WO2003061343A2

WO2003061343A2 - Surround-sound system

Info

Publication number: WO2003061343A2
Application number: PCT/GB2003/000001
Authority: WO
Inventors: Andrew Matheson; Paul Thomas Troughton; Anthony Hooley
Original assignee: 1... Limited
Priority date: 2002-01-04
Filing date: 2003-01-02
Publication date: 2003-07-24
Also published as: GB0200149D0; WO2003061343A3; AU2003201020A1

Abstract

An audio system is described for producing a plurality of surround-sound channels in response to an audio input signal, comprising a first stand-alone amplifier system (103) adapted to receive a left and right channel signal from a source and to output signal levels of said left and right channel signals to drive left and right channel loudspeakers (101, 102); a second amplifier system (111) including a surround-sound decoder (212) to split said audio input signal into said left and a right channel signal and into at least three surround-sound channels and an amplifier section to increase the signal levels of said at least three-surround channels to drive surround-sound channel loudspeakers and maintaining signal levels of said left and right channel signals at input levels for said first amplifier stage. Both amplifier systems are minimally linked and preferable volume-controlled through the stereo system.

Description

SURROUND-SOUND SYSTEM FIELD OF THE INVENTION This invention relates to a surround-sound system having multiple channels for immersing a listener in a multi-directional sound field. More specifically, the invention pertains to a system that is capable of integrating a second, preferable preexisting, audio/video system into a system capable of producing surround-sound. BACKGROUND OF THE INVENTION Recent years have seen widespread use of multi-channel stereophonic sound in audio/visual systems. The trend in the technology has been away from conventional stereo sound reproduction systems, and towards "surround-sound" systems that reproduce sound field is dynamically (and intentionally) to the sides of and behind the listener.

To improve listener perceived characteristics, multi-channel sound reproduction systems are known which include one or more surround-sound channels (often referred to in the past as "ambience" or "special-effects" channels) in addition to left and right (and optimally, centre) sound channels. These systems are now relatively common in motion picture theatres and are becoming more and more common in the homes of consumers. A driving force behind the proliferation of such systems in consumers' homes is the widespread availability of surround-sound home video software, mainly surround-sound motion pictures (movies) made for theatrical release and subsequently transferred to home video media (e.g., digital video discs (DVDs), videocassettes, videodisks, and broadcast or cable television).

In the case where sound is reproduced in such a way as to provide a sound field expanding behind a listener or to localize a sound image behind a listener, two (front) loudspeakers are arranged to the left and right front of a listener for sound reproduction and at least one or two rear loudspeakers are additionally arranged behind the listener for surround reproduction. In addition, modern surround-sound systems may include a centre speaker arranged in front of the listener between the front left and the front right speaker. To improve sound quality, a low-frequency part of the audio signal may be directed to an additional sub-woofer. The exact position of the subwoofer with respect to a listener is not critical to the overall performance of the surround-sound system.

In ordinary homes, however, it is difficult to arrange five to six loudspeakers in a room. As new surround-sound systems are often incompatible with any existing stereo systems, a user is left with the choice of having two co-existing systems in one room (bringing the number of speakers up to seven or eight) or to discard the old system. This being clearly unsatisfactory, attempts have been made in recent years to reduce the number of speakers to generate surround-sound or to at least provide for a better integration between new surround-sound systems and any legacy stereo equipment.

The most advanced system aiming at reducing the number of hardware components is described in the commonly-owned published International Patent application No WO-0123104. In WO-0123104, an array of transducers generates a number of independently steerable sound beams. In operation the sound beams are directed at suitable locations of reflecting surfaces or walls left and right of a listener and towards the left and right corners of the wall behind the listener. The reflected sounds converge towards the listener position, the so-called "sweet spot", in very much the same manner as if projected from loudspeakers located at those positions. Hence, the system disposes of the need to have speakers in more than one location of the room.

On the other hand, efforts are known to provide upgrade equipment to integrate surround-sound capability with existing two-speaker stereo sets. However, the problems such equipment has to overcome is probably best described in the United States patent no. 6,026,168. In the '168 patent, an automatic volume synchronizing and regulating apparatus is included to match the output level of the legacy system with the output level of the additional surround-sound speakers without requiring further human intervention. However, the wiring between additional and legacy equipment according to the '168 patent requires a complex rewiring of the existing equipment to incorporate the new volume-tracking system. It is therefore an object of the invention to improve the apparatus and methods to combine existing stereo systems with surround-sound components.

