Single-pass constant- and variable-bit-rate MPEG-2 video compression

Abstract

Most real-time MPEG-2 encoders are designed to perform in a constant-bit-rate (CBR) mode, in which buffer constraints are imposed to circumvent large deviations from a desired rate at any instant in time. Although such streams are generally good-quality sequences, certain types of operations or environments call for a more efficient real-time CBR encoder. The first part of the paper describes how a better-quality CBR video stream can be produced by estimating the relative complexity of a picture in comparison with the average complexity of the partially encoded stream and using it to adjust the compression parameters in a single-pass mode of operation. Our CBR encoder is particularly attractive for digital broadcast and editing environments, in which representations of higher-fidelity video objects in both display and freeze modes are constantly pursued. The second part of the paper describes the real-time generation of video streams with a variable-bit-rate (VBR) encoder. This mode of operation is high ly desirable for home entertainment and recreational events. We propose a robust single-pass VBR video encoder algorithm which is capable of learning and adapting itself to the complexity of image segments and thereafter creating streams which have constant visual picture quality. The new VBR scheme displays a better performance than the CBR encoder, particularly when special effects such as scene transitions, fades, or luminance changes are to be compressed. Both CBR and VBR encoders are fully compliant with the MPEG-2 standard and are easily implementable with IBM encoder architecture. Compression results for the new single-pass encoding algorithms and comparisons with previous CBR schemes are provided. The result suggests the suitability of our VBR approach for record/playback in storage media such as digital video disc (DVD) players, disk-based camcorders, and digital videocassette recorders (DVCRs). It further reflects the importance of our single-pass CBR scheme for providers of broadcast services , for which it allows more video programs to be allocated to a selected communication link, and for in-studio applications, for which it greatly facilitates visual analysis of captured streams.