ATI AIW Radeon 7500 AGP Capture Levels and Histogram in VirtualDub

[theseeker2]

I'm capturing NTSC VHS in VirtualDub 1.9.11 Huffyuv YUY2 with an ATI AIW Radeon 7500 AGP (Rage Theater chip) in Windows XP with S-Video and a Panasonic DMR-ES10 for TBC-ish correction. I've been adjusting the brightness/contrast so that there's a minimum of red on the left and no red on the right to keep within the 16-235 range.

Question: is it necessary to adjust the brightness/contrast with an ATI AIW Radeon 7500 AGP? It seems like it's picking up 0-15 and 236-255 when capturing, as the histogram stretches all the way to the far left and far right when capturing using default levels. Am I going to harm anything by adjusting to be within the 16-235 range so there's just a little red on the left and none on the right? I don't want to lose details on capture by adjusting out things that the card could actually pick up. Hope that all made sense

[Skiller]

Whether or not it causes harm depends on where and how exactly the adjustment happens.
If it adjusts the interpretation of the analog signal's amplitude, it does not cause harm. This would be ideal, but is rarely the case.
If it's just the driver compressing and expanding the digital levels (so after ADC) it causes harm unless this is done in a bit depth which is significantly higher than 8 bits per channel.


It is quite easy to find out.
The ATI AIW Radeon 7500 AGP has an analog tuner (edit: or maybe not all of them, use a VCR then). In VirtualDub, select the analog tuner as input (with nothing connected to it).
You should now see the classic "snow storm" like on old TVs. Now enable the histogram. Play with the levels settings. If it creates any spikes or holes in the histogram, you know you should not touch those settings because all they do is introduce banding (and there would be nothing to be gained at this point anyways).

[Brad]

I've been adjusting the brightness/contrast so that there's a minimum of red on the left and no red on the right to keep within the 16-235 range.

Can't tell whether you know this, so I'll just add a reminder:

1. Crop all borders including head-switch noise (i.e. at least left, right, bottom)
2. Do the adjustment
3. Remember to disable the crop before capture

The borders are supposed to be in the "red" range.

[lollo]

And as a general rule capture in the range of your capture card, not 16-235. If you card can capture 2-254, for example, capture that.

If later in post-processing you need to change color space to RGB, then you will reduce the levels to 16-235 prior the conversion to RGB, or if you want your final output to be limited range.

The captured range of some capture cards is shown here: https://www.digitalfaq.com/forum/video-capture/9865-canopus-advc-110-a.html#post62640. Experiment with yours as Skiller and Brad suggested

[theseeker2]

Thans lollo. I've perused that thread you linked before, and I'd guess that my ATI AIW 7500 would perform similarly to the 9600 in that thread. If I'm following sanlyn's very long response at the end of the first page, sanlyn is saying that the 9600 will capture below 16 and above 235, but does clip some at the low end (LumaY loose minimum = 8) but not much at the high end (LumaY loose maximum =254). Does that mean I'm ok with this card to capture fully in the red on the right side of the histogram? Or is sanlyn saying to adjust it to avoid the clipping even if it's minimal? I did previously use a ATI AIW 600 USB, and I noticed I couldn't adjust that thing at all without the banding that skiller mentioned, so I ditched it.

Also, does it make sense that most of the professionally recorded tapes I've captured so far have needed the contrast turned down and sometimes brightness turned a bit? It seems fairly consistent that the contrast adjustment in particular is needed with commercial tapes to keep things out of the red on the right, but maybe I don't need to worry about that with my card.

Any and all input on what I typed above is more than welcome. I am not new to this but also am not anywhere near an expert, but I do like to learn.

[lollo]

The problem is that when you act on the procamp of the card you are in the "digital domain", and may introduce gaps or spikes in the histogram, together with banding and other defects.

On the other hand, if you do not adjust the levels you may crush the blacks lower than "what your card can capture" and clip the whites higher than "what your card can capture".

That's why IMO the best is to act on the levels through the procamp just to stay inside the range of the card, not pushing further.

I did previously use a ATI AIW 600 USB, and I noticed I couldn't adjust that thing at all without the banding that skiller mentioned, so I ditched it.

I suspect that the problem with the ATI USB 600 is that the procamp level correction arrives too late, when the signal is already digitized and the blacks < 16 were lost (it needs to be confirmed)

Any and all input on what I typed above is more than welcome. I am not new to this but also am not anywhere near an expert, but I do like to learn.

Post some of your experiments (video captures) here, and many experienced users will help you for sure!


edit: here an emulation created by jagabo of what may happen: output.mp4 and the specific thread: https://forum.videohelp.com/threads/402267-Adjusting-capture-card-levels-not-sure-if-it-s-right

[theseeker2]

Thanks for all that info, I really appreciate it.

I'll try to post some video captures once I resolve an issue with muffled audio in my captures (only muffled with linear audio and not Hi-Fi).

[Skiller]

once I resolve an issue with muffled audio in my captures (only muffled with linear audio and not Hi-Fi).

This could be the problem.

[theseeker2]

Thanks Skiller, that was exactly it! I cleaned the linear audio head a few times and now everything sounds good again.