Joint utilization of fixed and variable-length codes for improving synchronization immunity for image transmission

Robust transmission of images is achieved by using fixed and variable-length coding together without much loss in compression efficiency. The probability distribution function of a DCT coefficient can be divided into two regions using a threshold, so that one portion contains roughly equiprobable transform coefficients. While fixed-length coding, which is a powerful solution to the synchronization problem, is used in this inner equiprobable region without sacrificing compression, the outer (saturating) region is reserved for variable-length codes. The proposed image coder first encodes the bit allocated DCT coefficients using a fixed-rate quantizer bank, then the saturated values for these coefficients are encoded using an entropy constrained scalar quantizer, followed by an arithmetic encoder. Our simulations show that the proposed encoder is appropriate for applications in which an acceptable quality must always be maintained in any channel condition