Rate Control In Variable Transmission Rate Image Coders
Herbert Holzlwimmer · Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE · 1990
In conventional constant transmission rate image coders the image quality is varied in dependence on the current activity in the image sequence in order to obtain a constant data rate. Flexible transmission rate will be available in the future along with packetized networks and a constant image quality could be achieved. Using a 2 Mbit/s constant transmission rate codec for video conference we have studied three modifications of this coder to adapt it to variable transmission rate image coding: open loop, closed loop and constant MSE distortion. It is shown that none of these modes yields satisfying results neither from the image quality nor from the channel rate requirements point of view. After a thorough discussion of the pros and cons of above modifications the concept of a combined quality and bit rate controller is introduced. It is capable of satisfying both the viewer by a constant perceived image quality and the network by suitable coder output bit rate statistics. For a constant image quality a quality measure was developed reflecting the perceived image quality very well. Using this quality measure we are able to adapt quantization to the visibility of errors and thus prevent waste of bits.