Patent GB-1084573-A (GB)
Title: Frequency compensating system for television receivers
IPC: H04N5/44
Filed: 1964-09-17
Abstract: 1,084,573. Double superhet television receivers; multiplex radio signalling. BRITISH TELEMETER HOME VIEWING Ltd. Sept. 17, 1964 [Nov. 14, 1963], No. 38083/64. Headings H3Q and H4L. Oscillator drift (and inaccuracy of setting) in television receivers is compensated by passing the I.F. to two further mixers, the first of these receiving the I.F. and the output of a second (stable) local oscillator, and the second receiving the I.F. and the output of the first mixer; the second mixer thus delivers a signal at the (stable) second local oscillator frequency but carrying the modulation of the original signal. The invention also provides for elimination of ghost images due to phase modulation in the first mixer by adding an amplifier having a flat response each side of the video carrier issuing from the second mixer. In Fig. 2 (not shown), a tuner provides an I.F. of frequency F containing undesired fluctuations or tolerance dF. This is mixed with a stable frequency F 0  and the difference signal FdF-F 0  selected. After amplification this signal is further mixed in a second mixer with (FdF) so that the output is the stable frequency F 0 . If the signal from the amplifier is considerably greater than that from the tuner, the second mixer will reject the modulation from the amplifier and pass on only the modulation from the tuner. This rejection can be improved by including between the amplifier and the second mixer an amplitude limiter, e.g. a locked oscillator or a beam deflection tube. The amplifier can be placed in front of the first mixer (Fig. 4, not shown) so that the I.F. from the tuner is much greater than the local oscillator signal at the first mixer; the output of the latter is then substantially free from modulation. In application to a television receiver in accordance with the invention (Fig. 5) the amplifier 48 preceding the first mixer 50 passes only the video I.F. centred on 45À75 Mc/s. and rejects the audio I.F. centred on 41À25 Mc/s., together with the colour subcarrier I.F. (if present) at 42À17 Mc/s. The output of the mixer 50, which is a substantially unmodulated signal centred at 18À95 Mc/s., is passed through an amplifier 53 of the same bandwidth as 48 and applied to the second mixer 46 together with the whole of the original signal from 42. The signal from 53 is much greater than that from 44 so that only the modulation on the latter is passed on. The mixer 46 provides a video signal centred on 26À8 Mc/s. and an audio signal centred on 22À3 Mc/s. The bandwidths of the amplifiers 44, 48 and 53 are sufficient to deal with any frequency deviation from the tuner 42 due to drift or faulty re-setting of the local oscillator therein so that no fine tuning control is necessary on the tuner. In addition, any frequency modulation of the tuner local oscillator by vibrations from the loud-speaker is eliminated, so that inter-carrier sound detection is unnecessary, though it can be used if desired. If the final video I.F. stage 54 has a sloping amplitude/frequency characteristic around the carrier frequency of 26À8 Mc/s., a ghost image may be obtained due to phase modulation in the mixer 50. To avoid this the flat portion of the characteristic can be extended to 0À75 Mc/s. beyond the carrier. This results in a demodulated video signal having the portion 0-0À75 Mc/s. at twice the amplitude of the higher frequency signals. The detector circuit of Fig. 11 restores the amplitude of the lower frequency signals by a correction network of resistors 70, 72 and capacitor 74.