG. 12 is a conceptual diagram illustrating processing/operation for 5.1

surround format playback in the context of a surround sound system such as shown, for

example, in FIG. 6 or 1 1.

[0030] FIGS. 13 and 14 are graphs illustrating examples of frequency response

and phase transfer functions for a sound processing system having particular spectral

weighting and other characteristics.

[0031] FIGS. 15-1 , 15-2, and 1 5-3 are graphs illustrating examples of gain and/or

phase transfer functions for a sound processing system in accordance with FIG. 9-2.

[0032] FIG. 16 is a diagram of a sound processor employing a linear spectral

weighting filter.

[0033] FIG. 1 7 is a diagram of a preferred automobile sound system in

accordance with one or more embodiments as disclosed herein.

[0034] FIG. 18 is a diagram of a surround sound system for an automobile or

other vehicle.

[0035] FIGS. 19-1 , 19-2 and 19-3 are diagrams illustrating possible placement of

a pair of center speakers.

[0036] FIG. 20-1 is a front cut-away view of a preferred speaker enclosure for a

pair of stereo speakers.

[0037] FIG. 20-2 is a top cross-sectional view of the speaker enclosure shown in

FIG. 20-1. [0038] FIG. 20-3 is an oblique front view of the speaker enclosure shown in

FIGS. 20-1 and 20-2.

[0039] FIG. 20-4 is a diagram illustrating sound reflection from a downward

oriented speaker, such as a speaker in the speaker enclosure of FIGS. 20-1 through 20-3.

[0040] FIG. 21 is a block diagram illustrating an example of an automobile sound

system for providing potentially improved extreme right/left sound, in connection with

the pair of closely spaced center speakers.

[0041] FIG. 22 is a graph illustrating a relationship between speaker separation in

various embodiments as disclosed herein and difference channel gain.

[0042] FIG. 23 is a diagram of another embodiment of a surround sound system

for an automobile or other vehicle.

[0043] FIGS. 24-1 and 24-2 are diagrams comparing the audio effect of speaker

placement and sound processing between the prior art and various embodiments as

disclosed herein.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS [0044] According to various embodiments as disclosed herein, a preferred sound

reproduction system comprises, in one aspect, a pair of speakers located in close

proximity, and a sound processor which provides audio signals to the pair of speakers.

The sound processor preferably acts to "spread" the sound image produced by the two

closely spaced speakers by employing a cross-cancellation technique wherein a

cancellation signal is derived, for example, from the difference between the left and

right channels. The resulting difference signal is scaled, delayed (if necessary) and spectrally modified before being added to the left channel and, in opposite polarity, to

the right channel, thereby enlarging the perceived area and stability of the stereo sound

image. Further details of preferred sound processing techniques are described later

herein.

[0045] Some advantages of various embodiments disclosed herein can be

appreciated by way of contrast and comparison with conventional surround/multi¬

channel sound systems. FIG. 1 , for example, is a diagram illustrating playback of a

surround-encoded soundtrack in a 5.1 surround system 100. As shown in FIG. 1 , the

5.1 surround system 100 includes a front left speaker 104, a front right speaker 105, a

center speaker 102, a sub-woofer 109, a surround left speaker 1 14, and a surround right

speaker 1 15. In the example shown in FIG. 1 , the surround left and right speakers 1 14,

1 1 5 are both dipolar speakers, which distribute sound in multiple (typically opposite)

directions and are thereby provide improved ambient sound. The surround left and

right speakers 1 14, 1 1 5 are typically widely spaced on opposite sides of a room (or

other listening space), at positions which are above and slightly to the rear of the desired

listening position.

[0046] The speakers 102, 104, 105, 109, 1 14, and 1 15 in the 5.1 surround

system 100 are generally arranged to provide optimum sound for a listener 107

positioned in the approximate center of the speaker arrangement. However, a 5.1

surround system lacks an effective directional component to the immediate left and right

sides and to the rear of the listener 107. Therefore, a 6.1 or 7.1 surround system, both

of which have a rear speaker component, is generally capable of providing superior

sound and audio effects in certain contexts. A 6.1 surround system, as previously indicated, adds a single rear surround speaker, while a 7.1 surround system adds two

rear surround speakers typically spaced relatively far apart from one another.

[0047] FIG. 2 is a diagram of a 7.1 surround system 200, illustrating playback of a

5.1 surround-encoded soundtrack. As shown in FIG. 2, the 7.1 surround system 200

includes front left and right speakers 204, 205, a center speaker 202, a sub-woofer 209,

a surround left speaker 214, a surround right speaker 21 5, a surround back left speaker

224, and a surround back right speaker 225. In the particular example of FIG. 2, as with

FIG. 1 , the surround left and right speakers 214, 215 are dipolar in nature. The

surround back left and right speakers 224, 225 are typically spaced relatively far apart

behind the listener 207. When a 5.1 encoded soundtrack is played on a 7.1 surround

system 200 such as shown in FIG. 2, the surround left and right speakers 214, 215

receive the left and right surround channel information, and the surround back left and

right speakers 224, 225 may or may not receive the left and right surround channel

information, depending upon how the user has programmed the system 200. In either

case, certain anomalies can occur. For example, if the left and right surround channels

are monaural, the left/right surround effect can seem to disappear and be replaced by a

single central "phantom" sound image 230 at the rear of the listener 207. This effect

can be mitigated by the use of adaptive de-correlation, which involves derivation of two

substantially de-correlated signals from the single monaural channel in order to provide

an improved enveloping surround effect.

[0048] FIG. 3 is a diagram illustrating 6.1 surround format playback in a 7.1

surround system. In FIG. 3, the speakers labeled 3xx generally correspond to the same

speakers labeled 2xx in FIG. 2. When a soundtrack in a 6.1 surround format is played on a 7.1 surround system 300 such as shown in FIG. 3, the surround back speakers 324,

325 are fed with identical monaural signals (derived from the single surround back

channel in the 6.1 encoding format), which may or may not be delayed with respect to

each other to compensate for unequal distances from the optimum listening position.

As illustrated in FIG. 3, the identical monaural signals being played through the

surround back speakers 324, 325 produces a central "phantom" sound image 330

when the listener is positioned approximately equidistant from them. Reported side

effects include "coloration" associated with the phantom sound image 330, which can

lead to listener confusion or an unnatural sound, a narrow "sweet spot" (see FIG. 4)

due to lack of sound image stability when the listener moves off center from the axis

which is equidistant from both surround back speakers 324, 325 (see FIG. 5), and

suppression of certain frequency ranges due to cancellations (i.e., nulls) caused by a

"comb filter" effect as the sound waves interfere with one another. As a result, the

sound quality of a 6.1 surround format soundtrack, when played back in a 7.1 surround

system 300, can suffer significantly, particularly for listeners that are not positioned in an

optimum listening position.

[0049] As previously indicated in the Background section hereof, replay of

soundtracks in other multi-channel formats (such as Dolby AC-3, DTS or DVD-Audio)

can also suffer from similar effects, depending upon the nature of the signals fed to the

different left/right and back surround speakers.

[0050] FIG. 6 is a diagram showing a speaker configuration for a surround sound

system 600 in accordance with a preferred embodiment as described herein. The sound

system 600 of FIG. 6 includes, similar to the systems 200 and 300 shown in FIGS. 2 and 3, respectively, front left and right speakers 604, 605, a front center speaker 602, a sub-

woofer 609, a surround left speaker 614, and a surround right speaker 615. The sound

system 600 further includes a surround back left speaker 624 and a surround back right

speaker 625, which are preferably positioned in close proximity to one another, possibly

even within the same speaker enclosure. The surround back left and right speakers 624,

625 are preferably identical and may be either dipolar or monopolar in nature, but are

shown in FIG. 6 as monopolar. The speaker configuration of the sound system 600

illustrated in FIG. 6, coupled with a preferred sound processing technique, can provide

improved sound quality when, for example, playing audio tracks recorded in any of the

surround sound or multi-channel formats.

[0051] When the sound system 600 of FIG. 6 is used to play a soundtrack

recorded in 7.1 surround format, the various left, right, center, and surround left/right

channel audio signals are fed to the appropriate individual speakers, as would normally

be done with a typical 7.1 surround speaker configuration. However, the surround back

left and right speakers 624, 625 preferably receive the surround back right channel

audio signal and surround back left channel audio signal after sound processing as

further described in more detail later herein.

[0052] When, on the other hand, the sound system 600 of FIG. 6 is used to play

a soundtrack recorded in 6.1 surround format, the various left, right, center, and

surround left/right channel audio signals are again fed to the appropriate individual

speakers, as would normally be done with a typical 7.1 surround speaker configuration.

Typically, assuming that Surround EX playback is properly selected (e.g., a Surround EX

flag is present), the surround back left and right speakers 624, 625 both receive and respond directly to the surround rear channel audio signal. The central rear sound

image produced by the closely spaced surround back left and right speakers 624, 625

from the monaural signal (i.e., the surround rear channel audio signal) is stable over a

much wider area, as compared to widely spread surround back left and right speakers,

and has significantly less "coloration" or unnaturalness than the audio sound produced

by such widely spaced rear surround speakers.

