WO1998054927A1

WO1998054927A1 - Method and system for enhancing the audio image created by an audio signal

Info

Publication number: WO1998054927A1
Application number: PCT/US1998/010977
Authority: WO
Inventors: Kenneth Dowes; William Craft
Original assignee: Desper Products, Inc.
Priority date: 1997-05-30
Filing date: 1998-05-29
Publication date: 1998-12-03
Also published as: TW413995B; AU7704798A

Abstract

The invention is a system for automatically enhancing the audio image derived from an audio signal. The sysem includes first and second signal lines each having a system input (103 and 107) and a system output (105 and 109). The system includes a first conditioning signal line comprising a subtractor (110) which subtracts the input on the second signal line from the input on the first signal line and a band pass filter (111) which with frequency contours and time delays the difference signal to obtain a first conditioning signal. The system includes a second conditioning signal line which includes an adder (115) which adds the signal on the first input signal line to the signal on the second input signal line to form a sum signal which is then passed through an all pass filter (118) to derive a second conditioning signal. The system further includes an adder (113) whose inputs are the first input signal, the first conditioning signal and the second conditioning signal and which derives the additive combination thereof which is communicated to a first system output. The system also comprises a subtractor (108) whose inputs are the input signal on the second signal line, the first conditioning signal and second conditioning signal, the output of which is equal to the input on the second signal line decreased by the first conditioning signal and decreased by the second conditioning signal, and which is communicated to the second system output. The invention also relates to a method for accomplishing audio enhancement of either a stereo or mono signal.

Description

METHOD AND SYSTEM FOR ENHANCING THE AUDIO IMAGE CREATED BZ AN AUDIO SIGNAL

BACKGROUND OF THE INVENTION

FIELD OF THE INVENTION This invention relates to an automatic sound image enhancement method and apparatus wherein the electronic signal which corresponds to the audio signal is electronically processed such that the listener perceives enhancements to music and sounds. The invention may be used in connection with the play of music and sound received from a broadcast signal or with the playback of music and sound which had been previously recorded (e.g., on tape, compact disc, phonograph records or any other media) to accomplish post production enhancement, or in connection with the recording or mixing of audio recordings or broadcasts (production enhancement) . The invention may be used in connection with either monaural or stereo sound sources to synthesize a stereo-like effect or to locate the perceived positions of such sources in positions beyond those normally found in the stereo sound stage.

PRIOR ART The prior art is replete with efforts in conjunction with an otherwise standard stereo system (i.e., two-speaker system) to synthesize a stereo-like effect from a monaural sound source and/or to make the perceived position of a sound source, whether derived from a monaural or a stereo audio signal, appear to come from a position beyond the space between two stereo speakers. Obviously, effects such as making a sound source appear to originate from various positions surrounding a listener can be achieved by using appropriately configured systems having three, four or more audio channels with corresponding speakers. However, the vast majority of users of sound systems are home users who either cannot justify spending the money required for systems having three or more audio channels and speakers or simply do not have the space to accommodate more than a two-speaker system. Therefore, a substantial amount of effort has been expended trying to achieve such effects with a two-channel, two-speaker system.

An example of a method and system used to synthesize stereolike effects from a monaural signal through a two-speaker system is illustrated in British Patent No. 942,459 which was published in 1963. An example of a method and system used to place the apparent position of a sound from a monaural source to any point around a listener through a two-speaker system is illustrated in United States Patent No. 4,219,696 issued to Kogure et al. in 1980. Examples of methods or systems used to enhance the apparent spread of sound sources represented by stereo signals are shown in

United States Patent No. 4,308,423, issued to Cohen in 1981 and

United States Patent No. 4,394,536 issued to Shima et al. in 1983.

A particularly pleasing sound image enhancement effect is obtained using the methods and systems taught in United States Patent No. 5,412,731 issued to Desper in 1995 and in United States Patent Application Serial No. 08/464,669 filed by Desper, both of which are owned by Desper Products, Inc., the assignee of the invention described herein. The disclosures of United States Patent No. 5,412,731 and United States Patent Application Serial No. 08/464,669 are hereby incorporated herein by reference.

