GB1307959A - Decoder for television signals - Google Patents

Abstract

1307959 Television INTERNATIONAL STANDARD ELECTRIC CORP 11 Nov 1971 [17 Nov 1970] 52378/71 Addition to 1307566 Heading H4F In a decoder for a PAL type chrominance signal 1, circuits 25 and 3-10 separately amplitude and phase demodulate the signal 1, the phase representative signal 18 (U#) is converted directly and after a one line delay 11 by respective halves of a phase coder 13 to pairs of signals 14, 15 and 17, 16 derived from triangular functions approximating the functions sin U# and cos U#, Figs. 2a and 2b (not shown), and after averaging the undelayed and delayed signals at 19, 20 and conversion to the sinusoidal or cosinusoidal function at 21, 22, they are fed together with the output of demodulator 25 to mixers 23, 24 to provide colour difference video signals 26, 27. It is said that in this way saturation errors resulting from the normal PAL averaging process are avoided, although some error in the average phase angle (hue) may arise near to 0 and 180 degrees or 90 and 270 degrees for the cosine and sine functions, respectively. As shown, in the phase demodulator, the signal 1 is limited, 4, and activates a Schmitt trigger 6. The output pulses are differentiated, 7, and applied, together with differentiated pulses similarly derived from a reference subcarrier frequency input 2 to a bi-stable 8 to produce a pulse width modulated phase representative signal. A PAL switch 9 alternately transmits positive and negative versions of this signal to a filter 10 providing the D.C. signal U#. A circuit suitable for converting the signal 18 to signals 14, 15 is described with respect to Fig. 4 (not shown). In variants of the embodiment shown, (a) the shaping circuits 21, 22 may occur before the respective adders 19, 20, or (b) the signal U# may be obtained by synchronously demodulating the output oflimiter 4 and passing the demodulated signal through a PAL switch.