Patent GB-931281-A (GB)
Title: Improvements relating to stereophonic sound transmission systems
Inventor: FRANKLIN KENNETH VINCENT
IPC: H04H20/88
Filed: 1958-10-13
Abstract: 931,281. Stereophonic systems. ELECTRIC & MUSICAL INDUSTRIES Ltd. Oct. 13, 1959 [Oct. 13, 1958], No. 32590/58. Class 40 (4). In a stereophonic transmission system in which directional information is transmitted by means of a control signal which represents the direction or position of the source of the sound being transmitted and is substantially independent of the loudness of the sound, spread control is effected by selectively modifying the control signal non-linearly with respect to the actual direction or position of the sound source. The invention is applied to a system of the kind described in Specification 871,992, the spread control means being at the transmitter, the receiver, or both. The receiving apparatus shown in Fig. 4 is a radio receiver which receives a carrier wave modulated with an audio signal VA (obtained at the transmitter (not shown) by adding the left and right directional signals after changing the phase of one by 90 degrees with respect to the other) and a control signal of the form V<SP>11</SP>L/(V<SP>11</SP>L + V<SP>11</SP>R). The input and demodulating circuits 21 separates V A  from the control signal and derive from the latter a control signal Vc in the form (V<SP>11</SP>L - V<SP>11</SP>R)/(V<SP>11</SP>L + V<SP>11</SP>R), the relation between which and the direction of the sound source is represented by the line 2 in Fig. 2. The signal Vc is applied to the spread control means 22 (Fig. 4) and the modified output (Vc)<SP>1</SP> is passed together with VA to the multiplying circuit 23. This circuit which may be constructed as described in Specification 927,252 derives two directional signals VL and VR, corresponding to the original directional signals, such that VL = VAV<SP>11</SP>L/ (V L <SP>11</SP> + V<SP>11</SP>R) and VR = VAV<SP>11</SP>R/(V<SP>11</SP>L + V<SP>11</SP>R). These signals are amplified at 24 and 25 and applied to left and right loud-speakers 26 and 27. There may be two groups of loud-speakers, or if desired only the higher audio frequencies may be used to produce VL and VR, the lower audio frequencies being applied directly to a single control loud-speaker. Spread control means.-This is shown in Fig. 1 and is capable of modifying the relation between the control signal and the source direction between the extremes represented by the lines 3 and 4 of Fig. 3. The resistors 6, 7, 9 and 10 are chosen so as to bias D1 and D2 non-conductive for an input signal at terminal 1 between + 16 volts and - 16 volts. Such an input signal therefore appears unattenuated at the point 12, whereas for an input signal of greater amplitude than Œ 16 volts, up to the maximum assumed of Œ 32 volts, the diodes D1 and D2 conduct and limit the signal appearing at the point 12 to the level Œ 16 volts, the relation shown by line 3. The diodes D3 to D6 constitute a bidirectional bridge limiter of known form which is such that in response to input signal of between + 16 volts and- 16 volts the point 16 is held at ground potential, the diodes conducting equally. However, when the applied signal exceeds Œ 16 volts the bridge becomes unbalanced and the signal is transferred to point 16 with attenuation. The relation between the potential at point 16 and the signal applied at point 1 is represented by the line 4 in Fig. 3. The points 12 and 16 are connected by a potentiometer 17 from the slider 18 of which the output control signal is taken. By choice of the slider position the relationship between the input and output signals may be varied between those of lines 3 and 4. a typical relationship being represented by the line 19. The central slider position will produce the linear relationship 20. Typical values of resistors and potentials are indicated in Fig. 1.