EP2457382B1

EP2457382B1 - A sound reproduction system

Info

Publication number: EP2457382B1
Application number: EP10742279.2A
Authority: EP
Inventors: Goh Kong San Gozali; Nandanahosur Sahadevappa Suresh; Frederick Jwee Koon Kwek
Original assignee: TP Vision Holding BV
Current assignee: TP Vision Holding BV
Priority date: 2009-07-24
Filing date: 2010-07-16
Publication date: 2013-09-11
Anticipated expiration: 2030-07-16
Also published as: WO2011010254A1; EP2457382A1; CN102484755A; JP2013500615A; RU2012106653A

Description

FIELD OF THE INVENTION

The invention relates to a sound reproduction system, and in particular, but not exclusively to sound reproduction for spatial audio.

BACKGROUND OF THE INVENTION

Spatial sound reproduction is widely used in many applications including consumer devices and other sound reproduction systems. For example, stereo sound reproduction is commonplace for many sound systems and is e.g. used for radio and television sound reproduction. However, a disadvantage with such systems is that they require multiple speakers in order to provide the desired spatial experience. Examples of speakers using multiple voice coils are disclosed in US 2 539 672 A ; US 5 295 194 A , US 5 682 436 A , and EP 0 341 926 A1 .
Indeed stereo devices require a speaker for each channel which is inconvenient for many small form factor devices. For example, location to speaker systems within a small television or portable radio is often difficult and results in an increased form factor. This is particularly disadvantageous because a relatively large speaker size is required in order to ensure low frequency operation of sufficient quality. Furthermore, the multiple speakers tend to add substantially to the cost and complexity of the device.
Accordingly, a system that allows smaller form factors, improved quality sound rendering, spatial sound rendering, improved low frequency performance, reduced cost, reduced complexity and/or improved performance would be advantageous.

