Compression technology
Apr 1, 2009 12:00 PM, By Aldo Cugnini
Quantization and transform coding help to assuage artifacts in video.
Transcoding
It is often necessary to convert video from one compression system to another, to convert bit rate or resolution within the same compression system, or both. For instance, when broadcast transmissions are carried on cable and satellite systems, the service operator may be taking a pre-encoded feed from the broadcaster. Rather than decode and encode, a higher-quality conversion can be achieved using transcoding. When transcoding an MPEG-2 stream to an MPEG-4 stream, the processor can use the motion vectors already derived in the MPEG-2 encoding as a starting point for generating the new motion vectors. To achieve higher efficiency, the new output processor can additionally use the enhanced toolkit offered by that compression scheme. Care needs to be taken when either of the compression systems runs at a low bit rate, because artifacts in the upstream video may become worsened by the subsequent transcoding.
Repurposing content from fixed broadcast to mobile receivers will also often require transcoding and bit rate reduction, so similar considerations apply — but viewing on small screens can render many artifacts less visible. An alternative to parallel coding or transcoding content is to code content in a scalable fashion, so simply dropping portions of the coded stream can yield versions of different qualities or resolutions. While possibly streamlining the production process, this approach works best when the same codec type (e.g., MPEG-4) is used for both decoders. However, when transmission systems multiplex different codec types, simple hierarchical coding is not possible.
Of course, archiving multiple versions of compressed video has storage implications; an alternative is to store only one compressed version and to play out the necessary compressed version(s) with transcoding done in real time. This will require a lot of faith that the end product can be transmitted without any intervening production inspection. However, the sophistication and quality of transcoders may already be at that point — a needed factor given the explosion of various forms of content distribution.
Aldo Cugnini is a consultant in the digital television industry.
Send questions and comments to: aldo.cugnini@penton.com
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