A Sliding-Scale Direct-Feedback PCM Coder for Television

Differential, direct-feedback, and delta-modulation pulse code modulation systems take advantage of the television viewer's tolerance to brightness errors, especially in high detail areas of the picture. 1-5 * Analog signals must be quantized into a finite number of levels for conversion to digital signals. This quantization introduces errors in the reconstructed picture. These errors are lumped together under the name of quantizing noise which for differential pulse code modulation (PCM) systems is a function of the quantizer step size(s), the sampling rate, channel capacity, and filter characteristics. Quantizing noise may be classified into six visually subjective catagories: granular * This family of coders are hereafter referred to as differential coders. 1537 1538 T H E BELL SYSTEM TECHNICAL JOURNAL, M A Y - J U N E 1969 noise, streaking, contouring, slope-overload, edge-busyness, and edgestepping. Granular noise is a high frequency noise, caused by individual sample errors, whose visibility is increased by amplitude differences from frame to frame. Contouring produces brightness steps in flat regions of the picture. Both of these defects may be decreased with proper filtering and decreasing the smaller step sizes. A reduction in contouring is usually made at the expense of increased granular noise. Streaking results from mistracking between the coder and the decoder. The length is determined by the decoder's time constant. Slope-overload, edge-busyness, and edge-stepping occur at large brightness boundaries which are not parallel to the scanning lines.