Transcoding of MPEG Bitstreams

This paper discusses the problem of transcoding as it may occur in, for instance, the following situation. Suppose a satellite transmits an MPEG-compressed video signal at say 9 Mbit/s. This signal must be relayed at a cable head end. However, since the cable capacity is only limited, the cable head end will want to relay this incoming signal at a lower bit-rate of, say, 5 Mbit/s. The problem is how to convert a compressed video signal of a given bit-rate into a compressed video signal of a lower bit-rate. The specific transcoding problem discussed in this paper is referred to as bit-rate conversion. Basically, a transcoder used for such a purpose will consist of a cascaded decoder and encoder. It is shown in the paper that the complexity of this combination can be significantly reduced. The paper also investigates the loss of picture quality that may be expected when a transcoder is in the transmission chain. The loss of quality as compared to that resulting in the case of transmission without a transcoder is studied by means of computations using simplified models of the transmission chains and by means of using computer simulations of the complete transmission chain. It will be shown that the presence of two quantizers, i.e. cascaded quantization, in the transmission chain is the main cause of extra losses, and it will be shown that the losses in terms of SNR will be some 0.5 ¿ 1.0 dB greater than in the case of a transmission chain without a transcoder

Improvement of MPEG-2 video compression based on rate distortion concepts.

This paper describes the principles of an approach to MPEG video compression referred to as rate-distortion optimal coding or in short R(D) optimal coding. Basically, with this approach MPEG compression is interpreted as a minimization problem. It is discussed how this can be done. This minimization problem, which is essentially a discrete constrained minimization problem, can be solved using the theory presented by Everett [4] and Shoham and Gersho [5] which is based on Lagrange multipliers. This paper shows results from experiments with R(D) optimal MPEG-encoding. It is concluded that this approach is worthwhile enough to pursue study of MPEG compression. It is furthermore concluded, that this approach relies in an explicit way on the definition of a measure for distortion, which is yet an incompletely solved problem. 1 INTRODUCTION Among the topics studied in RACE HAMLET WP2 was a study into `R(D)-optimal' coding, that is, rate-distortion optimal coding. In order to avoid confusion..