SUMMARY OF THE INVENTION

Accordingly, the Invention seeks to preferably mitigate, alleviate or eliminate one or more of the above mentioned disadvantages singly or in any combination.
According to an aspect of the invention there is provided a sound reproduction system comprising: a speaker unit comprising: a cone, a first transducer for converting a first electrical signal to movement of the cone, and a second transducer for converting a second electrical signal to movement of the cone; and a drive circuit for at least partially independently driving the first transducer and the second transducer by generating the first electrical signal to be different than the second electrical signal.
The invention may allow an improved sound reproduction system. In particular, a differentiated sound can be generated from a single speaker unit that allows a more flexible and diversified sound reproduction. The invention may in many embodiments allow reduced complexity, size and/or cost. For example, multiple sound source reproduction can be achieved by a single speaker unit. A spatial sound experience may be provided from a single speaker unit. The invention may allow improved audio quality in many scenarios. In particular, an improved low frequency performance can be achieved as the full cone area can be utilized by both transducers and thus by multiple sound sources. Thus, a low frequency extension can be achieved where the low frequency performance for both signals is given by a cone area corresponding to the full area of the cone but without requiring such a cone area to be provided for each signal.
The first and/or second transducer may specifically be electromagnetic transducers and may comprise a movable and a fixed part relative to a fixing element of the speaker unit. The movable part may be attached to the cone. The two parts may e.g. correspond to a permanent magnet and voice coil through which the electrical signal may be sent. The voice coil may be attached to the cone and the permanent magnet may be attached to a fixing structure for the speaker unit.
The partially independent driving may specifically mean that the first electrical signal and the second electrical signal may be non-identical. Specifically, the drive circuit is arranged to be capable of generating two non-identical signals to drive the two electromagnetic transducers.
In accordance with an optional feature of the invention, the drive circuit is arranged to generate the first electrical signal to correspond to a first sound source and to generate the second electrical signal to correspond to a second sound source.
The invention may allow multiple sound sources to be provided from a single speaker unit thereby allowing reduced cost, complexity, size and/or improved performance for a sound rendering system for multiple audio sources.
The drive circuit is arranged to generate the first electrical signal to correspond to a first channel signal of a first audio channel in at least a first frequency interval and to generate the second electrical signal to correspond to a second channel signal of a second audio channel in the first frequency interval.
The invention may allow multiple channels to be rendered from a single speaker unit thereby allowing reduced cost, complexity, size and/or improved performance for a multi-channel sound rendering system.
The first audio channel and the second audio channel are spatial channels.
The invention may be particularly advantageous for rendering multiple spatial channels. The first and second audio channels may specifically be left and right channels of a stereo signal.
Thus, the invention may allow stereo reproduction from a single speaker unit thereby providing a more compact and/or improved audio quality stereo system. The invention may for example allow improved and/or smaller stereo consumer devices such as televisions or radios.
In accordance with an optional feature of the invention, the drive circuit is further arranged to receive a low frequency effects channel signal and to generate both the first electrical signal and the second electrical signal in response to the low frequency effects channel signal.
This may provide an efficient reproduction of the additional channel without requiring additional speakers and may specifically allow a compact, low complexity and/or high performance 2.1 spatial audio reproduction system.
In accordance with an optional feature of the invention, the drive circuit is arranged to generate a combined signal by combining the first channel signal and the second channel signal in a second frequency interval, and to generate both the first electrical signal and the second electrical signal to correspond to the combined signal in the second frequency interval.
This may provide improved performance in many scenarios. In particular, it may allow signal components that are rendered by the full cone to be closely correlated between the two transducers. This may in many scenarios provide improved audio quality.
In accordance with an optional feature of the invention, the second frequency interval comprises lower frequencies than the first frequency interval.
This may provide improved performance in many scenarios. In particular, it may allow low frequency signal components that are rendered by the full cone to be closely correlated between the two transducers. This may in many scenarios provide improved audio quality. In particular a low frequency mono signal may be generated by combining the low frequency signals of the first and second audio channels.
The combination may for example be generated by a low pass filtering of the first and second channel signals followed by a combination, such as e.g. a summation.
In accordance with an optional feature of the invention, a first part of the cone is attached to a first moveable element of the first transducer and a second part of the cone is attached to a second moveable element of the second transducer.
Such an approach may provide advantageous operation, construction and/or performance. The first and second parts may in many scenarios advantageously be separated by a distance of at least 3 or 5 centimeters.
In accordance with an optional feature of the invention, the first part provides a first cone area having a movement which is fully determined by the movement of the first moveable element, the second part provides a second cone area having a movement which is fully determined by the movement of the second moveable element, and the cone further comprises a third cone area having a movement determined by the movement of both the first moveable element and the second moveable element.