[0053] In some instances, such as, for example, where the 6.1 Surround

soundtrack is matrix-encoded, or where Surround EX processing is not invoked for

whatever reason, a somewhat different type of playback may be experienced. In such a

case, the sound system may effectively treat the soundtrack as a 5.1 soundtrack, and

may send to the surround back left and right speakers 624, 625 the surround left and

right channel audio signals, which may be mixed with at least some portion of the

monaural channel information (if the soundtrack is matrix encoded). According to a

preferred sound system as disclosed herein, the surround back left and right speakers

624, 625 both receive and respond directly to the surround rear channel audio signal, if

such information is present, and, after suitable sound processing, as further described

herein, to the surround left/right channel audio signals. FIG. 7 illustrates the playback of

a 6.1 surround-encoded soundtrack in the sound system 600 of FIG. 6 in such a

situation. As shown in FIG. 7, a wide monaural sound image is projected from the

surround back left and right speakers 624, 625. The surround left and right channel

audio signals are fed to both the surround left and right speakers 614, 61 5, and to the

surround back left and right speakers 624, 625 after sound processing as further

described later herein. [0054] When the sound system 600 of FIG. 6 is used to play a soundtrack

recorded in 5.1 surround format, the various left, right and center channel audio signals

are fed to the appropriate individual speakers, as would normally be done with a typical

7.1 surround speaker configuration. Preferred operation with respect to the surround

left and right speakers 614, 615 and surround back left and right speakers 624, 625

depends in part upon the nature of the surround left/right channel audio signals. When

the surround left/right channel audio signals are monaural in nature, the sound system

600 preferably uses adaptive de-correlation to provide a de-correlated signal for the side

surround speakers 614, 615, and provides a direct feed to the surround back left and

right speakers 624, 625 to produce a superior rear central image. However, when the

surround left/right channel audio signals are stereo in nature, the surround left/right

channel audio signals are fed directly to the surround left and right speakers 614, 615

without adaptive de-correlation, and, if desired, after suitable sound processing as

further described herein, to the surround back left and right speakers 624, 625. The

surround left and right channel audio signals are processed such that the apparent rear

sound image size is increased, and its stability is improved at off-axis listening positions.

The appropriately apportioned and summed output of the two side surround speakers

614, 615 and the two surround back speakers 624, 625 creates a near-continuous rear-

half sound field, thereby improving the sound experience for listeners over a wider area.

[0055] FIG. 12 is a simplified diagram conceptually illustrating playback of a 5.1

surround format soundtrack in the sound system 600 of FIG. 6, when the sound system

600 is configured to apply the surround left and right channel audio signals 121 1 , 1212

to the rear surround speakers 1224, 1 125. As illustrated in FIG. 12, when the surround left and right channel audio signals 121 1 , 1212 are monaural, adaptive de-correlation

processing (as represented by blocks 1271 and 1272) is activated, and when they are

stereo in nature, adaptive sound processing for the rear surround speakers 1224, 1225

(as represented by block 1201 ) is activated.

[0056] More generally, the techniques described herein are capable of producing

potentially improved sound for a stereo signal in connection with a speaker

configuration that includes two speakers placed in close proximity. Whenever a stereo

signal from any encoded program (e.g., surround sound or multi-channel soundtrack), or

any audio product or source, is fed directly to the appropriate right and left speakers

(e.g., left and right surround speakers) and, after suitable sound processing as further

described herein, to the pair of speakers placed in close proximity (e.g., surround back

speakers). The pair of closely spaced speakers is thereby capable of generating a sound

image of improved stability and quality over a wider area, thus enlarging the optimum

listening area and providing greater satisfaction to the listeners.

[0057] Further details regarding preferred sound processing for closely spaced

speakers (such as rear surround speakers 624, 625 in FIG. 6) will now be described.

FIG. 8 is a generalized block diagram of a sound processing system 800 in accordance

with on embodiment as disclosed herein, as may be used, for example, in connection

with the speaker configuration illustrated in FIG. 6, or more generally, in any sound

system which utilizes multiple audio channels to provide stereo source signals. As

shown in FIG. 8, a left audio signal 81 1 and right audio signal 812 are provided to a

sound processor 810, and then to a pair of closely spaced speakers 824, 825. The left

audio signal 81 1 and right audio signal 812 may also be provided to left and right side (surround or non-surround) speakers, not shown in FIG. 8. In a preferred embodiment,

the sound processor 810 acts to "spread" the sound image produced by the two closely

spaced speakers 824, 825 by employing a cross-cancellation technique wherein a

right audio signals 81 1, 812. The resulting difference signal is scaled, delayed (if

necessary) and spectrally modified before being added to the left channel and, in

opposite polarity, to the right channel. The spectral modification to the difference

channel preferably takes the form of a low-frequency boost over a specified frequency

range, in order to restore the correct timbral balance after the differencing process which

causes a loss of bass when the low-frequency signals in each channel are similar. The

effect of the sound processor 810 is to enlarge the perceived area and stability of the

sound image produced by the speakers 324, 325, and provide an effect of stereo sound

despite the close proximity of the speakers 324, 325.

[0058] FIG. 9-1 is a more detailed diagram of a sound processing system 900 in

accordance with various principles as disclosed herein, and as may be used, for

example, in connection with the sound system 600 illustrated in FIG. 6, or more

generally, in any sound system which utilizes multiple audio channels to provide stereo

source signals. In the sound processing system 900 of FIG. 9-1 , a left audio signal 91 1

and right audio signal 912 are provided from an audio source, and may be fed to other

speakers as well (not shown in FIG. 9-1 ). A difference between the left audio signal 91 1

and right audio signal 912 is obtained by, e.g., a subtractor 940, and the difference

signal 941 is fed to a spectral weighting filter 942, which applies a spectral weighting

(and possibly a gain factor) to the difference signal 941. The characteristics of the spectral weighting filter 942 may vary depending upon a number of factors including

the desired aural effect, the spacing of the speakers 924, 925 with respect to one

another, the taste of the listener, and so on. The output of the spectral weighting filter

942 may be provided to a phase equalizer 945, which compensates for the phase

shifting caused by the spectral weighting filter 942 (if non-linear).

[0059] In FIG. 9-1 , the output of the phase equalizer 945 is provided to a cross-

cancellation circuit 947. The cross-cancellation circuit 947 also receives the left audio

signal 91 1 and right audio signal 912, as adjusted by phase compensation circuits 955

and 956, respectively. The phase compensation circuits 955, 956, which may be

embodied as, e.g., all-pass filters, preferably shift the phase of their respective input

signals (i.e., left and right audio signals 91 1 , 912) in a complementary manner to the

phase shifting performed by the phase equalizer 945 (in combination with the phase

distortion caused by the spectral weighting filter 924), such that the phase characteristic

of the central channel is substantially 180° degrees out-of-phase with the phase

characteristic of the left and right channels over the frequency band of interest. The

cross-cancellation circuit 947, which may include a pair of summing circuits (one for

each channel), then mixes the spectrally-weighted, phase-equalized difference signal,

after adjusting for appropriate polarity, with each of the phase-compensated left audio

signal 91 1 and right audio signal 912. The perceived width of the soundstage produced

by the pair of speakers 924, 925 can be adjusted by varying the gain of the difference

signal path, and/or by modifying the shape of the spectral weighting filter 942.

[0060] FIG. 9-2 is a diagram of a sound processing system 900' in general

accordance with the principles and layout illustrated in FIG. 9-1 , further showing typical examples of possible transfer function characteristics for certain processing blocks. As

with FIG. 9-1 , in the sound processing system 900' a left audio signal 91 1 ' and a right

audio signal 912' are provided from an audio source (not shown), and a difference

signal 941 ' is obtained representing the difference between the left audio signal 91 1 '

and the right audio signal 912'. The difference signal 941 ' is fed to a spectral

weighting filter 942', which, in the instant example, applies a spectral weighting to the

difference signal 941 ', the characteristics of which are graphically illustrated in the

diagram of FIG. 9-2. A more detailed graph of the transfer function characteristics (both

gain and phase) of the spectral weighting filter 942' in this example appears in FIG. 15-

1 . As shown therein, the spectral weighting filter 942' is embodied as a first-order shelf

filter with a gain of 0 dB at low frequencies, and turn-over frequencies at approximately

200 Hz and 2000 Hz. If desired, the gain applied by gain/ amplifier block 946' can be

integrated with the spectral weighting filter 942', or the gain can be applied

downstream as illustrated in FIG. 9-2. In any event, as previously noted, the turnover

frequencies, amount of gain, slope, and other transfer function characteristics may vary

depending upon the desired application and/or overall system characteristics.

[0061] A phase equalizer 945' is provided in the center processing channel, and

addition phase compensation circuits 955' and 956' in the right and left channels, to

ensure that the desired phase relationship is maintained, over the band of interest,

between the center channel and the right and left channels. As shown graphically in

both FIG. 9-2 and in more detail in FIG. 15-1 , the spectral weighting filter 942' in the

instant example causes a phase distortion over at least the 200 Hz to 2000 Hz range.

The phase equalizer 945' provides no gain, but modifies the overall frequency characteristic of the center channel. The phase compensation circuits 955' and 956'

likewise modify the phase characteristics of the left and right channels, respectively.

