GB1186163A - P.A.L. Colour Test Signal Generator. - Google Patents

Abstract

1,186,163. Colour television; automatic phase control systems. R.C.A. CORPORATION. 3 May, 1967 [13 May, 1966; 22 June, 1966], Nos. 21360/66 and 27947/66. Headings H3A and H4F. A signal suitable for testing P.A.L. receivers is produced by generating two phase-locked signals at frequencies F1 and F2, where F1 equals the colour sub-carrier frequency f sc minus the line scan freqency F H and F2 equals the frequency f sc plus line scan frequency F H , and applying the signals to the receiver alternately at line scan frequency. With reference to the receiver local demodulation signal at frequency f sc the signals Fl and F2 appear as signals which undergo a phase change of 360 degrees during the period of one line, the phase change being in opposite directions for the two signals so that the phase modulation appears to change in direction in alternate lines in accordance with the P.A.L. method of operation. The signals Fl and F2 may be produced by means by crystal oscillators 1 and 2, the signal Fl being passed through a gate which is operated at the beginning of lines at frequency F H (or alternatively at a frequency 2 F H ) to produce side-band signals including a signal at F1 + 2F H which phase locks oscillator 2 with the aid of control system 4. The signals are selected alternately at line frequency by switch 5. In an alternative arrangement (not described in detail) a circuit selects the side-band at frequency F1 + 2F H from the output of gate 3 and oscillator 2 is dispensed with. The signal 6 from switch 5 may be combined with scanning synchronizing signals and blanked in a manner such that a receiver displays the signal as a pattern of bars. No separate colour synchronizing " burst " is provided. A receiver " burst " gating circuit responds to signals F1 and F2 at the commencement of lines (when the phases are equal due to the phase lock operation) and produces a side band signal at frequency f sc to synchronize the local demodulation oscillator. A " swinging burst " (i.e. a " burst " alternating Œ45 degrees in successive lines with respect a reference axis) may be simulated by displacing F1 and F2 by 90 degrees. At the commencement of lines the receiver " burst " gating circuit will then provide a synchronizing signal which changes by 90 degrees in successive lines. The automatic phase control circuit will lock the local demodulation oscillator at the mean phase and produce a correction signal going positive and negative in alternative lines. This correction signal functions as the P.A.L. phase switching signal in the same manner as when the receiver responds to a standard signal.