Digital Coding of Color Picturephone Signals by Element-Differential Quantization

An element differential quantizer has been used to encode three baseband components of a color Picturephone® type signal in real time. The first part of this paper is concerned with an investigation of the extra bits required to transmit chrominance information over that required for a high-quality luminance signal (coded to 4 bits per picture element). We have found that a total allocation of 1 bit per picture element to the chrominance signals leads to a high-quality color display. The second part of the paper concerns an investigation of a more efficient coding format for color sigaals. In this regard, we have determined that the color signal can be packaged into a 6.3-Mbit/s rate by allocating 12 levels for the luminance component, 6 levels for one chrominance component, and 4 levels for the other chrominance component. Only one chrominance component is transmitted each line and the missing component is obtained by line averaging. The best results were obtained by coding chrominance signals that were matrixed such that their color axes lay between the
I
and
Q
and color difference axes. A scheme is suggested for simply combining this type of coder with an analog color signal that has the chrominance information compressed into the blanking portion of the signal.