It is another object of the invention to combine existing stereo systems with surround-sound components such that the surround-components can be installed with a minimum of interference with the existing system. It is yet a further object of the invention to exploit potential synergies between the legacy and the new components when producing surround-sound.

SUMMARY OF THE INVENTION In view of the above objects, the present invention provides a method and apparatus as claimed in the independent claims. According to a first aspect of the invention, there is provided an audio system for producing a plurality of surround-sound channels in response to an audio input signal, comprising a first stand-alone amplifier system adapted to receive a left and right channel signal from a source and to output signal levels of said left and right channel signals to drive left and right channel loudspeakers; a second amplifier system including a surround-sound decoder to split said audio input signal into said left and a right channel signal and into at least three surround-sound channels and an amplifier section to increase the signal levels of said at least three-surround channels to drive surround-sound channel loudspeakers and maintaining signal levels of said left and right channel signals at input levels for said first amplifier stage. Essentially the invention extends a conventional stereo system comprising for example an amplifier system that may include pre-amplifier and a power amplifier or just a single amplifier, input or peripheral stereo devices such as tuners, compact disc players, tape decks, phonographs etc. and a set of two loudspeakers into a full surround-sound system. In a surround-sound system the left and right channels/speakers of the conventional stereo equipment are augmented by three or more additional surround channels/speakers. The surround-sound extension system of the invention provides output signals for these additional channels, which are generally referred to as the centre channel, the surround (rear) left and the surround (rear) right channel, respectively. In addition to these channels, other channels may be added to generate a fuller surround-sound, such as subwoofer channels for low frequency sound or vertically stacked channels to generate a truly three-dimensional sound field.

To drive the additional channels the surround-sound extension system of the invention includes a surround-sound decoding section. Such decoder sets are readily available to reproduce Dolby Pro Logic(TM), Dolby Digital(TM) and Digital Theatre Systems or DTS surround-sound, Sony Dynamic Digital Sound or SDDS(TM) or the Direct Stream Digital(TM) encoding. Also included in the surround-sound extension system is an amplifier section for driving the surround speakers. The amplifier section can be by-passed for the left and right channels. Preferably, the decoder section and the surround signal amplifier share a housing and mains supply to facilitate its installation and integration into an existing stereo system.

The surround-sound extension system of the invention preferably transfers the audio signals exclusively between the L/R output ports of the extension system and L/R input ports of the stereo system. Hence, the only electrically conductive signal path between the two systems is through those ports, though a further cable may provide a common grounding if required. Any input signals from conventional stereo peripherals continue to be exclusively fed into the stereo system.

In a variant of the invention, the volume or gain of the extension system is set through the volume control of the stereo system and an automatic gain adjusting device in the amplifier section of the extension system. Thus, the extension system can be operated without an independent volume control interface. Preferably, the automatic gain adjusting system includes a signal generator mixing a test signal with one or both channels of the stereo system. The signal could be audible and, hence, used between or prior to the replay of surround-sound audio signals. However, in a preferred embodiment of this variant, the test signal does not interfere with the perception of the audio signal by a listener and is continuously or quasi-continuously added to it. By constantly tracking the volume level of the test signal, the volume of the surround channels can be instantaneously re-adjusted to match the user control setting of the stereo system. In a further embodiment of the invention, the low frequency content of the surround-sound signal is filtered from the remaining channels to be fed into the left and right channel or into an additional subwoofer channel. The cross-over frequency at which the filter suppresses low frequency content in the surround channel is a matter of design consideration. In a particular advantageous embodiment the surround-sound system includes a sound beam system similar to systems described in the commonly-owned published International Patent application No WO-0123104 which is incorporated herein by reference. Such systems comprise an array of transducers that, when appropriately controlled, project sound beams. Using reflection from walls or ceilings, virtual audio sources can be generated at any desired positions without having to place loudspeakers at such positions. Hence, including a sound beam system into the extension system reduces the number of components to add when upgrading an existing stereo system to surround-sound capability. Furthermore, the transducer array of the existing sound beam systems can be made smaller and less expensive when feeding the low frequency content to the left and right channels of the stereo system. Given that the stereo left and right channels will reproduce bass frequencies in a manner satisfactory to the user, it is possible to reduce the sound beam system in an extension system to even a one-dimensional or line array of transducers without compromising sound quality and/or steerabilty of the sound beam. These and other aspects of inventions will be apparent from the following detailed description of non-limitative examples making reference to the following drawings.