The phase compensation is preferably selected, in the instant example, such that the

phase characteristic of the center channel (that is, the combined phase effect of the

spectral weighting filter 942' and the phase equalizer 945') is approximately 180°

out-of-phase with the phase characteristic of the left and right channels, over the

frequency band of interest (in this example, over the 200 Hz to 2000 Hz frequency

band). At the same time, the phase characteristic of the left and right channels are

preferably remains the same, so that, among other things, monaural signals being played

over the left and right channels will have identical phase processing on both channels

(and thus maintain proper sound characteristics). Therefore, the phase compensation

circuits 955' and 956' preferably are configured to apply identical phase processing to

the left and right channels.

[0062] More detailed graphical examples of gain and phase transfer functions

(with the gain being zero in this case when the components are embodied as all-pass

filters) are illustrated for the center channel phase equalizer 945' in FIG. 15-2 and for

the left and right channel phase compensation circuits 955', 956' in FIG. 1 5-3. In

these examples, the phase equalizer 945' is embodied as a second-order all-pass filter

(with F = 125 Hz and Q = 0.12), and the phase compensators 955', 956' are each

embodied as second-order all-pass filters (with F = 3200 Hz and Q = 0.12). A higher

Q value may be used to increase the steepness of the phase drop-off, reducing the extent

to which the center channel is out-of-phase with the left and right channels at low

frequencies (thus minimizing the burden imposed upon the speakers 924', 925'). [0063] FIG. 1 1 illustrates another implementation of the sound system 900

shown in FIG. 9-1 , where all-pass filters 1 157, 1 158 are used in phase compensation

blocks 1 155 and 1 156, respectively, to provide phase equalization and/or

compensation. In FIG. 1 1 , elements labeled with reference numerals "1 1 xx" generally

correspond to their counterparts labeled "9xx" in FIG. 9-1.

[0064] FIG. 10 is another diagram of a sound processing system 1000, in

accordance with the general principles explained with respect to FIGS. 3 and 9,

illustrating representative transfer functions according to an exemplary embodiment as

described herein. In the sound processing system 1000 shown in FIG. 10, input audio

signals X1 and X2 (e.g., left and right audio signals) are processed along two parallel

paths, and the resultants individually summed together and provided as output signals

Y1 and Y2, respectively (which may be fed to a pair of speakers, e.g., left and right

speakers located in close proximity). A difference between the input audio signals X1

and X2 is obtained from a subtractor 1040, which provides the resulting difference

signal 1040 to a processing block 1060 having a transfer function -B. The first input

audio signal X1 is also fed to a processing block 1055 having a transfer function A, and

the output of processing block 1055 is added together with the output of processing

block 1060 and fed as the first output signal Y1 . Likewise, the second input audio signal

X2 is fed to a processing block 1056 having a transfer function -A (i.e., the inverse of

the transfer function A of processing block 1055), and the output of processing block

1056 is inverted and added together with the inverted output of processing block 1060,

then fed as the second output signal Y2. The overall relationship between the inputs

and the outputs of the FIG. 10 sound processing system 1000 can be expressed as:

In a preferred embodiment, the transfer function -B of processing block 1060 represents

the combined transfer functions of a spectral weighting filter of desired characteristics

and a phase equalizer, such as illustrated by the difference path in the sound processing

system 400 of FIG. 4. Also in a preferred embodiment, the transfer functions A and -A

of processing blocks 1055 and 1056, respectively, each represent the transfer function

of a phase compensation network that performs a complementary phase shifting to

compensate for the phase effects caused by the processing block 1060. The polarities in

FIG. 10 are selected so that appropriate cross-cancellation will be attained.

[0065] In a preferred embodiment, input signals X1 and X2 represent the Z-

transforms of the left and right audio channel inputs, and Y1 and Y2 represent the

corresponding Z-transforms of the left and right channel outputs which feed the pair of

speakers (e.g., left and right speakers) located in close proximity. The transfer functions

A, -A, and B may be represented in terms of z, and are determined in part by the

sampling frequency Fs associated with processing in the digital domain. According to a

particular embodiment, blocks 1055 and 1056 are each second-order all-pass filters with

f = 3200 Hertz, Q = 0.12, and may, in one example, possess the following transfer

function characteristics based upon representative examples of the sampling frequency [0066] For Fs = 48 KHz,

0.2578123 -0.6780222z^_1 + z^-2

A(z) = l-0.6780222z" -0.2578123z

[0067] ForFs = 44.1 KHz,

_ - 0.2944196 - 0.633509Z^"1 + z^"2 l-0.633509z^_1 -0.2944196Z^-2

[0068] For Fs = 32 KHz,

_A/ -0.4201395 -0.469117z^_1+z^" A(z) =

1-0.4691 Hz^"1 -0.4201395Z^-2

In this particular embodiment, block 1060 may be a first-order shelf having a gain of 0

dB at low frequencies and turn-over frequencies of 200 Hertz and 2 KHz in cascade

with a second-order all pass filter, with f = 125 Hz, Q = 0.12, and may, in one

example, possess the following transfer function characteristics based upon

representative examples of the sampling frequency Fs:

[0069] For Fs = 48 KHz,

„, . _ 0.1116288 -0.0857871Z^-1 0.8723543 -1.872104z^_1 +z^" B(z) = G x l-0.9741583z^_1 l-1.872104z^_1 + 0.8723543z -2 [0070] For Fs = 44.1 KHz,

„, „ 0.1126427 - 0.0845478Z^"' 0.8618468 - 1.861552z^"A z^" B(z) = G x x

1 - 0.9719051Z^'1 l -1.861552z^_1 + 0.8618468z^"

[0071] For Fs = 32 KHz,

„, „ 0.1173312 - 0.0788175z^~' 0.814462 - 1.813915z^"A z^"2 B(z) = G x x

1 - 0.9614863Z^"1 l - 1.813915z^-1 + 0.814462z^"2

A gain factor may also be included in block 1060, or else may be provided in the same

path but as a different block or element. The gain may be determined for a particular

application by experimentation, but is generally expected to be optimal in the range of

10-15 dB. In one embodiment, for example, the gain factor is 12 dB.

[0072] FIGS. 13 and 14 are graphs illustrating examples of frequency response

and phase transfer functions for a sound processing system in accordance with FIG. 10

and having particular spectral weighting, equalization and phase compensation

characteristics. FIG. 13 illustrates a frequency response transfer function 1302 and

phase transfer function 1305 for — B/A, which represents the transfer function of the

difference channel (-B) and the first input channel (X1) with + 12 dB of gain added. As

shown in FIG. 13, the frequency response transfer function 1302 exhibits a relatively flat

gain in a first region 1320 of bass frequencies (in this example, up to about 200 Hertz), a

decreasing gain in a second region 1321 of mid-range frequencies (in this example, from

about 200 Hertz to about 2 KHz), and then a relatively flat gain again in a third region 1322 of high frequencies (in this example, above 2 KHz). The phase response transfer

function 1305 indicates that in the second region 1321 of mid-range frequencies (i.e.,

between about 200 Hertz and 2 KHz) the output signal remains substantially in phase.

[0073] FIG. 14 illustrates a frequency response transfer function 1402 and phase

transfer function 1405 for — B/—A, which represents the transfer function of the

difference channel (-B) and the first input channel (X2) with + 12 dB of gain added. In

FIG. 14, as with FIG. 13, the frequency response transfer function 1402 exhibits a

relatively flat gain in a first region 1420 of bass frequencies (in this example, up to about

200 Hertz), a decreasing gain in a second region 1421 of mid-range frequencies (in this

example, from about 200 Hertz to about 2 KHz), and then a relatively flat gain again in

a third region 1422 of high frequencies (in this example, above 2 KHz). The phase

response transfer function 1405 indicates that in the second region 1421 of mid-range

frequencies (i.e., between about 200 Hertz and 2 KHz) the output signal is substantially

inverted in phase (i.e., at 180 degrees).

[0074] As noted, the output signals Y1, Y2 are preferably provided to a pair of

speakers located in close proximity. The transfer functions A, -A, and B are examples

selected for the situation where the speakers are located substantially adjacent to one

another. However, benefits may be attained in the system 1000 of FIG. 10, or other

embodiments described herein, where the pair of speakers are not immediately

adjacent, but are nevertheless in close proximity with one another.

[0075] FIG. 16 is a diagram of a sound processing system 1600 in accordance

with an alternative embodiment as described herein, employing a linear spectral

weighting filter. In the sound processing system 1600 of FIG. 16, a left audio signal 161 1 and right audio signal 1612 are processed to derive a pair of processed audio

signals 1648, 1649 which are applied to a pair of closely spaced speakers 1624, 1625

(e.g., left and right speakers). The left and right audio signals 161 1 , 1612 are operated

upon by a subtractor 1640, which outputs a difference signal 1641 representing a

difference between the left and right audio signals 161 1 , 1612. The difference signal

1641 is fed to a spectral weighting filter 1642 having a linear phase characteristic. The

spectral weighting filter 1642 may have frequency response characteristics in general

accordance, for example, with the transfer function illustrated in FIG. 7A or 7B.

Because the spectral weighting filter 1642 has a linear phase characteristic, phase

equalization and compensation are not necessary. Therefore, the output of the spectral

weighting filter 1642 may be provided directly to a cross-cancellation circuit 1646,

which then mixes the spectrally weighted signal with each of the left and right audio

channels before applying them to the speakers 1624, 1625. To compensate for the

delay caused by the spectral weighting filter 1642, delay components 1655 and 1656

may be added along the left and right channel paths, respectively. The delay

components 1655, 1656 preferably have a delay characteristic equal to the latency of

the linear spectral weighting filter 1642.

