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Encode your content at a variable bit rate when you intend for the content to be downloaded and then played back locally. Windows Media Player might experience pauses or glitches when streaming clips that were encoded at a variable bit rate over the Internet. This is, in part, caused by the bit rate of variable bit rate content varying significantly from scene to scene. As a result, variable bit rate clips are currently suitable for download-and-play applications only. Future versions of Windows Media Services are expected to stream variable bit rate clips properly, but streaming of variable bit rate clips is not supported currently.
As with constant bit rate content, you can encode variable bit rate content in one or two passes. These passes are called quality-based variable bit rate and bit-rate-based variable bit rate, respectively. In the variable bit rate mode that is based on quality, mode 2, all frames are encoded at the image quality specified and no frames are dropped. Use this mode when you want to guarantee the image quality. When encoding variable bit rate content in mode 2, you must specify the desired image quality setting using the –v_quality parameter.
Use the variable bit rate mode that is based on bit rate, mode 3, when you want to set an average bit rate. During the first pass, the encoder evaluates the complexity of the video to determine the image quality setting that can be achieved uniformly and then encodes the content during the second pass. When encoding variable bit rate content in mode 3, you must specify the desired bit rate using the –v_bitrate parameter. No frames are dropped in this mode.
Note The content generated in the variable bit rate modes should be played in Windows Media Player version 7 and above; variable bit rate content does not play well in Windows Media Player versions 6.4 and earlier.