BRIEF DESCRIPTION OF THE DRAWTNGS In the drawings: Fig. 1 shows a block diagram illustrating elements of an apparatus in accordance with an example of the present invention;

Fig. 2 shows a block diagram illustrating further details of the apparatus of Fig l;

Figs. 3A-3D illustrate various configurations of the bass management system included in the apparatus of Fig 2; Fig. 4 is a schematic diagram of an automatic gain calculation and adjustment system;

Fig. 5 is a flow diagram illustrating the operation of the automatic gain calculation and adjustment system of Fig. 4; and Figs. 6A,B are variants of a surround-sound channel reproduction system for use in the present invention.

DETAILED DESCRIPTION The present invention provides a method and apparatus to enhance existing stereo audio equipment for the reproduction of an increased number of audio channels. On the other hand, it facilitates and reduces the required technical specification for a surround-sound system based on the apparatus described in WO- 0123104.

In the block diagram of Fig.1, there is shown a stereo system 100 including a set of loudspeakers 101 and 102 to reproduce the left and right audio signal channel of a conventional stereo audio signal. Each speaker may include one, two, three or even more acoustic transducers (or the stereo system may even include a sub-woofer) to reproduce the signal over the desired frequency range using techniques well known in the art. The loudspeakers 101 and 102 receive a left and right channel signal from a stereo amplifier system 103. The amplifier system may or may not consist of a pre-amplifier and power amplifier or may have power amplifiers dedicated to each loudspeaker again in a manner well known.

The amplifier system 103 has inputs dedicated to the various known sources of stereo (and mono) audio signals, such as compact disk players 104, turntable (phono), tuner, and others (often referred to as auxiliary or AUX) (not shown) left and right channel inputs.

While a stereo system as described above is well known as such, the invention provides an extension block 110 to be described in detail making reference to this Fig. 1 and the following figures.

The extension block 110 provides a surround amplifier system 111 for multi- channel audio signal input 112. Within the scope of this invention multi-channel signal is understood to encompass audio recordings that provide signal or data for more than the conventional left and right channel. Such multi-channel signals are often referred to as surround-sound and typically enable the reproduction of, but not limited to, not only a left and right channel but in addition of a center and surround (rear) left and surround (rear) right channel. For example, the above SDDS enables the reproduction of up to eight channels. Various surround-sound decoding systems are currently known under the various trademarks as mentioned above.

The use of the multi-channel audio signal input 112 is illustrated in Fig. 1 by connecting it from a DVD player 113 assumed to generate Dolby Digital 5.1 digital signals.

Outputs of the surround amplifier system 111 include a left and right audio channel output 114, 115 directly connected via a low- or zero-loss connection, e.g. via cable or wire, to inputs of the stereo amplifier system 103 and thus ultimately to ^• the left and right loudspeakers 101, 102. In addition, the amplifier system has the capability of feeding additional audio channel signals to a surround-sound reproduction system 116 that in turn reproduces some or all of the additional audio channels of the multi-channel input.

An optional sub-woofer output 117 is integrated into the amplifier system 111 to enhance bass frequency reproduction. Further details of the surround amplifier system 111 are shown in Fig. 2.

There the multi-channel audio signal is assumed to arrive as an encoded digital bitstream. The digital data enter a decoder system 212 that separates the signal into the various audio channel signals, to generate the various channels including left (L), right (R), the surround channels (center, surround left, surround right etc.) and the low frequency effect (LFE) channel.

The thus decoded signal enters a bass management system 213 that controls the distribution of low frequency content among the various audio channels. The bass management system 213 is a system including mainly low and high pass filters depending on the configurations to be described below. A second component of the amplifier system is a gain detection system 214. The gain detection system is designed to facilitate the control of the audio signal levels between the stereo amplifier system 103 and the surround amplifier system 111. In principle, a user could adapt the levels of both system using an independent volume control system on each system and his or her subjective impression of the sound field. However, it is desirable to provide a system where the volume control is determined by the setting of the volume control system of one of the system, and more preferably through the volume control of the stereo amplifier 103.

Hence, the gain detection system 214 includes a system 215 to automatically increase or reduce the gain of the surround channels, details of which are described below.