[0076] The amount of cross-cancellation provided by the sound processing in

various embodiments generally determines the amount of "spread" of the sound image.

If too much cross-cancellation is applied, then the resulting sound can seem clanky or

echoey. If, on the other hand, too little cross-cancellation is applied, then the sound

image may not be sufficiently widened or stabilized. [0077] The pair of speakers (e.g., speakers 824 and 825 in FIG. 8, or closely

spaced speakers in other embodiments described herein) which receive the sound

processed information are preferably located immediately adjacent to one another;

however, they may also be physically separated while still providing benefits of

enlarged sound image, increased stability, and so on. Generally, the maximum

acceptable separation of the pair of speakers can be determined by experimentation, but

performance may gradually decline as the speakers are moved farther apart from one

another. Preferably, the two speakers are placed no further apart than a distance that is

comparable with the wavelength of the highest frequency that is intended to be radiated

by the speakers. For a maximum frequency of 2 kHz, this separation would correspond

to a center-to-center spacing of about 6 inches between the two speakers. However,

ideally the two speakers are placed immediately next to one another, in order to attain

the maximum benefit from the sound processing techniques as described herein.

[0078] Certain embodiments of the invention may find application in a variety of

contexts other than home theater or surround sound systems. For example,

implementations of the invention may, in some circumstances, be applicable to personal

computer systems (e.g., configured to play audio tracks, multi-media presentations, or

video games with "three-dimensional" or multi-channel sound), automobile or

vehicular audio systems, portable stereos, televisions, radios, and any other context in

which sound reproduction is desired. Certain embodiments may find particular utility in

situations in which possible speaker locations are limited and/or the maximum spacing

between left and right speakers is severely limited, but where two adjacent or closely

spaced speakers could be achieved. For example, the pair of closely spaced left and right speakers may be part of an integrated portable stereo unit, or else may be located

atop or beneath a computer monitor, etc.

[0079] Automobile or vehicular audio systems, in particular, may benefit from

application of the inventive concepts disclosed herein. Audio systems are

commonplace in automobiles and certain other vehicles. Such systems generally utilize

program sources ranging from simple radios to relatively elaborate stereo or multi¬

channel systems with CD and cassette players together with multiple equalizers, pre¬

amplifiers, power amplifiers etc.

[0080] While there is a great variety in the configuration and components of

conventional automotive audio systems, most of them suffer to varying degrees from a

number of persistent problems in providing the highest sound quality. These problems

partially result from the unique sound environment of the automobile when compared

with a good listening room. Among the disadvantages are:

• Much smaller internal volume resulting in a reduced reverberation time and

lower modal density at low frequencies resulting in a lack of ambience and

an uneven bass response .

• The proximity of highly reflective surfaces (such as the windows) to highly

absorbtive areas such as the upholstery or the occupants clothing leads to a

great variability with frequency and head position of the direct to indirect

sound arriving at the listener. Consequently even small changes in head or

seating position can cause significant and undesirable changes in the timbral

quality of the music. • The listening positions are necessarily restricted to the seating positions

provided (usually 4 or 5) and all of these are very asymmetrically placed with

respect to the speaker positions. Space is always at a premium within a car

interior and as a result the speakers are often placed in physically convenient

positions, that are nevertheless very poor from an acoustic point of view,

such as the foot wells and the bottom of the front and rear side doors. As a

result the listener's head is always much closer to either the left or right

speaker leading directly large inter-channel time differences and different

sound levels due to the 1/r law.

• Additionally, the angles between the axes from the listeners ears to the axes

of symmetry of the left and right speakers is quite different for each occupant.

The perceived spectral balance is different for each channel due to the

directional characteristics of the drive units. Masking of one or more speakers

by the occupants clothes or legs can often result in the attenuation of the

mid- and high- frequencies by as much as 10dB.

All of the above adversely impact the ability to produce high quality stereo

reproduction, which ideally has the following attributes:

• A believable and stable image or soundstage resulting from the listener being

nearly equidistant from the speakers reproducing the left and right channels and

a sufficiently high ratio of direct-to-indirect sound at the listener's ears .

• A smooth timbral balance at all the listening positions . • A sense of ambience resulting from a uniform soundfield.

[0081] Some features are provided in automobile audio systems which can

partially mitigate the aforementioned problems. For example, an occupant can

manually adjust the sound balance to increase the proportional volume to the left or

right speakers. Some automobile audio systems have a "driver mode" button which

makes the sound optimal for the driver. However, because different listening axes exist

for left and right occupants, an adjustment to the balance that satisfies the occupant

(e.g., driver) on one side of the automobile will usually make the sound worse for the

occupant seated on the other side of the automobile. Moreover, balance adjustment

requires manual adjustment by one of the occupants, and it is generally desirable in an

automobile to minimize user intervention.

[0082] Another modification made to some automobile audio systems is to

provide a center speaker, which reduces the image instability that occurs when the

listener is closer to either the left or right speaker when both are reproducing the same

mono signal, with the intention of producing a central sound image. Other potential

approaches which might be taken in an attempt to mitigate the foregoing automotive

sound problems include adding more speakers in a greater variety of positions (e.g., at

the seat tops). While such techniques can sometimes provide a more pleasing effect,

they cannot provide stable imaging as the problems associated with asymmetry

described above still remain. The considerable additional cost of such design

approaches is usually undesirable in the highly cost sensitive and competitive automotive industry. Moreover, as previously noted, space is usually at a premium in

the automobile interior, and optimal speaker positions are limited.

[0083] According to one or more embodiments as disclosed herein which are

designed to overcome one or more of the foregoing problems, drawbacks, or

disadvantages, a preferred automobile sound reproduction system comprises a pair of

speakers placed close together and located in the front of the console or dashboard with

their geometric center on (or as near as possible to) the central axis of symmetry of the

vehicle. The sound reproduction system further preferably comprises a sound processor

which provides audio signals to the pair of speakers. Because the left and right center

speakers are effectively adjacent to one another, the difference in time of arrival of the

sound information becomes minimal, and the relative volume level of both speakers

remains approximately the same. Moreover, both the right and left occupant experience

approximately the same volume level from the center pair of speakers and the ratio of

direct to indirect sound is minimized.

[0084] According to a preferred embodiment, the sound processor acts to

"spread" the sound image produced by the two closely spaced speakers by employing

a cross-cancellation technique in which, for example, the cancellation signal is derived

from the difference between the left and right channels. The resulting difference signal

is scaled, delayed (if necessary) and spectrally modified before being added to the left

channel and, in opposite polarity, to the right channel. The spectral modification to the

difference channel preferably takes the form of a low-frequency boost over a specified

frequency range, in order to restore the correct timbral balance after the differencing

process which causes a loss of bass when the low-frequency signals in each channel are similar. Additional phase-compensating all-pass networks may be inserted in the

difference channel to correct for the extra phase shift contributed by the usually

frequency range.

[0085] Alternatively, a linear-phase network may be employed to provide the

independent delay to both left and right channels.

[0086] In various embodiments, the pair of central speakers may be placed in a

common enclosure with a central dividing partition that is inserted into or else integral

with the front console or dashboard of the automobile. In certain embodiments, the

center speakers may be placed with their diaphragms facing down and in close

proximity to a rigid reflecting surface such that substantially all of the sound energy is

directed forward, towards the listener, via an arrow slot in the enclosure. The resultant

radiating system provides the dual benefit of occupying less dashboard area, where

space is always at a premium, and possessing a very wide directional characteristics due

to the slot having dimensions that can be made very small with respect to the

wavelength the radiated sound.

[0087] The use of a pair of central speakers in conjunction with sound processing

to provide improved sound quality may be employed in more than one location in the

automobile, to extend the foregoing concepts further. Thus, for example, a pair of rear

central speakers with similar sound processing may be added in the rear of the vehicle, for example in the center above the rear seatback, for use in the play back of program

with discretely encoded or simulated multi-channel surround sound. Likewise, for

larger vehicles (e.g., a limousine), a pair of front central speakers may be used in both

the driver compartment and the passenger compartment, the latter having applications

for rear seat video presentations of films or music videos having multi-channel surround

sound.

[0088] FIG. 1 7 is a diagram of a preferred automobile sound system 1 700 in

accordance with one or more embodiments as disclosed herein. In FIG. 17, two

speakers 1 714, 1 715 are positioned in close proximity to one another, and receive and

respond to audio signals 1 732 and 1 733, respectively, from a sound processor 1 708.

The speakers 1 714, 1 715 are preferably left and right speakers, may (but need not) be

nominally identical, may be separated by a distance ΔD from one another as further

described herein, and may be of any suitable size and type provided that they fit within

the size constraints of the available automotive compartment(s) or other space. Further,

the speakers 1 714, 1 715 may be positioned along or near the central axis of the interior

of the automobile, such as, for example, in the center console, or atop the center of the

dashboard, or in a central island between the driver and passenger seats.

