We describe two techniques for digital coding of the chrominance components of a color television signal. Both techniques make use of an observation that in color pictures most of the locations of large spatial changes in the chrominance are coincident with large spatial changes in the luminance. This allows us to predict the chrominance samples more efficiently using the previously transmitted chrominance and luminance samples, and the present luminance sample. In general, we determine which of the previous luminance samples best represents the present luminance sample and use the corresponding previous chrominance sample to represent the present chrominance sample. We present results of computer simulations of two such coding schemes. The first scheme, in which the chrominance components are coded by a DPCM coder, uses adaptive prediction of the chrominance components based on the luminance. In the second scheme, the chrominance signal is adaptively extrapolated from its past using the luminance signal for adaptation. Only those chrominance samples where the extrapolation error is more than a threshold are transmitted to the receiver. The addresses of such samples are derived from the luminance signal and therefore need not be transmitted. Our computer simulations on videotelephone type of pictures, indicate that, for the predictive coding, the entropy of the coded chrominance signals can he reduced by about 15 to 20 percent by adaptation. This results in a bit rate of 0.55 bits/ luminance pel, for transmission of chrominance information. Using adaptive extrapolation, only about 20 percent of the chrominance samples need to be transmitted which results in a bit rate of approximately 0.58 bits/luminance pel.

Visibility functions measure the relative visibility of noise added to a picture at those points where some measure of local activity exceeds a given threshold. The functions are obtained from a series of subjective experiments and vary with the content of the picture. Visibility functions have been used to design quantizing characteristics for DPCM coding of monochrome and color signals and for three-dimensional transform coding. We consider an alternative approach to determining the visibility function that obviates the need for repeated picture-dependent subjective tests. The visibility function is assumed to consist of two parts, a picture-dependent component and viewer-dependent component (referred to as the masking function). The visibility function may be approximated by the quotient of a probability density function raised to a power and the masking function. The role of probability is found to be weaker where the viewer has more opportunity to scrutinize the picture.

In the Candy design procedure for DPCM quantizers, the subject is given a specific task to perform. We test the hypothesis that the form of this task influences the results that are obtained. In one experiment the viewers were permitted unlimited time to evaluate noise visibility; in another test using a different testing procedure viewing time was reduced to one second. Visibility functions were obtained from these tests, and quantizers were designed and then evaluated on the basis of impairment ratings of coded pictures. Our results indicate that the design procedure is indeed sensitive to the viewing conditions.

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.

Plateau coding is a method for efficiently coding the chromatic component of color television signals. The signal is divided into regions of approximately constant chromaticity which are transmitted by sending one set of chrominance values for the whole region. Since most changes in chrominance are accompanied by luminance changes, the luminance signal (which must be transmitted quite accurately) is used at the transmitter and receiver to indicate changes in the two chrominance signals; thus no addresses need be sent to define the boundaries of the region at the receiver. Simulations have shown that plateau coding can give pictures of high quality for chrominance bit-rates in the range 0.25-0.5 bits per luminance sample, assuming straightforward pulse-code modulation (PCM) coding of the chrominance amplitudes.

The statistical dependence (redundancy) between three differentially coded color components of a video-telephone signal are explored by means of an entropy study. We measure the entropy of the differentially coded color baseband signals both jointly and as three separate signals. The source material consisted of a number of portraittype pictures. The color signal is treated in a format containing a baseband luminance channel and two baseband chrominance channels. Various orthogonal and nonorthogonal transformations, including the Karhunen-Loeve transform, are applied to the chrominance signals to study the effect on the entropy. With a well-chosen format for the color signal, our results show that the redundancy resulting from using a separate encoding of the signals rather than a joint encoding ranges between 0.2 and 0.5 bits. The possible savings accruing from considering the relationship between coded color differential components was only about 1 percent of the total bit rate for most pictures and increased to a maximum of 4 percent for one particular picture.