GB2209110A - Sound reproduction system - Google Patents

Abstract

A sound reproduction system for producing a 'surround-sound effect' uses three loudspeakers. The signal fed to a third loudspeaker 28 is derived from the signals supplied to the left and right speakers but is subject to a variable delay 25. The third signal is the difference between the left and right signals where there is such a difference above a threshold, and otherwise corresponds to the, or the sum of the, equal signals fed to the other loudspeakers in the system. The delay 25 can be variable and use an analogue bucket brigade device. <IMAGE>

Description

SOUND REPRODUCTION SYSTEM This Invention relates to a sound reproducton system in which an audio programme fs 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 conver.t onal two. Such systems have -r,cluded quadrophonic systems in which either four separately recorded channel 5 are fed te 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 reccrded material.One simple method of feeding an additional one cr two loudspeakers placed to the rear of the listener is to obtain a "differerce" signal by connecting the additional speaker cr speakers between the left and right speaker terminals of a conventional stereo system. A resistance may be connected in series to vary the frcnt-to-rear balance.

This method has the disadvantage that it seriously affects the impendance presented to the amplifier outputs and can degrade the performance of the existing front pair of speakers.

It is also grown te deprive a difference signal which is then amplified to feed a rear speaker or speakers as shows for example in the period-cal Hi-Fi News and Record Review January 1971 (pages 58 to 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 cf volume, tone and balance controls for the difference channel, and they are difficult to connect to existing stereo apparatus.

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

According to this inventions a sound reproduction system includes 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 se that in a first switching state the difference signal is fed to the output element via delay means, and 4n a second switching state a signal representative of the 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 via delay means, and sensing means arranged te sense the presence of a difference between the input signals and te control the switching device whereby in the presence of such difference it is caused to adopt the first swItchIng state. In this way, if the output element, normally an output amplIfIer, is connected to a loudspeaker at the rear of the 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 or weak stereo content is enhanced by the reproduction of the same or a sImIlar signal by the rear speaker as Is reproduced by the front speakers, the difference signal under the latter condItIons beIng zero or virtually zero. Also, by Introducing delay means prior to the output element he acoustics of a large auditorium are simulated for both mono and stereo systems.

The system of the InventIon may be constructed as a separate auxiliary unit for connection to a conventional stereo system (preferably to the loudspeaker terminals) cr it may be Incorporated with conentIonal stereo circuitry in a single amplifier unit.

The invention will now be described by way of example with reference to the drawings in which:- Figure 1 is a block diagram of a preferred sound reproduction unit in accordance with the invention coupled to a loudspeaker; Figure 2 is a cIrcuIt dIagram of the unit and loudspeaker of Figure 1; Figure 3 is a block diagram of a variable delay circuit; and, Figure 4 is a circuit diagram of an 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 12 for 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 cf the difference signal, and if the signal is above a predetermIned threshold a gate 22 in a signal path between the input and the output amplifier 24 feeds a left channel signal from Input 12 to a second analogue subtractor 26 which is already fed with a right channel signal from input 10 so that in the presence cf a difference signal above the threshcld, a dIfference signal from the second subtractcr 2b Is supplied te the output amplifier 24 via a variable delay stage 25. In the absence of a difference sIgnal of sufficient amplitude, only the right charnel signal is fed through the variable delay stage 25 to the output amplifier 24.

Thus, a stereo signal applIed to inputs 10 and 12 results in an amplified difference signal appearing at the output of the unit, whilst a mono signal or a signal with a very weak stereo content results in the same or approximately-the same signal, but amplIfIed, at the output as that applied at the two inputs. With the output connected to a rear loudspeaker 28 and with the signals fed to the inputs 10 and 12 being amplified and applied to a pair of front loud-speakers (nct shown), a surround effect is obtained with both stereo and mccc programme inputs.