This may provide advantageous operation, construction and/or performance. The arrangement may specifically provide for partially independent audio generation by three areas, namely the first cone area, the second cone area and the third cone area. The first cone area may predominantly be controlled by controlling the movement of the first moveable element whereas the second cone area may predominantly be controlled by controlling the movement of the second moveable element. The third cone area is controlled by both the first and second moveable element (and is typically symmetrically dependent on the movement of these).
The first and second cone areas may specifically correspond to areas of relatively small dimensions whereas the third cone area may specifically correspond to a relatively large area.
The approach may specifically allow higher frequencies to be relatively independently radiated from the first and second areas with lower frequencies being combined and radiated predominantly by the third cone area. Thus, high frequency components which comprise the majority of directional cues for a listener may predominantly be radiated from separate and distinct areas of the cone whereas the lower frequencies which contain fewer directional cues are radiated from the entire cone thereby providing improved low frequency performance.
The first cone area is fully determined by the movement of the first moveable element such that if the exact knowledge of the first moveable element is known, the movement of the first cone area is fully given and does not require consideration of the movement of the second moveable element. This is independent of whether the movement of the first moveable element results from the first transducer or e.g. is a function of movement of the second moveable element propagating via the cone.
Similarly, the second cone area is fully determined by the movement of the second moveable element such that if the exact knowledge of the second moveable element is known, the movement of the second cone area is fully given and does not require consideration of the movement of the first moveable element. This is independent of whether the movement of the second moveable element results from the second transducer or e.g. is a function of movement of the first moveable element propagating via the cone.
In contrast, the movement of the third area is a function of the movement of both the first and second moveable element and cannot be determined from information about the movement of only one of these.
In accordance with an optional feature of the invention, a channel separation between the first cone area and the second cone area is at least 10 dB.
This may provide improved performance in many scenarios. In some embodiments, the channel separation may advantageously be no less than 20 dB or 30 dB.
The channel separation may be determined as the relative difference in the sound level produced by one of the first and second areas relative to the other when one of the first and second transducers is not fed any electrical signal whereas the other is actively driven.
In many embodiments, the maximum displacement of the first area when no electrical signal is fed to the first transducer is less than half the maximum displacement of the second area when the second electrical signal comprises a rendered audio signal (i.e. is not zero). Such a measurement may e.g. be made for a given sound level from the second area.
In accordance with an optional feature of the invention, the first moveable element and the second moveable element are voice coils and the first cone area and second cone area are areas of the cone within perimeters of the voice coils.
This may provide a practical implementation and/or improved performance. The first and/or second cone areas may specifically correspond to a cone dome or dust cap of the speaker unit.
In accordance with an optional feature of the invention, the third part has a higher efficiency for the first electrical signal than the first cone area below a crossover frequency and a lower efficiency for the first electrical signal than the first cone area above the crossover frequency.
Similarly, the third part may have a higher efficiency for the second electrical signal than the second cone area below a crossover frequency and a lower efficiency for the second electrical signal than the second cone area above the crossover frequency.
This may provide improved sound quality. The crossover frequency may in many applications advantageously be between 150 Hz and 600 Hz and specifically between 200 Hz and 400 Hz.
In accordance with an optional feature of the invention, a cone perimeter of the cone has dimensions of less than 5 cm by 20 cm.
The invention may allow a relatively high quality audio to be generated by a compact speaker arrangement while at the same time providing a spatial sound rendering. In particular, the speaker unit may have a height of no more than 5 cm and a width of no more than 20 cm when viewed from the front (along the main central axis). The invention may furthermore allow a speaker to be used which has a shape that more closely approaches a rectangular shape than a circular shape as known from conventional speakers. This may provide more flexibility when implementing the audio system.
In accordance with an optional feature of the invention, a largest cross section distance between points of a cone perimeter of the cone is no more than 20 cm.
The invention may allow a relatively high quality audio to be generated for a compact speaker arrangement.
According to an aspect of the invention there is provided a method of driving a speaker unit comprising a cone, a first transducer for converting a first electrical signal to movement of the cone, and a second transducer for converting a second electrical signal to movement of the cone; the method comprising: at least partially independently driving the first electromagnetic transducer and the second electromagnetic transducer by generating the first electrical signal to be different than the second electrical signal, the driving generating the first electrical signal to correspond to a first channel signal of a first spatial audio channel in at least a first frequency interval and generating the second electrical signal to correspond to a second channel signal of a second spatial audio channel in the first frequency interval, the first spatial audio channel being different than the second spatial audio channel.
These and other aspects, features and advantages of the invention will be apparent from and elucidated with reference to the embodiment(s) described hereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will be described, by way of example only, with reference to the drawings, in which

