Patent GB-1075124-A (GB)
Title: Improvements in receiving arrangement for the reception of colour television signals
IPC: H04N11/18
Filed: 1964-11-30
Abstract: 1,075,124. Colour television. COMPAGNIE FRANCAISE DE TELEVISION. Nov. 30, 1964 [Dec. 3, 1963], No. 48651/64. Heading H4F. Relates to an arrangement for suppressing the colour sub-carrier in a complex video signal which includes a wide band luminance signal, the sub-carrier occupying a colour channel lying within the frequency band of the luminance signal, to obtain a complete luminance signal, that is, a luminance signal in which the luminance components lying within the colour channel are not suppressed. The subcarrier may be phase or frequency modulated but the invention is of particular importance when the complex video signal is according to the SECAM system in which the frequency band of the frequency modulated sub-carrier is comparatively wide. As shown, the complex video signal is fed at 1 to a filtering arrangement 2 which delivers at output 3 a " base " signal containing practically only luminance components, and at output 4 a " compound " signal whose frequency bandwidth coincides approximately with the colour channel, these two signals being complementary i.e. their sum represents the original complex video signal. The " base " signal is applied to adder 9 via a delay equalization device 5. The " compound " signal is also applied to adder 9 via a device 6 which delivers an approximate partial luminance signal and a filter 6<SP>1</SP> which limits the bandwidth of said partial luminance signal to that of the " compound " signal. In adder 9 the partial luminance signal is added to the " base " signal to form the complete luminance signal. Device 6 comprises an amplitude limiting means, which delivers an " amplitude limited compound signal," and means for delivering the partial luminance signal which is proportional to the difference between the compound signal and the amplitude limited compound signal multiplied by Ao/B, where B is the limitation threshold of the amplitude limiting means and Ao the amplitude of the sub-carrier. In one embodiment, Fig. 2 (not shown), device 6 has two channels one containing a limiter (61) which delivers the compound signal with constant amplitude (B) and the other containing a delay device (62), the outputs of the two channels being fed to a matrix (65) which delivers a signal whose instantaneous value is a predetermined linear combination of the instantaneous values of the inputs. The Specification includes a mathematical analysis of the operations involved. For correct operation of the arrangement the colour sub-carrier must have a constant amplitude Ao, but if the sub-carrier is a " coded colour sub-carrier " as in the SECAM system then a decoding filter must be inserted at the input of device 6 or limiter (61) and a coding filter identical to the transmitter filter at the output of device 6 or limiter (61). For correct operation of the arrangement of Fig. 2, not shown, the relationship between the amplitude Ao of the sub-carrier (if need be " decoded ") and the amplitude B has to be fixed, so that if for any reason Ao varies then device 6 e.g. amplitude B, has to be adjusted. In another embodiment, Fig. 3 (not shown), the limiter output B is controlled electrically as a function of the mean amplitude Ao of the subcarrier over an appropriate time interval by replacing limiter (61), Fig. 2 (not shown), by a modulator limiter (61<SP>1</SP>) its threshold being controlled by a D.C. voltage from an amplitude detector (66) fed by input 4. In a further embodiment, Fig. 4 (not shown), device 6 includes an adding frequency converter (601) fed with a locally generated oscillation, a subtractive frequency converter (603), a device (604) which suppresses the carrier oscillation so that its output signal is a suppressed carrier amplitude modulation and a second subtractive frequency converter (605). In a further embodiment, Fig. 5 (not shown), device 6 includes an amplitude demodulator (613), a high-pass filter (614) and a balanced modulator (615). The Specification explains that to reduce spurious components in the partial luminance signal the upper limit of the bandwidth of the compound signal is made less than that of the useful band of the colour channel, and also those components of the luminance signal whose frequencies are higher than the upper limit of the compound signal may be excluded from the base signal.