The foregoing methods and systems use one or more of the following techniques in various forms and combinations to obtain their desired enhanced audio effect: splitting a monaural signal into two separate channel signals and modifying one such signal by delaying, phase shifting, frequency contouring and/or varying the amplitude of the signal in a different manner from the way the signal in the other channel is modified, or subtracting one of a pair of input stereo signals from the other to derive a difference signal and then delaying, phase shifting, frequency contouring and/or amplifying the difference signal relative to the input stereo signals and then adding the modified difference signal to the input stereo signal which formed the minuend when the difference signal was derived and subtracting the modified difference signal from the other input stereo input signal to create new first and second output signals.

By virtue of the new differences between the new first and second output signals with respect to their amplitudes, delays, phase shifts and frequency contours, and by virtue of the elimination of "cross-talk" (i.e., the mixing of left channel signals with right channel signals at the listener's right ear and the mixing of right channel signals with left channel signals at the listener's left ear), the listener perceives an "audio illusion" that a monaural signal is a stereo signal or that a sound is positioned other than at or between the sound system's two stereo speakers, or that a sound "wraps around" the listener. As mentioned above, the methods and systems disclosed in United States Patent No. 5,412,731 and United States Application Serial No. 08,464,669 achieve a particularly pleasing result. One of the embodiments disclosed therein has been incorporated into integrated circuits (ICs) under licenses from Desper Products, Inc. The circuit in one of the ICs is shown in Figure 1. All elements shown in Figure 1 are embedded in the IC except for the capacitors and except for the resistors between pins 13 and 17. (The IC ' s pin numbers are shown in the circles . )

The circuit of this IC may be described as follows, with reference being made to the circuit identifying references in the circuit drawing of Figure 1. Left and right channel input signal Lin and Rin are applied to the system at pins 1 and 19, respectively. Rin is subtracted from Lin at Amp 1 which is configured as a subtractor. The output of Amp 1, (Lin - Rin), is fed to a bandpass filter formed by an RC network consisting of resistors internal to the IC at pins 16 and 18 and external capacitors connected to the same pins of the IC. The band pass filter's peak response is at 500 Hz, with 6db per octave roll offs. The bandpass filter also introduces a delay to the (Lin - Rin) signal. The filtered and delayed difference signal (referred to hereinafter as "C") is passed through voltage control amplifier (VCA) and is added to input signal Lin at Amp 3, which is configured as an adder, so that the left channel output of the system at pin 3, Lout, is Lin + C. At Amp 4, which is configured as a subtractor, the signal C is subtracted from input signal Rin so that the right channel output of the system at pin 5, Rout, is Rin - C. Lout and Rout are fed back to the system at a saturation detection circuit formed by RMS detectors RMS1 and RMS2. At the input of RMS1, Lout and Rout are added together, and at the input of RMS2, Rout is subtracted from Lout. The outputs of RMS1 and RMS2 are fed to differential amplifier Amp 7, the output of which regulates the VCA.

The system provides audio "image enhancement" to a stereo signal. By way of example, if a listener is listening to an orchestra concert on an otherwise ordinary two-channel, two- speaker stereo system, with the IC added to it, the listener may perceive that he is surrounded by the orchestra instruments disposed along an arc running through the two speakers, with some of the instruments to his left and some to his right, rather than merely having the orchestra in front of him.

The image enhancement is brought about by the C signal which may be thought of as a conditioning signal. The regulating feedback circuit through the RMS detector prevents the stereo signal from being "over-enhanced," such as when the source material has already been enhanced. In addition, the amount of enhancement can be controlled by the listener by adjusting the potentiometer at pin 13. If the wiper of the potentiometer is set to ground, the VCA passes none of the conditioning signals, and Lout = Lin and Rout = Rin. That is, the audio program will be heard in an unenhanced condition. As the wiper is moved away from ground, the enhancement effects are increased. The foregoing describes the IC when it is configured to receive a stereo signal, or at least, when it is configured to receive a left input signal which is different from the right input signal. When a monaural signal is received by the IC when it is configured as shown in Figure 1, the left and right output signals are identical and there is no audio enhancement.

