![]() Because MPEG-2 encoders code I- and P-pictures at a higher quality than B-pictures, better transcoding efficiency can be achieved if the H.264 encoder can align the picture type with that of the input stream. Reusing the picture type (I, P or B) information from the MPEG-2 bit stream can provide substantial improvement in transcoding quality. However, here the relevant information available from the MPEG-2 bit stream is reused.Īlthough there are significant differences between MPEG-2 and H.264, including block shapes for motion compensation, block sizes for transformation and motion search ranges, there is still useful information available in the input MPEG-2 bit stream that can be exploited by the H.264 encoder to improve transcoding quality and reduce computational complexity. Similar to the previous approach, the incoming MPEG-2 stream is decoded and then re-encoded using an H.264 encoder. Transcoding can result in up to 20 percent loss in compression efficiency. The bottom plot shows the performance of transcoding where the video is originally coded with MPEG-2 at 4Mb/s, decoded and then re-encoded with the same encoder used for direct encoding. The top plot shows the performance of direct encoding using an H.264 encoder. The PSNR numbers are obtained by averaging the results over 18 different sequences of varying content type and complexities. The figure shows the PSNR (a measure of mean square error between the input and decoded output) values computed at different bit rates. Figure 3 shows a comparison between direct encoding and transcoding. The quality of transcoding with this simple approach will not be high. System issues, such as handling SCTE-35 digital program insertion (DPI) messages, will require that the decode and encode operations be tightly coupled. ![]() The decode operation can be performed either externally or as a part of the H.264 encoder. The simplest approach to transcoding is to completely decode the MPEG-2 bit stream and then re-encode it with an H.264 encoder. The approach to transcoding MPEG-2 to H.264 is expected to progressively follow three approaches, which are presented below. ![]() The use of spatial prediction in I-slices in H.264 makes the transcoding of the MPEG-2 I-frame substantially more complex than the simple re-quantization techniques that have been used by the MPEG-2 rate transcoders. Several new features available in H.264, such as multiple reference frames, smaller block shapes and spatial intra prediction, have no corresponding information in the MPEG-2 bit stream. Although MPEG-2 and H.264 use similar techniques of motion compensation, transformation, quantization and entropy coding, there are several basic differences between the two standards that make the transcoding operation challenging. This article primarily focuses on the issues and challenges associated with format transcoding from MPEG-2 to H.264. Resolution transcodingThis involves the conversion of coded spatial resolution - for example, converting a standard-resolution bit stream to common intermediate format (CIF) resolution for a mobile application.Format transcodingThis entails converting the compression format - for example, converting an MPEG-2 bit stream to an H.264 bit stream.MPEG-2 bit rate transcoders, also called rateshapers, have been widely deployed today, and they employ efficient, high-density rateshaping by primarily operating in the discrete cosine transform (DCT) domain. Bit rate transcodingThis process changes the bit rate of the compressed bit stream while keeping the resolution, frame rate and the encoding format the same.Several forms of transcoding are possible, depending on the specific parameters of the compressed bit stream that are modified during the transcoding process. ![]() Finally, improvements in programmable processor technologies, such as the DSPs and FPGAs, have made it possible for video processing vendors to field products that can handle multiple existing formats and field-upgrade the products for future emerging standards. ![]()
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