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Codec implementation

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The Windows Media Audio and Video codecs are implemented as COM objects. Typically, a codec is implemented as a pair of COM objects: one for the encoder and one for the decoder. The encoder has a class identifier (CLSID) and the decoder has a different CLSID. For example, the encoder portion of the Windows Media Audio 9 codec has a CLSID represented by the constant CLSID_CWMAEncMediaObject, and the decoder portion of that same codec has a CLSID represented by the constant CLSID_CWMADecMediaObject.

In some cases, more than one encoder is included in a single COM object. For example, the Windows Media Video 9 encoder and the Windows Media Video 9.1 encoder are both part of the same COM object. Consequently, they both have the same CLSID, which is represented by the constant CLSID_CWMV9EncMediaObject. Similarly, some COM objects include more than one decoder.

Each encoder or decoder object exposes the IMediaObject interface so that the object can be used as a DirectX Media Object (DMO) and the IMFTransform interface so that the object can be used as a Media Foundation Transform (MFT).

For most encoders, regardless of whether you use the encoder as a DMO or an MFT, you use the same CLSID to create an instance of the encoder. For example, to create an instance of the Windows Media Video 9 encoder, you use CLSID_CWMV9EncMediaObject, regardless of whether you intend to use the encoder as a DMO or an MFT. Similarly, for most decoders, each decoder has a single CLSID regardless of whether you use the decoder as a DMO or an MFT.

Note

There are some exceptions to the preceding statement about using a single CLSID for both the DMO and the MFT. For example, the MPEG-4 Part 2 decoder has one CLSID when it is acting as a DMO and a different CLSID when it is acting as an MFT.

 

In addition to the core interfaces, each encoder or decoder object implements two similar interfaces for working with codec properties, IPropertyBag and IPropertyStore. Older versions of the encoder and decoder objects used IPropertyBag, which identifies each property by a string value containing a property name. IPropertyStore is a newer interface that identifies properties with a unique property key value. Support for IPropertyStore was added to provide support for MFTs. Most IPropertyBag property name strings have a corresponding IPropertyStore property key GUID and most of the GUIDs have a corresponding IPropertyBag name string, with a few exceptions.

This documentation lists the properties by property key constant, but each entry includes the property-name string constant for use with IPropertyBag when appropriate.

Windows Media Codecs