However, the external connections to the IC can be changed so that audio enhancing effects can be applied to monaural signals. In one such embodiment, illustrated in Figure 2, the left and right inputs of the IC are connected together through a balance control (or panning) potentiometer to a monaural signal source, and otherwise the circuits are the same.

In another embodiment, illustrated in Figure 3, the left and right inputs are connected together directly to a monaural signal source, and the saturation detection circuit is disabled. The conditioning signal is derived from the difference between ground and the monaural signal at the right channel input (i.e., it is derived essentially from -Rin) , which is passed through the bandpass filter and through an amplifier with a fixed gain. The conditioning signal is added to the left channel input signal to form the left output signal and an attenuated version of the conditioning signal is subtracted from the right channel input signal to form the right output signal.

Note that all of these configurations can be obtained without using an integrated circuit, such as by using discrete analog components. Note also that these configurations can be used both with respect to post production enhancement (i.e., creating enhancement of audio signals received from a broadcast signal and the like or received from playback devices such as, for example compact disc players, tape players, and phonograph records) and production enhancement (i.e., creation of enhanced audio signal for stereo recording and the like onto any media or for stereo broadcast to be reproduced on a standard stereo system) . In both instances, the audio illusion perceived by the listener would be the same.

While the foregoing system shown in Figures 1-3 may be configured for enhancing either a monaural signal or a stereo pair of signals, it cannot be configured to do both at once or to be able to do either one in a single commercial embodiment without using substantial extra circuitry and switching.

It is an object of the present invention to provide a method and system for providing automatic audio enhancement of both a monaural signal and a stereo pair of signals without having to switch between various configurations.

SUMMARY OF THE INVENTION

The invention is a system for automatically enhancing the audio image derived from an audio signal. The system includes first and second signal lines each having a system input and a system output. The system includes a first conditioning signal line comprising a subtractor which subtracts the input on the second signal line from the input on the first signal line and a band pass filter which frequency contours and time delays the difference signal to obtain a first conditioning signal. The system includes a second conditioning signal line which includes an adder which adds the signal on the first input signal line to the signal on the second input signal line to form a sum signal which is then passed through an all pass filter to derive a second conditioning signal. The system further includes an adder whose inputs are the first input signal, the first conditioning signal and the second conditioning signal and which derives the additive combination thereof which is communicated to a first system output. The system also comprises a subtractor whose inputs are the input signal on the second signal line, the first conditioning signal and second conditioning signal, the output of which is equal to the input on the second signal line decreased by the first conditioning signal and decreased by the second conditioning signal, and which output is communicated to the second system output. The invention also relates to a method for accomplishing audio enhancement of either a stereo or mono signal.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 is a simplified schematic diagram of a prior art circuit which provides automatic sound image enhancement to a stereo pair of signals.

Figure 2 is a simplified schematic diagram of a prior art circuit which provides sound image enhancement to a monaural signal.

Figure 3 is a simplified schematic diagram of a prior art circuit which provides automatic sound image enhancement to a monaural circuit.

Figure 4 is a block diagram of an embodiment of the invention which provides automatic sound image enhancement to both monaural signals and to stereo pairs of signals. Figure 5 is a simplified schematic diagram of an embodiment of the invention which provides automatic sound image enhancement to both monaural signals and to stereo pairs of signals. DETAILED DESCRIPTION OF THE INVENTION

The preferred embodiments of the subject invention are illustrated in Figures 4 and 5 of the attached drawings. The same reference numeral will be used to identify identical elements throughout the drawings.

