Interfacing intraframe d.p.c.m. with conditional replenishment coding for viewphone signals

Conditional replenishment is a coding technique that is particularly efficient at exploiting the large frame-to-frame redundancies that exist in visual telephone or visual conferencing signals. However such techniques are necessarily complex and their cost of implementation may, in the case of lightly loaded routes, outweigh the saving in transmission capacity. It is therefore possible that on such routes, cheaper, less efficient, techniques such as differential p.c.m. (d.p.c.m.) will be used, thereby creating an interface between links employing different methods of source coding. The compatibility problems that exist at this interface are discussed and an experimental system is described that accepts a d.p.c.m. signal at 8 Mbit/s and uses a novel interfacing technique to take advantage of frame-to-frame correlation and the statistical distribution of the samples within the moving areas for transmission at 2 Mbit/s