ES350150A1 - Improvements in or relating to colour television - Google Patents

Abstract

A colour television system wherein a colour carrier (i.e. sub-carrier) having a frequency equal to either a whole number (m) multiple of the line frequency f H or a whole number multiple plus or minus an nth part of the line frequency n, being a whole number, is derived from a crystal oscillator 6, Fig. 1, which oscillates at the colour carrier frequency, the oscillator is synchronized by a control voltage obtained by phase comparing 5 its output with a colour carrier oscillation sequence derived from a startstop (or pull-in) oscillator 4 arranged to be started by line frequency pulses 1 either directly or via a frequency divider. Where the colour carrier equals mf H i.e. whole line shift, as in the SECAM (Trade Mark) system, divider 3 is not required and on the arrival of each pulse 1 oscillator 4 produces a colour carrier oscillation sequence of approximately only 10 oscillations. The phase comparator 5 therefore receives colour carrier oscillation sequences having the same phase on each line, and controls oscillator 6 so that at any particular instant during a line the colour carrier at output 9 always has the same phase position. Where the colour carrier equals (m + )f H i.e. half-line shift (n = 2), as in the NTSC system, oscillator 4 produces a colour carrier oscillation sequence only for every second line, oscillator 6 being arranged to have a resonance frequency such that an uneven number of colour carrier oscillations occur during two lines. Where the colour carrier equals (m + )fH i.e. quarter-line shift (n = 4), as in the PAL system, oscillator 4 produces a colour carrier oscillation sequence only for every fourth line, oscillator 6 being arranged to have a resonant frequency such that an uneven number of colour carrier oscillations occur during four lines. In a modification for quarter-line shift (n = 4) Fig. 3, frequency divider 3 has a division factor of 2: 1 so that the phase comparator 5 receives a colour carrier oscillation sequence for every second line. Oscillator 4 feeds its output through a transformer 10 having a push-pull secondary associated with a switch 11 actuated by a one quarter line frequency switching waveform 13, and thus the phase of the colour carrier oscillation sequence fed to comparator 5 in each second line is switched over through 180 degrees between one oscillation sequence and the next. By removing frequency divider 3 from the circuit of Fig. 3 (Fig. 4, not shown), oscillator 4 is activated for each line and switch 11 actuated by a half-line frequency switching waveform, so that the colour carrier oscillation sequence fed to comparator 5 differs by 180 degrees from line to line i.e. half-line shifted. In a further embodiment for quarter-line shift Fig. 5 (not shown) the colour carrier sequences reach the phase comparator at a phase which differs by 90 degrees from one line to the next. The present invention may be used to produce simultaneously from the line frequency (derived by frequency division from a PAL carrier frequency produced by a quartz oscillator with a 25Hz shift in addition to the quarter-line shift) an NTSC colour carrier (half-line shift) and a SECAM colour carrier (whole line shift).