To Configure Quality-Based VBR

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 It has been superseded by Source Reader and Sink Writer.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

You can use quality-based variable bit rate (VBR) encoding on a stream to specify a quality level that will be maintained for the entire stream, regardless of the bit-rate requirements that result.

For quality-based VBR video streams, you must specify a quality level from 1 to 100, with 100 representing the highest quality. At present there are only 30 discrete quality settings. The following quality levels equate to discrete quality settings: 1, 4, 8, 11, 15, 18, 22, 25, 29, 33, 36, 40, 43, 47, 50, 54, 58, 61, 65, 68, 72, 75, 79, 83, 86, 90, 93, 97, 100. Numbers between two consecutive values in the preceding list equate to the same quality setting as the lower number. For example, 1 and 4 are listed, so 2 and 3 both result in the same quality setting as 1.

For audio streams, you can enumerate the available modes and retrieve a stream configuration object. For more information, see To Enumerate Codec Formats.

When using quality-based VBR video, you must set the dwBitrate member of the WMVIDEOINFOHEADER structure to a positive value. This value is not used by the writer, but passing zero or a negative number can cause errors when writing.

To configure a stream in a profile to be encoded with quality-based VBR, perform the following steps.

  1. Create a profile manager object by calling the WMCreateProfileManager function.
  2. Open an existing profile to which you want to add VBR support. For more information about opening profiles, see Working with Profiles.
  3. Get a stream configuration object for the stream you want to use by calling either IWMProfile::GetStream or IWMProfile::GetStreamByNumber.
  4. Get a pointer to the IWMPropertyVault interface of the stream configuration object by calling IWMStreamConfig::QueryInterface.
  5. Enable VBR for the stream by calling IWMPropertyVault::SetProperty for the g_wszVBREnabled property.
  6. Set the quality level for the VBR stream by calling IWMPropertyVault::SetProperty for the g_wszVBRQuality property.
  7. Set g_wszVBRBitrateMax and g_wszVBRBufferWindowMax both to zero with IWMPropertyVault::SetProperty.
  8. Save the changes made to the stream by calling IWMProfile::ReconfigStream.
  9. Save the profile, or pass it to the writer object and start writing.

Configuring VBR Streams