Fig. 1 illustrates an example of a sound reproduction system in accordance with some embodiments of the invention;
Fig. 2 illustrates an example of a dual motor speaker unit;
Fig. 3 illustrates an example of a sound reproduction system in accordance with some embodiments of the invention; and
Fig. 4 illustrates an example of a sound reproduction system in accordance with some embodiments of the invention.

DETAILED DESCRIPTION OF SOME EMBODIMENTS OF THE INVENTION

Fig. 1 illustrates an example of a sound reproduction system in accordance with some embodiments of the invention. The system comprises a speaker unit 101 which is a dual motor speaker unit. Specifically, the speaker unit 101 radiates sound from a cone which can be moved by two different actuators. The speaker unit 101 comprises a first (electromagnetic) transducer which can receive a first electrical signal and convert this into mechanical movement of the cone. Further, the speaker unit 101 comprises a second (electromagnetic) transducer which can receive a second electrical signal and convert this into mechanical movement of the cone. Each of the transducers may for example comprise a permanent magnet coupled to a moving voice coil which is attached to the cone. Thus, the speaker is a multi-motor speaker unit which can be driven by two different actuators attached to different parts of the cone.
The speaker unit 101 1 is coupled to a drive unit 103 which is arranged to generate the first and second electrical signals. The drive unit 103 specifically generates the first and second signals such that they may be independent and specifically may correspond to two different sound sources. Thus, the dual motor speaker unit 101 is not merely used to extend the low frequency performance of a single sound source but is rather used to provide two different sound sources thereby allowing a single speaker unit to provide sound reproduction for more than one sound source. Furthermore, the two sound sources will appear spatially separated as they (at least partly) originate from different parts of the cone of the speaker unit 101.
In the example, the two sound sources are used to represent different channels and specifically to represent different spatial channels. Thus, a single speaker unit 101 is used to provide more than one spatial channel with the spatial channels being perceived as originating from different directions despite being created by the same cone.
In the specific example, the drive unit 103 comprises a first driver 105 which receives a first audio channel and generates the first electrical signal therefrom. It furthermore comprises a second driver 107 which receives a second audio channel and generates the second electrical signal therefrom. In the example, the first and second drivers 105, 107 are completely independent of each other and there is no interaction between the signals of the different audio channels (except for via the cone of the speaker unit 101). The drivers 105, 107 may for example include suitable audio filters, amplifiers, volume controls etc as will be well known to the skilled person.
The two audio channels are spatial channels such as the two channels of a stereo signal. Thus, the drive circuit may receive a left and right channel of a stereo signal and may therefrom generate a drive signal corresponding to a left channel and a separate drive signal corresponding to a right channel. However, the drive signals are not fed to different speakers or speaker drivers but are fed to different transducers/actuators of the same speaker unit 101. Thus, in the system, the same cone is driven to reproduce both the left channel and the right channel.
The dual motor drive circuit is arranged such that this dual driving results in the radiated sound corresponding to not just the combined mono signal but rather the radiated sound also has a spatial distribution and comprises different spatial cues for the right and the left channel.
In more detail, for higher frequencies, the sound reproduction will predominantly be generated by the cone area proximal to the actuation point and thus the higher frequency content of the left signal will predominantly be radiated from the cone area around the attachment point of the transducer to which the left electrical signal is fed. Similarly, the higher frequency content of the right signal will predominantly be radiated from the cone area around the attachment point of the transducer to which the right electrical signal is fed. Thus, the higher frequency content of the radiated sound for the left and right channels are radiated by different parts of the cone thereby providing directional cues that allow a spatial experience by the listener.
For reducing frequencies the sound radiation is from increasingly large areas of the cone and for low frequencies the sound radiation is typically from the whole cone area. Thus, at the lower frequencies, the radiated sound does not correspond to individual separate channels but rather provides a combined sound of the two channels. Thus, for low frequencies a mono sound is radiated. However, as the sound at low frequencies for both channels can be radiated by the entire cone area a much improved efficiency and low frequency performance can be achieved.
In essence, the dual motor speaker unit allows the low frequency performance for each channel that corresponds to the total cone area while not requiring that this area is provided independently for each channel. Thus, the total cone area of the system in order to provide a given low frequency performance is effectively halved (i.e. normally two speakers with such a cone area is required). Although this may be at the expense of directional cues for the lower frequencies, this is typically acceptable and even insignificant as human perception is predominantly sensitive to high frequency directional cues but is insensitive to low frequency directional cues. Thus the approach may allow a single speaker unit to provide both a spatial experience and an efficient low frequency response while maintaining a very compact structure. Furthermore, cost may be substantially reduced as only a single speaker unit is required.
The system may for example advantageously be used in compact consumer devices such as flat screen televisions or portable radios.
Fig. 2 illustrates the speaker unit 101 in more detail and specifically shows a cross section of the speaker unit 101 mounted in an enclosure 201
The speaker unit 101 comprises a mounting frame 203 which is used to fixate the speaker unit 101. In the specific example, the speaker unit 101 is mounted in an enclosure 201 which for example may be mounted in a flat screen television or a portable radio. In the example, the enclosure 201 further comprises a bass reflex port 205 which supports the speaker unit 101 to provide an improved low frequency response. The mounting frame 203 provides a rigid support for the elements of the speaker unit 101. The mounting frame 203 is fixedly and rigidly mounted in the enclosure 201, e.g. via suitable fixation points (e.g. the mounting frame 203 can comprise screw holes that can be used to fix the loudspeaker unit 101 to the enclosure 201 by screws).
