GB1395534A - Decoder circuit for a multi-system colour television receiver - Google Patents

Abstract

1395534 Colour television; transistor amplifying circuits; limiters PHILIPS ELECTRONIC & ASSOCIATED INDUSTRIES Ltd 1 June 1972 [4 June 1971] 25713/72 Headings H4F and H3T A switching system for a television receiver for the selective reception of PAL and S.E.C.A.M. signals comprises a decoder circuit 85 to which an attenuated 91 and a one line period delayed 89 chrominance signal are applied and (B-Y) and (R-Y) chrominance signals are obtained at outputs 83 and 65 respectively and fed to demodulators 123 and 133. Decoder 85 includes system (i.e. PAL/S.E.C.A.M.) changeover switches 93, 95, 111, 115 operated by hand or automatically in response to a system dentification signal derived from the received chrominance signals by a system change-over voltage generator 117. The decoder also includes a half-line frequency change-over switch 113, an adder 101 and subtractor 103 for PAL signals, and amplitude limiters 97 and 99 for S.E.C.A.M. signals. In a second embodiment, Figs. 3 and 4, not shown, the half-line frequency change-over switch signal and the system change-over switching is effected in the decoder prior to the PAL signals being fed to the adder and subtracter and the S.E.C.A.M. signals being fed to the two limiters. A detailed transistor circuit for the decoder 85 (which forms the subject matter of the claims) is described with reference to Fig. 1 and includes three transistors 1, 5 and 17 connected as shown via resistors 9 and 15 to a current source 11, a frequency modulated signal (e.g. S.E.C.A.M. signal) or an amplitude modulated signal (e.g. an amplitude and phase modulated PAL signal) being coupled to input 3. In operation when transistor 5 is rendered conductive by a control signal at its base an amplitude limited signal is derived at the collector of transistor 5 whereas if transistor 17 is rendered conductive an amplified signal is derived at the collector of transistor 17. Transistor 5 is rendered conductive when a FM signal is received and transistor 17 when an AM signal is received. The collectors of transistors 5 and 17 may be coupled together, Fig. 3 (not shown), and the output signals may be derived from the collector of transistor 1 instead of from the coupled together collectors.