Patent GB-932557-A (GB)
Title: Improvements relating to stereophonic sound reproducers
Inventor: PERCIVAL WILLIAM SPENCER
IPC: H04H20/88
Filed: 1958-10-11
Abstract: 932,557. Stereophonic systems. ELECTRIC & MUSICAL INDUSTRIES Ltd. Oct. 2, 1959 [Oct. 11, 1958], No. 32523/58. Class 40 (4) In a stereophonic system wherein there are provided an audio signal representing the waveshape of sound and a control signal, substantially independent of the loudness of the sound, representing the direction or position of the sound source, the audio signal is applied to three groups of loud-speakers in relative proportions dependent upon the instantaneous value of the control signal, the arrangement being such that the audio signal is applied to a single and different one of the loud-speaker groups for different discrete values of the control signal and is proportional between the respective two loudspeaker groups for values of the control signal intermediate the discrete values. It is assumed that the antenna 1 (Fig. 1), receives a broadcast carrier-wave modulated by the two signals. The audio signal is denoted by VA and the control signal, which is of the form (V<SP>11</SP>L - V<SP>11</SP>R)/(V<SP>11</SP>L+V<SP>11</SP>R), by Vc. Vc has the value + 1 for sound coming from the extreme left, - 1 for that coming from the extreme right, and zero for that corning from the centre of the stereophonic base. The signals VA and Vc may be obtained at the transmitter as described in Specification 932,556 or Specification 890,833. The amplifying and demodulating circuits 2 separate the audio and control signals as voltage signals, the polarity of the former being such that it is represented by - VA. The signal -VA is applied to a resistor 3, which will be referred to later, and to a voltage-to-current amplifier 4 which produces a current +iA. Signal Vc, the relation between which and the direction of the sound source is shown in diagram 5, is applied to a bridge network of four diodes 6, 7, 8, 9 so that signals representing left of centre produce a positive voltage at the junction of 6 and 7, while those representing centre and right of centre produce no output voltage, whereas right-of-centre signals give a negative output voltage at the junction of 8 and 9 while leftof-centre and centre signals give no output voltage, as indicated in the diagrams 10 and 11. The voltage outputs from the junctions 6, 7 and 8, 9 are amplified by the non-phase inverting amplifier 12 and the phase-inverting amplifier 13, respectively, the outputs of which are fed to the field windings 14 and 15 of two Hall multiplifier circuits, through the Hall crystals of which circuits the current iA flows. The output voltages from the Hall crystals 16 and 17 are applied to amplifiers 19 and 22 respectively, which feed the left and right-hand loud-speaker groups 20 and 23, and also to resistors 18 and 21 respectively. The other ends of these resistors are connected to that of the resistor 3 (all three resistors being of the same value) and any voltage appearing at their junction is applied via the phase-inverting amplifier 25 to the central loud-speaker group 26, this amplifier having three times the gain of amplifiers 19 and 22. When the sound source is fully left the voltage output from crystal 16 will equal VA, and that of crystal 17 will be zero, so that no voltage will appear at the input of amplifier 25 or at that of amplifier 22. Similarly, for fully right conditions only amplifier 22 will have an input. For a central source the output of both crystals is zero and only amplifier 25 will have an input. For intermediate positions the audio signal is shared between the two relevant amplifiers. Modification.-In the modification shown in Fig. 2, one of the Hall crystal multiplying circuits and one of the audio amplifiers are dispensed with. Vc is applied directly to amplifiers 12 and 13 (the latter being phase inverting as in Fig. 1), the outputs of which are applied to the diodes 30 and 31 which have a common load resistor 32. The voltage across resistor 32 represents the modulus of Vc as indicated in the diagram 33 and this voltage is applied to the field winding 34 of the single Hall crystal multiplier 35. The output of the Hall crystal is applied to one end of resistor 36 (corresponding to 18 or 21 of Fig. 1), the other end of which is connected to that of resistor 3, and to an audio amplifier 37. The output of this amplifier is applied by switch 38 to either the left or right-hand loud-speakers 20 or 23 under control of the polarized relay 39 according as Vc has a positive or negative value. The gain of amplifier 25 is adjusted relative to that of 37 to suit the attenuation of the network 3, 36. Specification 931,281 also is referred to.