ES293512A1 - Improvements in systems for the generation of special effects in colour television - Google Patents

Abstract

The invention relates to a system for use with colour television signals of the type which comprise a luminance signal and at least one sub-carrier which is modulated by a colour signal and occupies a frequency band which is included at least partially within the band of the luminance signal, and is concerned with the generation of special effects by which is intended (a) the fade-over from one signal to another or (b) the substitution of one signal for another over selected parts of the picture area. Reference is made generally to signals of the N.T.S.C. type but the invention is described in detail only as applied to SECAM signals where the subcarrier is frequency modulated by two colour signals which alternate at line rate. The broad form of the invention is illustrated in Fig. 1 where two synchronized sources of colour signals are applied to terminals 101, 201 and an output signal is derived from an adding circuit 303. The signals are separated into their luminance and colour sub-carrier components by low-pass filters 104, 204 and high-pass filters 105, 205, and the sub-carriers are demodulated in stages 106 and 206. The two luminance signals are applied as inputs to a first " special effects " switch 600 and the two demodulated colour signals are applied as inputs to a second similar switch 602. The switches are controlled conjointly as indicated by dotted line to introduce the desired special effect. The colour signal passed by switch 602 is remodulated on a subcarrier in circuit 302 and is then combined with the luminance signal passed by switch 600 in adding circuit 303 to produce the final output. Switches 103, 203 and 304 are included so that the " special effects " facility may be removed and one or other of the input signals (as determined by the setting of switch 600) passed directly to the output without the band-width limitation which is imposed on the luminance signals by filters 104 and 204. Delay units 102 and 202 equalize the delays in the luminance and colour channels prior to switches 600, 602, and delay unit 305 equalizes the delays after the switches. A first modification, Fig. 2 (not shown), dispenses with low-pass filters 104, 204, delay units 102, 202 and switches 103, 203 and substitutes a common low-pass filter and common delay unit after switch 600 with provision for by-passing the filter when the special effects facility is not required. A second modification, Fig. 3 (not shown), is somewhat similar but uses both a low-pass and a high-pass filter in parallel in the common luminance channel. Both filters are operative when the " special effects " facility is dispensed with and only the low-pass filter when the facility is used. In addition provision is made for improving the luminance response when only the low-pass filter is in circuit by a " crispening " technique based on the addition of differential components to the original signal. Fig. 4 (not shown) illustrates a system in which unequal time delays in the channels leading up to switches 600 and 602 are allowed for by delaying a switching control signal which is applied to one switch relative to a switching control signal which is applied to the other. Fig. 5 (illustrates) a modification of the Fig. 3 arrangement described above in which the low-and-high-pass filter combination is replaced by low-pass filter in combination with a subtractor circuit. In a further system, Fig. 6 (not shown), the bandwidth limitation of the luminance component during use of the " special effects " facility is effected by signal cancellation of the high frequency components by a component added to the colour channel.