A 36-Mbit/s Television Codec Employing Pseudorandom Quantization

The paper describes an experimental system constructed to allow investigation of pseudorandom dither signals for coarse quantization of 625-line monochrome television siginals. The system employs a separate encoder/decoder for the television synchronizing pulses so that the picture signal can be allowed to occupy the full nonsaturating range of the video encoder. Dither samples are added to the video signal before coding and subtracted at the decoder and are negatively correlated to maximize the picture signal-to-noise ratio according to a subjective noise weighting function. The power spectral distribution of the quantizing noise is derived and, assuming this to be picture independent and random, pre- and deemphasis networks are used to increase the signal-to-noise ratio. Empirical methods are used to determine the network parameters. The results of preliminary subjective experiments with the 3 bits/sample pseudorandom codec are given for a sampling frequency of 12 MHz and indicate the practical limitations of a theory which predicts a high standard of picture quality.