WO2009138936A1

WO2009138936A1 - A surround sound reproduction system

Info

Publication number: WO2009138936A1
Application number: PCT/IB2009/051910
Authority: WO
Inventors: Gerrit De Poortere; Frédéric ROSKAM; Werner P. J. De Bruijn
Original assignee: Koninklijke Philips Electronics N.V.
Priority date: 2008-05-15
Filing date: 2009-05-08
Publication date: 2009-11-19

Abstract

A surround sound reproduction system comprises a receiver (111) for receiving a surround sound signal comprising at least a centre channel signal and a plurality of spatial channel signals. A processor (113, 115) and an audio transducer (101) generates a centre sound signal from the centre channel signal in a centre audio beam and a set of audio transducers (103, 105, 107, 109, 113) generates spatial sound signals from the spatial channel signals. The centre audio beam (117) has a beam width of less than 30° in a frequency interval from 2 kHz to 8 kHz for 10 dB attenuation relative to a maximum gain of the audio beam. The use of a very narrow beam for the centre channel may allow a high quality surround sound experience in a listening zone (119) while at the same time reducing disturbance in a non-disturbance zone (121) by reducing speech components and increasing non- speech masking components in the non-disturbance zone (121).

Description

A surround sound reproduction system

Field of the invention

The invention relates to a surround sound reproduction system for reproducing central and spatial channels.

Background of the Invention

In recent years, the popularity of multi-channel surround sound systems has increased substantially and in particular surround sound systems for providing music or audiovisual experiences have become commonplace consumer products.

A surround sound system generates sound representing a plurality of locations typically including a front central position, front left/right positions and rear left/right positions. In addition, a low frequency (subwoofer) speaker is often included. The use of surround sound systems create an enhanced listening experience and has therefore become widely implemented. However, a problem with sounds systems and in particular surround sound systems is that the generated sound may be disturbing to other people in the sound environment. For example, if a group of people are enjoying a home cinema experience, other people in the same room tend to be highly disturbed when performing other activities.

Hence, an improved surround sound system would be advantageous and in particular a system allowing reduced disturbance to other users, high audio quality for users listening to the surround sound system, facilitated and/or improved implementation 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 surround sound reproduction system comprising: means for receiving a surround sound signal comprising at least a centre channel signal and a plurality of spatial channel signals, audio producing means for producing a centre sound signal from the centre channel signal in a centre audio beam; and a set of audio transducer means for generating spatial sound signals from the spatial channel signals; wherein the centre audio beam has a beam width of less than 30° in a frequency interval from 2 kHz to 8 kHz for a 10 dB attenuation relative to a maximum gain of the centre audio beam.

The invention may provide an improved listening experience in many scenarios. In particular, the invention may in many situations provide a high quality surround sound experience to listeners in a listening zone while reducing the perceived disturbance to users located in a non-disturbance zone.

In particular, the inventors have realized that speech is perceived as more of a disturbance for users than other types of sound, such as music, background noise, etc. In addition, the inventors have realized that speech signals in surround sound signals tend to be focused in the centre channel. The inventors have furthermore realized that radiating the centre channel signal in a narrow beam can provide high surround sound quality in a listening zone while substantially reducing the perceived disturbance outside the listening zone. Furthermore, by using a narrow beam for the centre channel to limit radiated speech signals outside the narrow beam, a substantial reduction of the perceived disturbance may be achieved not only due to the reduced speech level itself but also by an increasing masking effect achieved by the signals from the set of audio transducers.

In some embodiments, particularly efficient performance may be achieved for a beam width of less than 20° in a frequency interval from 2 kHz to 8 kHz for a 10 dB attenuation relative to a maximum gain of the audio beam.

In accordance with an optional feature of the invention, surround sound reproduction system of claim 1 wherein the audio producing means comprises: a plurality of sound radiating elements for radiating the centre sound signal; and a beam former coupled to the plurality of sound radiating elements and arranged to generate the centre audio beam by generating signals for the plurality of sound radiating elements from the centre channel signal.

This may allow improved performance and/or may facilitate implementation. The beam former may be arranged to change a direction of the audio beam e.g. in response to a user input. The feature may for example facilitate or allow a variation in a location of a listening area and/or non-disturbance area.

