Patent GB-932556-A (GB)
Title: Improvements relating to stereophonic sound transmission systems
Inventor: PERCIVAL WILLIAM SPENCER
IPC: H04S1/00
Filed: 1958-08-26
Abstract: 932,556. Stereophonic systems. ELECTRIC & MUSICAL INDUSTRIES Ltd. Aug. 21, 1959 [Aug. 26, 1958], No. 27275/58. Class 40 (4). In a system wherein, as in that described in Specification 871,992, there are derived from sound an audio frequency signal representative of the waveshape of the sound and a control signal representative of the direction or position of the source of the sound, left and right signals VL and VR are derived from the sound and components of each of these signals in different frequency bands are individually processed to provide different parts of the control signal. A pair of crossed directional microphones 1, 2 provide the signals VL, VR which pass via preamplifiers and tone and volume control circuits to amplifiers 7, 8 of adjustable gain, the outputs of which are summed in amplifier 9 and differenced in amplifier 10. The sum and difference signals pass through phase shift networks 11 and 12 which produce a relative delay of 90 degrees between components of the respective signals over their full frequency range, the resulting signals being added in amplifier 13 to produce the composite signal VA representative of the waveshape of the sounds picked up by the microphones 1 and 2. Components of the signal VL having frequencies greater than 1,000 c/s. also pass via high-pass filter 14 to five narrow band channels, centred on the frequencies shown in Fig. 1, which are all of similar construction. The first such channel has a filter and amplifier 16 centred on 1,300 c/s., the output of which is rectified at 17. The filter 18 removes audio components from the resulting envelope signal which is then applied by a cathode follower 19 to a filter 20 which emphasizes high-frequency components corresponding to transients in the original sound. The pulses produced by filter 20 are elongated by the elongator 21 having a time constant of the order of 100 ms. The individual envelope signals produced by the five narrow band channels are fed by summing resistors 22 to 26 to an amplifier 27. Components of-VL having frequencies of less than 1,000 c/s. pass via lowpass filter 15 to an amplifier 28 and thence to a suppressed carrier modulator 29 fed by a 5,000 c/s. oscillator 29a, the filter and amplifier 30 selecting the lower-side band from the modulation products. This side-band is applied to six narrow band channels similar to that described above, the outputs of which pass via summing resistors 37 to 42 to the amplifier 27. The output of 27 passes to a further transient-emphasizing filter 43 and an elongator 44 with a time constant of the order of 35 ms., the output of which is applied by way of resistor 45 to an amplifier 46. VL is also applied to an amplifier 47 the output of which is rectified (48), filtered (49) and applied via resistor 50 to the amplifier 46. The proportions in which the envelope of VL and the output of elongator 44 are added is determined by the ratio of resistors 50 and 45, the output of amplifier 46 being the desired processed control signal port V<SP>11</SP>L. The signal VR is processed in like manner to derive V<SP>11</SP>R and V<SP>11</SP>L and V<SP>11</SP>R are fed to an encoding circuit, 51, which may be as described in the above-mentioned Specification, to produce the control signal. Transient emphasizing filter and elongator.- The filter 20 and elongator 21 are shown in Fig. 2. For falling transients the diode 62 conducts and produces substantial attenuation, but for a rising transient a pulse of relatively large amplitude is applied to the elongator, charging the capacitor 67 rapidly to nearly that amplitude. The capacitor 67 discharges slowly, after the impulse ends, by way of resistors 65 and 66.