Clocks

For playback and preview, it is important to maintain the correct playback rate, and to synchronize audio and video streams. To this end, each media sample carries a time stamp, which specifies the presentation time for that sample. The Filter Graph Manager maintains a clock for the entire graph. This clock is called the reference clock, and the time that it reports is called the reference time. Reference time is measured in units of 100 nanoseconds.

In some filter graphs, a filter can provide the reference clock. For example, the audio renderer can usually provide an accurate clock based on the sound card. If no filter can provide a clock, the Filter Graph Manager uses a default clock (based on the system time).

For more information, see Time and Clocks in DirectShow.