In accordance with an optional feature of the invention, each of the set of audio transducer means is arranged to radiate a spatial sound signal in an audio beam having a beam width of more than 60° in a frequency interval from 2 kHz to 8 kHz for a 10 dB attenuation relative to a maximum gain of the audio beam. This may allow particularly advantageous performance. In particular, the combination of a narrow centre channel beam and substantially wider audio beams (including possibly a substantially omni-directional radiation) for other channels may allow a particularly efficient masking of speech components thereby reducing the perceived distortion.

In accordance with an optional feature of the invention, the surround sound reproduction system further comprises a dynamic range compressor for compressing a dynamic range of the centre channel signal.

This may provide improved performance and may in particular in many embodiments reduce the perceived disturbance in a non-disturbance zone. The compression may in particular provide a synergistic effect with the narrow audio beam for the centre channel resulting in improved masking and reduced perceived disturbance by speech components.

In accordance with an optional feature of the invention, the dynamic range compressor comprises an absolute signal level compressor and a relative signal level compressor.

The feature may provide improved performance in many embodiments and scenarios. In particular, the system may provide an improved trade-off between suppression of sound components that may disturb other people in the environment and audio quality for listeners. In particular, the feature may allow improved suppression of short term and longer term sound peaks.

The particular compression may provide particularly advantageous operation with the narrow centre channel audio beam as the perceived disturbance in the non- disturbance zone may be effectively reduced. The absolute signal compressor may apply compression which depends on a single signal level estimate for the signal but does not depend on any other signal characteristics. The relative signal compressor may apply compression which depends on at least two signal characteristics which specifically may be two signal level estimates for the centre channel. In accordance with an optional feature of the invention, the relative signal level compressor comprises: first signal level estimation means for generating a first signal level estimate; second signal level estimation means for generating a second signal level estimate, the first signal level estimate being a longer term signal level estimate than the second signal level estimate; and means for adjusting a compression characteristic of the relative signal level compressor in response to a comparison of the first signal level estimate and the second signal level estimate.

This may provide particularly advantageous operation in many embodiments and may in particular allow an improved trade-off between audio quality degradation in a listening zone and perceived disturbance reduction in a non-disturbance zone. The feature may in many scenarios allow facilitated implementation.

In accordance with an optional feature of the invention, the relative signal level compressor is arranged to apply compression only if a difference between the second signal level estimate and the first signal level estimate exceeds a threshold. This may provide particularly advantageous operation in many embodiments and may in particular allow an improved trade-off between audio quality degradation in a listening zone and perceived disturbance reduction in a non-disturbance zone. The feature may in many scenarios allow facilitated implementation.

In accordance with an optional feature of the invention, the relative signal level compressor is arranged to perform compression of the centre channel signal subsequent to compression of the centre channel signal by the absolute signal level compressor.

In accordance with an optional feature of the invention, the relative signal level compressor is arranged to apply a higher compression ratio than the absolute signal level compressor.

In accordance with an optional feature of the invention, the dynamic range compressor comprises: means for classifying signal segments of the centre channel signal into content categories of a plurality of content categories, means for determining a signal level estimate for signal segments of a first content category of the plurality of content categories; and adjusting means for adjusting a compression characteristic of the dynamic range compressor in response to the signal level estimate. The feature may provide improved performance in many embodiments and scenarios. In particular, the system may provide an improved trade-off between suppression of sound components that may disturb other people in the environment and audio quality for listeners. The feature may in many scenarios allow facilitated implementation. The particular compression may provide particularly advantageous operation with the narrow centre channel audio beam and in particular the perceived disturbance in the non-disturbance zone may be effectively reduced.

The plurality of content categories may specifically contain one category corresponding to a desired content type and one category representing all other content types. The signal segments may correspond to predetermined time intervals or may e.g. be dynamically generated in response to characteristics of the centre channel signal. For example, a signal segment may be initiated in response to a detection that the centre channel signal matches a content criteria for the first content category and may be terminated in response to a detection that the content criteria is no longer met. In accordance with an optional feature of the invention, the adjusting means is arranged to set a threshold for the dynamic range compressor applying compression in response to the signal level estimate.

In accordance with an optional feature of the invention, the first content category is a speech content category.

