Low Bit Rate Differential PCM for Monochrome Television Signals

This paper contains an examination of the performance of low bit rate differential PCM systems when used to encode monochrome National Television System Commission (NTSC) television pictures. 12 different encoders with various sampling rates and numbers of quantizing levels operating at bit rates from 9 to 24 Mb were considered. The differential PCM systems were implemented by using an 8-b PCM A/D converter followed by digital logic that performed the differential operation in a simple previous sample feedback loop. The subjective performance of these encoders was determined using an anchored seven-point quality rating scale and equipreference contours were plotted in a plane whose axes are sampling rate and quantizing bits. Entropy measurements were made on the output of the differential PCM encoders. These entropy measures were compared with the subjective performance of encoding systems operating at the same transmission rate. The encoding systems studied produced television pictures of medium quality in which quantizing noise and bandwidth limitations were apparent.