Throughput, Bandwidth, etc for Video Capture

dvogel

Junior Member
Dec 31, 2001
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Lately I've been thinking about building a system for use as a DVR (among other things) in my living room. I've been looking at lower-end video capture cards, reading up on television signaling standards, and coming up with more questions than answers.

First and foremost (as it affects every other question), I live in Madison, WI (USA). The signals broadcast in my area should be NTSC, correct? If I am incorrect, stop reading, because so far I have assumed NTSC encoding. Assuming NTSC is in fact what is used in my area, what resolution is used and/or how would I go about determining/verifying this? Does any of this change for cable television services?

For calculations I have used a resolution of 525x480 (mostly because it was what I saw mentioned most frequently, and it was below the 576 vertical resolution of PAL signals). I assume video capture cards transmit the captured data over the PCI bus as 24-bit, RGB packed pixels. Also, the only figure I found for NTSC requency was 30 frames/second. Using these figures I get:

( ( 525 * 480 ) * 24 ) * 30 = 181440000 bits/sec or 173 Mbit/sec

Obviously we can't push 173 Mbit/sec over your average 33MHz 32-bit PCI bus, so the 5:1 compression ratio I see mentioned in the product summaries for most capture cards must take place on the card, before it is transfered over the PCI bus. 173 Mbit / 5 = 34.6 Mbit, so now we have overcome the PCI bus.

Higher end cards like the Pinnacle Systems DC2000 have a sustained data rate of 50 MBit/sec. Obviously these can capture all 30 frames/sec of an NTSC signal. What about the lower end cards, such as the Pyro ProDV, which seem to all have a sustained data rate substantially lower than 34.6 Mbit/sec; how do they account for their handicap? I would hope they don't just drop frames at random. Are my calculations flawed? I can only imaging it getting worse with PAL signals if my calculations are correct.

Are standard desktop AGP video cards, like the ATI Radeon 8500 DV, able to achieve the sustained data rate required for full-quality video capture? If not, which capture cards would you reccomend for DV capture from a TV signal?

Regards,
Drew Vogel
 

rbV5

Lifer
Dec 10, 2000
12,632
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You are correct, NTSC for USA.

Instead of confusing yourself, figure 640X480 for NTSC video full resolution video. For simplicity, and because I'm too lazy/not smart enough, I open VirtualDub(excellent freeware capture/editor) and just plug the numbers in.

For full resolution NTSC video with 44.10 KHz 16 bit audio at 29.97 fps, you get 18,155 KB/sec sustained data rate. Interestingly enough, full 1080i HDTV transport streams(OTA broadcasts) are almost the same data rate due to compression!

You just need a faily fast IDE drive(s) (large btw) and yes, a card like The 8500dv can capture full NTSC uncompressed easily(the IDE subsystem is the workhorse here...not the card) AND compress it to MPEG-2 (or other formats) in real time, dependant on the CPU (the real workhorse here...not the card again)
If not, which capture cards would you reccomend for DV capture from a TV signal?
DV capture is a whole different animal. With NTSC broadcast signals...we are talking analog capture, not digital transport.
 

videobruce

Senior member
Nov 27, 2001
971
1
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NTSC for North America.

Video from a PCI capture card isn't a whole lot better than video from a USB 1.1 camera (haven't seen USB 2 video yet).
A AGP card with VIVO is the way to go. Problem being everything I have seen stinks. Smeary video that I doubt passes 2MHz! Not any better than Beta 3!

I feel computers and video have a long way to go, at least at a level most people could afford. Data rates are very high for analog video, that's why tape is really the only proven method that doesn't suffer from digitial artifacts!

Look at solutions from ATI (9500Pro) or nVidea (T14200).
 

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