COST 211-a European collaborative research project in redundancy reduction techniques for visual telephone signals-has involved laboratories in Belgium, France, Holland, Italy, Sweden, the United Kingdom, and West Germany since 1977. A broad review is offered of progress made in the empirical optimization of conditional replenishment coding and the parallel development of subjective test and computer simulation techniques with which to assess this and other picture coding methods. An international network of 2 Mbit/s transmission is provided by the European satellite OTS and interest is being generated in further collaboration for a European Videoconference Experiment under the auspices of the CEPT.

This paper reviews the field of the efficient coding of color television signals. Because this paper is perhaps the first review on this topic, some background is given in the areas of colorimetry, visual perception of color and color television systems. We assume that the reader has some familiarity with luminance encoding techniques. Coding techniques themselves are divided into two broad groups: component coding methods in which each component (usually three) is coded separately, and composite coding methods in which the composite television signal with its "color" modulated subcarrier is processed as a single entity. Both approaches are covered in detail. The field is still growing, pushed primarily by the desire in the television area to find digital coding standards accepted by both broadcasters and carriers and suitable for use with NTSC, PAL and SECAM television systems. We discuss this aspect by comparing composite and component coding methods.

A digital coding technique which interfaces efficiently with composite color signals is clearly desirable for connections formed from a chain of analog and digital transmission links. This paper reviews a number of such techniques, and develops a predictive encoding algorithm, termed chrominance-corrected prediction, which is efficiently matched to the nature of the composite signal, and which avoids the need for prior separation of the signal into luminance and chrominance components. The application of composite predictions to PAL and NTSC signals is discussed, and experimental results are described which indicate that a saving in excess of 2 bits/sample can be achieved over PCM for similar picture quality.