Abstract:
In order to permit three-component (left, right and back) audio signals (LS, RS, BS) to be conveyed by a three-channel (left, right and mono) transmission system and still permit stereo and mono compatibility, the signals are encoded so that the three channels carry LS-1/2BS, RS-1/2BS and LS+RS+BS, respectively. Upon receipt, the signals can be decoded back to LS, RS and BS respectively. In a mono receiver without such a decoder, the signal LS+RS+BS on the mono channel is employed, and in a stereo receiver the signals LS-1/2BS and RS-1/2BS on the stereo channels are employed.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to three-channel (stereo plus mono) audio signal transmission or conveyance systems and to three-component (stereo plus back) sound reproduction systems. 
     2. Description of the Prior Art 
     There are several known systems which convey, on three channels (LC, RC, MC), left and right stereo signals (LS,RS) and a mono signal (MS=LS+RS), such as terrestrial television systems with &#34;NICAM&#34;728 stereo and frequency modulated (FM) mono, and &#34;Video 8&#34; systems with pulse code modulated (PCM) stereo and FM mono. There is also known a system of sound reproduction using three sources which produce left and right stereo signals (LS,RS) and a background or &#34;back&#34; signal (BS), these signals subsequently being used to reproduce left and right stereo sounds and a back sound. 
     The present invention is concerned with reproducing three-component stereo/back signals conveyed using a three channel stereo/mono system. A simple way of doing this would be to convey the left and right stereo signal components (LS,RS) on the left and right stereo channels (LC,RC), respectively, and to convey the back signal component (BS) on the mono channel (MC). However, this presents a problem that mono compatibility is lost, because a mono receiver would receive merely the back signal (BS), whereas for satisfactory mono reproduction the reproduced mono signal should primarily be the sum of the left and right signals (LS+RS). 
     OBJECTS AND SUMMARY OF THE INVENTION 
     An object of the present invention is to provide an improved three-channel audio signal transmission and/or conveying system. 
     Another object of the present invention is to provide a system for reproducing three-component stereo/back signals conveyed using a three channel stereo/mono system without loss of mono compatibility. 
     A further object of the present invention is to provide a three-channel audio signal processing system with a minimum of circuit complexity. 
     According to one aspect of the present invention, the above problem is overcome and the above objects are achieved by conveying: 
     (i) on the mono channel (MC) a composite mono signal (MS&#39;) formed by an addition of the left and right stereo signals (LS,RS) and the back signal (BS), for example in generally equal proportions (MS&#39;=LS+RS+BS); 
     (ii) on the left channel (LC) a composite left signal (LS&#39;) formed by a difference between the left signal (LS) and the back signal (BS), for example with the back signal generally half-weighted (LS&#39;=LS-1/2BS); and 
     (iii) on the right channel (RC) a composite right signal (RS&#39;) formed by a difference between the right signal (RS) and the back signal (BS), again for example with the back signal generally half-weighted (RS&#39;=RS-1/2BS). 
     According to another aspect of the invention, there is provided an encoder for encoding a three component signal comprising left and right stereo signals (LS,RS) and a back signal (BS) for conveyance on a three-channel system, having left and right stereo channels (LC,RC) and a mono channel (MC), wherein the encoder is operable: to provide for the left stereo channel (LC) a composite left signal (LS&#39;) in which the left stereo signal (LS) and the back signal (BS) are weighted generally in the ratio 1:-1/2; to provide for the right stereo channel (RC) a composite right signal (RS&#39;) similarly formed from the right stereo signal (RS) and the back signal (BS); and to provide for the mono channel (MC) a composite mono signal (MS&#39;) in which the left and right stereo signals (LS,RS) and the back signal (BS) are generally equally weighted. 
