Patent GB-973034-A (GB)
Title: Improvements in stereophonic systems for television broadcasting
IPC: H04N7/06
Filed: 1961-10-27
Abstract: 973,034. Stereophonic broadcasting systems. GENERAL ELECTRIC CO. Oct. 27, 1961 [Oct. 31, 1960], No. 38530/61. Heading H4L. Relates to an arrangement for transmitting compatible stereophonic signals in a conventional television system. As shown in Fig. 1, at a television transmitter, left microphone 2 and right microphone 4 supply audio frequency signals via conventional preemphasis networks 6, 8, which increase the amplitude of the higher frequencies, to a matrix 10 producing output signals L+ R and L - R respectively. The L+R signal is supplied via a delay network 14 and amplifier 22 to frequency modulate a transmitter 12 and the L - R signal is supplied to a balanced modulator 34 to amplitude modulate a sub-carrier, e.g. 23.625 kc/s., half-way between the line-scanning frequency of 15.75 kc/s. and its second harmonic of 31.5 kc./s. supplied via a lead 36. The subcarrier frequency is so chosen to avoid interference from video components in the audio signals when television receivers of the intercarrier type are used. The suppressed carrier signal from the modulator 34 is supplied via a bandpass filter 38 and amplifier 46 to frequency modulate the transmitter 12. Synchronizing pulses from the source 54, of frequency 15.75 kc./s. are frequency tripled at 56 and halved at 58 to provide the 23.625 kc/s. sine wave sub-carrier which is supplied via a buffer amplifier 60 to the modulator 34. A pilot carrier of 39.375 kc./s. is also transmitted so that it can be combined at the receiver with a wave of line scanning frequency to produce the sub-carrier frequency and is derived by supplying the 23.625 kc./s. sub-carrier together with the line synchronizing pulses from the source 54 to a mixer 62 producing the sum of the two frequencies. The pilot carrier is supplied via an amplifier 72 to the frequency modulation transmitter 12. The video portion is shown as an amplitude modulation transmitter 80 supplied with video signals from 82. In a modification the synchronizing pulse frequency is quadrupled and the sub-carrier frequency subtracted to provide the required pilot carrier frequency. Fig. 3 shows an intercarrier sound television receiver in which the audio and video carriers are converted into respective intermediate frequencies at 84, amplified at 86 and supplied to the second detector 88. The video signals are supplied via a circuit 92 to the C.R. tube 90 and the beat frequency between the frequency modulated audio carrier and the video carrier (the audio I.F.) is selected in known manner by a circuit 94, giving a 4.5 mc./s. frequency modulated signal which is supplied to a frequency discriminator 96. The resulting L + R signal is de-emphasized by circuit 100, 102, which also blocks the L - R sidebands and pilot carrier, and supplied to potentiometers 148, 158. A circuit 108, 110 resonant at the sub-carrier frequency 23.625 kc./s. selects the L - R sidebands, de-emphasis the sidebands being effected by suitable selection of the Q value of this circuit. The pilot carrier is selected by a circuit 118, 120 and supplied to the grid of a mixer valve 126 receiving line synchronizing pulses via a resistor 130, and the resulting sub-carrier via capacitor 146 is combined with the L - R sidebands. This L - R signal is supplied to oppositelypoled detectors 147, 156 producing signals L - R and R - L respectively and combination of these signals with the L + R signal in the respective potentiometers 148, 158, produces the required L and R signals. In a modified receiver the fourth harmonic of the linescanning frequencies is used to derive the subcarrier from the pilot carrier in order to avoid audible beat frequencies. In an alternative receiver, Fig. 6, the audio I.F. is derived at 94, as before, and supplied to a frequency discriminator 206 providing as outputs L + R across coupling impedance 228, 230, and - L - R across coupling impedance 233, 234. The L+R signal is supplied via a de-emphasis network 100, 102 to a potentiometer 148 and the - L - R signal is supplied via a de-emphasis network 302, 304 to a potentiometer 158. The output of the discriminator 206 which includes the L - R sideband signals is supplied to a network 242, 244, 246 tuned to the sub-carrier frequency and having a Q value arranged to provide the required de-emphasis. The fourth harmonic of the line-scanning frequency is selected by a circuit 250, 252, 254 and supplied to the cathode of a mixer valve 260, and the pilot carrier is selected by a circuit 266, 268 and supplied to the grid, the difference frequency providing the sub-carrier which is supplied to the balanced demodulator 282, 288 via capacitor 278. The balanced detector circuit ensures that any unwanted modulation components present on the sub-carrier are balanced out. A filter 292, 294, 296 provides further protection against unwanted beats between the sub-carrier and the line scanning frequency if they should be present. The resulting L - R signal is combined in the potentiometer 152 with the L+R signal and in the potentiometer 164 with the - L - R signal to produce the required L and R signals. Specification 965,705 is referred to.