Format Negotiation

[The feature associated with this page, Windows Media Player SDK, is a legacy feature. It has been superseded by MediaPlayer. MediaPlayer has been optimized for Windows 10 and Windows 11. Microsoft strongly recommends that new code use MediaPlayer instead of Windows Media Player SDK, when possible. Microsoft suggests that existing code that uses the legacy APIs be rewritten to use the new APIs if possible.]

For Windows Media Player and a DSP plug-in to share data, both programs must agree on the format of the data they are processing. The DSP plug-in implements methods that the Player calls to determine which formats the plug-in supports. The plug-in also implements methods that the Player calls to set the current format.

If the plug-in is acting as a DirextX Media Object (DMO), Windows Media Player discovers and sets media formats by calling methods of the IMediaObject interface. For example, the Player calls the plug-in's IMediaObject::GetInputType repeatedly to get a list of all input formats supported by the plug-in. DMO plug-ins use the DMO_MEDIA_TYPE structure to organize the information that specifies a media format. For more information about how DMO plug-ins and the Player negotiate format, see About IMediaObject.

If the plug-in is acting as a Media Foundation Transform (MFT), Windows Media Player discovers and sets media formats by calling methods of the IMFTransform interface. For example, the Player calls the plug-in's IMFTransform::GetInputAvailableType repeatedly to get a list of all input formats supported by the plug-in. MFT plug-ins and the Player use the IMFMediaType interface to organize and exchange format information.

Windows Media Player will use an audio DSP plug-in only if the plug-in supports the same bit depth as the digital audio being played. For instance, if the digital audio is 20-bit, the plug-in must be written to process 20-bit audio. For CD audio, DSP plug-ins must support 20-bit processing.

During format negotiation of multi-channel content on a computer configured for use with stereo speakers, Windows Media Player first attempts to connect to an audio DSP plug-in using the existing input and output format by calling IMediaObject::SetInputType and IMediaObject::SetOutputType. Once this initial negotiation occurs, the Player then enumerates the formats the plug-in supports and attempts to negotiate the best format combination for the Player and the plug-in. If the plug-in accepts stereo audio (defined by a WAVEFORMATEX structure) as the input format during the initial negotiation, and then subsequently accepts only multi-channel audio (defined by a WAVEFORMATEXTENSIBLE structure), the Player will provide multi-channel audio as the input format to the plug-in. This behavior during format negotiation is available for use in the Microsoft Windows XP operating system. It may be altered or unavailable in subsequent versions.

Related topics

DSP Plug-in Developer Overview