The inventors have realized that the speech components are of particular high importance both for providing an acceptable experience to listeners and for reducing disturbance to other people, and that particular attractive performance can be achieved by specifically adapting the compression to the speech components of the surround sound signal rather than to the signals as a whole. It may also improve efficiency and improve the seamlessness of switching between applying compression or not. The means for classifying may specifically comprise a speech detector which detects speech content in the centre channel signal. E.g. if the speech detector detects speech in a signal segment in accordance with any suitable category, the signal segment is included in the first content category and otherwise it is not included in the first content category. In accordance with an optional feature of the invention, the first content category is a music content category.

The means for classifying may specifically comprise a music detector which detects music content in the centre channel signal. E.g. if the music detector detects music in a signal segment in accordance with any suitable category, the signal segment is included in the first content category and otherwise it is not included in the first content category. In accordance with an optional feature of the invention, the dynamic range compressor is arranged to adjust a compression characteristic of the dynamic range compressor in response to a source of the surround sound signal.

This may provide particularly advantageous operation in many embodiments and may in particular allow automatic adaptation of the operation to the specific use of the surround sound reproduction system.

According to an aspect of the invention there is provided a method of operation for a surround sound reproduction system, the method comprising: receiving a surround sound signal comprising at least a centre channel signal and a plurality of spatial channel signals, producing a centre sound signal from the centre channel signal in a centre audio beam; and generating spatial sound signals from the spatial channel signals; wherein the centre audio beam has a beam width of less than 30° in a frequency interval from 2kHz to 8 kHz for a 10 dB attenuation relative to a maximum gain of the audio beam.

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 surround sound reproduction system in accordance with some embodiments of the invention;

FIG. 2 illustrates an example of an angular amplitude response for a centre channel of a surround sound reproduction system in accordance with some embodiments of the invention;

FIG. 3 illustrates an example of an audio transducer for a centre channel of a surround sound reproduction system in accordance with some embodiments of the invention;

FIG. 4 illustrates an example of a dynamic range compressor for a centre channel of a surround sound reproduction system in accordance with some embodiments of the invention;

FIG. 5 illustrates an example of a compression characteristic for a dynamic range compressor for a centre channel of a surround sound reproduction system in accordance with some embodiments of the invention;

FIG. 6 illustrates an example of a compression characteristic for a dynamic range compressor for a centre channel of a surround sound reproduction system in accordance with some embodiments of the invention; and

FIG. 7 illustrates an example of a dynamic range compressor for a centre channel of a surround 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 surround sound reproduction system in accordance with some embodiments of the invention.

The exemplary surround sound reproduction system is a five channel surround system which comprises five audio transducers 101-109. Each sound transducer 101-109 may e.g. be an individual loudspeaker or may e.g. be a combination of several loudspeakers. Furthermore, the audio transducers 101-109 may be considered to include functionality for processing the individual channels including for example amplification, equalization etc.

In the example, the surround sound reproduction system comprises a centre channel audio transducer 101, a front left channel audio transducer 103, a front right audio transducer 105, a rear left audio transducer 107 and a rear right audio transducer 109. It will be appreciated that in other embodiments, a different number of channels may be included including for example side audio transducers and/or a low frequency audio transducer (sub woofer). It will also be appreciated that although the exemplary surround sound reproduction system of FIG. 1 uses a dedicated audio transducer for each channel, other embodiments may use virtual surround sound techniques. E.g. in some embodiments, the rear (surround) signals may be generated from directional audio signals radiated from audio transducer(s) in front of the listener and being reflected by obstacles in the audio environment to arrive at the listener from a sideways or rearwards direction.

The surround sound reproduction system comprises a signal receiver 111 which receives a surround sound signal comprising at least a centre channel signal and a plurality of spatial channel signals. In the example, the surround sound signal is a 5 channel surround sound signal corresponding to the five spatial positions of the audio transducers 101-109.

It will be appreciated that the surround sound signal may be received from any internal or external source and may e.g. be received from a DVD player, a television, a cable or satellite receiver etc.

In the specific example, the surround sound signal is a digital surround sound signal but it will be appreciated that in other embodiments the surround sound signal may be an analog surround sound signal.

