GB2220818A - Stereo systems - Google Patents

Abstract

This system provides means to create realistic stereo sound reproduction through the earphones of a headset or a similar appliance. In the stereo system left and right audio channel inputs are fed via delay lines 2 to adders 3. The second half of the circuit comprises two delay lines 4A, 4B, two attenuators 5, two filters 6 and two adders 7. The function of this second half is to add a delayed, attenuated and filtered portion of the signal from one channel to the other channel. This mimics the difference in sound received by the left and right ears in terms of time, loudness and spectrum. The delay lines 4A, 4B should produce delay times of about 0.5ms. The attenuators 5 should produce attenuation of about 12 dB. The filters 6 should mimic the difference in the spectra received by the ears from a distant sound source. The outputs 8 for the two channels are connected to the speakers of a headset. <IMAGE>

Description

"Improvements relating to Stereo Systems" This invention relates to stereo equipment which aims to produce a realistic output equivalent to that which might be produced by a live band or orchestra.

Standard equipment comprises means for producing the two stereo outputs which are fed to speakers which are spaced apart some distance in front of the listener.

With suitable speaker spacings the perceived sound can be very realistic. However, there is a problem if the loudspeakers are placed too close to one another (for instance, due to lack of space within a room) the effect is substantially diminished. Furthermore, if the stereo outputs are fed to the earphones of a headset, the stereo effect is virtually lost and the sound seems to come from the centre of the listener's head.

It is an object of this invention to provide means whereby the problem discussed above can be reduced, particularly in the case of the use of stereo equipment with a headset.

Accordingly, this invention provides a circuit for processing stereo audio signals produced by left and right audio channels for transmission to stereo loudspeakers, comprising direct feeder lines to each loudspeaker and respective delay lines from each channel, and mixer circuits for adding each delay line output to the respective opposite direct feeder line, prior to transmission to the respective loudspeaker.

With suitable timing control, the effect of the delay lines is to supply to the opposite ear a signal which is delayed in time by a split second, this timing being equivalent to that which would be experienced by the listener if he were listening to a conventional stereo system with suitably placed loudspeakers.

Ideally the delay lines will incorporate suitable attenuators and filters.

Preferably, each feeder line will incorporate an additional delay device disposed in parallel to and fed back into the feeder line in advance of the delay line.

This can further enhance the realism of the system. In one such arrangement, a single delay device is fed from a mixed output from the two direct feeder lines and the output of this delay device is fed back into the two direct feeder lines.

In a further modification, the extent of delay in each delay line is controlled by a monitoring device which monitors the angular displacement of the head of a listener from a predetermined straight-ahead position.

Such a monitoring device may comprise a unit which determines the extent of displacement of a compass needle with respect to a mounting for the needle. The compass needle can carry a transparent block through which is directed a light beam, displacement of the light beam due to refraction being registered by a lateral effect diode.

The invention may be complemented in various ways and preferred embodiments thereof will now be described with reference to the accompanying drawings, in which: Figure 1 is a circuit diagram of one form of stereo processing circuit in accordance with this invention; Figures 2 and 3 illustrate modifications of parts of t-he -circuit of Figure 1; Figure 4 is a representation of a stereo headset; and Figures 5 and 6 illustrate parts of equipment carried by the stereo headset of Figure 4, in two differing conditions.

Figure 1 shows a block diagran of an electrical circuit giving a semi-realistic effect to stereo speakers of a headset. The circuit has inputs for left and right audio channels. These are connected to a first half of the circuit, comprising two delay lines 2 and two adders 3 which add the delayed signals to themselves, as shown. The delay lines 2 should produce equal delay times of about 4 ms. The purpose of this first half of the circuit is to provide a degree of ambience to the processed audio signals.

The second half of the circuit comprises two delay lines 4A, 4B, two attenuators 5, two filters Gand two adders 7. The function of this second half is to add a delayed, attenuated and filtered portion of the signal from one channel to the other channel. This mimics the difference in sound received by the left and right ears in terms of time, loudness and spectrum. The delay lines 4A,4B should produce delay times of about 0.5 ms.

The attenuators 5 should produce attenuation of about 12 dB. The filters 6 should mimic the difference in the spectra received by the ears from a distant sound source.

