GB1510316A - Processing circuit for chromatic television signals - Google Patents

Abstract

1510316 Colour television; transistor amplifiers INDESIT INDUSTRIA ELETTRODOMESTICI ITALIANA SpA 1 April 1975 [3 April 1974] 13309/75 Headings H4F and H3T In a colour television receiver for receiving signals which require a switching operation to occur at line frequency, e.g. as in PAL or SECAM (Trade Mark) system, line frequency pulses are passed to a first bi-stable, which provides a signal at half line frequency to control the switching, and to a second bi-stable which controls the phase of the first bi-stable. If the phase of the switching operation is incorrect, the phase of the bi-stables is corrected. In the embodiment shown in Fig. 1 chroma signals C, which include a colour burst either with every line or with alternate lines, pass via a gate 203, which removes the bursts, to demodulators 210, 212 which provide the blue and red colour difference signals. The chroma signals also pass to a gate 204 which is opened at half the line frequency so as to pass only the burst associated with alternate lines to phase locked loop 217-219. Oscillator 219 provides sub-carrier of + 90 degrees phase relative to the B-Y signal. The sub-carrier is shifted - 90 degrees at 222 and passed to B-Y demodulator 210 and also to a demodulator 224. A switch 220 passes the sub-carrier direct or via phase inverter 225 to R-Y demodulator 212, switch 220 being controlled by half line frequency pulses from a bi-stable 226 which is fed with line frequency pulses at input H via a gate 232 which is inhibited if bi-stable 226 operates in the wrong phase. The subcarrier also passes via a second switch 221 direct or inverted (228) to a demodulator 227. Switch 221 is controlled by a bi-stable 229 which is phase-locked to bi-stable 226 and receives line frequency pulses. The output of bi-stable 229 also enables gate 204 which selects alternate colour bursts. Should bistable 226 have the wrong phase so also will bistable 229, and consequently demodulators 224, 227, which receive the colour components of the chroma signals via a.g.c. unit 208 and which normally provide B-Y and R-Y signals respectively, will generate Œ(R-Y) and Œ(B-Y) signals respectively. The half line frequency alternating component in these signals is detected by units 238-242 to provide the inhibit signal to gate 232 until the phase of bi-stable 226 is correct. Units 238, 239 are half line frequency filters. The filters are each as shown in Fig. 4 and each includes a positive feed-back loop 488 to increase the gain and a negative feed-back network to increase the gain at the half line frequency. The network includes resistors 489-492 and 495 and capacitors 493, 494. Resistor 492 is variable to adjust the preferential frequency of the filter.