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Quality Messages

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[The feature associated with this page, DirectShow, is a legacy feature. It has been superseded by MediaPlayer, IMFMediaEngine, and Audio/Video Capture in Media Foundation. Those features have been optimized for Windows 10 and Windows 11. Microsoft strongly recommends that new code use MediaPlayer, IMFMediaEngine and Audio/Video Capture in Media Foundation instead of DirectShow, when possible. Microsoft suggests that existing code that uses the legacy APIs be rewritten to use the new APIs if possible.]

Quality messages are defined with the Quality structure. This structure contains the following members:

  • Type: Defined by the QualityMessageType enumeration; either Famine, indicating that the filter is receiving too little data, or Flood, indicating that the filter is receiving too much data.
  • Proportion: The requested adjustment in the data rate, from a baseline of 1000. For example, 750 indicates 75% and 1500 indicates 150%.
  • Late: Reference time indicating how late the most recent sample arrived. The value is negative if the sample arrived early.
  • TimeStamp: The time stamp on the most recent sample.

For example, suppose that a sample with a time stamp of 240 milliseconds (ms) reaches the renderer at 280 ms, stream time. The renderer creates a quality message of type Famine. The sample arrived 40 ms late, so the Late member is 400000. (All reference times are in 100-nanosecond units.) The TimeStamp member is 2400000.

For the Proportion member, the renderer might use a running average to calculate the value. Perhaps samples have been arriving on time, and this sample is an anomaly. In that case the renderer might request only a small correction. On the other hand, if samples are consistently late, the renderer might request a larger correction.

Quality control is handled through the IQualityControl interface. It contains two methods.

  • Notify: Sends a quality message.
  • SetSink: Specifies a custom quality manager.

An object that implements IQualityControl receives quality messages through its Notify method. It can either handle the message or pass the message to another object. If the application calls the object's SetSink method, the object should delegate quality control to the specified quality manager.