The outputs of the complete circuit for the two channels are provided at 8 and transmitted to the speakers of the headset. In order that the outputs 8 of the circuit do not overload other circuitry connected to the outputs, the overall gain of the circuit for each channel should be unity.

The main effect of the circuit of Figure 1 is supplied by the second half. The first half can comprise a reverb effect unit as illustrated in Figure 2, instead of the first half circuit shown in Figure 1.

This appears to provide a slightly more realistic effect, when used with the second half, as compared to use of the second half alone. The circuit comprises filters 9, adders (mixers) 10,11 and 12, attenuators 13 and 14 and a delay line 15. The adders 10,11,12 together feed back the delayed signal to itself to give a reverb effect (giving ambience), the aim being to simulate the effect of sound transmission through air.

The reverb effect circuit of Figure 2 uses one delay line 15 and adds components of the left channel to the right and vice versa. It is possible that a reverb circuit 11,13,15 used on the individual channels (keeping them separate) gives a better effect. The first half of the circuit would then appear as in Figure 3.

The first half of the circuit, as described, is optional, since a semi-realistic effect can be obtained by using only the second half of the circuit, as described.

Additionally a more realistic effect may be obtained by attaching to the head of the listener a device which gives a signal indicating the number of degress of turn to the left or right by the listener's head from the looking-straight-ahead position. In Figures 1 to 3 the right hand channel is associated with the upper half of the circuit, and the left with the bottom half. The signal from the additional device may be used to alter the delay times of the delay lines 4A,4B (Figure 1) so that, as the head is turned towards the right, the delay time of delay line 4A is reduced compared to that of delay line 4B. For a turn of the head towards the left, the delay time of delay line 4B is reduced compared to that of delay line 4A. This is achieved by- attaching a unit 16 onto the top of the headset band 17 (Figure 4).

Details of the head direction monitoring unit 16 are illustrated in Figures 5 and 6. A constant intensity light source 18 (for example a light emitting diode), emits a light ray 19 which passes through a collimating lens 20. A magnetic compass 21, free to pivot, carries a rectangular block 22 of optically transparent material (such as perspex) so that the incident light of the ray 19 enters the block directly above the compass pivot. A second collimating lens 23 directs the light onto a lateral effect diode 24 (also known as a position sensitive device, or PSD). This is a specially fabricated silicon photo-diode which produces small photo-currents that are proportional to the distance from the chip centre of an incident light spot. These devices (one-dimensional in this case) are readily available, and are often used for measuring small displacements by optical means. The output from the diode 24 is fed to a differential amplifier 25.

Figure 5 shows the case of the listener facing due (magnetic) north. Figure 6 shows the case where the listener has turned the head towards the right. The light path through the block 22 is displaced by refraction and a different position of the light spot is recorded by the PSD 24. The output of the amplifier 25 is used to adjust a voltage controlled oscillator (not shown) which controls the delay in the delay line 4A or 4B of Figure 1.

Claims (9)

1. A circuit for processing stereo audio signals produced by left and right audio channels for transmission to stereo loudspeakers, comprising direct feeder lines to each loudspeaker and respective delay lines from each channel, and mixer circuits for adding each delay line output to the respective opposite direct feeder line, prior to transmission to the respective loudspeaker.

2. A circuit according to claim 1, wherein the stereo loudspeakers form part of a headset.

3. A circuit according to claim 1 or claim 2, wherein the delay lines incorporate attenuators and filters.

4. A circuit according to any one of claims 1 to 3, wherein each feeder line incorporates an additional delay device disposed in parallel to and fed back into the feeder line in advance of the delay line.

5. A circuit according to claim 4, wherein a single delay device is fed from a mixed output from the two direct feeder lines and the output of this delay device is fed back into the two direct feeder lines.

6. A circuit according to any one of claims 1 to 5, wherein the extent of delay in each delay line is controlled by a monitoring device which monitors the angular displacement of the head of a listener from a predetermined straight-ahead position.

7. A circuit according to claim 6, wherein the monitoring device comprises a unit which determines the extent of displacement of a compass needle with respect to a mounting for the needle.

8. A circuit according to claim 7, wherein the compass needle carries a transparent block through which is directed a light beam, displacement of the light beam due to refraction being registered by a lateral effect diode.

9. A circuit for processing stereo audio signals substantially as herein described, with reference to the accompanying drawings.