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Old 30th September 2002, 19:47   #21  |  Link
Xesdeeni
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Join Date: Aug 2002
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aldonix - That's the Nyquist frequency (like ronnylov said). But it applies to frequency, not lines of resolution. However, you can equate lines of resolution to 2x the frequency. This is because for each cycle the frequency in question contains, there would be a high (white) and a low (black) part, which is equivalent to two lines. This is the 2 in the equations used above.

ronnylov - You were going along fine until you said:
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So if VHS is maximum 240 lines horisontal resolution this would be equal with the on screen visual picture width of 320 pixels. Adding the overscan and you can be safe to reproduce the whole resolution in 352 pixels width as used in half D1 (like 352x576 PAL or 352x480 NTSC).
You made the same mistake pointed out above.

PIXELS != LINES OF RESOLUTION

Reread the above explanation and post again if this doesn't make sense. But your 320 number above is actually the number of lines of (horizontal) resolution which VHS can produce, and this is an analog measurement. You must use the Extended Kell Factor (which is somewhat debatable but we'll stick with sqrt(2) or 1.41) to convert from lines of resolution to digital pixels in order to ensure you can capture that number of lines with all phase relationships. This means you need in the neighborhood of 450 digital pixels to represent what a VHS recording can produce.
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But capture cards are not perfect. The Nyqvist theorem also requires to remove all frequencies above the sampling frequency before the sampling. The required capturing resolution to get all the needed 240 lines resolution is probably higher than 352 width. I suggest capturing at 704 pixels width, or at least 480 pixels width and then resizing down to 352 pixels width after capturing before encoding.
What you are referring to is called aliasing, and it is not a result of an imperfect capture card.

Sampling is theoretically done in an infinitely small time (in reality it takes a finite time that is extremely short, but it's definitely close enough for our purposes). So if the frequency is not limited, it is possible to get a sample that is not representative of the overall signal. [ex. Sample 00411422433444433422411 at points 3,6,9, etc., and you would completely lose the baseline wave/frequency.]

To avoid aliasing, the input signal must be low-pass filtered. [The above sequence would end up being something like 01211222333344333211, and the baseline wave/frequency is not lost.] The maximum frequency of that filter is determined by the Nyquist frequency. But the important point here is that the appropriate filtering frequency varies with the sampling frequency.

The problem with real-world sampling devices (as I mentioned above) is that most of them have only a single filter frequency. It is more complex to build multiple frequency filters, and most people won't notice if you don't. And of course, that single frequency chosen will not be for the lowest sampling rates, but for the highest, because otherewise the higher sampling rates would be a complete waste. So to sample at lower frequencies, these devices leave in undesirable frequencies that will cause aliasing.

This is why it is always best to sample at the highest frequency that the device allows, making the assumption that the filter is matched to this rate. Then using software, you can scale the image down with appropriate filtering, to get the desired sampling rate (or in our discussion, resolution).
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I have done half D1 resolution DVD's (352x576 PAL) from VHS source and can't see any loss in detail compared to the original.
As I mentioned above, I have also used half-D1 resolution, but I have definitely seen the difference. The above information was obtained during my research into why everyone said that half-D1 was sufficient, when in fact my equipment and my eyes told me it was not. Perhaps your equipment and your eyes do not see the difference, but mine did, so I was intrigued as to how this was possible. The information I've found and shared above closely matches my actual experience, which makes me feel much more comfortable :-)

Xesdeeni
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