This invention is concerned with colour television systems.
In the existing broadcast television standards, when colour pictures are transmitted, the colour information is modulated onto a subcarrier within the luminance band. The chrominance and luminance signals thus share the same frequency space, so there can be crosstalk between them. To reduce this crosstalk, most receivers restrict the bandwidths of both chrominance and luminance signals. Thus receiver design involves a compromise between four impairments:
(i) crosscolour, the demodulation of high frequency luminance as spurious chrominance patterns; PA1 (ii) crossluminance, the high frequency luminance pattern resulting from imperfect suppression of the chrominance subcarrier PA1 (iii) reduction of luminance resolution; and PA1 (iv) reduction of the chrominance resolution.
FIG. 1 shows three frequency spectra in a conventional television system. FIG. 1(a) shows the spectrum of a conventional PAL signal by way of example. The luminance signal spectrum extends to a maximum video frequency f.sub.v which in the UK System I is 5.5 MHz, but the upper part of the band is shared with modulated chrominance signals, as shown by the cross-hatched portion. A conventional receiver has an i.f. filter characteristic which falls off towards high frequencies, and a notch filter to attentuate plain area subcarrier at the subcarrier frequency f.sub.sc which would cause crossluminance. Thus the displayed luminance characteristic is similar to FIG. 1(b), where the cross-hatched area indicates the part of the spectrum affected by crossluminance. Similarly the receiver incorporates a chrominance bandpass filter giving a modulated chrominance spectrum similar to FIG. 1(c). The whole of this area is affected by crosscolour.
A demand now exists for a broadcast television system of higher quality than the conventional standards. The introduction of satellite broadcasting and cable television would permit the improvement of broadcasting standards and in particular would reduce the importance of transmitting the whole signal within the existing bandwidth.
It has been proposed to repeat the modulated chrominance signal at an out-of-band frequency. A high-quality receiver would use this out-of-band chrominance signal in place of the in-band chrominance components. In this way the high-quality receiver would not suffer from crosscolour. A normal receiver would simply ignore the out-of-band signal and would receive a conventional signal.