GB1500614A - Circuit for the tuning of a television receiver - Google Patents

Abstract

1500614 AFC systems INDESIT INDUSTRIA ELETTRODOMESTICI ITALIANA SpA 15 Jan 1975 [25 Jan 1974] 1670/75 Heading H3A In or for a TV receiver an intercarrier sound channel comprises a demodulator 4 for separating the "intercarrier" signal 14 from the received signal 1 and a limiting amplifier 22 with symmetrical outputs, and a video channel provided with a suppression filter 3 tuned to the sound intermediate frequency and a video detector 5, signals from 22 and 5 are supplied to a phase comparator 21 in the form of a double symmetrical demodulator whose output 12 controls a high frequency oscillator of the receiver. The signals from detector 5 preferably pass via a second limiting amplifier 20 having symmetrical outputs to comparator 21, and the signals from amplifier 22 are also fed directly and via a phase shifter 26 to a second symmetrical coincidence demodulator 23 from which the demodulated sound signal 11 is fed to an audio frequency amplifier. Two signals at the intercarrier frequency are fed to comparator 21 via 4, 22 and 3, 5, 20; filter 3 imposes a phase shift on the signal passing through it if the received sound IF differs from the tuned frequency-i.e. if the tuning is not exact. This results in a control voltage 12 which is applied to, e.g. a varicap diode in the oscillator to adjust its frequency. Each pair 20-21 and 22-23 may be formed as an integrated circuit 24, 25 as shown in Fig. 4. Here phase shifter 26 is formed by coil 102 and capacitor 103 which provide phase modulation at the rate of audio modulation of the signal. The oscillator control voltage 12 is obtained from pin 8" of IC24 via a circuit which includes an AFC on/off switch 137, a switch 143 for selecting Bands I, III or IV and V, and channel select switches 146, 150.