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Old 28th February 2006, 03:32   #10  |  Link
tedkunich
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Join Date: Mar 2003
Posts: 219
Quote:
Originally Posted by jmac698
From the capture guide:
This makes your final sample rate 704 samples / 52 µs = 13.54 MHz.

This is sample rate. According to the Nyquist theorem, you can record frequencies of half the sample rate, making the bandwidth 6.77MHz. Modern video DAC's have excellent filters and I expect to reach very near this value. If you read my post, I said rate, so while I was correct, I should have compared bandwidth MHz numbers together, I apologize for the mistake. In any case, the luma bandwidth is still greater, in one estimation (approx.), 6.77/3.57=1.88 times better.
Not trying to bust your chops bud, just trying to point out discrepancies. Everyone loves to quote Nyquist as if that was a holy grail or something... sure you can sample at that frequency... at a piss poor accuracy! Nyquist only determines MAXIMUM frequency that you can sample and NOT get aliasing - it does not state anything about the accuracy of that signal, i.e. voltage, which happens to be very important in video as it determines the brightness, contrast and detail of video.

Also do not forget that all cards on the market have some intended and unintended filtering happening in the analog domain before it ever reaches the ADC (not a DAC BTW). Since MFG's typically use a simple, ie inexpensive, 3db or 6db filter prior to the ADC, the cut-off frequency will be significantly lower than Nyquist such that the rolled off frequencies will be reduced below the ADC's sensitivity @ Nyquist. I would be willing to bet that your card is limiting the analog to something less than 4.5MHz, so no matter what you do RE sampling, you are never going to get better than that.

Since it sounds like you have a PVR (I assume since you mention a three pass capture), why not hack the box or remove the HD to get access to the files?



Peace,

T
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