     According to a further aspect of the present invention, there is provided a decoder for decoding composite left, right and mono signals (LS&#39;,RS&#39;,MS&#39;) received on left, right and mono channels (LC,RC,MC) of a three-channel signal conveying system to produce left and right stereo signals (LS,RS) and a back signal (BS), wherein the decoder is operable: to produce the left stereo signal (LS) by combining the composite left, mono and right signals (LS&#39;,MS&#39;,RS&#39;) with weightings generally of 3:1:-1, respectively; to produce the right stereo signal (RS) by combining the composite right, mono and left signals (RS&#39;,MS&#39;,LS&#39;) with weightings generally of 3:1:-1, and to produce the back signal (BS) by combining the composite mono, left and right signals (MS&#39;,LS&#39;,RS&#39;) with weightings generally of 2:-2:-2. 
     As will be appreciated from the following specific description of a preferred embodiment of the invention, the decoded left and right stereo signals and back signal can be formed so as to be equivalent to the originating left and right stereo signals and back signal. Further, a mono receiver on the mono channel (MC) will receive the composite mono signal (MS&#39;=LS=RS+BS), which provides good mono reproduction. 
     Moreover, a stereo receiver on the left and right channels (LC,RC) will receive the signals (LS-1/2 BS) and (RS-1/2 BS), having the effect of stereo reproduction with the back component inverted and placed centre stage. This provides reasonable stereo reproduction, especially if the back signal (BS) is independent of the left and right stereo signals (LS,RS) because the listener would be unaware of the inversion. 
     The above, and other objects, features and advantages of this invention will be apparent from the following detailed description of illustrative embodiments which is to be read in connection with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a circuit diagram of a three-channel encoder and transmitter embodying an aspect of the invention. 
     FIG. 2 is a circuit diagram of a three-channel receiver and decoder embodying another aspect of the invention. 
     FIG. 3 is a schematic diagram of a stereo receiver when receiving signals from the transmitter shown in FIG. 1. 
     FIG. 4 is a schematic diagram of a mono receiver when receiving signals from the transmitter shown in FIG. 1. 
     FIG. 5 is a schematic diagram of a three-channel encoder. 
     FIG. 6 is a schematic diagram of a three-channel decoder. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to FIG. 1 of the drawings, an encoder 10 is supplied with left and right stereo signals LS,RS and a back signal BS, and is operable to encode these signals to provide composite left, right and mono signals LS&#39;,RS&#39;,MS&#39; which are transmitted on three channels LC,RC,MC of a transmitter 12. The encoder 10 includes a summing amplifier 14 which sums all three input signals LS,RS,BS with equal weight to produce the composite mono signal MS&#39;=LS+RS+BS for the mono channel MC. The encoder 10 also includes a -6dB attenuator 16 which receives the input back signal BS to produce a signal 1/2BS, which is supplied to the inverting inputs of two differencing amplifiers 18,20. The left input signal LS is supplied to the non-inverting input of the differencing amplifier 18, which thus produces an output signal LS-1/2 BS, which is provided as the composite left signal LS&#39; for the left channel LC. Similarly, the right input signal RS is supplied to the non-inverting input of the differencing amplifier 20, which thus produces an output signal RS-1/2BS, which is provided as the composite right signal RS&#39; for the right channel RC. 
     Referring to FIG. 2, a three channel receiver 22 receives the signals from the transmitter 12 (FIG. 1), and respectively supplies the composite signals LS&#39;,RS&#39;,MS&#39; on the three channels LC,RC,MC to a decoder 24. The decoder 24 includes a summing amplifier 26 which sums the composite left and right signals LS&#39;,RS&#39; with equal weighting to produce a first intermediate signal S1, such that: 
     