Figure 4 shows a block diagram of a preferred embodiment 101 of the invention. In the following respects, it is very similar to the circuit shown in Figure 1. Where the prior art circuit shown in Figure 1 has a first (or left) signal line comprised of a left system input means (the left input buffer), an adder for additively combining the first (or left) input signal with a conditioning signal and a first (or left) system output (pin 3), the present invention comprises a first (or left) signal line 102 comprising a first (or left) input 103 for receiving a-first input signal, an adder 104 for additively combining a first conditioning signal with the first input signal and a first (or left) system output 105 for outputting the result of adding the first conditioning signal with the first input signal. Where the prior art circuit in Figure 1 has a second signal line including a second input system (the right input buffer) and a subtractor for subtracting the conditioning signal from the second (or right) input signal and a second (or right) system output, the present invention comprises a second (or right) signal line 106, a second (or right) system input 107 for receiving a second input signal, a subtractor 108 for subtracting the first conditioning signal from the second input signal and a second (or right) system output 109, for outputting the results of subtracting the first conditioning signal from the second input signal. Where the prior art circuit in Figure 1 has a conditioning signal line formed of a subtractor which receives the first and second input signals from the first and second system inputs and subtracts the second signal from the first signal to derive a difference signal, a bandpass filter for delaying and frequency contouring the difference signal, and a voltage controlled amplifier whose input is the delayed and frequency-contoured difference signal and whose output is the conditioning signal which is fed to the adder in the first signal line and the subtractor in the second signal line, the present invention as shown in Figure 4 has a first conditioning signal line comprised of subtractor 110 whose inputs are the first and second input signals received by the first and second system inputs 103 and 107, respectively, and whose output is a difference signal equal to the second input signal subtracted from the first input signal, a filter device 111 whose input is the difference signal and whose purpose is to modify the difference signal by shifting its phase, by time delaying it and/or by frequency contouring it, a variable gain device 112 whose input is the modified difference signal and whose output is the first conditioning signal 120 which is connected (i) to adder 104 where the first conditioning signal is added to the first input signal, the result of which is passed on to first output means 105, through adder 113 and (ii) through adder 114 to subtractor 108 where the first conditioning signal is subtracted from the second input signal and the result of which is passed on to second system output 109.

Up through this point, the system of Figures 1 and 4 are very similar and both would provide automatic audio enhancement of stereo pairs of signals in much the same way with much the same result. However, where the particular configuration of Figure 1 could not provide automatic audio enhancement of a monaural signal (i.e., identical signals being inputted at the first and second (or left and right) system inputs), the system of Figure 4 can. That is because the present invention includes a second conditioning signal line 130 which includes an adder 115 whose inputs are the first and second input signals and whose output is the sum signal equal to the additive combination of the first and second input signals, a low pass filter 116 whose input is the sum signal and whose output is fed to a variable gain device 117, and an all pass filter 118 whose input is the output of the variable gain device 119, which is the amplified version of the sum signal, and whose output is the second conditioning signal. The second conditioning signal is shown to be (i) added to the combination of the first input signal and the first conditioning signal at adder 113, the output of which is fed to first system output 105, and (ii) added to the first conditioning signal at adder 114, the output of which is subtracted from the second input signal at subtractor 108, the output of which is fed to the second system output 109. In reality, the second conditioning signal is added to the first input signal and subtracted from the second input in the same way that the first conditioning signal is. Therefore, adders 104 and 113 could be a single adder with three positive inputs (the first input signal, the first conditioning signal and the second conditioning signal) and adder 114 and subtractor 108 could be a single subtractor with a single positive input (the second input signal) and two negative outputs (the first and second conditioning signals). Viewed still another way, the first and second conditioning signals are added together to form a "super" conditioning signal which is added to the first input signal to derive the first output signal and subtracted from the second input signal to derive the second output signal.

The all pass filter creates a comb filter effect when a monaural signal is present to provide a stereo-like effect. So, even when only a monaural signal is present, audio enhancement is automatically achieved with only a single circuit, and without having to throw switches or make additional adjustments when a monaural signal is being received after a stereo signal has been received.

