GB2187068A - An enhanced stereophonic sound reproduction system - Google Patents

Abstract

In an enhanced stereo sound reproduction system, at least one rear loudspeaker (28) is coupled to the output of difference signal generating means (26) arranged to generate a signal representing the difference between a left channel signal at a first input (10) of the system and a right channel signal at a second input (12) of the system. A switching device (22) is connected to means (14, 16, 18, 20) for sensing the level of the difference between the left and right channel signals, and is coupled in the signal path between the inputs (10, 12) and the rear loudspeaker (28) to interrupt one of the input signals. Interruption occurs when the difference between the input signals falls below a predetermined threshold, with the result that the rear loudspeaker (28) receives one only of the input signals. In an alternative embodiment, a signal representing the sum of the two input signals is fed to the rear loudspeaker when the difference falls below the threshold. <IMAGE>

Description

SPECIFICATION Sound reproduction system This invention relates to a sound reproduction system in which an audio programme is reproduced through a plurality of loudspeakers to provide the listener with improved realism compared with a conventional stereo reproduction system.

In the past a number of attempts have been made to extend the advantages of stereo reproduction over mono reproduction by providing three or more loudspeakers instead of the conventional two. Such systems have included quadrophonic systems in which eitherfour separately recorded channels are fed to respective loud-speakers, or four signals derived from a coded stereophonic recorded medium are resolved and then fed to four loudspeakers. Both systems require special transducers and amplification equipment, and specially prepared recorded material. One simple method of feeding an additional one ortwo loudspeakers placed to the rear of the listener is to obtain a "difference" signal byconnectingthe additional speaker or speakers between the left and right speakerterminals of a conventional stereo system.A resistance may be connected in series to vary the front-to-rear balance. This method has the disadvantage that it seriously affects the impedance presented to the amplifier outputs and can degrade the performance of the existing front pair of speakers.

It is also known to derive a difference signal which is then amplified to feed a rear speaker or speakers as shown for example in the periodical Hi-Fi News and Record Review January1971 (pages 58to 67).

The prior systems suffer from a number of drawbacks in that they provide no surround-sound or enhancement effects with mono programme material, they often require an additional set of volume, tone and balance controls for the difference channel, and they are difficultto connect two existing stereo apparatus.

It is an object of the present invention to provide an improved and relatively inexpensive surround-sound or enhancement system.

According to this invention, a sound reproduction system includes an electronic circuit arrangement having first and second signal inputs, means coupled to the inputsforgenerating a difference signal representing the difference between signals fed to the inputs, an output element, a switching device coupled between the inputs and the output element so that in a first switching state the difference signal is fed to the output element, and in a second switching state a signal representative of the inputsignalfed to one ofthe inputs or ofthe sum ofthe two signals fed to the inputs is fed to the output element, and sensing means arranged to sense the presenceofa difference between the inputsignals and to control the switching device whereby in the presence of such difference it is caused to adoptthe first switching state. In this way, if the output element, normally an output amplifier, is connected two a loudspeakerattherearofthe listening area, a normal stereo programme fed to a front pair of loudspeakers is enhanced by the reproduction of the difference signal by the rear speaker, whilst a programme with a mono orweakstereo content is enhanced by the reproduction of the same or a similar signal bythe rear speaker as is reproduced by the front speakers, the difference signal underthe latter conditions being zero or virtually zero.

The system of the invention may be constructed as a separate auxiliary unit for connection to a conventional stereo system (preferablyto the loudspeakerterminals), or it may be incorporated with conventional stereo circuitry in a single amplifier unit.

The invention will now be described byway of example with referencetothedrawingsinwhich:- Figure lisa block diagram of a preferred sound reproduction unit in accordance with the invention coupledtoa loudspeaker; Figure2 is a circuit diagram ofthe unit and loudspeaker of Figure 1; and Figure3 is a circuit diagram of a alternative unit in accordance with the invention.

Referring to Figure 1, the preferred embodiment of the invention comprises a unit having right (R) and left (L) signal inputs 10 and 12for connection to right and left channel outputs of a stereo source (not shown). From signals fed to these inputs, a difference signal is produced by an analogue subtractor 14.A rectifier 16, integrator 18, and a level detector 20 monitor the level ofthe difference signal, and if thesignal isabovea predeterminedthreshold a gate 22 in a signal path between the inputandthe output amplifier 24 feeds a left channel signal from input 12to a second analoguesubtractor 26 which is alreadyfedwith a right channel signal from input 10 so that in the presence of a difference signal above the threshold, a difference signal from the second subtractor26 is supplied to the output amplifier 24.

