GB1005290A - Improvements in or relating to decoder circuit arrangements in colour television receivers - Google Patents

Abstract

1,005,290. Colour television receivers. MULLARD Ltd. April 28, 1964 [Aug. 9, 1963], No. 31583/63. Heading H4F. The invention relates to a demodulator circuit for a colour television signal of the type in which two colour components I, Q, Fig. 3 (b), are modulated in quadrature on a sub-carrier signal, the phase of one of the components being reversed in alternate lines as indicated at IN and I R . In accordance with the invention the subcarrier signal is demodulated with the aid of two synchronous demodulations A and B, Fig. 3 (a), at phase angles A and B, Fig. 3 (b), which are symmetrically disposed with respect to the subcarrier component (Q) which is not subject to phase reversal during alternate lines, switch means 3 operating at line rate being provided either to reverse the connections of the subcarrier reference signals to the two demodulators (as illustrated), or to reverse the output connection 12 and 13 of the demodulators to the output signal channels. The switch operation is controlled by a square-wave signal applied at terminal 4 which is synchronized from the line time-base, and is established in operation in the correct phase in relation to the line sequential changes in the received signal by means of a special synchronizing signal included during the field blanking period. The invention is described more particularly as applied to the XZ demodulation system (see " Colour Television " by Carat & Townsend, 1961, p. 241 et seq.), two examples being given where the demodulation axes A and B are respectively Œ 17 degrees and Œ19À5 degrees with respect to the - Q axis of 213 degrees. A detailed circuit utilizing diode switching elements is described for switch 3 with reference to Fig. 4 (not shown).