GB2145307A - Sound reproduction system - Google Patents

Abstract

Enhanced stereo effects are provided by an audio processing unit 18 which is connectible to left ad right channel signal outputs of a stereo amplifier 12. These outputs may be the loudspeaker terminals of the amplifier 12. A differencing stage 22 in the audio processing unit has a high input impedance to avoid coloration of the output signals of amplifier 12 and has dual capacitor coupled inputs which are insensitive to polarity so that the unit can be connected by unskilled persons. The difference signal obtained from differencing stage 22 is amplified and fed to a rear loudspeaker 20 to produce an enhanced sound field. <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 either four separately recorded channels are fed to respective loudspeakers, 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 or two loudspeakers placed to the rear of the listener is to obtain a "difference" signal by connecting the additional speaker or speakers between the left and right speaker terminals of a conventional stereo system. A resistance may also 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 an object of this invention to provide a system which does not require the special equipment and material of known quadrophonicsystems butwhich avoids degradation of an existing reproduction system.

According to a first aspect of th is invention, a sound reproduction system comprises the combination of a stereo reproduction system having a stereo signal source, a stereo amplifier and a pair of loudspeakers, wherein signal outputs of the amplifier are connected to an audio processing unit including an auxiliary amplifier, the audio processing unit being arranged to produce a difference signal at an output thereof, the output being coupled to one or more additional loudspeakers. The audio processing unit comprises means for combining signals Land R from respectively the left channel and right channel of the stereo amplifier to produce, for example, a difference signal |L| IL |R| - IR or IRI - ILl, where IL and IRI are the normalised moduli of the amplitudes of Land R.This difference signal is then amplified and applied to one or more loudspeakers placed preferably to the rear of the listener. In a preferred embodiment of the invention, the audio processing unit has two pairs of input terminals, one for receiving a left channel output signal and one for receiving a right channel output signal. The terminals of each pair are connected by a respective series impedance element to an input of a differential amplifer, so that the polarity of connection of each pair of terminals is unimportant. The impedance elements may be resistors, but are advantageously capacitors to provide d.c. isolation as well as input symmetry.

In accordance with the invention, the system may be incorporated with conventional stereo amplifier circuitry in a single unit, or it may be constructed as a separate auxiliary unit for connection to a preexisting stereo system. Thus, the invention also includes an audio processing unit for connection to a stereo reproduction system, comprising a differential amplifier having a first input coupled to a first pair of input terminals for connection to a left signal output of the system, and a second input coupled to a second pair of input terminals for connection to a right signal output of the system, and an output for connection to an auxiliary loudspeaker or loudspeakers for reproducing a signal corresponding to the difference between a signal at the left output and a signal at the right output.

The unit has the advantage that it can be added to a stereo reproduction system at relatively low cost, and can enhance the audio effect obtained from conventional stereo programme sources such as disc, cassette, reel-to-reel tape and VHF radio. It does not depend in any way on the use of special source material, and the user can revert to conventional stereo or mono by inhibiting the enhanced effects which the system provides.

The invention has particular application in domestic and personal stereo systems, public address systems and vehicle stereo equipment.

The invention will now be described by way of example in the accompanying drawings, in which: Figure l is a simplified block diagram of a sound reproduction system in accordance with the invention; Figure2 is a diagrammatic plan view of a typical listening arrangement suitable for the system of Figure 1; Figure 3 is a circuit diagram of part of a first audio processing unit embodiment for use in the system of Figure 1; and Figure 4 is a circuit diagram of a second audio processing unit embodiment.

Referring to Figure 1, a sound reproduction system in accordance with the invention typically includes firstly a conventional stereo reproduction system having a stereo source 10 such as a disc player, tape recorder or radio tuner, a power amplifier 12 having a left channel and a right channel, and a pair of loudspeakers 14 and 16, and secondly an audio processing unit 18 feeding a third loudspeaker 20. The processing unit 18 in this case has an input and differencing stage 22 which is coupled to a power amplifier 24forfeeding the third loudspeaker 20. The input and differencing stage 22 is typically connected to the loudspeaker terminals of the amplifier 12, but may also be connected to other stereo signal outputs such as a headphone output or tape recorder output.Preferably, however, outputs which are subject to the operation of the volume control of amplifier 12 are used so that balance between the signal levels of the third loudspeaker 20 and the pair of loudspeakers 14 and 16 is maintained when the volume control is adjusted.

By arranging for the audio processing unit 18 to have a relatively high input impedance, for example in excess often times the output impedance of the stereo amplifier 12, and preferably much higher, the additional loading on the amplifier 12 is negligible and no significant degradation of the signals fed to the existing pair of speakers 14 and 16 occurs.

It will be appreciated that the audio processing unit 18 can be an auxiliary unit which can be connected to a separate existing stereo amplifier, or it may be a permanent part of a single piece of apparatus.

Referring to Figure 2, by feeding a difference signal to an additional loudspeaker or loudspeakers, an enhanced stereo effect can be obtained compared with the conventional two-speaker system. In a conventional stereo system the locus of apparent sound sources is a straight line ACB and the maximum angular separation of those sources (ml) is 180 , (with the listener at point C). With the additional speaker 20 at D a sound present only in the left stereo channel will be reproduced by both speakers 14 and 20 at A and at D, so that it will seem to the listener to originate from a position A' which is located along AD at a point determined bythe relative amplitudes of the signals fed to the two speakers 14 and 20. (For equal amplitudes, A' is halfway along AD.As the amplitude of D is increased, A' tends towards D). Similarly a sound present only in the right channel will seem to originate from B'. A sound common to both stereo channels will be suppressed in the extra speaker 20 at D (which is reproducing the difference between the channels) and will seem to originate from C, as in a conventional system. For each point on the conventional sound locus ACB there is a corresponding point on the new increased length locus A'CB' such that a sound moving from left to right across the stereo field will seem to the listener to follow the curved line A'C B', with a consequent 'surround sound' effect.

