Patent GB-1218015-A (GB)
Title: Improvements in or relating to systems for transmitting television signals
Inventor: THOMPSON JOHN EDWARD
IPC: H04N7/26
Filed: 1967-03-13
Abstract: 1,218,015. Television. NATIONAL RESEARCH DEVELOPMENT CORP. 12 March, 1968 [13 March, 1967], No. 11750/67. Heading H4F. [Also in Division G4] In a system for transmitting video signals, e.g. luminance information, by P.C.M., of the type in which a pseudo-random noise or "dither" signal is added to the video signal before quantization and is subtracted at the receiver after the video signal has been decoded (to minimize the effects of quantizing), the noise signal generator is synchronized with the sampled video input signal and includes a network supplied with pseudo-random binary noise signals which are each thereby weighted to provide a multilevel "dither" signal which by autocorrelation techniques is designed to give at the output of the system a power spectrum of increasing power density towards the high frequency end of the frequency band of the video signal. As shown in Fig. 1, a video signal from source 10 is applied, after suitable correction at 11, to an adder 12 where it is combined with a dither signal DS from generator 13, the combined output being supplied to a 2-bit 4-level quantizer P.C.M. encoder 14. The binary coded signal is transmitted via link 15 to a decoder 16 and the resulting analogue signal is supplied to an adder 17 which also receives the inverted dither signal from a generator 22 or alternatively over a separate channel 23, 24. The output of the adder consisting of the original video signal is supplied via conventional correction circuits 18 to a display device 19. The dither signal generator 13 includes a square wave generator 20 which is synchronized by the frame sync. pulses related to the source 10 and provides an output signal of amplitude equal to half a quantizing step at a frequency, for example, of 3MHZ. which is an odd multiple of half the line scan frequency of the video signal. The output signal level is arranged to average zero over two frames and is also arranged so that its phase is advanced and retarded alternately by 90 degrees at the start of each field so that interference is minimized. A shift register 21 controlled by a 6MHZ clock generator associated with the source 10 is provided with feedback, the output stages 10 and 11 in co-operation with resistance network 3R, 3R, 3R, R providing synchronized pseudorandom binary noise signals which are combined with the output of generator 20 to form the six level dither signal DS. In a modification of the pseudo-random noise generator 21, Fig. 2, the shift register is clocked at 1À5mHz. obtained from a 6mHz. clock signal via bi-stables 30, 31, and the outputs from stages 10 and 9 are modulated by the 3mHz. and 1À5mHz. square waves respectively from bi-stables 30, 31. The outputs of stages 9, 10, 11 are combined in a resistive network to provide a 6-level dither signal which is stated to have an improved noise spectrum. In a further arrangement, Fig. 3 (not shown), high-frequency pre-emphasis precedes the adder 12 and corresponding de-emphasis follows the adder 17 and a four-level dither signal is used. This is stated to provide a further reduction of quantizing noise.