The loudspeaker unit 101 furthermore comprises a cone 207 which is flexibly attached to the mounting frame 203. The cone 207 is in the example of Fig. 2 attached to the mounting frame 203 by a resilient or flexible mounting that allows the cone 207 to move relative to the mounting frame 203. Specifically, the cone 207 is fixed to the mounting frame via a rubberized surround 209.
The speaker unit 101 comprises two transducers that are arranged to convert electrical signals into movement of the cone 207. The first transducer comprises an element fixed to the mounting frame 203 and a moveable element which is attached to the cone 207 and which can move relative to the fixed element and thus the mounting frame 203. In the example, the fixed element is a permanent magnet 211 and the moveable part is a voice coil 213. Thus, when an electrical signal is fed to the voice coil 213 the resulting magnetic field causes the voice coil 213 and thus the cone 207 to move relative to the permanent magnet 211 and thus the mounting frame 203.
The second transducer similarly comprises a fixed element in the form of a permanent magnet 215 and a moveable element in the form of a voice coil 217 attached to the cone 207.
The first and second voice coils 213, 217 are attached to different elements of the cone 207 and thus will impart movement to the cone 207 differently.
The cone 207 has a relatively large cone area and may be arranged to have a perimeter which is non-circular. Specifically, the perimeter of the cone 207 (corresponding to the part of the cone which connects to the surround 209) has a non-circular shape which is closer to an oval or rectangular shape. Specifically the width as measured between the opposite perimeter points along the line through the central points of the two transducers (i.e. between the center points of the voice coils 213, 217) is longer than the height measured between the opposite perimeter points along a line perpendicular to this (e.g. measured at the center of the transducers or at a point between these centers).
Indeed, in many embodiments the width is at least twice that of the height and often it may advantageously be at least three, four or five times longer. This arrangement may provide a shape which is practical for integration in many consumer electronic devices. It may further in many scenarios ensure an improved spatial differentiation between the different sound sources rendered by the speaker unit.
Indeed, the circular shape of traditional loudspeakers have often been found to be impractical and it has been attempted to create loudspeakers with similar bass response but with a more rectangular or oval outline. A problem with such speakers has been found to be that it is more difficult to drive the cone symmetrically and sufficiently accurately to avoid distortion from non-symmetric movement of the cone in different areas (e.g. the transducer is not able to effectively move the furthest regions of the cone along the longest axis without causing movement to be distorted for the cone along the shortest axis. It has been proposed to provide a more uniform cone movement by moving the cone identically at two different cone points. Such a speaker may comprise two transducers attached to the cone at different points with both transducers being fed the same signal in order to drive the cone uniformly.
However, the system of Fig. 1 uses a completely different approach where different signals are fed to different transducers such that a single cone is used to provide multiple spatially separated sound sources. Indeed, the current approach explicitly seeks to drive the cone non-uniformly and with different movement at different parts of the cone in order to provide such multiple sound source positions from a single speaker element.
In the speaker unit of Fig. 2, the cone 207 can be considered to comprise three different parts.
A first part is the part of the cone which can be fully determined by the movement of the first voice coil 213. In the specific example, this part corresponds to the area of the cone which is within the perimeter of the attachment of the first voice coil 213, i.e. in the specific example, the first part corresponds to the dust cap or dome cone 219 attached to the first voice coil 213. The first part/ area of the cone will henceforth be referred to as the first dust cap 219.
It will be appreciated that as the first dust cap 219 is attached to the first voice coil 213 along the whole circumference of the first dust cap 219, exact knowledge of the movement of the first voice coil 213 will also allow the exact movement of the first dust cap 219 to be determined. For example, if an electrical signal is applied to the first voice coil 213 this will control the movement of the first voice coil 213 and this will further control the movement of the dust cap 219. If no signal is being applied to the first voice coil 213, movement may reach the first dust cap 219 from the movement of the second voice coil 217 and may cause the first dust cap 219 to move. However, even in this case, the movement of the first dust cap 219 can be determined from the knowledge of the movement of the first voice coil (213) as this will be made to move correspondingly.
It will be appreciated that the same comments can equally be applied to the second transducer and thus the cone 207 comprises a second part/ area which has a movement that can be fully determined from knowledge of the movement of the second voice coil 217. This second part corresponds to a second dust cap 221 in the specific example.
The cone 207 furthermore comprises a third part 223 for which movement cannot be fully determined only from knowledge of the movement of just one of the voice coils 213, 217. This third part 223 specifically comprises the remaining part of the cone 207, i.e. the part which is not part of the dust caps 219, 221. It will be clear that the movement any point of this third part 223 is not exclusively determined by the movement of only one of the voice coils 213, 217 but rather is dependent on the movement of both the voice coils 213, 217.
When in use, the high frequency components of the signal fed to the first transducer will predominantly be radiated by the first dust cap 219 and a small area of the third part 223 around the first voice coil 219. Similarly, the high frequency components of the signal fed to the second transducer will predominantly be radiated by the second dust cap 221 and a small area of the third part 223 around the second voice coil 221. However, the low frequencies of both the first and second signals are predominantly radiated by the third part 223 which typically constitutes the major area of the cone 207.
