Forthcoming interactive video services such as video on demand will use pre-encoded bit streams for storage and transmission. Video Transcoding is intended to provide transmission flexibility to pre-encoded bit streams by dynamically adjusting the bit rate of these bit streams according to new bandwidth constraints that were unknown at the time of encoding.
In this paper, we propose a new bitrate reduction algorithm from 1.5Mbit/sec MPEG-1 to 384Kbit/sec MPEG-4 Simple Profile. For rate reduction, we use both requantization and frame skipping method. Proposed algorithm has two important schemes. Previous requantization methods of homogeneous transcoder cannot be used directly for the MPEG-1 to MPEG-4 heterogeneous transcoder due to the mismatch in quantization parameters between MPEG-1 and MPEG-4 syntax. In order to solve this problem, we propose a gradual approach method with which MPEG-4 bitrate follows MPEG-1 bitrate. For the bit rate change by frame skipping to find a new motion vector, the FDVS(Forward Dominant Vector Selection) method is modified to induce the effect of skipped frame’s macroblock type.
A simple bitrate reduction system is simulated, and the gradual approach method is tested and compared for the proposed FDVS and for the conventional FDVS. It is shown that the proposed algorithm gives much better performance in picture quality than conventional FDVS method at the tested heterogeneous transcoder system.
