GB1192876A - Synchronous Demodulator - Google Patents

Abstract

1,192,876. Synchronous demodulator. PHILIPS ELECTRONIC & ASSOCIATED INDUSTRIES Ltd. 22 Aug., 1968, No. 40219/68. Heading H4F. A synchronous demodulator for use in PAL systems is constructed in the general manner shown in Fig. 1 wherein elements 1 to 10 denote active devices and elements 11 and 12 denote load impedances and is such that it is particularly suitable for use with integrated circuit techniques. The demodulator is for use in systems wherein the quadrature colour components are each demodulated separately, the demodulator of this invention operating only on the alternating component. Elements 4 to 8 form a balanced demodulator, while elements 9 and 10 form a multivibrator operating at line frequency to switch the supply from terminal 14 alternately to elements 1, 3, 5 and 6 and 2, 4, 7 and 8 on successive lines. Elements 1 and 2 are fed with a first phase of the quadrature modulated signal while elements 3 and 4 are fed with the opposite phase of said signal. Similarly a second phase of a reference signal, derived from the colour burst signal, is fed to elements 5 and 7 while elements 6 and 8 are fed with a reference signal of opposite phase. Alternatively elements 1, 2, 3 and 4 may be fed with the reference signal and elements 5, 6, 7 and 8 fed with the quadrature modulated signal in a similar manner to that above. In this way, the reversal of phase on successive lines of one of the colour difference signals is taken care of. The circuit also includes a colour killer comprising a transistor (22) (Fig. 2, not shown) interposed between the supply terminal 14 and the multivibrator which is cut off when no colour signal is present.