Fixed Distortion, Variable Rate Subband Coding Of Images

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Abstract
Most image coding systems are designed for transmission over constant bit rate channels. Such systems usually achieve varying coded image quality, depending on the inherent "compressibility" of the transmitted image. From a user's perspective it is desirable to have a codec which achieves a prescribed level of fidelity regardless of the image sent. The encoder output in this case is necessarily variable rate, making it difficult to reconcile with constant bit rate channels. However, the impending availability of high performance packet switched networks compatible with variable rate sources will make variable rate coding feasible. The codec design in this paper is based on a new method of allocating distortion rather than bits among the subbands. Judicious selection of subband quantizers compatible with the allocation procedure produces a simple four band encoder structure. Several four band structures are then nested in a hierarchical fashion for better compression performance; the resulting image codec achieves mean square distortion within 1.5 dB of the user specified value for a variety of images and over a wide range of distortions. Its rate-distortion performance rivals fixed rate systems of similar complexity. The design can be configured to take best advantage of datagram networks or prioritized packet-switched networks and the encoder output is suitable for progressive image transmission applications.

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