[0089] The sound processor 1 708 receives audio input signals 1 702 and 1 703

from a suitable audio signal source 1 705, from any typical automotive audio

components (e.g., CD player, cassette player, radio, etc.) that may be included

therewith. The audio input signals 1 702, 1 703 may be derived from any audio product,

including any prerecorded medium (such as a cassette, CD, or DVD), any digital audio

file, or any wireless (e.g., radio) broadcast received by the audio system. The sound processor 1 708 preferably processes the stereo sound signals 1 702, 1 703 according to

techniques described in more detail herein, and provides the processed signals 1 732,

1 733 (after any desired amplification or level shifting) to the pair of closely spaced

speakers 1 714, 1 715. The stereo signals 1 702, 1 703 may also optionally be fed, either

directly or via the sound processor 1 718 (if certain additional or complementary sound

processing is desired) to additional speakers, if any, such as left speaker 1724 and right

speaker 1 725 shown in FIG. 1 7.

[0090] In a preferred embodiment, the sound processor 1 708 acts to effectively

"spread" the sound image by, in a broad sense, taking the difference between the two

audio channels 1 702, 1 703, spectrally modifying the intermediate difference signal, and

then, after scaling, adding it in appropriate polarity to the left and right channels. When

the speakers 1714, 1715 are placed close together, side-by-side, the resulting

phenomenon causes an apparent expansion of the stereo sound image despite the fact

that the speakers 1 714, 1 71 5 are located in close proximity.

[0091] The bass lifting or spectral weighting carried out by the sound processor

1 708 may cause phase shifting, which can be compensated for using phase

equalization. Complementary phase compensation can be provided along each of the

audio channels 1 702, 1 703 prior to mixing (i.e., cross-cancellation) so that the left and

right audio channels 1 702, 1 703 are substantially in phase with the spectrally modified

difference signal. Where the bass lifting or spectral weighting is accomplished using

linear phase filtering, however, no phase equalization may be needed or desired,

although equal delays are preferably added to both the left and right audio channel

paths in order to compensate for the additional delay produced by the linear-phase equalizer in the difference channel. The primary purpose of the speakers 1714, 1715 is

not necessarily to provide only monaural information, as with a conventional centrally

positioned speaker (although monaural information may be fed to the speakers 1 714,

1 71 5), but rather, when combined with suitable mid- to high-frequency processing and

mixing (via the sound processor 1 708), to produce a symmetrical spreading of stereo

information, which results in a better stereo presentation for both left and right

occupants regardless of the listening axis.

[0092] Because the two center speakers 1 714, 1 71 5 are closely spaced with

respect to one another, the difference in time of arrival of the sound information to a

given listener becomes minimal, and the relative volume level of both speakers, as

perceived by a given listener, is approximately the same. Moreover, both the right and

left occupant will generally experience approximately the same volume level from the

center pair of speakers 1 714, 1 715. In the event that the closely spaced speakers are

unable to radiate potentially large out-of-phase, low-frequency components resulting

from the cross-cancellation process, the very low frequencies can be isolated by means

of a low-pass filter and directed to a separate sub-woofer, while a corresponding high-

pass filter may be utilized to prevent high-level, low-frequency signals from overloading

the smaller speakers. For any bass audio components that might be difficult for the

relatively small center speakers 1 714, 1 715 to handle, the left and right audio channels

1 702, 1 703 can be fed to left and right bass speakers 1 721 and 1 722, respectively,

possibly in conjunction with attenuation at mid/high frequencies and/or boosting at

low/bass frequencies as provided by the sound processor 1 708 or any other suitable

means. In embodiments in which mid/high frequencies are output by the center pair of closely spaced speakers and bass or low frequencies are output by left and right door-

mounted speakers, advantages in amplifier efficiency may be achieved because less

power will generally be needed to obtain higher volume levels.

[0093] When the speakers 1 714, 1 71 5 are placed in the front console or

dashboard, or otherwise on or near the center axis of the automobile, they may (but

need not be) mounted at a sufficient height so as to have a relatively unobstructed

pathway to the listeners' ears, thus eliminating muffling or damping associated with

obstructions such as seats and occupant bodies. In such embodiments, the speakers

1 714, 1 71 5 are located at an ideal or at least preferably acoustical position, being less

obstructed and less reflected, and allowing more space for the sound to unfold.

[0094] The pair of speakers (e.g., speakers 1 714 and 1 715 in FIG. 1 7) which

receive the sound processed information are preferably located immediately adjacent to

one another; however, they may also be separated by some distance ΔD while still

providing benefits of enlarged sound image, increased stability, and so on. Generally,

the farthest maximum separation of the speakers 1 714, 1 71 5 can be determined by

experimentation, but performance may gradually decline as the speakers 1714, 171 5 are

moved farther apart from one another. Preferably, the pair of speakers 1 714, 171 5 are

placed no further apart than a distance that is comparable with the wavelength of the

highest frequency that is intended to be radiated by the speakers 1 714, 1 715. For a

maximum frequency of 2 kHz, this would correspond to a center-to-center spacing of

about 6 inches between speakers 1 714 and 1 715. However, ideally the speakers 1 714,

1 71 5 are placed immediately next to one another, in order to attain the maximum

benefit from the sound processing techniques as described herein. [0095] When the pair of speakers 1 714, 1 715 are closely spaced, they may be

placed in a common enclosure, with a central (preferably airtight) dividing partition, that

may, for example, may be inserted into or else integral with the front console or

dashboard of an automobile, or placed elsewhere near the central axis of the

automobile. FIGS. 20-1 , 20-2, and 20-3 illustrate one example of an enclosure 2001,

particularly suited to applications where space is limited, housing a pair of speakers

2014, 2015 which can receive and respond to sound processed signals from left and

right audio channels in accordance with the various techniques described herein. FIG.

20-1 is a front cut-away view of the exemplary speaker enclosure 2001 housing the pair

of speakers 2014, 2015; FIG. 20-2 is a top cross-sectional view of the speaker enclosure

2001 shown in FIG. 20-1 ; and FIG. 20-3 is an oblique front view of the speaker

enclosure 2001 shown in FIGS. 20-1 and 20-2. As shown perhaps best in FIG. 20-3, the

speaker enclosure 2001 in this example is preferably substantially rectangular in shape,

and is preferably designed with dimensions so as to slide into or otherwise fit within a

standard "DIN " slot (approximately 8" by 2 ¹/₂") in the front console space of an

automobile. The speaker enclosure 2001 may include a front panel 2032, a pair of side

panels 2030, a top panel 2035, a bottom panel 2039, and possibly a back panel 2031.

To achieve isolation between the two speakers 2014, 2015, an interior wall 2016 such

as illustrated in FIG. 20-1 and 20-2 may be placed between the speakers 2014, 2015,

thus creating two separate speaker chambers, one housing each of the two speakers

2014, 2015.

[0096] The pair of speakers 2014, 2015 may be pointed directly frontwards;

however, in the instant example, the speakers 2014, 201 5 are oriented downwards, as illustrated in FIG. 20-1. When so oriented, a slot 2019 may be located at the bottom of

the speaker enclosure 2001 , to allow the sound from the speakers 2014, 2015 to radiate

outwards towards the direction of the listeners in the automobile. Effectively, then, the

speakers 2014, 2015 only take up an amount of surface space corresponding to the size

of the slot 2019. In an automobile environment, front console/dash space is typically

extremely valuable since it is scarce, and thus the ability to position two speakers 2014,

2015 in the front console/dash while minimizing the amount of surface space consumed

can be extremely advantageous. Audio system controls/display(s) or other conventional

console accouterments (controls, LCD or other displays, air vents, etc.) can be attached

to or integral with the front panel 2032 of the speaker enclosure 2001 , so the available

surface space on the front panel 2032 is valuably utilized.

[0097] Moreover, when so oriented, the speakers 2014, 2015 may be potentially

larger in size (assuming console space is limited); for example, each speaker may be

about 4" (for a total of 8" across collectively), which fits into the 8" DIN space, whereas

the speakers would otherwise generally have to be under 2 ¹/-;" to fit within the DIN

space, if oriented in a frontwards direction. The ability to place larger speakers in the

center speaker unit allows better bass reproduction and, hence, reduces or potentially

dispenses with the need for side (e.g., door-mounted) bass speakers to carry the bass

information of the left and right channels.

[0098] The effect of orienting the speakers 2014, 2015 in a downward direction

is conceptually illustrated in FIG. 20-4, which shows a generic speaker 2090 pointing

downwards towards a surface 2091. The sound output from the speaker 2090 radiates

outward from the centerpoint along the surface 2091 in essentially all directions (i.e., a complete 360-degree circle). Thus, as shown in FIGS. 20-1 and 20-3, a slot 2019 is

preferably located at the bottom of the speaker enclosure 2001 , to allow the sound from

the speakers 2014, 2015 to radiate outwards towards the direction of the listeners in the

automobile. A layer of insulation 2012 (e.g., foam) preferably matching the outer

contours of the speakers 2014, 2015, as illustrated in FIG. 20-2, may be placed within

the speaker enclosure 2001 , so that the sound does not reflect on the back panel 2031

(if any) of the speaker enclosure. In the resulting speaker enclosure configuration, sound

emanating from the speakers 2014, 2015 is cleanly projected through the slot 2019 to

the listeners in the automobile.

[0099] In an alternative embodiment, the speakers 2014, 2015 might be directed

upwards instead of downwards, with the slot 2019 being located at the top of the

speaker enclosure 2001 , to achieve a similar effect. The speakers 2014, 2015 may

alternatively be positioned sideways, either facing towards are away from each other,

with a pair of slots (one for each of the speakers 2014, 201 5) being vertical in

orientation rather than horizontal, as with slot 2019. In such an embodiment, the

speaker enclosure may be taller but narrower in size.

