Patent GB-1027825-A (GB)
Title: Improvements in or relating to colour television standards changing apparatus
IPC: H04N11/20
Filed: 1964-01-13
Abstract: 1,027,825. Colour television. TELEFUNKEN PATENTVERWERTUNGSGESELLSCHAFT. Jan. 13, 1964 [Feb. 27, 1963], No. 1368/64. Heading H4F. A colour television standards changer, for transforming colour television signals according to the NTSC system into signals according to the PAL system or vice versa, comprises means for periodically adding to the complete video signal for the times during which the change is to be introduced an additional colour subcarrier signal offset in phase by 180 degrees in relation to the phase of the colour carrier vector (I<SP>1</SP> or Q<SP>1</SP>) to be changed, and having an amplitude equal to double, the instantaneous value of the colour carrier vector to be changed. As shown, the complete video signal 2 is fed on lead 1 to an amplifier 6 and then via an adjustable delay line 7 to adding combining stage 8 and also on lead 9 to a band filter 10 which filters the colour subcarrier with its side bands out of the complete video signal. The colour subcarrier is fed via amplifier 12 into a synchronous demodulator 13 which is supplied with a local reference carrier from oscillator 15 through adjustable phase shifter 16 which is adjusted so that the demodulator yields the colour difference signal - I<SP>1</SP>; in this embodiment the standards changer changes a NTSC signal to a PAL signal, I<SP>1</SP> being the colour carrier vector (difference signal) which is periodically reversed through 180 degrees. To change a PAL signal to a NTSC signal the phase shifter is adjusted so that the demodulator yields the signal +I<SP>1</SP>. The demodulated signal is applied via a lowpass filter 18 to a modulator 19 in which it is modulated with the reference carrier supplied from unit 16 in suppressed carrier modulation. The gain of amplifier 12 is such that the output of modulator 19 is - 2I<SP>1</SP>. When electronic switch 21 is closed there thus appears in the output lead of adding stage 8 a complete video signal in which the phase of the colour carrier vector I<SP>1</SP> is reversed to -I<SP>1</SP>. Switch 21 is operated by a square-wave switching generator 24 at line, field or picture frequency, e.g. line synchronizing pulses 27 may be fed over line 26. A receiver receiving the output on line 23 will also require a signal which indicates when the signal must be demodulated along the axis + I<SP>1</SP> or -I<SP>1</SP> and such a signal is provided by source 29 which has the reference carrier oscillation and the switching wave applied thereto. Source 29 may add an additional colour synchronizing pulse to the complete video signal or produce a periodic change in phase. Alternatively, every other colour synchronizing pulse may be eliminated or additional impulses may be transmitted during the vertical flyback. In a further embodiment for transforming signals according to the PAL system into signals according to the normal NTSC system no demodulation and remodulation (for producing the additional colour subcarrier signal) is required, the colour subcarrier signal of one line being added to the delayed colour subcarrier signal of the preceding line with such sign that the non-reversed modulation axis is eliminated and only the reversed modulation vector remains from which the additional colour subcarrier signal is obtained.