GB1198986A - Improved Colour Television Systems and Apparatus - Google Patents

Abstract

1,198,986. Colour television. COMMUNICATIONS PATENTS Ltd. 27 July, 1967 [9 Aug., 1966], No. 35646/66. Heading H4F. Colour television signals are transmitted with the aid of a single carrier wave which is amplitude modulated in respect of luminance information and is angle modulated in respect of hue information which is obtained from the interaction of luminance and chrominance information. The chrominance information may comprise line sequential red and blue colour difference signals, reference being made, however, to signals which alternate at a more rapid rate. The arrangement at a transmitter may be as shown in Fig. 2 where red, green and blue signals from a camera 18 are combined to form a luminance signal in matrix 22 and are also formed into a line sequential chrominance signal by circuits 24, 26, 28 and switch 30. After band-width restriction in filters 34, 36, the signals are then amplitude modulated on quadrature phase carriers in modulators 35 and 36 whose outputs are combined in adder 42. Oscillator 38 provides the carrier signal, being connected directly to modulator 37 and via a 90 degrees phase shifter 39 to modulator 35. Modulator 37 operates to produce negative modulation, whilst modulator 35 is a balanced modulator which produces a double-sideband, suppressed carrier, output signal. The output from adder 42 is amplitude limited in circuit 44 and passed through a band-pass filter 45 to provide a carrier signal which is of constant amplitude but angle modulated, i.e. specifically phase modulated, in respect of a function of both the luminance and chrominance signals. The carrier signal is converted in frequency by mixer 47 and oscillator 48, and is then applied to a modulator 50 where it is amplitude modulated by the original luminance signal from matrix 22. The final doubly modulated signal passes via a vestigial sideband filter 53 to an aerial 52 for transmission. A circuit 54 responsive to the signal from filter 45 may predistort the luminance signal to compensate for distortion due to the vestigial sideband transmission. Provision may also be made for delay equalization of the luminance signal. The carrier signal provided by oscillator 38 may be in the band 4-10 Mc/s. in order that the signal from filter 45, together with the luminance signal on a separate carrier, may be transmitted by cable to a remote transmitter or may be recorded on magnetic tape. At a receiver, Fig. 3, the received carrier signal is envelope detected at 62 to provide the luminance information which is applied through amplifier 63 as one input of matrix 64 which furnishes the colour signals for cathode-ray tube 68. The other matrix inputs comprise the colour difference signals. These are derived by applying the carrier signal through a band-pass filter 69 to an amplitude limiter 70 and then obtaining continuous trains of carrier signals on leads 73, and 74, corresponding respectively to the red and blue colour difference signal modulations, with the aid of a one-line duration delay 72 and a line sequential change-over switch 71. The signals are multiplied in circuits 75 and 76 by a luminance signal and then amplitude detected in detectors 84 and 85. The luminance signal for multiplication is obtained via low-pass filter 78 and amplitude modulated in modulator 79 on a signal having the same frequency as the carrier but in phase quadrature therewith which is derived from a locked oscillator 80 via a phase shifter 81. In a modified receiver, Fig. 4 (not shown), the simultaneous carrier signals from switch 71 are first phase detected and then multiplied by the luminance signal as a normal video signal. Specification 1,086,352 is referred to.