Patent GB-1333888-A (GB)
Title: Hue selection in colour television
IPC: H04N9/75
Filed: 1972-03-20
Abstract: 1333888 Chroma-keying PYE Ltd 20 March 1972 12813/72 Heading H4F Relates to the production of an output signal, for use in a chroma-keying system, when a selected hue (e.g. blue) appears in a PAL colour signal with quarter line offset. As shown a PAL signal at input 1 is fed via a high-pass filter 2 which passes the chroma signal and the high frequency components of the luminance signal to a comb filter 3 which includes a two-line period delay 6, the output from difference circuit 9 comprising the chroma signal still with the quadrature relationship of the two colour subcarrier signals U and V. This signal is then fed to a second comb filter which includes a delay line 14, having a delay time of one line period less one quarter of the colour subcarrier period, the V chroma subcarrier signal being produced at the output of summation circuit 19 and the U signal at the output of difference circuit 20, both signals retaining their quadrature relationship. These signals are demodulated 23 and 24 to obtain chrominance signals R-Y and B-Y which are passed via low pass filters 29, 31 and clamp circuits 30, 32 to voltage controlled amplifiers 35 and 36, to which a hue selector control 37 formed by a sine/cosine potentiometer is connected, and the outputs of these amplifiers are applied to a keying input 42 of a video mixer circuit 43 via an amplifier 40 and an adjustable delay unit 41. In addition to the PAL signal at input 1 being fed to mixer 43 via delay lines 44 and 45, a further signal from a second television camera or video tape recorder is fed thereto via input 48, and when the hue selected by control 37 is present in the signal at input 1 this further signal is gated to output 49 instead of the signal at input 1. Delay 44 is included to compensate for the chroma keying signal propagation delay to input 42 (of the order of 1 microsecond), fine adjustment being made by delay 41, and thus prevent horizontal fringing of the colour used (e.g. blue) to generate the keying signal along the left hand side of the display inset. Two line period delay 45 similarly removes colour fring- ing at the top of the display inset. Oscillator 25 is locked in frequency and phase by the burst components of the composite video signal, and clamping generator 34 is triggered by synchronizing pulses derived from the composite signal in synchronizing pulse separator 33. A switchable phase inverter 50 is included in the V chroma signal path and is controlled by a half line frequency pulse generator 51 itself controlled by the output of synchronizing pulse separator 33 and ident. signals derived from oscillator 25. lnverter 50 may alternatively be provided in line 28 or between the output of demodulator 23 and the input of filter 29. If the signal content in a field does not change to any degree from line to line then delay line 6 may have a delay equal to (4n-2)l where n is an integer and l is the line period, and delay line 14 may have a delay equal to (xl - P/4) where x is an odd integer and P is the period of the colour subcarrier. Both these delays may be increased by m(f-¢l) where f is the field period and m is an integer if the signal content does not change in the vertical direction from field to field.