Thus, the speaker unit 101 will tend to radiate different sounds and from different parts of the cone 207 for different frequencies. At higher frequencies a localized and spatially differentiated sound for the two input channels are radiated from small areas within or adjacent to the voice coils 213, 217. For reducing frequencies, the effective area of the cone(s) 207 may gradually increase and for low frequencies the whole cone area is used for combined sound radiation for both channels. Thus, for increasing frequencies the spatial differentiation is gradually increased and for reducing frequencies the effective cone area is gradually increased. Thus, the behavior of the single speaker unit 101 is particularly advantageous for rendering of multiple spatial audio channels, such as a stereo signal, since the human perception is sensitive to directional cues for high frequencies but not for low frequencies whereas high audio quality at lower frequencies requires a larger cone area.
In the system, the sound pressure level provided by the third part 223 is thus higher than that of the dust caps 219, 221 at lower frequencies whereas the sound pressure level provided by the dust caps 219, 221 is higher than that of the third part 223 at higher frequencies. Thus, the third part 223 has a higher efficiency for the first electrical signal than the first cone area (the first dust cap 219) below a given crossover frequency and a lower efficiency for the first electrical signal than the first cone area (the first dust cap 219) above the crossover frequency. In many embodiments, the crossover frequency is advantageously between 150 and 600Hz and more advantageously between 200Hz and 300Hz (both endpoints of the intervals included).
In the system, the speaker unit has a channel separation of 10 dB or more between the two cone areas corresponding to the first part and the second part, i.e. between the first and second dust caps 219, 221 in the specific example. The channel separation may be measured as the difference between the sound levels being generated by the cone areas (dust caps) for the situation wherein only one of the transducers is driven.
For example, a test signal may be applied to the first voice coil 213 whereas no signal is applied to the second voice coil 217. The frequency may be selected relatively high, e.g. above 1 kHz. The sound level generated by the first dust cap 219 may then be determined (e.g. by analysis or measurement). However, in the scenario the movement of the first voice coil 213 may propagate through the cone 207 to the second dust cap 221 resulting in this part of the cone 207 also moving and thus generating sound. This sound level may then be determined (e.g. by analysis or measurement). The construction of the speaker unit 101 (e.g. the stiffness of the cone) is such that the difference between these sound levels is at least 10 dB, and in many embodiments even more advantageously at least 20 dB or even 30 dB. This may ensure an improved spatial experience from the single cone 207.
In some situations, the channel separation may be determined based on the relative movement/excursions of the two dust caps 219, 221 when only one of the voice coils 219, 221 is actively driven. For example, the excursion to the first dust cap 219 caused by movement of the second voice coil 217 is substantially less than that of the second dust cap 221. In many scenarios, the excursion of the first dust cap 219 may advantageously be less than a fifth or a tenth the excursion of the second dust cap 221.
The system can thus provide multiple spatially separated sound sources and high quality low frequency reproduction from a single compact speaker unit 101. Indeed, the approach has been found to provide a good user experience for very small speaker units 101 and in many embodiments, the width of the speaker unit 101 may be less than 20cm (and thus the maximum cross sectional distance may be less than 20 cm. Furthermore, the height may be less than 5 cm. Thus, the approach may allow high quality multi channel sound reproduction from an area less than 20 cm by 5 cm.
Practical implementations have been found to provide highly advantageous performance for heights of around 30 mm and widths between 135 mm and 180 mm. For these implementations, the dust caps had a diameter of around 15 mm. The implementations were found to provide both a perceptible stereo effect and a bass that extended to around 80 Hz in the specific enclosure. It was found that for a similar sized enclosure used with conventional speaker, the bass would typically only extend to around 200 Hz. Thus a very substantial low frequency extension was found to be achieved.
In the system of Fig. 1, the first electrical signal corresponds to an audio channel, and specifically the left audio channel of a stereo signal, whereas the second electrical signal corresponds to a second audio channel, and specifically the right audio channel of the stereo signal. In the example, this is the case in the entire frequency interval since the first and second electrical signals are generated to represent the signal in the first audio and second audio channels respectively over the entire frequency range. Thus, in the example, the combination of the signals at lower frequencies occurs in the cone 207 itself.
In some embodiments, the electrical signals driving the transducers may only correspond directly to the respective input audio channels in a limited frequency range. In such embodiments, the input audio channels may for example be combined in other frequency ranges when generating the electrical signals.
Fig. 3 illustrates an example of such an approach. The example corresponds to that of Fig. 1 but with a modified drive circuit 103. In particular, the input stereo channels are preprocessed to generate a combined signal in a frequency interval with the electrical signals driving the transducers being generated from this combined signal in this frequency interval. Outside the frequency interval, the electrical drive signals are predominantly generated from the input signals of the audio channels.
In the system of Fig. 3, the signal from the left input audio channel is fed to a low pass filter 301 which generates a low pass filtered signal having a cut-off frequency which may specifically be in the range from 300-600 Hz. Similarly, the signal from the right input audio channel is fed to a low pass filter 303 which also generates a low pass filtered signal having a cut-off frequency which may specifically be in the range from 300-600 Hz. In the system of Fig. 3, the two low pass filters 301, 303 are substantially identical. The low pass filtered signals are then combined in a combiner 305 (which in the specific example is a simple adder) thereby generating a combined signal. The combined signal is thus a mono signal generated from the input stereo signal in a low frequency interval.