The signal receiver 111 is coupled to a surround processor 113 which is arranged to decode the surround sound signal to generate the individual channels of the surround sound signal. In the specific example, the surround processor 113 decodes the digital surround signal to generate the five individual audio channel signals. The surround processor 113 is further coupled to the side and surround (rear) speakers 103-107 which are each fed the appropriate sound signal. Thus, spatial signals are generated by the surround processor 113 and fed to the spatial audio transducers 103-109 resulting in these radiating the corresponding sound signals. It will be appreciated that the surround processor 113 may comprise amplifiers, equalizers etc for processing the spatial signals before these are applied to the audio transducers 101-109.

In addition, the surround processor 113 generates a centre channel signal which is fed to a centre channel processor 115 coupled to the surround processor 113 and the centre channel audio transducer 101. The centre channel processor 115 is arranged to drive the centre channel audio transducer 101 to generate a centre sound signal corresponding to the centre channel signal.

Typically in conventional surround sound systems, the speakers are designed to radiate an audio signal which as far as possible corresponds to an omni-directional pattern. In particular, the centre speaker is designed to provide a substantially omni-directional sound signal in order to provide suitable surround sound signals in the audio environment.

However, in direct contrast to this approach, the centre channel processor 115 and/or the centre channel audio transducer 101 are designed to generate a centre channel sound signal which is emitted in a very narrow beam from the centre channel audio transducer 101. In particular, the centre channel audio signal is emitted in an audio beam which has a beam width of less than 30° in a frequency interval from 2 kHz to 8 kHz for a 10 dB attenuation relative to a maximum gain of the audio beam. The attenuation of the emitted sound signal is above 10 dB outside the audio beam (in the frequency interval from 2 kHz to 8 kHz) thereby resulting in the centre channel audio transducer 101 generating a sound signal which is emitted in a very narrow angle. An example of a generated audio beam is shown in FIG. 2 (for a frequency of 2067 Hz).

It will be appreciated that the directivity of the emitted centre channel sound signal will tend to be highly frequency dependent and that the centre channel audio beam may have wider beam widths outside the frequency range from 2 kHz to 8 kHz.

The surround sound reproduction system of FIG. 1 provides a relatively limited listening zone 119 wherein a high quality surround sound experience is achieved while at the same time providing a very efficient reduction of the disturbance to people outside the listening zone 119. Specifically, the inventors have realized that speech signals tend to be perceived as substantially more disturbing than other sound signals including e.g. music and background noise. Specifically, it has been found that people are very distracted by even low level speech signals as it is part of human nature to try to understand what is being said. Therefore, even at low sound levels, people will tend to subconsciously concentrate on the speech in order to understand the spoken words. Furthermore, the inventors have combined this realization with a realization that by reducing the signal level from a centre channel of the surround sound reproduction system, a particularly high attenuation of speech components can be achieved. Specifically, speech tends to be concentrated in the centre channel and therefore the signal level of any speech components received outside the listening zone 119 is reduced by using a narrow audio beam for the centre channel.

Furthermore, the inventors have realized that an improved masking of the residual speech components can be achieved by the sound signals emitted from the spatial speakers (103- 109). Accordingly, a very substantial reduction of the perceived disturbance to other people can be achieved while maintaining a high audio quality surround sound experience for listeners in the listening zone 119. Furthermore, this efficient disturbance reduction can be achieved by low complexity and can easy be implemented.

In the example of FIG. 1 the spatial audio transducers 103-109 i.e. the non- centre channel audio transducers are wide-angle audio transducers that radiate sound in a wide angle. In the system of FIG. 1, the spatial audio transducers 103-109 generate sound signals in an audio beam which has a beam width of at least 60° in a frequency interval from 2 kHz to 8 kHz for a 10 dB attenuation relative to a maximum gain of the audio beam. In some embodiments, the spatial audio transducers 103-109 are substantially omni-directional audio transducers. The use of wide-angle spatial audio transducers 103- 109 allows the radiated sound signals to provide efficient masking of speech components outside the listening zone 119. For example, the surround sound reproduction system of FIG. 1 may create an effective non-disturbance zone 121 wherein speech components are efficiently attenuated by the narrow audio beam used for the centre channel while at the same time being effectively masked by the sound signals from the spatial audio transducers 103-109 which typically contain background noise, music etc that tend to be perceived as much less of a disturbance. The system can thus create both an "experience" zone in which listeners can enjoy a full surround sound experience and a "non-disturbance" zone in which other people are less distracted by the generated sound. It will be appreciated that in some embodiments, the central audio transducer

101 may comprise a single sound element such as a single loudspeaker, and the narrow beam may be generated by the design of the sound element, i.e. a directional sound element may be used.

