Patent GB-912040-A (GB)
Title: Improvements relating to stereophonic sound transmission systems
Inventor: PERCIVAL WILLIAM SPENCER
IPC: H04S1/00
Filed: 1958-03-11
Abstract: 912,040. Stereophonic transmission systems. ELECTRIC & MUSICAL INDUSTRIES Ltd. March 6, 1959 [March 11,1958; Aug. 26, 1958], Nos. 7738/58 and 27276/58. Class 40 (4). In a system of the type disclosed in Speci. fication 871,992, in which a control signal indicative of the direction or position of a sound source is obtained from the envelopes of two directional signals derived from the sounds to be transmitted and is employed, in conjunction with a signal representative of those sounds, to energize spaced loud-speakers, the two directional signals are individually transposed to frequency ranges higher than and spaced substantially from their respective original frequency ranges, the transposed signals are then individually rectified, the rectified signals are filtered to remove signals in the higher frequency ranges, and the envelope signals thus obtained are used to derive the control signal. As shown in Fig. 1, left and right signals VL and VR are derived from the two microphones 1 and 2 of a pair of directional microphones having their axes of maximum response mutually at right-angles in a horizontal plane and each at approximately 45 degrees to the plane containing the sound sources. These signals are applied respectively to pre-amplifiers 3 and 4 and thence to tone and volume control circuits 5 and 6, and to amplifiers 7 and 8, the outputs of which are summed in an amplifier 9 and differenced in an amplifier 10, the sum and difference signals being applied respectively to phase shift networks 11 and 12 to produce between corresponding components of the respective signals a phase difference of 90 degrees. The outputs of these networks are added in an amplifier 13 to produce a composite signal VA representative of the sounds to be transmitted. The signal VL from circuit 5 is also applied to an amplifier 14, the response of which rises approximately linearly with frequency. The output signal V<SP>1</SP>L, which is thus weighted in favour of the higher frequency components, is applied to a suppressed carrier modulator 15 fed with a 100 kc/s. carrier wave from carrier source 16 and the lower side band is removed by the highpass filter 17. The upper side-band is then peak-rectified at 18, side-band components being removed by filter 20, and the envelope signal V<SP>11</SP>L thus obtained is applied to a cathode follower stage 21. Further filter circuits may be employed, if desired, for purposes such as weighting V<SP>11</SP>L in favour of transients. A similar signal V<SP>11</SP>R is derived in like manner. V<SP>11</SP>R is applied to a logger circuit 27 via resistor 30, and V<SP>11</SP>L to the two logger circuits 26 and 27 via equal resistors 24 and 25. The output of logger circuit 27 is phase-inverted at 31 so that signals representing log V<SP>11</SP>L and log 1/(V<SP>11</SP>R+ V<SP>11</SP>L) are added by way of cathode followers 32 and 33 and resistors 34 and 35 to produce a control signal representing log (V<SP>11</SP>L/V<SP>11</SP>R + V<SP>11</SP>L) which is applied via a D.C. coupled amplifier 36 to an amplifier 37 having an exponential response characteristic which gives an output representing (V<SP>11</SP>L/V<SP>11</SP>R +V<SP>11</SP>L) which is applied via cathode follower 38 to a ring modulator 39 fed with a 15 kc/s. carrier from, oscillator 40. The modulator output is amplified at 40a, and added to VA in an adding circuit 41, the resulting signal controlling the cutting stylus of a gramophone recorder 42 in known manner. To avoid the difficulties inherent in providing high-pass filters such as 17 having a sufficiently sharp cut-off to separate the sidebands having maximum frequency 99, 970 c/s. from that having minimum frequency 100,030 c/s., the arrangement of Fig. 2 may be employed. In this the output of amplifier 14 is applied to a low-pass filter 23 passing frequencies up to 1000 c/s. and to a high-pass filter 22 passing those above 1000 c/s. The output of 23 modulates a 5 kc/s. carrier supplied by source 16a in modulator 15a, the upper side-band being selected by the high-pass filter 24 and peakrectified at 18a. The output of filter 22 is directly peak-rectified at 18b, and is passed together with the signals from 18a into the filter 20.