Differences in how Windows 2000 and Windows XP handle Serial Digital Interface signals (834727)

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SDI (or SMPTE 259M) is a standard for serial transmission of digital video. The SDI standard transmits plain video data and provides space for ancillary data, including time-code data, user-specific control data, and digitized audio. The SMPTE 291M standard defines the generic format and location of ancillary data. Audio embedding is specified in the SMPTE 272M standard. A data rate of 270 megabits per second (Mbps) is standard for conventional TV formats (4:2:2 video). This rate is equal for 525-line systems (29.97 Hertz (Hz)) and for 625-line systems (25 Hz). New standards with higher data rates have been developed after the introduction of new image formats and of HDTV.

The Microsoft Digital Video (MSDV) camcorder driver (Msdv.sys) uses a fixed mechanism for timing DV streams. A problem may occur in how the correct stream rate is maintained. The Msdv.sys driver will insert empty common isochronous packet (CIP) header packets every nth isochronous packet. For example, this pattern might be an alternating pattern like 15, 15, 15, 15, 16, repeat. This pattern would cause correct DV transmission, but the timing would be imprecise.

In Windows XP, the Msdv.sys driver sits on top of the IEC-61883 protocol driver (61883.sys). 61883.sys, not Msdv.sys, specifies a time value that tells how long it should take to send one data block. The time value is specified in terms of 1394 ticks. For more information, see the following formula:

1394 ticks = 8000 cycles/second * 3072 ticks/cycles = 24,576,000
		  ticks/second 

This time information and the number of ticks that have actually occurred are tracked to derive the correct time to insert empty CIP packets. The result is a much more accurate data stream rate in the Windows XP Msdv.sys driver than the data stream rate in the Windows 2000 Msdv.sys driver may provide.