A fourth component of the surround amplifier system is a surround channel reproduction system 216. In the present example, this system is an amplifier and transducer panel capable of generating independently steerable narrow beams of sound to be reflected from walls and/or ceiling. By placing the transducer panel at the same location where a center loudspeaker would be placed and steering two beams such that the reflected sound appears to stem from the rear corners of a room, a complete surround-sound is created in the middle of the room. Such systems are available and can be purchased, for example, from the applicant 1... Limited. Further details of a surround channel reproduction system 216 are described below. The surround channel reproduction system 216 is preceded by an equalization stage 217 to compensate the surround-sound audio channel signals levels (separately from the L, R and SUB signal channels) according to the frequency response of the surround channel reproduction system 216 that follows.

Before the signal is fed to the various outputs of the surround amplifier system, delays stages 218, 219 are introduced into the output of the L, R signal path and the optional SUB output. These delay sections compensate for the processing and acoustic delays in the surround channel reproduction system 216 and hence ensure that all the audio channel signals from the loudspeakers and transducers arrive at approximately the same time at a given listener position. Without the delay stages 218, 219, the L and R (and SUB) signals are likely to arrive earlier at a listener position than the surround signals, thus distorting the sound reproduction.

When a user is prepared to tolerate adding further loudspeakers to the system, the surround channel reproduction system 216 can be replaced by a conventional surround-sound audio amplifier system having distinct outputs for each of the center and surround-sound channels.

In Figs. 3A-3D, various configurations of the bass management system 213 are illustrated. The bass management system 213 receives L, R, Surround and optional LFE signals as inputs.

In a first configuration, illustrated in Fig. 3 A, there is no subwoofer attached to the surround channel reproduction system 216 and it is assumed that the surround channel reproduction system has a good bass response or low frequency reproduction capability. In such a configuration, the LFE signal after some gain adjustment 311 is added to the L and R channel. The number n of surround channels are passed through after being gain-adjusted. In a second configuration, illustrated in Fig. 3B, there is no subwoofer attached to the surround channel reproduction system 216 and it is assumed that the surround channel reproduction system has a poor bass response or low frequency reproduction capability. In such a configuration, the n surround channels are band split using a high pass and a low pass filter 312, 313 and their low frequency content is added together with the LFE signals to the L and R channels. The high frequency content is passed on to the surround channel reproduction system.

In the third and fourth configuration as illustrated by Fig. 3C and Fig. 3D, respectively, a subwoofer is connected to the surround channel reproduction system 216 for bass reproduction. Hence, low frequency content is fed into the subwoofer channel SUB.

In the third configuration, the surround channel reproduction system is assumed to have a good bass response or low frequency reproduction capability. In such a configuration, the n surround channels and the L,R channels are both band split using high pass and low pass filters 312, 313 and their low frequency content is added together with the LFE signals to the SUB channel. The high frequency content is passed on.

In the third configuration shown in Fig 3C, the surround channel reproduction system is assumed to have a good bass response or low frequency reproduction capability. In such a configuration, the n surround channels and the L,R channels are both band split using high pass and low pass filters 312, 313 and their low frequency content is added together with the LFE signals to the SUB channel. The high frequency content is passed on.

In the fourth configuration shown in Fig 3D, the surround channel reproduction system is assumed to have a poor bass response or low frequency reproduction capability. In such a configuration, the n surround channels enter a first band-split system of high and low filters 312, 313 and their low frequency content is added the L, R channels. The combined L, R signals is then band split in a second system of high and low filters 312, 313 and fed together with the LFE signals to the SUB channel. After establishing the final (frequency) content of the L and R channels through the bass management system 213, these channels input into the gain detection system 214. As illustrated by Fig 4, the gain detection system adds noise to the L,R channel signals. The noise generator 411 is capable of generating additional signals which are designed to be largely imperceptible to the audience, and typified by low level pseudo-random noise sequences, which are superimposed on the programme audio signals. The noise can be white or pink, or shaped to the human hearing threshold, for example using the Fletcher-Munson curves, or constrained to the frequency bands where the human ear is known to have a low sensitivity.

The noise is inserted into the L,R channels at intervals and at a power level Gi set by a microprocessor 412 that is used to calculate Go. The control signal Go determines the respective gains of the surround (and optional SUB) outputs of the surround-sound amplifier system 111.