The right low pass filtered signal from the first low pass filter 301 is furthermore fed to a first subtractor 307 which subtracts the right low pass filtered signal from the input right signal to generate a right residual signal. Similarly, the left low pass filtered signal from the second low pass filter 303 is furthermore fed to a second subtractor 309 which subtracts the left low pass filtered signal from the input left signal to generate a left residual signal.
The combiner 305 and the first subtractor 307 are coupled to a first combiner 311 which combines the right residual signal and the common combined signal to generate a right modified signal. Similarly, the combiner 305 and the second subtractor 309 are coupled to a second combiner 313 which combines the left residual signal and the common combined signal to generate a left modified signal.
Thus, a left and right modified signal is generated which corresponds to the original left and right input signals at the higher frequencies but which are identical for lower frequencies. Thus, a stereo signal is generated which corresponds to the original input stereo signal at higher frequencies but to a common mono signal at lower frequencies. The first and second combiners 311, 313 (which specifically may be summers) are coupled to the first and second drivers 105, 107 which generate the left and right drive signals from these modified signals.
Thus, in the example of Fig. 3, the spatial user experience is provided by rendering of the high frequency components from different parts of the cone 207 of the loudspeaker unit 101. However, at the lower frequencies, a common mono signal is provided resulting in a uniform mono sound being radiated. This may provide improved performance in many scenarios as a low frequency common driving of the cone may provide a more consistent movement of the cone 207 thereby e.g. requiring the cone 207 to flex less. Hence, cone distortions may be reduced resulting in improved low frequency performance.
In some embodiments, the drive circuit 103 may also receive a low frequency effects (LFE) channel. Thus, rather than a stereo signal, the input signal may be a 2.1 spatial signal. In this case, the LFE channel may also be rendered by the single speaker unit 101. Specifically, the drive circuit may generate both the first and second electrical signals not only from the left and right input signals but also in response to the signal of the LFE channel.
An example of a system implemented to render a 2.1 signal from the single speaker unit 101 is illustrated in Fig. 4. The example corresponds to the system of Fig. 3 but instead of generating the combined signal from the input signals, the LFE signal is used directly. Thus, the input to the first driver 105 is generated by combining the LFE signal and the right signal and the input to the second driver 107 is generated by combining the LFE signal and the left signal. Thus, the LFE signal is uniformly driving the two transducers of the speaker unit 101 resulting in an efficient and high quality rendering of the low frequency signal components forming the LFE signal.
It will be appreciated that the systems of Figs. 3 and 4 can easily be combined simply by combining the LFE channel signal and the combined signal from the combiner 305 and feeding the resulting signal to the combiners 311, 313.
It will be appreciated that the above description for clarity has described embodiments of the invention with reference to different functional units and processors. However, it will be apparent that any suitable distribution of functionality between different functional units or processors may be used without detracting from the invention. For example, functionality illustrated to be performed by separate processors or controllers may be performed by the same processor or controllers. Hence, references to specific functional units are only to be seen as references to suitable means for providing the described functionality rather than indicative of a strict logical or physical structure or organization.
The invention can be implemented in any suitable form including hardware, software, firmware or any combination of these. The invention may optionally be implemented at least partly as computer software running on one or more data processors and/or digital signal processors. The elements and components of an embodiment of the invention may be physically, functionally and logically implemented in any suitable way. Indeed the functionality may be implemented in a single unit, in a plurality of units or as part of other functional units. As such, the invention may be implemented in a single unit or may be physically and functionally distributed between different units and processors.
Although the present invention has been described in connection with some embodiments, it is not intended to be limited to the specific form set forth herein. Rather, the scope of the present invention is limited only by the accompanying claims. Additionally, although a feature may appear to be described in connection with particular embodiments, one skilled in the art would recognize that various features of the described embodiments may be combined in accordance with the invention. In the claims, the term comprising does not exclude the presence of other elements or steps.
Furthermore, although individually listed, a plurality of means, elements or method steps may be implemented by e.g. a single unit or processor. Additionally, although individual features may be included in different claims, these may possibly be advantageously combined, and the inclusion in different claims does not imply that a combination of features is not feasible and/or advantageous. Also the inclusion of a feature in one category of claims does not imply a limitation to this category but rather indicates that the feature is equally applicable to other claim categories as appropriate. Furthermore, the order of features in the claims do not imply any specific order in which the features must be worked and in particular the order of individual steps in a method claim does not imply that the steps must be performed in this order. Rather, the steps may be performed in any suitable order. In addition, singular references do not exclude a plurality. Thus references to "a", "an", "first", "second" etc do not preclude a plurality. Reference signs in the claims are provided merely as a clarifying example shall not be construed as limiting the scope of the claims in any way.

