I have a engine to check the H264 video is compliance with AVCHD or BDMV spec, the SPEC mentions the MAX system data rate is up to 24 Mbit/s, I want to know how to calculate the system data rate? Does it mean the average of whole file? Or does it mean the average of 1 second?
+1
A:
The maximum specifies that you that you will never exceed 24Mbps so you will never send more than one bit in any 42nS (approximately) period. You can scale that to any time frame you want by simple multiplication to the point when you will never burst beyond 24M bits in one second (and you will still never send more than one bit in any of the 24M 42nS periods that make up that second).
When you calculate an average for any time period, it MUST be below the specified maximum burst, but is simply considered an average. Those of us in the CATV industry spend a lot of time trying to make the transmission system behave as if the average rate is a constant rate, because if you have a certain throughput (in bits) for video, you don't want to waste any of it. We "rate shape" the video as well as using adaptive buffering in the digital set-top boxes that receive the signal.
A single QAM256 channel on the U.S. broadband cable system will support 40Mbps and usually between 10 and 12 normal definition signals with an average bit rate of approximately 4Mbps. These channels will burst to 9Mbps when there is a lot of change in the picture of an unpredictable nature. As you can imagine, a boxing match (with a lot of movement) takes significantly more bandwidth, than a network news anchor reading from their desk, so we also try to match channels to fill this available bandwidth.
Typically, we can only fit 3 high-definition channels in the same 40Mbps channel and these have an average bit rate of about 12.5Mbps and as you've noted above, are limited to 24Mbps.
Hope this helps!
Steve Moyer
2009-02-26 03:14:35