However, in the specific example, the central audio transducer 101 comprises a plurality of sound radiating elements 301 as illustrated in FIG. 3. The sound radiating elements 301 may for example be relatively small loudspeakers that are coupled to a beamforming unit 303 which receives the centre channel signal (the beamforming unit 303 may e.g. be considered part of the centre channel processor 115 and/or part of the centre channel audio transducer 101). The beamforming unit 303 is arranged to control the phase/delay of the signal being fed to each of the sound radiating elements 301 such that a narrow audio beam is generated. Furthermore, in the specific example, both the beam width and direction of the audio beam can be controlled by the beamforming unit 303. Thus, the systems allows for the direction of the audio beam 117 to be controlled thereby allowing the system to effectively move the listening zone 119 in the environment. In particular, the beamforming unit 303 is provided with a parameter indicating the desired beam width and direction and in response to this, a beamforming algorithm sets the phase/delay parameters for each sound radiating element 301 such that the desired audio beam is generated. It will be appreciated that many suitable beamforming algorithms are known to the skilled person and that any such algorithm can be used without detracting from the invention.

In the system of FIG. 1, the centre channel processor 115 comprises a dynamic range compressor which is arranged to compress a dynamic range of the centre channel signal. Specifically, the centre channel processor 115 can reduce the signal level of the centre channel signal when the signal level exceeds a given signal level threshold. For example, a gain factor for the centre channel signal may be reduced when the threshold is exceeded. The dynamic range compression may further reduce the perceived disturbance outside the listening zone 119.

Specifically, a listener in the listening zone 119 will typically adjust the audio volume of the system such that it is perceived to the listener to be at a comfortable level which often corresponds to a level that is not loud but allows speech dialog to be easily understandable. The described system reduces the disturbance to listeners outside the listening zone 119.

However, loud peaks or outbursts in the generated sound (e.g. explosions, loud shouts, etc) often still cause disturbance to people in the non-disturbance zone 121 and as the system reduces the perceived typical disturbance in this zone 121 this will tend to be perceived even more clearly than for conventional surround sound systems. However, such disturbing effects may be effectively reduced by introducing efficient dynamic range compression to the centre channel signal.

However, dynamic range compression may in some scenarios introduce some distortion or artifacts and the trade off between reducing the perceived disturbance to people in the non-disturbance zone 121 and reducing the quality degradation to listeners in the listening zone 119 is improved by using the following specific dynamic range compression techniques.

In some embodiments, the centre channel processor 115 can specifically comprise an absolute signal level compressor 401 followed by a relative signal level compressor 403 as illustrated in FIG. 4.

The absolute signal level compressor 401 is arranged to reduce the overall dynamic range of the input signal and specifically reduces the gain of the centre channel signal when the signal level of this exceeds a threshold. A specific example of the amplitude transfer characteristic of the absolute signal level compressor 401 is illustrated in FIG. 5 which shows the relationship 501 between an input signal level and an output signal level. In the specific example the compression ratio is 2:1, the threshold for applying the compression is -3OdBFS and the make-up gain is +5dB. Thus, for input signal levels below -3OdB of the full scale value, no compression is performed and an output gain of 5 dB is applied. For input signal values above -3OdB of the full scale value compression is applied such that a 1OdB input level increase results in only a 5dB increase in the output level. Thus, the absolute signal level compressor 401 provides a relatively low compression which is based only on the input signal level and without considering any other signal parameters.

The absolute signal level compressor 401 specifically comprises a variable gain amplifier 405 which applies an input signal level dependent gain to the centre channel signal. The absolute signal level compressor 401 furthermore comprises a level detector 407 which generates a signal level estimate for the input signal. The level detector 407 may for example be an amplitude peak detector, an RMS detector or any other suitable amplitude/power/signal level detector.