The present invention achieves unexpectedly good results. With a stereo pair of signals, the resulting enhancement includes the exceptional spatial expansion achieved by the circuit of Figure 1 plus an additional enhancement of the monaural signal that virtually all stereo signal pairs carry. This combined effect is surprisingly pleasant. Further in this regard, the inventor has found that when the all pass filter 118 is a third order all pass filter, the results are inspiring.

It should be noted that the circuitry of the invention is symmetrical because, otherwise, certain frequencies would be lost when there is only a monaural input. Figure 5 is a schematic diagram of another preferred embodiment of the invention. This embodiment can be implemented using either a single semi-conductor chip or by means of discrete analog components. In Figure 5, the circuit 200 has two system inputs, 201 and 202, which may correspond to a left input and a right input respectively. These inputs are communicated respectively to input buffers 203 and 204. The input signal at input buffer 203 is communicated to two operational amplifiers, namely amplifiers 205 and 206, and to combination adder and low pass filter 207. The input at input buffer 204 is communicated to two operational amplifiers, namely operational amplifier 206 and 208, and to the combination adder and low pass filter 207. The left and right input signals are subtracted from each other at amplifier 206 with the left input signal forming the minuend and the right input signal forming the subtrahend to produce at the amplifier's output the difference signal L-R. That difference signal is communicated to potentiometer 209 which communicates a portion (depending upon the position of the potentiometer 209) of the difference signal through a band pass filter 210 formed by a conventional capacitor and resistor network. Filter 210 serves to frequency contour the difference signal L-R and preferably has a center frequency of approximately 500 Hz with 6dB/octave slopes. However, the center of frequency can fall within the range of about 300 Hz to 3,000 Hz. Filter 210 also provides frequency dependent time delay to the difference signal. The output of the band pass filter is fed to the input of amplifier 211. The output of amplifier 211 is first conditioning signal Cl. First conditioning signal Cl is added to the first (or left) input signal at amplifier 205 and subtracted from the second (or right) input signal at amplifier 208 (points A-A should be viewed as being shorted together). The output of amplifiers 205 and 208 are audio enhanced first (left) and second (right) system outputs 212 and 213 respectively.

In addition to the foregoing, the first and second (or left and right) input signals are added together and passed through a low pass filter at adder/low pass filter 207 which produces at its output a sum signal. The sum signal is equal to L+R and is communicated to variable gain amplifier 214, the output of which is fed through potentiometer 215 to all pass filter 216 which is formed of a network of resistors, capacitors and amplifiers. In the most preferred embodiment, the all pass filter is a third order all pass filter. Its output is second conditioning signal C2 which, like first conditioning signal Cl, is added to the first input signal at amplifier 205 and subtracted from the second input signal at amplifier 208. This means that the signal at first (or left) system output 212 is Lin+Cl+C2, while the output at second (or right) system output 213 is Rin-Cl-C2. The outputs at 212 and 213 may then be conveyed (i) to the inputs of the power amplifier of a consumer audio apparatus such as a stereo radio, computer, television receiver and the like and then on to paired loud speakers in the usual fashion, in connection with doing post production enhancement or (ii) to the inputs of professional recording equipment in connection with doing production enhancement. The listener or the producer, as the case may be, controls the amount of enhancement added by adjusting potentiometers 209 and 215. If the wiper of potentiometers 209 and 215 are put to the ground side, then the audio program, be it stereo or monaural, would be heard in its unenhanced mode. However, as the wipers of potentiometers 209 and 215 are adjusted to communicate more and more of the L-R signal and L+R signals respectively, more and more additional spatially enhanced and synthesized stereo is perceived by the listener. Assuming for the moment that the potentiometers were set at their highest settings, if the signals at the system inputs are mono, then spatial enhancement as normally achieved by a circuit such as that shown in Figure 1 would not be achieved but there would still be stereo synthesis by virtue of the signal which is passing through the second conditioning signal line formed of adder/low pass filter 207, variable gain amplifier 214 and all pass filter 216. The circuitry of Figure 5 is shown as essentially discrete components. Those skilled in the art will appreciate that all, or most, of the circuit could be reduced to a single silicon chip if desired. Those skilled in the art will also appreciate that the capacitors in the filters may be rather large if implemented on a chip and therefore it may be desirable to have appropriate pin- outs from the chip for connecting to discrete capacitors.