The maximum angular separation provided by this system (2 in Figure 2) depends on the relative amplitudes of the signals fed to the speaker at D compared to those at A & B, but can be up to 360 when the amplitude at D is very much greater than that at A and B, and is of the order of 270 when the amplitudes are equal.

Whilst the listener is shown in Figure 2 facing the two speakers 14 and 16, the effect described above are equally realisable if he faces the opposite direction. With the two dimensional field created by the system, effective stereo perception can be less dependent on listener position than with a conventional stereo system.

Referring to Figure 3, the input and differencing stage 22 has an operational amplifier IC1 (in this case, type No.TL091 manufactured by Texas Instruments) having nominal 100 kQ input resistances R1 and R2 connected between the two input pins 2 and 3 and respective pairs of d.c. isolating capacitors C1, C2 and C3, C4. The gain of the amplifier is set by a 100kin feedback resistor R3, and the d.c. voltage of the input pins is set by further 1 00kQ resistors R4 and R5 coupled to input pin 3. Each capacitor pair comprises two 0.1 FF capacitors coupled in series across a respective pair of input terminals 26 or 28, the capacitors acting as a potential divider having a tapping point coupled to a respective differential amplifier input.The symmetrical arrangement of the capacitors enables the inputs of each pair to be connected to a respective pair of outputs on the stereo amplifier 12 (Figure 1) either way round, so that the unit can be connected by unskilled persons without requiring a particular polarity. It is assumed that the amplifier IC1 and the stereo amplifier 12 connected to the input terminals 26 and 28 have a common earth connection. The output pin 8 of the op-amp IC1 feeds a power amplifier (not shown) via a 22nF capacitor C5 and 470kQ shunt resistor R6.

Resistor R2 is variable to allow adjustment of the relative levels of the right and left signals fed to the differential amplifier.

Referring to Figure 4, a modified processing unit which is particularly suitable for vehicle installation has a combined differencing and poweramplifica- tion stage based on integrated circuit IC2 (type No.

LM380N manufactured by National Semiconductors). The input configuration is similar to that of the embodiment shown in Figure 3, having 0.1 F capacitors C11 to C14 connected in pairs. In this case, however, the tap of each pair is coupled to a respective half of a ganged potentiometer VR1A, VR1 B for level adjustment relative to the left and right output signals from the stereo amplifier. The sliders of the potentiometer VR1 are coupled directly to the high impedance inputs of IC2, but are shunted to earth buy a common potentiometer VR2 which acts as a left to right balance control. VRl is a ganged 1 MQ logarithmic potentiometer, and VR2 is a 150kQ linear potentiometer. IC2 feeds loudspeaker 20 from pin 8 via shunt capacitor and resistor C15 (0.1 off) and R7 (2.7Q) and series capacitor C16 (470iF). Pin 1 is coupled to earth via a 0.47F capacitor C17. Pins 3,4, 5,7, 10, 11 and 12 are connected directly to earth.

Claims (13)

1. A sound reproduction system comprising the combination of a stereo reproduction system having a stereo signal source, a stereo amplifier and a pair of loudspeakers, wherein signal outputs of the amplifier are connected to an audio processing unit including an auxiliary amplifier, the audio processing unit being arranged to produce a difference signal at an output thereof, the output being coupled to one or more additional loudspeaker.

2. A system according to claim 1, wherein the audio processing unit has two pairs of input terminals each connected to a respective pair of output terminals of the stereo amplifier, the terminals of each input pair being coupled to a respective series connected pair of capacitors.

3. A system according to claim 2, including a differential amplifier having a pair of differential inputs each connected to a tapping point between the capacitors of a respective one of the pairs of capacitors.

4. A system according to any preceding claim, having one additional loudspeaker, the three loudspeakers being placed at the vertices of a triangle.

5. An audio processing unit for connection to a stereo reproduction system, comprising a differen tial amplifier having a first input coupled to a first pair of input terminals for connection to a left signal output of the system, and a second input coupled to a second pair of input terminals for connection to a right signal output of the system, and an output for connection to an auxiliary loudspeaker or loudspeakers for reproducing a signal corresponding to the difference between a signal at the left output and a signal at the right output.

6. A processing unit according to claim 5, wherein the input terminals are d.c. isolated from inputs of the differential amplifier.

7. A processing unit according to claim 6, wherein each pair of input terminals is connected to opposite ends of a respective series-connected pair of capacitors, and a respective one of the amplifier inputs is connected to a tapping point between the capacitors.

8. A processing unit according to claim 7, wherein an additional impedance element is connected in series between the capacitors of each pair.

9. A processing unit according to claim 8, wherein the additional impedance element is a potentiometer, the slider of which is coupled to the respective amplifier input.

10. A processing unit according to any of claims 5 to 9, further comprising a balance potentiometer coupled across the differential amplifier inputs.

11. A processing unit according to claim 8 or claim 9, further comprising a connection between one end of one of the impedance elements and one end of the other of the impedance elements, and a balance potentiometer coupled both to the said connection and to leads carrying signals from the input terminals to the amplifier inputs.

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

13. An audio processing unit constructed and arranged substantially as herein described and shown in the drawings.