Also, agaIn for both stereo and mono programme inputs, by using the variable delay stage 25 in the sound reproductIon unit connected to the one or more rear loudspeakers, the acoustics of a large auditorium are simulated. Other elements of the preferred unit Include input d.c. isolation stages 30,32, input buffer amplifiers 34,36 and a volume control 38 for contrclllng the level of the rear speaker output. It should be noted that this volume control is connected so as nct 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 compcnents of the circuit blocks of FIgure 1 are here shown in more detail, and the reference numerals correspond to thcse 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 pair of input terminals so that the inputs are independent of the polarity of the sources. The buffer amplIfIers 34,36 comprise simple op-amp voltage follcwers coupled to a first subtractor amplifier 14 as well as, via capacitors C5, C6 and volume control 38, to a second subtractor amplifier 26.

The buffer amplifiers and subtractor amplifiers 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 resistor R9, a 1 uF capacitor C8, and a 10 Mohm resistor R10. The time constant of the integrator, determined by R9 and C8 Is about 1 second, so that short term variations in the difference signal do not affect the output of the difference signal sensing stages. Schmitt trigger integrated circuits STla and STib (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 output amplIfIer is conventional, and uses an integrated circuit power amplIfIer type No.LM380. As In the embodiment of Figure 1, a variable delay stage 25 is connected between subtactor amplifier stage 26 and amplifier 24. The variable delay stage 25 is shown in more detail in Figure 3 -and comprises an analogue multistage variable bucket brigade delay device 46 with pre-and post-amplifiers 44 and 48. The delay device 46 is clocked by a clock driver 50.

An example of an analogue variable bucket brIgade delay device is the MN 3011 integrated circuit (avaIlable from RS Components Limited) in whIch capacitors are used to provide the analogue storage in each of 3328 delay stages. Six tap outputs make available signals of different delays from each output.

The cverall delay can be varied either by switching between the varIous outputs of the bucket brigade device, each output correspondIng to a different number of delay stages, or by adjusting the oscillation frequency of a clock driver 50, which may be an integrated circuit type number MN 3101 (also avaIlable from RS Components Limited). An alternative embodiment of the invention is shown Figure 4. This has the same input isolation stages 30,32, but these 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 similar to that cf Figure 2 to control two transmIssIon gates G1 and G2 in opposite phase by means of an inverter B1. Thus, in this case, the output amplifier 24 and the rear speaker 28 are supplied alternatively with the sum (R + L) or the difference (L - R) dependIng on the absence cr presence respectively cf a sIgnificant difference signal. AgaIn a variable delay device 25 is inserted prior to the input to amplifier 24 to provide simulated large enclosure acoustics. 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)

CLAIMS:

1. A sound reproduction system comprising 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 put out element so that in a first switching state the difference signal is fed to the output element via delay means, and in a second switching state a signal representative of the 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 via delay means, 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 adopt the first switching state.

2. A sound reproduction system according to claim 1.

wherein the delay means has a selectively variable delay period.

3. A sound system according to claim 1 or 2, wherein the delay means is an analogue 'bucket brigade' delay device.

4. A sound reproduction system according to any one of the preceding claims, further comprising first, second and third loudspeakers, the input to said first loudspeaker corresponding to the signal applied to the first signal input, the input to said second loudspeaker corresponding to the signal applied to the second signal input, and the output element being or being connected to said third loudspeaker.

5. A sound reproduction system for connection to a sound source having a stereo output providing left and right signals for left and right loudspeakers, the system further comprising means for deriving a third signal from said left and right signals, delay means, and means for connecting said third signal through said delay means to a third loudspeaker.

6. A system as claimed in claim 5, wherein the means for deriving the third signal comprises sensing means for detecting the difference (if any) between the left and right signals, and means for outputting as said third signal the detected difference signal when a difference above a predetermined threshhold is sensed.

and for outputting as said third signal the left or right signal or the sum thereof otherwise.

7. A sound reproduction system substantially as herein described with reference to the accompanying drawings.