The invention also is a method for automatically enhancing the audio image derived from an audio signal. The method includes the following steps: deriving first and second input signals from a stereo pair of signals or from a monaural signal, subtracting said second input signal from said first input signal to obtain a difference signal, passing said difference signal through a band pass filter to obtain a first conditioning signal, adding said first and second input signals together and passing them through a low pass filter to derive a sum signal, passing said sum signal through an all pass filter to obtain a second conditioning signal, adding said second conditioning signal to said first input signal and said first conditioning signal to obtain a first output signal and subtracting said second conditioning signal and said first conditioning signal from said second input signal to obtain a second output signal. This invention has been described with reference to a number of embodiments, and it is clear that it is susceptible to numerous modifications, modes and embodiments within the ability of those skilled in the art and without the exercise of inventive faculty once the foregoing disclosure has been read. By way of example, the conditioning signals in all embodiments are shown as being added to the left channel and subtracted from the right channel, with the right channel being subtracted from the left channel in order to derive the difference signal. This convention may be reversed, if desired, although it is believed that the electronics industry will follow the convention described herein for consistency. Other modifications are well within the skill of those skilled in the art, such as accomplishing the invention by using computer software, or such as providing an enhancement bypass mode. Accordingly this invention is not limited to the disclosed embodiments. Further in this regard, it will be understood that various changes of the details, steps, arrangements of parts and uses which have been herein described and illustrated in order to explain the nature of the invention will occur to and may be made by those skilled in the art and such changes are intended to be included within the scope of this invention.

Claims

WHAT IS CLAIMED IS

1. An automatic audio image enhancement apparatus comprising: first and second signal lines, said first signal line comprising a first system input and a first system output, said second signal line comprising a second system input and a second system output, said first system input receiving a first input signal and said second system input receiving a second input signal, wherein said first and second input signals may be identical or they may be different, a first conditioning signal line comprising a first subtractor connected to said first system input and said second system input member, said first subtractor having an output which is a derived difference signal equal to the difference of the second input signal being subtracted from said first input signal, a frequency contouring and time delay device connected to said first subtractor so as to receive the derived difference signal, said frequency contouring and time delay device having an output which is a first conditioning signal, a first adder having its inputs connected to the first system input and said second system input, said second adder deriving at its output a sum signal equal to the sum of the first and second input signals, a low pass filter connected to the first adder so as to receive the sum signal, an all pass filter connected to the low pass filter's output, said all pass filter deriving a second conditioning signal at its output, a second adder adapted to receive as its inputs said first input signal, said first conditioning signal and said second conditioning signal, said second adder having an output equal to the additive combination of said first input signal, said first conditioning signal and said second conditioning signal and which is connected to said first system output, and a second subtractor having as its inputs the second input signal, said first conditioning signal and said second conditioning signal and deriving as its output the subtractive combination of the second input signal decreased by the first conditioning signal and decreased by said second conditioning signal, the output of which second subtractor is communicated to the second system output.

2. A method for automatically enhancing the audio image derived from an audio signal, said method including the following steps : deriving first and second input signals from a stereo pair of signals or from a monaural signal, subtracting said second input signal from said first input signal to obtain a difference signal, passing said difference signal through a band pass filter to obtain a first conditioning signal, adding said first and second input signal together to derive a sum signal, passing said sum signal through an all pass filter to obtain a second conditioning signal, adding said second conditioning signal to said first input signal and said first conditioning signal to obtain a first output signal, and subtracting said second conditioning signal and said first conditioning signal from said second input signal to obtain a second output signal.