Minidrivers should time stamp data packets to synchronize multiple data streams. Kernel-mode clocks begin counting time when they first transition out of the KSSTATE_STOP state. Thereafter, clocks should increment time stamps in regular intervals of 100 nanosecond units until the stream transitions to the KSSTATE_STOP state.
Each data packet that is transferred corresponds to a single frame or field of video or ancillary data. Video capture driver writers that are concerned with frame-accurate video capture can choose to provide a clock that all other filters can use as a reference. Digital video minidrivers are an example of minidrivers that should provide clocks to use in a filter graph. Alternatively, video capture minidrivers that run asynchronously to other multimedia streams, such as USB and IEEE 1394 conferencing cameras, should time stamp their data packets with a clock provided by another component, such as an audio digitizer.
If a Stream class minidriver provides master clock, it should specify the following values in the HW_STREAM_OBJECT structure:
PHW_STREAM_OBJECT *pStreamObject; PStreamObject->HWClockFunction = (PHW_CLOCK_FUNCTION)StreamClockRoutine; PStreamObject->ClockSupportFlags = CLOCK_SUPPORT_CAN_READ_ONBOARD_CLOCK | CLOCK_SUPPORT_CAN_RETURN_STREAM_TIME;