Television receiving apparatus.

摘要

A television receiver has an a.f.c. arrangement for ensuring the correct tuning of a tuner unit (4) which is tuned to the nominal carrier frequency by a tuning voltage (VT). When the tuner unit (4) is incorrectly tuned to the actual carrier frequency the a.f.c. system operates in a first mode in which a sweep current produced by a current source (15) is applied by way of a closed switch (16) to an a.f.c. capacitor (17) to produce a swept a.f.c. voltage for the tuner unit (4). The sweep direction is controlled by an RS flip-flop (13) by way of a multiplexer (12) in response to voltage comparators (19, 20) connected to the a.f.c. capacitor (17). When the tuning is sufficiently close such that the received transmission can be correctly decoded the a.f.c. arrangement goes into a second mode in which the output of a frequency demodulator (6) is applied via multiplexer (12) to control the current source (15) and hence the voltage across a.f.c. capacitors (17). For this second mode gate pulses (G1, G2) from a data processor (11) control the multiplexer (12) paths via a monostable (14) and only cause switch (16) to be conductive during a given portion of each line period.