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Video transcoding
1. Introduction
Some people use the term Transcoding in Digital Video for any transformation from one video format to another video format. Like others we only use it here to describe the process to transfrom a compressed stream into another compressed stream.
Before reading this article we would recommend first to read our article MPEG explained which contains some information about MPEG compression.
There are many reasons to transcode a video stream, but one of the most important reasons is to reduce the bitrate of the stream. This may be necessary for example if the stream has to be broadcasted through a medium with limited bandwith or if the filesize has to be reduced to fit it on a CD or DVD. Transcoding is therefor very relevant for DVD to DVD copying and is very useful to copy an original dual layer DVD fit on a single layer DVD-R. One of the first programs on the market that makes use of this principle is DVD2One. Let's have a look at transcoding an MPEG stream to another MPEG stream.
2. Re-encoding
The first method is very straightforward and is shown in Picture 1. Using a decoder, the existing MPEG stream is decompressed and the decompressed stream is then fed into an MPEG encoder to compress it with settings as desired.
This process is for example used if you rip a DVD to create an (S)VCD and programs like DVDx and others that were written for this purpose make use of this process.
3. Compressed Domain Transcoding
The second method is shown in Picture 2. An MPEG transcoder that takes the existing MPEG stream and directly transforms it to another MPEG stream. The original stream is not decompressed completely and the transcoding actually happens while the stream remains compressed. It is therefor called Compressed Domain Transcoding.
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