A microphone 413 is used to register the full audio signal present in the room. The various noise or test signals used are distinguishable from other sound emitted by the left and right channels by means of a cross-correlator 414. After a calibration step to be performed when setting up the system and using only test signals with no other sound input, the cross-correlator is capable of measuring the signal level of the noise signals together with a confidence associated with the gain measurement. Using the measured gain Gm, the confidence and the known input level Gi of the noise signals, a gain calculator 412 can establish the current gain or volume setting of the stereo amplifier system 100 and adjust the gain Go of the surround channels accordingly without user intervention.

The operating of the gain calculator is illustrated in the flow chart of Fig. 5. The process makes use of a confidence interval [CmimCmax] in which the inserted noise level is calculated with sufficient confidence. For each block of noise fed into the L, R channels, the confidence calculated using the cross-correlator 414, is established to be within this interval and or outside the boundaries of it. The gain level Go is set equal to the ratio of Gm over Gi. If the confidence is higher than Cmax, the gain level Gi of the injected noise is lowered. If the confidence is lower than Cmin, the gain level Gi of the injected noise is raised provided the level of Gi remains lower than a pre-set Gmax. When Gi exceeds Gmax, the system assumes that the stereo amplifier system 103 is either switched off or set to an input different to the L,R signal of the surround-sound amplifier system 111, e.g. the CD input 104. In addition, the cross-correlation can be used to determine and adjust any delays between the stereo system 100 and the surround system 110.

Once the gain of the surround-sound channels are established and set, they are fed into a surround channel reproduction system 116, 216. It is seen as advantageous to use in combination with the surround-sound amplifier system Ili a surround- sound reproduction system 116, 216 as described for example in WO-0123104. These known surround channel reproduction systems use beams generated using an array of transducers located at a single location to create a sound field usually requiring the presence of at least one or two separate loudspeaker, e.g., right and left surround speakers, located behind a listener.

The basic elements of the surround channel reproduction system generating beams are illustrated in Fig. 6 A. In Fig. 6A, the surround channels 63 provide the input to a multi-channel sample rate converter 64 for conversion to a standard sample rate and bit length. The outputs of the sample-rate-converter stage 64 are combined into a single high-speed serial signal comprising all six channels. In case of a conventional stereo input, only two of these may contain valid data.

The serialized data enters Digital Signal Processing (DSP) unit 65 to further process the data. The unit comprises a pair of commercially available Texas Instruments TMS320C6701 DSPs running at 133MHz and performing the majority of calculations in floating point format. The first DSP performs filtering to compensate for the irregularities in the frequency response of the transducers used. It provides four-times over-sampling and interpolation to remove high-frequency content generated by the oversampling process.

The second DSP performs quantization and noise shaping to reduce the word length to nine bits at a sample rate of 195kHz.

The output from the second DSP is distributed in parallel using bus 651 to eleven commercially available Xilinx XCV200 field programmable gate arrays (FPGAs) 66. The gate arrays apply a unique time delay for each channel and for each transducer. Their output is a number of different versions or replicas of the input, the number being equal to the number of transducers times the number of channels. As the number of transducers 611-1 to 611-n in this example is 132, several hundred different versions or replicas of the input are generated at this stage. The individual versions of the channels are summed at adders 67-1 to 67-n for each transducer and passed to pulse width modulators (PWM) 68-1 to 68-n. Each pulse width modulator drives a class-D output stage 69-1 to 69-n whose supply voltage can be adjusted to control the output power to the transducers 611-1 to 611-n.

System initialisation is under the control of a micro-controller 691. Once initialised the micro-controller is used to take direction and volume adjustment commands from the user via an infrared remote controller (not shown), display them on the system display, and pass them to the third DSP 692. The third DSP in the system is used to calculate the required time delay for each channel on each transducer to be able to steer, for example, each channel into a different direction. For example, a first pair of channels can be directed to the right and left side- walls (relative to the position of the surround-sound reproduction system ) of a room while a second pair is directed to the right and left of the rear-wall to generate a surround-sound. The delay requirements, thus established, are distributed to the FPGAs 66 over the same parallel bus 651 as the data samples. Most of the above steps are described in more detail in WO-0123104.

In Fig. 6B, an additional filtering process 61 is added to the signal path of Fig. 6 A. It should be noted that in order to put emphasis on the changes introduced by the present invention the same reference numerals and characters designate like parts in Figs. 6A and 6B, respectively.