[0100] In some circumstances, high frequencies (such as over 2 KHz) might

become lost or reduced in the speaker enclosure configuration illustrated in FIGS. 20-1

through 20-3. Therefore, one or more additional speakers 201 7 of small size (e.g.,

tweeters) may be advantageously placed above the "bell" of the speakers 2014, 2015

and in the front panel 2032 of the speaker enclosure 2001 , to radiate the higher

frequencies. [0101] While the speaker enclosure 2001 shown in FIGS. 20-1 through 20-3 has

certain advantages for placement in a standard DIN space of an automobile, it should be

understood that the closely spaced speakers 1 714, 1 71 5, whether or not contained in a

speaker enclosure 2001 , may be positioned in other areas of the automobile as well,

such as atop the front dashboard, above the rear seatback, or in a center console or

island located between the front seats or between the front and back seats. Preferably,

the closely spaced speakers 1 714, 1 715 are located on or near the center axis of the

automobile, so as to provide optimal sound quality evenly to occupants on both sides.

[0102] Because of space constraints within an automobile, the centrally located

speakers (e.g., speakers 1 714, 1 71 5 in FIG. 1 7) may be of limited size. Smaller

speakers, however, tend to suffer losses at low frequencies. To compensate for the loss

of low frequency components where the central pair of speakers are small, left and right

bass speakers (e.g., speakers 1 724, 1 725) may be provided in a suitable location - for

example, built into the automobile doors. The left and right audio channels fed to the

left and right door speakers can be processed to attenuate the mid/high frequencies

and/or boost the bass audio components,. Providing bass frequencies through the door

speakers will not destroy the stereo effect of the mid/high frequencies provided by the

central pair of speakers, since it is well known that frequencies below about 100 Hz are

not normally localized by the human listener.

[0103] In addition, as previously noted, a sub-woofer may be added in a suitable

location within the automobile to further enhance very low frequency bass audio

components. The sub-woofer may be located, for example, in the rear console of the

car above the rear seatback, or in any other suitable location. [0104] In certain situations, passengers in the back seat area of the automobile

may actually experience even better sound quality than the front occupants, because

they are farther away from the stereo source (that is, the pair of central front speakers).

Various modifications may be made to provide even further improved sound for the

front occupants as well. For example, a similar pair of closely spaced speakers to those

placed in the front console or area can also be placed in the rear of the automobile, for

example, atop the rear seatback on or in the rear parcel shelf. The same signals that are

used to feed the front pair of closely spaced speakers can be used to feed the rear pair of

closely spaced speakers. If desired, a speaker enclosure 2001 , such as shown in FIGS.

20A - 20C, containing the pair of closely spaced speakers may be placed in the rear of

the vehicle to house these rear speakers.

[0105] FIG. 19-1 is a simplified top view of an automobile 1900 illustrating an

example of placement of a pair of closely spaced speakers 1905 (whether or not in a

speaker enclosure) in the front section of the automobile 1900 (e.g., in the front console

or the front dash), with the addition of two door-mounted speakers 1907, 1908 for, e.g.,

providing added bass or low frequency audio components. FIG. 19-2 illustrates an

example similar to FIG. 19-1 , but adding a pair of closely spaced speakers 1930

(whether or not in a speaker enclosure) in the rear of the automobile 1920. FIG. 19-3

illustrates an example of placement of speakers in a large vehicle such a limousine, with

separate driver and passenger compartments. In the driver compartment 1941 , the

layout is similar to FIG. 19-1 , with a pair of closely spaced speakers 1945 in the front

area (e.g., console, dash, or the like) of the vehicle 1940, and pair of door-mounted left

and right speakers 1947, 1948. In the passenger compartment 1942, the layout is similar to FIG. 19-2, with two pairs of closely spaced speakers 1955, 1960, one in the

front area and one in the rear area of the passenger compartment 1942, with a pair of

right and left door-mounted speakers 1957, 1958 also. Of course, in any of these

examples, any number of additional speakers and audio components may be added

based upon individual need and preference, subject to spatial limitations of the vehicle,

cost, etc.

[0106] In certain applications, it may be desirable to provide surround sound or

other multi-channel capability in a vehicular automotive system, in conjunction with the

closely spaced speaker arrangement described previously herein. For example, a van or

other large vehicle may have a DVD system which allows digital audio-visual media to

be presented to the passengers of the vehicle, with the sound potentially being played

through the vehicle audio system. In other cases, it may be desirable to allow for

extreme right and left directional sound, which may originate by the existence of left

and right surround channels on the recorded medium, or simply by the presence of an

extreme and intentional disparity in the relative volumes of the left and right channel.

[0107] A block diagram illustrating an example of an automobile sound system

2100 for providing potentially improved extreme right/left sound, in connection with the

pair of closely spaced center speakers 21 14, 21 15, is illustrated in FIG. 21. The system

2100 shown therein operates much as described with the FIG. 1 7 sound system 1 700

with respect to the closely spaced center speakers 21 14, 21 1 5, producing the illusion of

a widened stereo sound image for the occupants of the vehicle. In addition, the sound

system 2100 illustrates the feed of left and right audio signals 2102, 2103 to left and

right door-mounted speakers 2124, 2125, optionally through low pass filters 2181 , 2182, respectively, to emphasize the bass tones. To produce extreme left/right sounds,

which may be difficult with the closely spaced speakers 21 14, 21 1 5, some portion

(dictated by a gain factor k) of the difference between the left and right audio channels

2102, 2103 is mixed in to each of the signals fed into the left and right door mounted

speakers 2124, 2125. When the left and right audio channels 2102, 2103 are close in

amplitude (and frequency), the signals mixed into the left and right channels fed to the

door mounted speakers 2124, 2125 are negligible. However, as the difference between

the left and right audio channels 2102, 2103 grows, the signal fed into the left or right

channel (depending on which channel is larger) grows proportionately (in a linear or

non-linear fashion, depending upon preference). A large difference between the left and

right audio channels 2102, 2103 indicates an extreme left or right sound, which, in the

sound system 2100 of FIG. 21 , can be successfully reproduced in the left or right door-

mounted speakers 2124, 2125.

[0108] Another embodiment, directed to a surround or multi-channel sound

system 1800 as may be utilized in a vehicle, is illustrated in block form in FIG. 18. As

shown therein, the sound system 1800 may include an audio signal source 1805 which

provides a source for left and right audio channels 1802, 1803, which are fed to a sound

processor 1808 which functions in a manner similar to sound processor 1 708 shown in

FIG. 1 7, or various other sound processor embodiments described herein with respect to

closely spaced left/right central speakers. The left and right audio signals 1802, 1803

may, in the present example, comprise front left and front right audio signals of a

surround sound formatted medium. A center audio signal of the surround sound

formatted medium may be mixed into the signals 1832, 1833 provided to the closely spaced speakers 1814, 1815, and may also be provided to additional center speakers

181 7 (e.g., tweeters), if provided. The closely spaced speakers 1814, 1815 and

additional speakers 181 7 may be embodied and arranged, for example, in the form of

the speaker enclosure and arrangement illustrated in FIGS. 20A - 20C. A surround left

and surround right audio channel 1871, 1872 may be fed into surround left and right

speakers 1824, 1825, which may be dipolar or monopolar in nature. The surround left

and right speakers 1824, 1825 may be generally used to provide ambient sound. When

the surround left and right audio channels 1871 , 1872 are monaural in nature, adaptive

decorrelation may be employed, as well understood in the art, to prevent signal

cancellation or similar sound deterioration.

[0109] Left and right speakers 1834, 1835, which may be, e.g., door-mounted

speakers, may be directly fed the left and right audio channels 1802, 1803, or else may

be fed only the bass/low frequency tones, possibly mixed with extreme right or left

sound components, such as described previously with respect to the sound system of

FIG. 21.

[0110] In addition, the sound system 1800 of FIG. 18 may further be provided

with an additional pair of closely spaced speakers (not shown) located at the rear of the

vehicle. The additional pair of closely spaced speakers may be fed the same processed

left and right audio channel signals 1832, 1833 as provided to the front closely spaced

speakers 1814, 1815, or may be fed similarly processed signals derived from the

surround left and right audio channel signals 1871 , 1872, or alternatively, surround

back left and back right audio channel signals (not shown), if the audio product is

encoded in a 7.1 surround or similar multi-channel format. [0111] FIG. 23 is a diagram of a surround or multi-channel sound system 2300

similar to the sound system 1800 shown in FIG. 18, but illustrating the presence of a

pair (right and left) of closely spaced surround back speakers 2394, 2395. In the

embodiment shown in FIG. 23, a rear surround processor 2398 receives as inputs two

surround back channels 2392, 2393 provided from the audio signal source 2305. The

rear surround processor 2398 preferably provides sound processing to the two surround

back channels 2392, 2393 for the closely spaced rear surround speakers 2394, 2395 in

a manner similar to that for the closely spaced front right/ left speakers 2314, 2315,

using any of the sound processing techniques described herein for closely spaced

speakers. The sound processing for the surround back speakers 2394, 2395 need not be

identical to that of the closely spaced front right/left speakers 2314, 231 5, but may differ

in terms of spectral weighting, gain, etc., to account for the fact that the surround back

speakers 2314, 2315 may serve a different purpose to some degree than the front

right/left speakers 2314, 2315.