The level detector 407 is coupled to a gain setting processor 409 which is further coupled to the variable gain amplifier 405. The gain setting processor 409 compares the detected input signal level to the threshold and sets the appropriate gain. The gain setting processor 409 may e.g. be implemented using a look-up table relating input signal level estimates to appropriate gain values to achieve the transfer function 501 of FIG. 5.

The relatively mildly compressed centre channel signal is then fed to the relative signal level compressor 403. Thus, in the example, the relative signal level compressor 403 performs compression of the centre channel signal subsequent to compression of the centre channel signal by the absolute signal level compressor 401. The compression performed by the relative signal level compressor 403 depends on at least two parameters of the centre channel signal and in the specific example depends on the difference between a short term signal level estimate and a long term signal level estimate. Specifically, the relative signal level compressor 403 comprises a variable gain amplifier 411 which is arranged to apply a variable gain to the centre channel signal in order to implement the compression.

Furthermore, the relative signal level compressor 403 comprises a first signal level estimator 413 and a second signal level estimator 415. The first signal level estimator 413 is arranged to generate a slower, longer term signal level estimate whereas the second signal level estimator 415 is arranged to generate a faster, shorter term signal level estimate. Thus, the time constants, averaging or filtering performed by the two signal level estimators 413, 415 are different such that both a slower (longer averaging/ more low pass filtered) and a faster (shorter averaging/ less low pass filtered) signal level estimate are generated.

The first signal level estimator 413 and the second signal level estimator 415 are coupled to a comparator 417 which compares the slow and the fast signal level estimates. In the specific example, the comparator 417 is a simple subtractor generating the difference between the fast and the slow signal level estimates but it will be appreciated that in other embodiments other and more complex functions relating the two estimates to each other may be used. The difference signal is fed to a second gain setting processor 419 which is further coupled to the variable gain amplifier 411 and which is arranged to set the gain of the variable gain amplifier 411 in response to the difference signal.

In the specific example, the second gain setting processor 419 is arranged to generate a gain which does not apply any compression as long as the difference signal is below a given threshold. However, if the threshold is exceeded, a relatively heavy compression is applied.

A specific example of the amplitude transfer characteristic of the relative signal level compressor 403 is illustrated in FIG. 6 which shows the relationship 601 between an input signal level and an output signal level for a specific value of the slow signal level estimate of -3IdBFS. In the specific example, the difference threshold for applying the compression is 5dB corresponding to a compression threshold of -26dBFS for the input signal level (which typically corresponds to the fast signal level estimate).

In the specific example the compression ratio is 8:1, i.e. much higher than for the absolute signal level compressor 401. An output gain of 3 dB is applied below the compression threshold. Thus, the relative signal level compressor 403 provides a relatively high compression which is based on two signal level estimates having different filtering/averaging characteristics. Specifically, the slow signal level estimate may correspond to an average signal level estimate and the fast signal level estimate may correspond to a current signal level estimate. Thus, the relative signal level compressor 403 provides a relatively high compression to signal segments which sufficiently exceed the average signal level.

In some embodiments, the dynamic range compressor of the centre channel processor 115 is arranged to adapt the compression in response to characteristics of specific content segments of the centre channel signal rather than of the centre channel signal as a whole. Specifically, the centre channel processor 115 may divide the centre channel signal into different segments corresponding to different content types (speech, music, background noise etc) and can then set the compression based specifically on the characteristics of segments belonging to a specific content type. Thus, in this example the dynamic range compression is applied to the centre channel signal to reduce the signal level when this exceeds a signal level estimate of a certain class of content (for example speech). This may effectively reduce the disturbance of such signal segments while reducing the perceived quality impact to listeners in the listening zone 119. FIG. 7 illustrates an example of such a dynamic range compressor. The dynamic range compressor comprises a variable gain amplifier 701 which adjusts the gain applied to the centre channel signal to apply the compression.

The dynamic range compressor furthermore comprises an automatic speech classifier 703 which is arranged to classify signal segments of the centre channel signal into a content category of a plurality of content categories.

Specifically, the automatic speech classifier 703 can apply a continuous speech content detection algorithm to the centre channel signal and when the algorithm indicates that the centre channel signal predominantly comprises speech, the signal is included in a speech signal segment. As another example, the centre channel signal may be divided into signal segments of a predetermined duration and the speech detection algorithm may be applied to each signal segment to identify the segments that predominantly contain speech.

