Originally Posted by Delta
to avoid clipping for the brightest of whites and darkest of the blacks from my tapes?
This question is asked from the wrong stance. It infers that the capture card is doing something wrong, and harming the video. But that's not accurate.
Anything out 16 and 235 are illegal values for NTSC. Most capture cards therefore only capture the 16-235 legal range.
If your tapes are being "clipped", it's because the tapes have the problem, not the capture card. This is addressed with an external proc amp prior to the capture card.
The capture card's "proc amp" isn't actually a proc amp whatsoever. It corrects alreadty-digital values, and is not any different from post-capture NLEs for color correction. (I"m not 100% sure about every single card doing this, as it doesn't have to be this way, but I've yet to see one does any differently.)
Even if you capture the blacker-than-black and whiter-than-white levels, you still must correct those in post. It's not NTSC, and formats like DVD-Video will truncate them. Even playing a streaming version will be truncated to 16-235 by the television. The only "safe" place to view these badly-made videos is the computer.
The ATI AIW is one of the few cards that capture the entire range. (sanlyn used to harp on this topic quite a bit, and showed his VC500 to properly get the illegal values. However, my VC500 did not, as it's a variable card.)
I sometimes think too much is made of this issue. It mostly becomes important when you get into more nth gen materials, where even the slightest loss is magnified. Or if your video is fully overexposed or underexposed. For general home movies, you're unlikely to see major difference when watching for enjoyment. In most cases, it's the difference between seeing a head of brown hair, and the individual hairs, when pausing the screen and looking for flaws. Or fine details in clouds. This error rarely "jumps out at you" like wiggle lack-of-TBC errors, bad capture cards, etc. Your TV set and computer monitor likely monkeys with contrast values far more than anything the capture card does.
See also: DO NOT "correct" with a non-calibrated monitor. Because then you're just ruining video to look good on a single monitor.
PAL values differ, "clipping" less of a problem.