GB1099616A - Improvements in or relating to circuit arrangements for producing a colour television colour sub-carrier - Google Patents

Abstract

1,099,616. Colour television; transistor pulse circuits. PHILIPS ELECTRONIC & ASSOCIATED INDUSTRIES Ltd. Aug. 15, 1966 [Aug. 14, 1965], No. 36375/66. Headings H3T and H4F. The invention relates to a colour television circuit arrangement for producing a colour subcarrier of frequency f h to which are locked line and field synchronizing signals, f h being equal to a multiple of a quarter of the line frequency f H plus or minus an odd multiple of one half the af H mf v field frequency f v i.e. f<SP>h</SP> = -Π-. The 4 2 arrangement comprises, Fig. 1, a stable freerunning crystal-controlled oscillator 1 which produces frequency f h , an oscillator 5, 6 which produces a frequency 2f H , a first frequency divider 8, 16 arranged to give a signal of mf v frequency -, a second frequency divider 7, 2 10, 12 arranged to give a signal of frequency f H -, a mixer stage 2 arranged to beat subcarrier 4 frequency f h from oscillator 1 with the a<SP>th</SP> f H harmonic of frequency - to produce an output 4 mfv of frequency - which is compared in a phase 2 discriminator 3 with the output of the first frequency divider, 8, 16, the output of which is supplied to the reactance means of oscillator 5, 6 to control the frequency thereof so that the af H mf v equation f h = - Π- is satisfied. In the 4 2 detailed embodiment described a = 1135 and m = 1 and mixing stage 2 is a pulse mixing stage, Fig. 3 (see later) arranged to compare the subcarrier signal with the signal of frequency f H -converted to needle pulses having a recur- 4 f H rence frequency equal to - the output circuit 4 of the pulse mixing stage including a filter arranged to cut off all frequencies above fv/2 = f o - If mixing stage 2 is a pulse mixing stage, dividing stages 7, 10 and 12 may be multivibrators, and frequency multiplication of fH/4 by a may be omitted. Line synchronizing and 4 scan signals may be taken off via lead 9, line colour switching signals may be taken off via lead 11 an field synchronizing and scan signals may be taken off via lead 15. If the circuit responds incorrectly, i.e. oscillator 5, 6 produces a line frequency corresponding to equation f h = af H fv - - - instead of a line frequency corresponding 4 2 af H fv to equation f h = -+- the circuit may be 4 2 automatically interrupted by an interruptor 37, e.g. a transistor, which is cut off by a signal derived from a frequency discriminator 39 when frequency f H is incorrect. This restoring circuit 37, 39 is arranged to have a short time constant so that the blocking voltage for interrupter 37 disappears after a certain time period. In pulse mixing stage 2, Fig. 3, signal 14 of frequency fH/4 is differentiated 18, 19 and the resulting negative-going needle pulses applied to the base of transistor 21, the positive-going pulses being suppressed by diode 20. The resulting pulsed collector current of transistor 21 causes, in conjunction with the parasitic capacitance of winding 22, ringing which only occurs in the negative-going direction since the positive direction is suppressed by diode 26. Winding 22 is connected to the collector of a second transistor 27 and backcoupled via a capacitor 28 and a secondary winding 29 to the base of transistor 27. This backcoupled circuit results in a very sharp current peak in winding 22, which is induced in winding 30 causing transistor 31 to conduct. The subcarrier of frequency f h is supplied through capacitor 32 and since the 1135th harmonic of frequency fH/4 differs by exactly f v /2 from frequency f h the required beat frequency of f v /2 occurs at the collector of tran- 2 sistor 31. Lowpass filter 40 suppressed any fv frequencies higher than -. 2