About the Fast Transcode Feature

Transcoding converts a digital media file from one format to another. The source media data is decompressed (or decoded) to an uncompressed format, and then the data is recompressed (or encoded) to the target format. Compatible decoders and encoders (codecs) for audio and video streams are used to decompress and compress digital data. Microsoft provides many built-in codecs with Windows 7 that you can use to perform format conversions.

Transrating is a special type of transcoding that converts a higher bit-rate file to a lower bit-rate file without changing formats. Transrating is useful in scenarios such as synchronizing media content from a PC to a portable device that does not support a higher quality or resolution media.

To create a transcoding application using Media Foundation in Windows Vista, you had to enumerate the suitable decoders and encoders, create the media sinks, and build and connect the pipeline objects in a topology that was set on the Media Session to generate the output file. See Media Foundation transforms. It was the responsibility of the application to choose the right encoder by setting the correct properties that governed the quality and type of encoding. The format conversions relied on the built-in media sinks that were focused on Advanced System Format (ASF) files. For more information, see Topologies and ASF Encoding in Media Foundation.

In Windows 7, Media Foundation introduces the fast transcode feature. The process of creating transcoding applications in C/C++ that perform media format conversions is simplified. The advantages of using the fast transcode feature are highlighted in following list:

• Simpler programming model. The fast transcode objects validate the specified source and create a topology by connecting the appropriate codecs and media sinks.
• Improved format support. The fast transcode objects support ASF, MP4, and 3GP formats.
• Faster transcoding. Hardware accelerated encoders and decoders are added to the topology to provide faster transcoding.

The fast transcode feature is useful in device synchronization scenarios where transcoding speed and supported format is important. You can provide the transcoding criteria based on the device's capabilities. The fast transcode objects build a transcode topology by adding the required pipeline components. By using the transcode feature, an application can transcode the media content on the PC based on output requirements, for example, transrated from a high bit-rate file to a lower bit-rate file to be played on a cell phone. The application can then transfer the transcoded file to the target device, such as a cell phone that reads 3GP format.

The fast transcode feature has the following limitations:

• Supports Constant Bit Rate Encoding where target bit rate is known before the encoding session.
• Supports only one audio stream and one video stream.
• Does not support protected content other than ASF files protected by using WMDRM.
• Does not support live streaming scenarios, such as live-to-file streaming and live-to-live streaming.

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Build date: 2/5/2010