Transmission of images using an adaptive ternary delta modulator

Constant-area quantisation (CAQ), a predictive encoding scheme specifically designed for the transmission of images over narrowband channels, is discussed, and modifications to both the predictor and quantiser are described. These modifications result in improved image quality at a lower transmitted bit rate while still maintaining the relative simplicity of hardware implementation of the CAQ encoder. It is shown that transmission errors can lead to severe degradation of the received images in CAQ-based communications systems, resulting in both loss of line synchronism and random shifts in grey level. Solutions to both of these problems are given allowing usable reconstructed images to be obtained at bit error rates in excess of 10¿3.