Media Quality
The goal of most codecs is to retain the quality of media content while compressing the data as much as possible. In this context, high quality means that decompressing a stream that was compressed with the codec yields media that is hard to distinguish from the original. All of the Windows Media Audio and Video codecs, with the exception of Windows Media Audio Lossless, use lossy encoding. This means that during the process of compressing and then decompressing the content, some of the data is lost.
The important questions about lossy encoding are:
- Which part of the data is lost?
- How much data is lost?
- Can the human ear or eye tell the difference?
Other factors come in to play in the design of codecs, such as whether the media content retains the ability to seek to arbitrary points in the data, or recover from missing data. This is, however, beyond the scope of this documentation.
Codec developers attempt to optimize their algorithms so that the data that is lost in compression won't noticeably degrade the quality of the final content. Some of this optimization is simple. If, for example, audio content contains frequencies outside of the human range of hearing, that data can be discarded without perceptively altering the content.
However, the designers of a codec must decide to optimize for a certain type of content. For general-use codecs like Windows Media Audio 9 and Windows Media Video 9, this means that the characteristics of the original media can greatly affect the quality of the encoded content. It is very important to understand that some content may result in a poor-quality stream, even if other content encoded with the same settings yields good results.
If an encoded stream is lower-quality than expected, you can sometimes change the configuration slightly and get better results. Of course, this can take some experimentation. Specific tips about how to get the best quality output from the codecs are given in the Working with Audio and Working with Video sections.
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Build date: 4/7/2010