With the advancement of time, video coding technologies are becoming more widely used. Most digital video contents exist in the coded form for saving storage (for example, MPEG-2 technology defined by the Moving Picture Experts Group (MPEG)). Transcoding coded video contents from one format to another, from one bit rate to another and from one resolution to another is an indispensable operation in many multimedia applications, such as digital video broadcasting, video on demand (VOD), teleconferencing, long distance learning, and so on.
The simple way to perform video transcoding is to cascade a decoder and an encoder together. The cascaded decoder and the encoder, first performs a full decoding of an input stream and then performs a full encoding of a decoded raw video data. Since the encoder can optimize video quality at the target bit rate by estimating another set of motion vectors and re-allocating bits of each picture, the cascaded transcoder typically achieves better video quality than other transcoders. The transcoder is called a cascaded pixel domain transcoder (CPDT).
FIG. 1 illustrates a conventional transcoder 100 for converting MPEG-2 to VC1 format. The transcoder 100 receives incoming source video streams in a fully decoded form, and then re-encodes with desirable bit rate or format (VC1) with minimal degradation in the visual quality due to the transcoding. Transcoding algorithms which do not have a feedback loop in a reconstruction process suffer from a drift error. A cascaded pixel domain transcoder (CPDT) on the other hand, has the feedback loop present in a reconstruction path and hence does not suffer from the drift error. Systems which do suffer from the drift error (i.e. open loop systems) have to employ algorithmic modifications to eliminate the drift error. Hence, only for such cases, it depends on efficiency of such algorithms to eliminate drift.
However, due to the need of complete blinded motion estimation, this approach leads to a computational complexity. Due to the need of reordering twice the decoded pictures, a memory is required and also results in processing latency.
FIG. 2 illustrates a block diagram of a conventional transcoder 200. The transcoder 200 employs a restricted reuse of decoded data which consist mainly of decoded pixel data and also reuse motion vectors.
In one approach of the transcoder 200, only the MPEG-2 decoded pixel data and the motion vector information are reused by the H.264 encoder. All the other decisions pertaining to output coding mode for each MB, such as MB modes, a prediction direction, a picture type, a GOP structure, a DCT type, etc., is done by the encoder itself which is a highly time consuming process.
Some of the other conventional transcoders that exist decide the output macroblock types depending on the input macroblocks while downsizing (resolution downsampling) an input MPEG-2 HD video bit stream to an MPEG-2 SD output. This is done by estimating the output macroblock mode based on the spatial weight (based on sizes of overlapping areas of each of the input macroblocks with a correlation area that is correlated with the output macroblock) and a DCT weight (based on a number of zero DCT coefficients of each of the input macroblocks that overlap the correlation area) and hence is restricted to same input and output macroblock types.
Recently, there is a wide deployment of the MPEG-2 format for broadcast and consumer applications owing to the advent of new technologies in digital television (TV) and other digital technologies. A lot of MPEG-2 coded material in the form of DVD movies or personal videos from portable digital cameras has become ubiquitous in recent times. For all the latest applications like high definition TV (broadcast), video streaming on small networks (home), subscription services, video on demand over IP, web broadcast, new advanced codecs like VC1 with fundamental advantage of compressing video sequences much better than old standards, are the choices for coming years. Contents compressed with VC1 are being consumed by a wide range of wired and wireless consumer electronic devices, for example, mobile phones, DVD players, portable music players, car stereos, etc. Windows media content can also be delivered to consumers in physical formats. In fact, DVD forum has adopted the windows media video 9 (WMV-9) codec as mandatory for the next generation high definition (HD) DVD format. At the same time, all previously created, broadcasted or stored MPEG-2 material should be taken into account and provide compatibility for all transmissions still using MPEG-2 standard so that they do not become obsolete in future.
Therefore, there is a need of a system and methodology for transcoding data from one video standard to another video standard.