         S1=LS&#39;+RS&#39;                                                 (1) 
    
     The first intermediate signal S1 and the composite mono signal MS&#39; are supplied to the inverting and non-inverting inputs, respectively, of a differencing amplifier 28, which produces a second intermediate signal S2, such that: ##EQU1## 
     The second intermediate signal S2 is supplied to a -6dB attenuator 30, which provides the output back signal BS&#34; from the decoder, such that: ##EQU2## 
     Bearing in mind that in the encoder in FIG. 1: 
     
         LS&#39;=LS-1/2BS                                               (6) 
    
     
         RS&#39;=RS-1/2BS                                               (7) 
    
     
         MS&#39;=LS+RS+BS                                               (8) 
    
     it follows from (5) to (8) that: 
     
         BS&#34;=1/2(LS+RS+BS-LS+1/2BS-RS+1/2BS)=BS 
    
     Therefore the output back signal BS&#34; from the decoder 24 of FIG. 2 is equivalent to the input back signal BS to the encoder 10 of FIG. 1. 
     The decoder 24 also includes a -6dB attenuator 32 which receives the back signal BS&#34; (=BS) and provides a third intermediate signal S3, such that: 
     
         S3=1/2BS&#34;                                                  (9) 
    
     Furthermore the decoder includes: a summing amplifier 34 which sums the third intermediate signal S3 and the composite left signal LS&#39; with equal weight to produce the output left stereo signal LS&#34; from the decoder; and similarly a summing amplifier 36 which sums the third intermediate signal S3 and the composite right signal RS&#39; with equal weight to produce the output right stereo signal RS&#34; from the decoder. Accordingly: ##EQU3## 
     Therefore, the output left and right stereo signals LS&#34;,RS&#34; from the decoder 24 of FIG. 2 are equivalent to the input left and right stereo signals LS, RS, respectively, to the encoder 10 of FIG. 1. 
     In the case where the three-channel signal transmitted by the transmitter of FIG. 1 is received by a compatible stereo receiver 38, as shown in FIG. 3, the receiver will output the composite left and right signals LS&#39;,RS&#39; on the left and right channels LC,RC. From equations (6) and (7) above it will be appreciated that these signals LS&#39;,RS&#39; are equivalent to LS-1/2BS and RS-1/2BS which will provide stereo reproduction with the back signal appearing inverted and centre stage. 
     In the case where the three-channel transmitted signal is received by a compatible mono receiver 40, as shown in FIG. 4, the receiver will output the composite mono signal MS&#39; on the mono channel MC. From equation (8) above, it can be seen that this signal MS&#39; is equivalent to LS+RS+BS, which will provide good mono reproduction. 
     It will be appreciated that many modifications and developments may be made to the encoder and decoder described above. For example, although the arrangement of summing amplifier, attenuator and pair of differencing amplifiers shown in FIG. 1 provides a preferred form of encoder, in view of the simplicity of the circuitry, other arrangements are possible which provide the transfer functions, shown in FIG. 5, of: 
     
         LS&#39;=a.(LS-1/2BS) 
    
     
         MS&#39;=a.(LS+RS+BS) 
    
     
         RS&#39;=a.(RS-1/2BS) 
    
     where &#34;a&#34; is an amplification constant. Furthermore, although the arrangement of summing amplifiers, differencing amplifier and attenuators shown in FIG. 2 is preferred, again in view of the simplicity of the circuitry, other arrangements are also possible which provide the transfer functions, shown in FIG. 6, of: 
     
         LS&#34;=b.(3LS&#39;+MS&#39;-RS&#39;) 
    
     
         BS&#34;=2b.(MS&#39;-LS&#39;-RS&#39;) 
    
     
         RS&#34;=b.(3RS&#39;+MS&#39;-LS&#39;) 
    
     where &#34;b&#34; is an amplification constant. 
     The invention is applicable to any system which provides at least three separate audio channels, such as the above-mentioned &#34;NICAM&#34; 728 or other three-audio-channel television systems, or &#34;Video 8&#34; systems. For practical purposes, the signal-to-noise ratios of the three channels should preferably be of comparable magnitudes. 
     Although illustrative embodiments of the invention have been described in detail herein with reference to the accompanying drawings, it is to be understood that the invention is not limited to those precise embodiments, and that various changes and modifications can be effected therein by one skilled in the art without departing from the scope and spirit of the invention as defined by the appended claims.