Setting the Playback Rate
To change the playback rate, call the IMediaSeeking::SetRate method. Specify the new rate as a fraction of the original rate. For example, to play at twice-normal speed, use the following:
Rates greater than one are faster than normal. Rates between zero and one are slower than normal. Negative rates are defined as backward playback, but in practice most filters do not support it. Currently none of the standard DirectShow filters support reverse playback.
Regardless of the playback rate, the current position and the stop position are always expressed relative to the original source. For example, if a source file is 20 seconds long at normal playback rate, setting the current position to 10 seconds will seek to the middle of the file. If the playback rate is 2.0, the stop position is 20 seconds, and you seek to the 10-second position, the file will play for 5 seconds of real time: 10 seconds' worth, at twice the normal playback speed. At a playback rate of 2.0, the current position increases at twice the rate of the reference clock.