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This Part describes subjective tests performed to establish the sampling rate and the number of quantizing levels or corresponding bits per sample which should be used to obtain broadcast-quality television pictures from a p.c.m. system handling PAL colour, 625-line, 5.5 MHz video signals according to the UK standard (CCIR Standard I). These tests included an investigation of adding `ditherÂ¿ signals to the video signals prior to the p.c.m. encoder as a means of reducing the required number of bits per sample. An effective dither signal is proposed which breaks up coherent quantizing errors such as `contouringÂ¿ effects and replaces them with less noticeable random quantizing errors. A theoretical estimate of the required number of bits per sample is derived by comparing calculated values of the magnitude of quantizing noise with the existing specification for random noise allowable on analogue links. The paper also gives brief details of the p.c.m. coder and decoder developed by the author and used in the tests carried out in the BBC Research Department.