[0112] The content of the surround back channels 2392, 2393 may depend upon

the format of the encoded audio product. In 5.1 surround format, for example, the

surround back channels 2392, 2393 may be the same as the right and left surround

channels 2371 , 2372. In 6.1 surround format, the surround back channels 2392, 2393

may be the same as the right and left surround channels 2371 , 2372, added or mixed

with the single surround back channel. In 7.1 surround format, the surround back

channels 2392, 2393 are preferably the independent left and right surround back

channels encoded in the audio product. [0113] In various embodiments as described herein, improved sound quality

results from a stereo sound image that has stability over a larger area than would

otherwise be experienced with, e.g., speakers spaced far apart without comparable

sound processing. Consequently, the audio product (e.g., soundtrack) can be enjoyed

with optimal or improved sound over a larger area, and by more listeners who are able

to experience improved sound quality even when positioned elsewhere than the center

of the speaker arrangement. Thus, for example, a home theater surround sound system

or automobile sound system may be capable of providing quality sound to a greater

number of listeners, not all of whom need to be positioned in the center of the speaker

arrangement in order to enjoy the playback of the particular audio product.

[0114] In any of the foregoing embodiments, the audio product from which the

various audio source signals are derived, before distribution to the various speakers or

other system components, may comprise any audio work of any nature, such as, for

example, a musical piece, a soundtrack to an audio-visual work (such as a DVD or other

digitally recorded medium), or any other source or content having an audio component.

The audio product may be read from a recorded medium, such as a DVD, cassette,

compact disc, CD-ROM, or else may be received wirelessly, in any available format,

from a broadcast or point-to-point transmission. The audio product preferably has at

least left channel and right channel information (whether or not encoded), but may also

include additional channels and may, for example, be encoded in a surround sound or

other multi-channel format, such as Dolby-AC3, DTS, DVD-Audio, etc. The audio

product may also comprise digital files stored, temporarily or permanently, in any format used for audio playback, such as, for example, an MP3 format or a digital multi-media

format.

[0115] The various embodiments described herein can be implemented using

either digital or analog techniques, or any combination thereof. The term "circuit" as

used herein is meant broadly to encompass analog components, discrete digital

components, microprocessor-based or digital signal processing (DSP), or any

combination thereof. The invention is not to be limited by the particular manner in

which the operations of the various sound processing embodiments are carried out.

[0116] While examples have been provided herein of certain preferred or

exemplary filter characteristics, transfer functions, and so on, it will be understood that

the particular characteristics of any of the system components may vary depending on

the particular implementation, speaker type, relative speaker spacing, environmental

conditions, and other such factors. Therefore, any specific characteristics provided

herein are meant to be illustrative and not limiting. Moreover, certain components,

such as the spectral weighting filter described herein with respect to various

embodiments, may be programmable so as to allow tailoring to suit individual sound

taste.

[0117] The spectral weighting filter in the various embodiments described herein

may provide spectral weighting over a band smaller or larger than the 200 Hertz to 2

KHz band. If the selected frequency band for spectral weighting is too large, then

saturation may occur or clipping may result, while if the selected frequency band is too

small, then the spreading effect may be inadequate. Also, if cross-cancellation is not

mitigated at higher frequencies, as it is in the spectral weighting filters illustrated in certain embodiments herein, then a comb filter effect might result which will cause nulls

at certain frequencies. Therefore, the spectral weighting frequency band, and the

particular spectral weighting shape, is preferably selected to take account of the physical

limitations of the speakers and electronic components, as well as the overall quality and

effect of the speaker output.

[0118] While certain system components are described as being "connected" to

one another, it should be understood that such language encompasses any type of

communication or transference of data, whether or not the components are actually

physically connected to one another, or else whether intervening elements are present.

It will be understood that various additional circuit or system components may be added

without departing from teachings provided herein.

[0119] In some embodiments, the pair of closely spaced speakers may be forced

to work harder than they would without cross-cancellation, because the cross-mixing of

left and right signals requires that the speakers reproduce out-of-phase sound waves. To

compensate for this effect, it may, for example, be desirable in some embodiments to

increase the size of the amplifier(s) feeding the audio signals to the pair of closely

spaced speakers. In any of the embodiments described herein, the speakers utilized in

the automobile sound system may be passive or active (i.e., with built-in or on-board

amplification capability) in nature. The various audio channels may be individually

amplified, level-shifted, boosted, or otherwise conditioned appropriately for each

individual speaker or pair of speakers.

[0120] While preferred embodiments of the invention have been described

herein, many variations are possible which remain within the concept and scope of the invention. Such variations would become clear to one of ordinary skill in the art after

inspection of the specification and the drawings. The invention therefore is not to be

restricted except within the spirit and scope of any appended claims.

Claims

CLAIMSWhat is claimed is:

1. A sound system, comprising:

a left speaker and a right speaker located in close proximity;

a left channel audio signal;

a right channel audio signal; and

a sound processor receiving as inputs said left channel audio signal and said right

channel audio signal, said sound processor configured to cross-cancel a spectrally

weighted stereo difference signal with said left channel audio signal and said right

channel audio signal prior to applying said left channel audio signal and said right

channel audio signal to said left speaker and said right speaker, respectively.

2. The sound system of claim 1 , wherein said sound processor is configured

to generate a difference signal representing a difference between said left channel audio

signal and said right channel audio signal, and to apply a spectral weighting to said

difference signal thereby generating said spectrally weighted signal.

3. The sound system of claim 2, wherein said sound processor comprises a

subtractor for generating said difference signal.

4. The sound system of claim 2 or 3, wherein said sound processor

comprises a spectral weighting filter for applying said spectral weighting to said difference signal, said spectral weighting filter being characterized by a first filter region

of relatively level gain, a second filter region having a generally decreasing gain with

increasing frequency, and a third filter region of relatively level gain.

5. The sound system of claim 4, wherein said spectral weighting filter is

further characterized by a roll-off from said first filter region to said second filter region

at approximately 200 Hertz.

6. The sound system of claim 5, wherein said spectral weighting filter is

further characterized by a boundary between said second filter region and said third

filter region at approximately 2 KHz.

7. The sound system of claim 2, 3, 4, 5 or 6, wherein said sound processor

comprises a linear filter for applying the spectral weighting to said difference signal.

8. The sound system of any of the preceding claims, wherein said sound

processor further comprises a phase equalizer for equalizing the phase of said spectrally

weighted difference signal prior to cross-cancellation, and a plurality of phase

compensators, having a phase characteristic complementary to said phase equalizer and

said spectral weighting filter over a frequency band of desired cross-cancellation, placed

in series along each of said left channel audio signal and right channel audio signal,

respectively, prior to cross-cancellation.

9. The sound system of claim 8, wherein said phase equalizer comprises a

plurality of all pass filters collectively having a first phase transfer function, and wherein

each of said phase compensators comprises a plurality of all pass filters collectively

having a second phase transfer function complementary to a combined phase

characteristic of said phase equalizer and said spectral weighting filter over a frequency

band of desired cross-cancellation.

10. The sound system of claim 8, wherein said phase equalizer comprises a

second order filter.

1 1. The sound system of any of the preceding claims, wherein said left

channel audio signal comprises a surround left channel audio signal coupled to a

surround left speaker, wherein said right channel audio signal comprises a surround

right channel audio signal which is coupled to a surround right speaker, and wherein

said left speaker and said right speaker comprise a surround back left speaker and a

surround back right speaker, respectively, for utilization in a surround sound stereo

system.

12. The sound system of any of the preceding claims, wherein said sound

processor is implemented in whole or in part in the digital domain.

13. The sound system of any of the preceding claims, wherein said left

speaker and said right speaker are positioned within a distance corresponding to a

wavelength of a highest frequency intended to be radiated by the left and right speakers

14. The sound system of any of the preceding claims, wherein said left

speaker and said right speaker are immediately adjacent.

1 5. The sound system of claim 1 , wherein said sound processor comprises:

a subtractor receiving as inputs said left channel audio signal and right channel

audio signal, and outputting a difference signal representing a difference between said

left channel audio signal and said right channel audio signal;

a spectral weighting filter receiving said difference signal as an input and

outputting a spectrally weighted signal; and

a cross-cancellation circuit for mixing said spectrally weighted signal with said

left channel audio signal and said right channel audio signal, thereby generating a first

speaker signal for said left speaker and a second speaker signal for said right speaker.

16. The sound system of claim 1 5, wherein said spectral weighting filter is

characterized by a first filter region of relatively level gain, a second filter region having

a generally decreasing gain with increasing frequency, and a third filter region of

relatively level gain.

1 7. The sound system of claim 15 or 16, further comprising a phase equalizer

interposed between said spectral weighting filter and said cross-cancellation circuit.

18. The sound system of claim 1 7, further comprising a first phase

compensator interposed between said left channel audio signal and said cross-

cancellation circuit, said first phase compensator having a phase characteristic

complementary to a combined phase characteristic of said phase equalizer and said

spectral weighting filter, and a second phase compensator interposed between said right

channel audio signal and said cross-cancellation circuit, said second phase compensator

having a phase characteristic complementary to said combined phase characteristic.

19. The sound system of claim 18, wherein said phase equalizer comprises a

plurality of all pass filters, and wherein said first phase compensator and said second

phase compensator each comprises a plurality of all pass filters having a substantially

identical phase transfer function.

20. The sound system of claim 1 7, wherein said phase equalizer comprises a

second order filter.