It will be appreciated that many suitable automatic speech classifier algorithms are known which are able to classify a signal segment as speech or non-speech by analyzing the signal itself. Examples may for example be found in United States of America patent US5878391.

The automatic speech classifier 703 is coupled to a signal level estimator 705 which determines a signal level estimate for the signal segments of a particular content category and specifically determines a signal level estimate for the speech signal segments detected by the automatic speech classifier 703. In the specific example, the signal level estimate is an average signal level but it will be appreciated that other signal level estimates may be used in other embodiments including e.g. a filtered peak power estimate, a filtered amplitude RMS estimate etc. The signal level estimator 705 is fed to a compression controller 707 which is further coupled to the variable gain amplifier 701. The compression controller 707 is arranged to adjust a compression characteristic of the dynamic range compressor in response to the signal level estimate generated by the signal level estimator 705. In the specific example, the variable gain amplifier 701 is arranged to not apply any compression below a given threshold and to apply compression above the threshold. In the example, the compression ratio is constant but the threshold at which the dynamic range compressor starts to apply compression is adjusted in response to the signal level estimate. Thus, the level at which compression is applied is dynamically modified depending on signal characteristics which are specifically related to speech components of the centre channel signal rather than to characteristics of the centre channel signal as a whole. Thus, the described dynamic range compressor may e.g. determine a long-term or cumulative average speech dialog level of the centre channel signal by first identifying the segments of the centre channel signal that contain (pure) speech dialog. The long-term or cumulative average level of these signal segments is then determined e.g. using very simple techniques (e.g. RMS) or more complex approaches.

As mentioned previously, a user of the surround sound reproduction system will typically use the level of speech as a reference when setting the volume level. At this volume level, input signal segments having a level equal to or lower than the average speech level will tend not to disturb people outside the listening zone 119 unacceptably. However, signals segments that are significantly louder than the average speech level will often be a possible disturbance. By setting the threshold parameter of the dynamic range compressor in relation to the average level of speech in the centre channel signal, the threshold may be set to more accurately reflect the perceived disturbance. The threshold may e.g. be set exactly at the average speech level or may be set with a suitable offset thereto (e.g. a few dB).

As an example, the system may have a high suppression such that (with the system set at a typical volume) even segments with a relative signal level x dB higher than the average speech level are still sufficiently suppressed in the non-disturbance zone 121. In this case, it may be suitable to set the threshold for the dynamic range compressor at a level that is x dB higher, so as to minimize the compression artifacts perceived by a listener within the listening zone 119. However, if the suppression in the non-disturbance zone 121 is more critical and only mild compression is used, it may be more beneficial to set the threshold a few dB's below the average speech level. It will be appreciated that the compression may not necessarily be in response to speech signal segment characteristics but may alternatively or additionally consider other content characteristics.

Thus, instead of speech, other types of content may be used for setting the threshold for applying compression. For example, a user who mostly listens to music may use music instead of speech as a reference for setting the volume of the system. Since music tends to be broadcasted louder than speech, it is in this case often more advantageous to use an average music level to set the threshold of the dynamic range compressor. This may allow reduced introduction of compression artifacts in lower-level signals, such as speech. In some embodiments, the adjustment of the compression characteristic of the dynamic range compressor may not only be in response to the signal level estimate of the class of content but may also be in response to a signal characteristic of the centre channel signal. Specifically, the compression characteristic may be adjusted in response to an estimated loudness level of the centre channel signal. Thus, the compression may be adapted in response to characteristics of both the whole signal and the specific content category.

In some embodiments, the signal level estimate may be an estimate which is a perceived signal level estimate. For example, the signal level estimate may be a perceived loudness estimate which is indicative of a perceived loudness of the signal segments in the selected content category. As a specific example, the signal segments may be weighted by a frequency-dependent sensitivity characteristic of the human hearing system when generating the signal level estimate. It will be appreciated that in other embodiments other processing, scaling, weighting, filtering etc of the signal segments may be included in the generation of the signal level estimate.

