Time Stamps

The time stamp defines a media sample's start and finish times, measured in stream time. The time stamp is sometimes called the presentation time.

When a renderer filter receives a sample, it schedules rendering based on the time stamp. If the sample arrives late, or has no time stamp, the filter renders the sample immediately. Otherwise, the filter waits until the sample's start time before it renders the sample. (It waits for the start time by calling the IReferenceClock::AdviseTime method.)

Source filters and parser filters are responsible for setting the correct time stamps on the samples they process. Use the following guidelines.

• File playback: The first sample is time stamped with a start time of zero. Subsequent time stamps are determined by the sample length and the playback rate, which itself is determined by the file format. The filter that parses the file is responsible for calculating the correct time stamps (for example, the AVI Splitter or the ASF Reader).
• Video and audio capture: Every sample is time stamped with a start time equal to the stream time when it was captured. Video preview frames are an exception. Because of graph latency, a preview frame that is time stamped with the current stream time will always arrive late at the video renderer. This may cause the renderer to drop frames, in an attempt at quality control. Therefore, preview frames should not be time stamped. For information about quality control, see Quality-Control Management.
• Mux filters: Depending on the output format, a mux filter may need to generate time stamps, or it may not. For example, the AVI file format uses a fixed frame rate with no time stamps, so the AVI Mux filter assumes that samples are arriving at approximately the right time. If the incoming time stamps show a gap larger than one frame, however, the AVI Mux writes an index entry with size zero, to indicate a dropped frame. On file playback, new time stamps are generated on the fly, as described previously.

To set the time stamp on a sample, call the IMediaSample::SetTime method.

Optionally, the filter can also specify a media time for the sample. In a video stream, media time represents the frame number. In an audio stream, media time represents the sample number in the packet. For example, if each packet contains one second of 44.1 kilohertz (kHz) audio, the first packet has a media start time of zero and a media stop time of 44100. In a seekable stream, the media time is always relative to the start time of the stream. For example, suppose you seek to 2 seconds from the start of a 15-fps video stream. The first media sample after the seek has a time stamp of zero but a media time of 30.

Renderer and mux filters can use the media time to determine whether frames or samples have been dropped, by checking for gaps. However, filters are not required to set the media time. To set the media time on a sample, call the IMediaSample::SetMediaTime method.