In the absence of a difference signal of sufficient amplitude, onlythe right channel signal reaches the output amplifier 24. Thus, a stereo signal applied to inputs 10 and 12 results in an amplified difference signal appearingattheoutputoftheunit,whilsta mono signal or a signal with a very weak stereo content results in the same or approximately the samesignal,butamplified,attheoutputasthat applied at the two inputs. With the output connected two a rear loudspeaker 28 and with the signalsfedto the inputs 10 and 12 being amplified and applied to a pair of front loud-speakers (not shown), a surround sound effect is obtained with both stereo and mono programme inputs.Other elements of the preferred unit include inputd.c. isolation stages 30,32, input buffer amplifiers 34,36 and a volume control 38 for controlling the level of the rear speaker output. It should be noted that thins volume control is connected so as not to affect the sensing of the difference signal by the chain of stages 14,16,18 and 20, and also that the circuit "tracks" the master volume tone and balance settings of the signal source apparatus.

Referring now to Figure 2,the cpmponents of the circuit blocks of Figure 1 are here $hown in more detail, and the reference numerals correspond to those of Figure 1. The input isolation stages 30,32 each comprise a pair of capacitors C1,C2 or C3, C4 coupled in series between a respective pairofinput terminals so thatthe inputs are independent of the polarity ofthe sources. The bufferamplifiers34,36 comprise simple op-amp voltage followers coupled to a firstsubtractor amplifier 14 as well as, via capacitors C5, C6 and volume control 38, to a second subtractor amplifier 26.The buffer amplifiers and subtractoramplifiers may all be contained in a single quad op-amp integrated circuit. The output of amplifier 14 is coupled to the difference signal detector comprising diode D1 and the integrator 18 comprising a 1 Mohm resistorR9,a 1 uFcapacitor C8, and a 10 Mohm resistor RIO. Thetime constant of the integrator, determined by R9 and C8 is about 1 second, so that shortterm variations in the difference signal do not affectthe output of the difference signal sensing stages. Schmitttrigger integrated circuits ST1 and ST1 b (e.g. type CD 40106) form the level detector 20 which controls the gate 22, an integrated circuit bilateral switch such as type number CD 4016.The outputamplifier is conventional, and uses an integrated circuit power amplifiertype No.LM380.

An alternative embodiment of the invention is shown in Figure 3. This has the same input isolation stages 30,32, butthese feed a summing amplifier circuit 40 and a difference amplifier circuit 42 in parallel. The output of the difference amplifier 42 is coupled to a rectifier, integrator and level detector arrangement similarto that of Figure 2 to control two transmission gates G1 and G2 in opposite phase by meansofan inverterBl.Thus,inthiscase,the output amplifier 24 and the rear speaker 28 are supplied alternatively with the sum (R + L) orthe difference (L - R) depending on the absence or presence respective of a significant difference signal.

If required, the invention can be embodied in a unit in which the majority of the circuitry is contained in a single custom built analogue integrated circuit.

Claims (7)

1. A sound reproductiop system including an electronic circuit arrangement having first and second signal inputs, means coupled to the inputs for generating a difference signal representing the difference between signals fed to the inputs, an output element, a switching device coupled between the inputs and the output element so that in a first switching state the difference signal is fed to the output element, and in a second switching state a signal representative ofthe input signal fed to one of the inputs or of the sum of the two signals fed to the inputs is fed to the output element, and sensing means arranged to sense the presence of a difference between the input signals and to control the switching device whereby in the presence of such difference it is caused to adaptthe first switching state.

2. An enhanced stereo sound reproduction system comprising a left channel input and a right channel input coupled respectively to a left channel amplifier and a right channel amplifier, means coupled to the said inputsforgenerating an amplified difference signal, a pair of front loudspeakers coupled respectively to an output of the left channel amplifier and to an output ofthe right channel ampiifier, and at least one rear Loudspeaker coupled to an output terminal connected to an output of the difference signal generating means, wherein the system further comprises a switching device coupled between the inputs and the rear loudspeaker or loudspeakers, sensing means arranged to sense the level or the difference between signals at the inputs and to control the switching device whereby, in the presence of a difference exceeding the said level, the switching device is caused to feed an output signal from the difference signal generating means to the rear loudspeaker, and in the absence of a difference exceeding the said level, the switching device is caused to feed a signal to the rear loudspeaker which corresponds to one of the signals at the inputs ortothesum ofthesignals at the inputs.

3. A system according to ciaim 1 orclaim2, wherein the switching device is coupled in series with an input of the difference signal generating means to interruptthe signal from one of the inputs in the absence of a difference orwhen the difference is below the said level.

4. A system according to claim 1 or claim 2, further comprises means for generating a sum of the signals present at the inputs, and wherein the switching device comprises a first signal switch coupled to an output ofthe difference signal generating means and a second signal switch coupled to an output of the sum generating means, the switches being operable in alternation to feed either a sum or a difference signal to the output terminal.

5. A system according to any preceding claim, wherein the sensing means comprises a rectifierfor rectifying a signal representing the said difference, and integrator and a level detector.

6. Asystem accordingto claim 4, including volume adjusting means coupled between the inputs and the difference signal generating means, on the input side ofthe switching device, the sensing means being arranged to receive a signal representing the difference between signals at the said inputs prior to their adjustment by the volume adjustment means.

7. A sound reproduction system constructed and arranged substantially as herein described and shown in the drawings.