In some embodiments, the dynamic range compressor may be arranged to adjust a compression characteristic of the dynamic range compressor in response to a source of the surround sound signal.

For example, the signal level estimator 705 may individually track the long- term or cumulative average speech levels for individual television or radio channels, set-top boxes, DVD players etc thereby allowing the threshold of the dynamic range compressor to be individually optimized for the individual source. This may in many scenarios provide a substantial advantage since loudness levels between different sources may often vary substantially.

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

CLAIMS:

1. A surround sound reproduction system comprising: means (111) for receiving a surround sound signal comprising at least a centre channel signal and a plurality of spatial channel signals, audio producing means (101, 115) for producing a centre sound signal from the centre channel signal in a centre audio beam; and a set of audio transducer means (103, 105, 107, 109, 113) for generating spatial sound signals from the spatial channel signals; wherein the centre audio beam (117) has a beam width of less than 30° in a frequency interval from 2 kHz to 8 kHz for a 10 dB attenuation relative to a maximum gain of the centre audio beam (117).

2. The surround sound reproduction system of claim 1 wherein the audio producing means (101, 115) comprises: a plurality of sound radiating elements (301) for radiating the centre sound signal; and a beam former (303) coupled to the plurality of sound radiating elements (301) and arranged to generate the centre audio beam (117) by generating signals for the plurality of sound radiating elements (301) from the centre channel signal.

3. The surround sound reproduction system of claim 1 wherein each of the set of audio transducer means (103, 105, 107, 109, 113) is arranged to radiate a spatial sound signal in an audio beam having a beam width of more than 60° in a frequency interval from 2 kHz to 8 kHz for a 10 dB attenuation relative to a maximum gain of the audio beam.

4. The surround sound reproduction system of claim 1 further comprising a dynamic range compressor for compressing a dynamic range of the centre channel signal.

5. The surround sound reproduction system of claim 4 wherein the dynamic range compressor comprises an absolute signal level compressor (401) and a relative signal level compressor (403).

6. The surround sound reproduction system of claim 5 wherein the relative signal level compressor (403) comprises: first signal level estimation means (413) for generating a first signal level estimate; second signal level estimation means (415) for generating a second signal level estimate, the first signal level estimate being a longer term signal level estimate than the second signal level estimate; and means (419) for adjusting a compression characteristic of the relative signal level compressor (403) in response to a comparison of the first signal level estimate and the second signal level estimate.

7. The surround sound reproduction system of claim 6 wherein the relative signal level compressor (403) is arranged to apply compression only if a difference between the second signal level estimate and the first signal level estimate exceeds a threshold.

8. The surround sound reproduction system of claim 5 wherein the relative signal level compressor (403) is arranged to perform compression of the centre channel signal subsequent to compression of the centre channel signal by the absolute signal level compressor (401).

9. The surround sound reproduction system of claim 5 wherein the relative signal level compressor (403) is arranged to apply a higher compression ratio than the absolute signal level compressor (401).

10. The surround sound reproduction system of claim 4 wherein the dynamic range compressor comprises: means (703) for classifying signal segments of the centre channel signal into content categories of a plurality of content categories, means (705) for determining a signal level estimate for signal segments of a first content category of the plurality of content categories; and adjusting means (701, 707) for adjusting a compression characteristic of the dynamic range compressor in response to the signal level estimate.

11. The surround sound reproduction system of claim 10 wherein the adjusting means (701, 707) is arranged to set a threshold for the dynamic range compressor applying compression in response to the signal level estimate.

12. The surround sound reproduction system of claim 10 wherein the first content category is a speech content category.

13. The surround sound reproduction system of claim 10 wherein the first content category is a music content category.

14. The surround sound reproduction system of claim 4 wherein the dynamic range compressor is arranged to adjust a compression characteristic of the dynamic range compressor in response to a source of the surround sound signal.

15. A method of operation for a surround sound reproduction system, the method comprising: receiving a surround sound signal comprising at least a centre channel signal and a plurality of spatial channel signals, producing a centre sound signal from the centre channel signal in a centre audio beam; and generating spatial sound signals from the spatial channel signals; wherein the centre audio beam (117) has a beam width of less than 30° in a frequency interval from 2kHz to 8 kHz for a 10 dB attenuation relative to a maximum gain of the audio beam (117).