In Fig. 6B, digital filters 61-1 to 61-n are applied after the signals have been separated according to channel and added together. The output of the digital filter stage is sent to the PCM stage 68-1 to 68-n of each of the transducers 611-1 to 611-n. The digital filters 61-1 to 61-n can be implemented by separate DSPs or gate arrays, or, in fact, may just be included into other signal processing devices 65, 66.

The additional filters can be used to generate beams with a frequency independent beam width as described in more details in the PCT application No. PCT/GB02/004605 which is incorporated herein by reference.

The use of a sound beam system facilitates the set-up of a system in accordance with the present invention. The stereo amplifier system 100 with its left and right channel loudspeakers, on the other hand, provides the possibility to reduce the technical requirements for the sound beam system. As the stereo system includes a left and right channel and likely to have a sufficiently good bass response, the sound beam system can be limited to reproduce the center and left and right surround channels. In this case, the number and the quality of the transducers in the sound beam system can be greatly reduced and rearranged. The reduced number of beams to be generated and the reduction of low-frequency content correlates directly with the required size or dimensions of the transducer array. For example, the sound beam syste when used in combination with the conventional stereo system 100 can be reduced to a very narrow two-dimensional array of transducers or even a (single) line array of transducers while maintaining the surround-sound quality offered by a much bigger two-dimensional array.

Claims

1. An audio system for producing a plurality of surround-sound channels in response to an audio input signal, comprising a first, stereo, amplifier system adapted to receive left and right channel signals from a source and to generate output signals of said left and right channel signals to drive left and right channel loudspeakers and a second surround-sound amplifier system including a surround-sound decoder to split said audio input signal into said left and a right channel signal and into at least three surround-sound channels and an amplifier section to increase the signal levels of said at least three- surround channels to drive surround-sound channel loudspeakers and maintaining signal levels of said left and right channel signal at input level for said first amplifier stage, wherein the stereo amplifier system is a unit adapted to independently reproduce stereo audio signals from a stereo audio source.

2. The audio system of claim 1 wherein the volume setting of the stereo amplifier determines the volume setting of the surround-sound amplifier system.

3. The audio system of claim 1 or 2 comprising an automatic gain detection system.

4. The audio system of claim 3 wherein the automatic gain detection system comprises a microphone to detect test signals in an output of the left and/or right channel loudspeakers.

5. The audio system of claim 3 wherein the automatic gain detection system comprises a cross-correlator to detect test signals in an output of the left and/or right channel loudspeakers.

6. The audio system of any one of the preceding claims having a direct electrical signal path between a left and right channel output of the surround-sound amplifier system and a left and right channel input of the stereo amplifier system.

7. The audio system of claim 6 wherein the signal path between a left and right channel output of the surround-sound amplifier system and a left and right channel input of the stereo amplifier system is the sole direct electrical signal path between both amplifier systems.

8. The audio system of any one of the preceding claims wherein the surround- sound amplifier system comprises a bass management system to distribute low frequency audio signals from an input surround-sound channel to the left or right channel or a subwoofer channel.

9. The audio system of any one of the preceding claims wherein the surround- sound amplifier system includes a surround-sound reproduction unit capable of generating steerable beams of sound and wherein said sound beams are used to reproduce at least one or two of the surround-sound channels using reflections off surfaces.

10. The audio system of claim 9 wherein the surround-sound reproduction unit comprises a two-dimensional array of transducers.

11. The audio system of claim 9 wherein the surround-sound reproduction unit comprises a one-dimensional array of transducers.

12. A surround-sound amplifier system adapted to be integrated as second amplifier system into an audio system in accordance with any one of the preceding claims.

13. A method to generate surround-sound by adding to a stereo amplifier system a surround-sound amplifier system in accordance with claim 12.

14. A method to generate surround-sound comprising the steps of connecting a first stereo amplifier system adapted to receive said left and right channel signals from a source and to generate output signals of said left and right channel signals to drive left and right channel loudspeakers and a second surround-sound amplifier system including a surround-sound decoder to split said audio input signals into said left and a right channel signals and into at least three surround-sound channels; and using an amplifier section to increase the signal levels of said at least three surround-sound channels to drive surround-sound channel loudspeakers while maintaining signal levels of said left and right channel signals at input level for said first amplifier stage.

15. The method of claim 14, further comprising the step of automatically controlling the output signal levels of the surround-sound channels in accordance with the output levels of the left and right channel signals.