Claims

A sound reproduction system comprising:
a speaker unit (101) comprising:
a cone (207),

a first transducer (211, 213) for converting a first electrical signal to movement of the cone (207), and

a second transducer (215, 217) for converting a second electrical signal to movement of the cone (207); and characterized by comprising:

a drive circuit (103) for at least partially independently driving the first transducer (211, 213) and the second transducer (215, 217) by generating the first electrical signal to correspond to a first channel signal of a first spatial audio channel in at least a first frequency interval and generating the second electrical signal to correspond to a second channel signal of a second spatial audio channel in the first frequency interval, the first spatial audio channel being different than the second spatial audio channel.
The sound reproduction system of claim 1 wherein the drive circuit (103) is further arranged to receive a low frequency effects channel signal and to generate both the first electrical signal and the second electrical signal in response to the low frequency effects channel signal.
The sound reproduction system of claim 1 wherein the drive circuit (103) further is arranged to generate a combined signal by combining the first channel signal and the second channel signal in a second frequency interval, and to generate both the first electrical signal and the second electrical signal to correspond to the combined signal in the second frequency interval.
The sound reproduction system of claim 3 wherein the second frequency interval comprises lower frequencies than the first frequency interval.
The sound reproduction system of claim 1 wherein a first part of the cone is attached to a first moveable element (213) of the first transducer (211, 213) and a second part of the cone is attached to a second moveable element (217) of the second transducer (215, 217).
The sound reproduction system of claim 5 wherein the first part provides a first cone area (219) having a movement which is fully determined by the movement of the first moveable element (213), the second part provides a second cone area (221) having a movement which is fully determined by the movement of the second moveable element (217), and the cone (207) further comprises a third cone area (223) having a movement determined by the movement of both the first moveable element (213) and the second moveable element (215).
The sound reproduction system of claim 6 wherein a channel separation between the first cone area (219) and the second cone area (221) is at least 10 dB.
The sound reproduction system of claim 6 wherein the first moveable element (213) and the second moveable element (215) are voice coils and the first cone area (219) and second cone area (221) are areas of the cone (207) within perimeters of the voice coils.
The sound reproduction system of claim 6 wherein the third part (223) has a higher efficiency for the first electrical signal than the first cone area (219) below a crossover frequency and a lower efficiency for the first electrical signal than the first cone area (219) above the crossover frequency.
The sound reproduction system of claim 1 wherein a cone perimeter of the cone has dimensions of less than 5 cm by 20 cm.
The sound reproduction system of claim 1 wherein a largest cross section distance between points of a cone perimeter of the cone (207) is no more than 20 cm.
A method of driving a speaker unit (101) comprising a cone (207), a first transducer (211, 213) for converting a first electrical signal to movement of the cone (207), and a second transducer (215, 217) for converting a second electrical signal to movement of the cone (207); the method characterized by comprising:
at least partially independently driving the first electromagnetic transducer (211, 213) and the second electromagnetic transducer (215, 217) by generating the first electrical signal to be different than the second electrical signal, the driving generating the first electrical signal to correspond to a first channel signal of a first spatial audio channel in at least a first frequency interval and generating the second electrical signal to correspond to a second channel signal of a second spatial audio channel in the first frequency interval, the first spatial audio channel being different than the second spatial audio channel.