Specifies the number of bidirectional predictive frames (B-frames).

Constant for IPropertyBag


Data Type


Default Value



By default, Windows Media Video 9 uses only intraframes (I-frames), also known as key frames or anchor frames, which are fully encoded frames, and predictive frames (P-frames), which are encoded as a difference from the previous I-frame. B-frames are different from P-frames because they store both the differences from the previous frame and the differences from the following frame.

When you configure the codec to uses B-frames, it will use the specified number of B-frames between each pair of frames of either I or P type.

For example, if a sequence of frames without B-frames is "IPPPPPPPPI" then the same sequence encoding with two B-frames would be "IBBPBBPBBI".

For most content either one or two B-frames are appropriate. At higher data rates, one B-frame is normally the optimal choice. Three or more are rarely useful.

The valid range of values for this property is 0 to 7.


Minimum supported client
Windows XP [desktop apps only]
Minimum supported server
Windows Server 2003 [desktop apps only]

See also

Media Foundation Properties