21 . The sound system of any of the preceding claims, wherein said left

speaker and said right speaker are located in a vehicle.

22. The sound system of claim 21 , wherein said left speaker and said right

speaker are positioned substantially on or near a center axis of the vehicle.

23. The sound system of claim 21 or 22, wherein said left speaker and said

right speaker are mounted within a common housing.

24. The sound system of claim 23, wherein said right speaker and left speaker

are oriented in a downwards or upwards direction towards a flat surface of said common

housing, and wherein said common housing defines at least one slot near an edge of

said flat surface allowing sound to emanate from said left speaker and said right speaker

without obstruction from an interior of said common housing.

25. The sound system of claim 24, further comprising an insulating material

residing within said common housing and opposite said slot, said insulating material

generally conforming to outer contours of said left speaker and right speaker.

26. The sound system of claim 23, 24 or 25, further comprising one or more

additional speakers mounted in or on said common housing, said one or more

additional speakers oriented directly towards the interior of the vehicle.

27. The sound system of claims 21 , 22, 23, 24, 25, or 26, further comprising a

pair of door-mounted left and right speakers.

28. A method for adaptively reproducing sound in a manner so as to enlarge

the perceived area and stability of a stereo sound image, the method comprising the

steps of:

placing a left speaker and a right speaker in close proximity;

receiving a left channel audio signal;

receiving a right channel audio signal; and

cross-canceling a spectrally weighted stereo difference signal with said left

channel audio signal and said right channel audio signal prior to applying said left

channel audio signal and said right channel audio signal to said left speaker and said

right speaker, respectively, said spectrally weighted difference signal derived from said

left channel audio signal and said right channel audio signal.

29. The method of claim 29, wherein said spectrally weighted difference

signal is generated by obtaining a difference signal representing a difference between

said left channel audio signal and said right channel audio signal, and applying said

difference signal to a spectral weighting filter.

30. The method of claim 29, wherein said spectral weighting filter is

relatively level gain.

31 . The method of claim 30, wherein said spectral weighting filter is further

characterized by a roll-off from said first filter region to said second filter region at

approximately 200 Hertz.

32. The method of claim 31 , wherein said spectral weighting filter is further

characterized by a boundary between said second filter region and said third filter

region at approximately 2 KHz.

33. The method of claim 29, 30, 31 , or 32, further comprising the step of

performing phase equalization on an output of said spectral weighting filter prior to said

step of cross-canceling said bass-enhanced stereo difference signal with said left channel

audio signal and said right channel audio signal.

34. The method of claim 33, further comprising the step of performing phase

compensation on each of said left channel audio signal and right channel audio signal to

compensate for said step of performing phase equalization on said output of said

spectral weighting filter.

35. The method of claim 34, wherein said step of performing phase

equalization on said output of said spectral weighting filter is carried out using a first

plurality of all pass filters, and wherein said step of performing phase compensation on

each of said left channel audio signal and right channel audio signal is carried out using

a second and third plurality of all pass filters

36. The method of claim 34, wherein said step of performing phase

equalization is carried out using a second order filter.

37. The method of claim 29, wherein said spectral weighting filter comprises

a linear filter.

38. The method of any of preceding claims 28 through 37, wherein said left

channel audio signal comprises a surround left channel audio signal which is coupled to

a surround left speaker, wherein said right channel audio signal comprises a surround

right channel audio signal which is also fed to a surround right speaker, and wherein

system.

39. The method of any of preceding claims 28 through 38, wherein said step

of placing left speaker and said right speaker in close proximity comprises the step of

placing said left speaker and said right speaker within a distance corresponding to a

40. The method of any of preceding claims 28 through 39, wherein said left

speaker and said right speaker are immediately adjacent.

41 . The method of claim 28, wherein said step of cross-canceling said

spectrally weighted stereo difference signal with said left channel audio signal and said

right channel audio signal comprises the steps of:

generating a difference signal representing a difference between said left channel

audio signal and said right channel audio signal;

applying a spectral weighting to said difference signal thereby generating a

spectrally weighted signal; and

cross-canceling said spectrally weighted signal with said left channel audio signal

and said right channel audio signal, thereby generating a first speaker signal for said left

speaker and a second speaker signal for said right speaker.

42. The method of claim 41 , wherein said step of generating said difference

signal is carried out using a subtractor.

43. The method of claim 41 or 42, wherein said step of applying said spectral

weighting to said difference signal is carried out using a spectral weighting filter, said

spectral weighting filter being characterized by a first filter region of relatively level gain,

a second filter region having a generally decreasing gain with increasing frequency, and

a third filter region of relatively level gain.

44. The method of claim 41 , 42 or 43, further comprising the steps of: performing phase equalization on said difference signal prior to said step of

cross-canceling said spectrally weighted signal with said left channel audio signal and

said right channel audio signal; and

performing phase compensation on each of said left channel audio signal and

right channel audio signal to compensate for the spectral weighting and phase

equalization performed on said difference signal.

45. The method of claim 44, wherein said step of performing phase

equalization on said difference signal is carried out using a first plurality of all pass filters

collectively having a first phase transfer function, and wherein said step of performing

phase compensation on each of said left channel audio signal and right channel audio

signal is carried out using a second and third plurality of all pass filters, said second

plurality of all pass filters and said third plurality of all pass filters each having a

collective phase transfer function complementary to a combined phase transfer function

of said first phase transfer function and a spectral weighting phase transfer function

associated with the step of applying spectral weighting to said difference signal.

46. The method of claim 44, wherein said step of performing phase

equalization is carried out using a second order filter.

47. The method of any of preceding claims 41 through 46, wherein said step

of applying said spectral weighting to said difference signal is carried out using a linear

filter.

48. The method of any of the preceding claims 41 through 46, wherein one or

more of said steps of generating said difference signal, applying a spectral weighting to

said difference signal, and cross-canceling said spectrally weighted signal with said left

channel audio signal and said right channel audio signal is carried out in whole or in

part in the digital domain.

49. The method of any of preceding claims 28 through 48, further comprising

the step of placing said left speaker and said right speaker in a vehicle.

50. The method of claim 49, further comprising the step of positioning said

left speaker and said right speaker substantially on or near a center axis of the vehicle.

51. The method of claim 49 or 50, further comprising the step of mounting

said left speaker and said right speaker within a common housing.

52. The method of claim 51 , wherein said right speaker and left speaker are

oriented in a downwards or upwards direction towards a flat surface of said common

without obstruction from an interior of said common housing.

53. The method of claim 52, wherein an insulating material resides within

said common housing and opposite said slot, said insulating material generally

conforming to outer contours of said left speaker and right speaker.

54. The method of claim 51 , 52 or 53, further comprising the step of

mounting one or more additional speakers in or on said common housing, said one or

more additional speakers oriented directly towards the interior of the vehicle.

55. The method of claims 50, 51 , 52, 53, or 54, further comprising a pair of

door-mounted left and right speakers.

56. A sound reproduction system for a surround sound stereophonic system,

comprising:

a surround left speaker;

a surround right speaker;

a pair of surround back speakers located in close proximity;

a surround left channel audio signal electrically connected to said surround left

speaker;

a surround right channel audio signal electrically connected to said surround

right speaker; and

channel audio signal, said sound processor configured to generate a difference signal

representing a difference between said surround left channel audio signal and said surround right channel audio signal, apply a spectral weighting to said difference signal

thereby generating a spectrally weighted signal, and cross-cancel said spectrally

weighted signal with said surround left channel audio signal and said surround right

channel audio signal, thereby generating a first speaker signal and a second speaker

signal for said pair of surround back speakers.

57. The sound reproduction system of claim 56, wherein said pair of surround

back speakers comprises a surround left back speaker and a surround right back speaker.

58. The sound reproduction system of claim 56, wherein said pair of surround

back speakers are located in a single speaker enclosure.

59. The sound reproduction system of claim 56, further comprising a left

speaker, a right speaker, and a center speaker.

60. The sound reproduction system of claim 56, further comprising a first

adaptive decorrelation circuit interposed between said surround left channel audio

signal and said surround left speaker, and a second adaptive decorrelation circuit

interposed between said surround right channel audio signal and said surround right

speaker.

61 . The sound reproduction system of claim 56, wherein said sound processor

increasing frequency, and a third filter region of relatively level gain.

62. The sound reproduction system of claim 61 , wherein said spectral

weighting filter is further characterized by a roll-off from said first filter region to said

second filter region at approximately 200 Hertz.

63. The sound reproduction system of claim 62, wherein said spectral

weighting filter is further characterized by a boundary between said second filter region

and said third filter region at approximately 2 KHz.

64. The sound reproduction system of claim 56, wherein said sound processor

further comprises a phase equalizer for equalizing the phase of said difference signal

prior to cross-cancellation, and a plurality of phase compensators complementary in

phase characteristics to a combined phase characteristic of said phase equalizer and said

spectral weighting filter, said phase compensators placed in series along each of said

surround left channel audio signal and surround right channel audio signal, respectively,

prior to cross-cancellation.

65. The sound reproduction system of claim 64, wherein said phase equalizer

comprises a plurality of all pass filters, and wherein each of said phase compensators

comprises a plurality of all pass filters.

66. The sound reproduction system of claim 56, wherein said sound processor

67. The sound reproduction system of claim 56, wherein said surround left

speaker and said surround right speaker are each dipole speakers.