Codec Overview
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The Windows Media Audio and Video codecs are a collection of objects that you can use to compress and decompress digital media data. Each codec consists of two objects, an encoder and a decoder. This part of the documentation describes how to use the features of the Windows Media Audio and Video codecs to produce and consume compressed data streams.
The term codec is an amalgamation of the terms compressor and decompressor. A codec is usually implemented as a pair of COM objects: one for encoding content, and another for decoding content. In some cases the COM objects occupy the same dynamically linked library (DLL).
Every codec object implements two separate but similar interfaces: IMFTransform which is compatible with the Media Foundation SDK and IMediaObject which is compatible with the DirectShow SDK.
Not only are there different codecs for audio and for video, but also different codecs for different kinds of content that you might want to put into an audio or video file. The algorithms used to compress and decompress data for spoken words (for example, the Windows Media Audio 9 Voice codec) differ from the algorithms used to compress and decompress music data (for example, the Windows Media Audio 9 codec).
Codec Descriptions
The following table describes the intended uses of the Windows Media codecs.
| Codec | Description |
| Windows Media Audio 9 | Audio codec for general use in encoding complex audio, such as music. |
| Windows Media Audio 10 Professional | Audio codec for encoding complex audio, such as music. Supports multi-channel and 24-bit encoding. |
| Windows Media Audio 9 Lossless | Audio codec for lossless encoding. |
| Windows Media Audio 9 Voice | Audio codec optimized for encoding the human voice at high compression ratios. This is the preferred codec for streams consisting mostly of spoken words. For content that is mixed music and speech, this codec can dynamically change the encoding algorithm used, to get optimal quality. |
| Windows Media Video 9 | Video codec for general use in encoding complex video, such as movies. |
| Windows Media Video 9 Advanced Profile | Video codec that includes advanced features such as interlaced encoding and mixed progressive and interlaced encoding. |
| Windows Media Video 9 Screen | Video codec optimized for encoding sequential screen shots from computer monitors. This codec is often used for software training or support by recording monitor images while computer applications are being used. |
| Windows Media Video 9.1 Image | Video codec for converting bitmap images (such as still photographs) with deformation information into compressed video. |
The most recent versions of the codec objects also enable access to some legacy codecs, including Windows Media Video 7 and 8, Windows Media Screen 7, the older Microsoft MPEG-4 codecs, and the Microsoft ISO MPEG-4 codecs.
Note This documentation does not cover these legacy codecs; it covers only the Windows Media 9 Series and later codecs.
For older codecs, use the same procedures as when using the Windows Media 9 Series codecs; however, remember that not all features are supported in all codecs. In addition, the MPEG-4 codecs are not supported in the video encoder object.
See Also