How to use the histogram in VirtualDub capture?

[ronnylov]

I have tried capturing with VirtualDub 1.6.4 which now supports WDM capture drivers and it works with my ATI AIW Radeon 32MB PAL card. But I noticed that the histogram show signals in the red areas to the left and the right. According to virtualdub homepage this seems to happen with my capture card and that this may degrade video quality:

Link: http://www.virtualdub.org/blog/pivot/entry.php?id=35

One of my pet peeves about consumer-level video capture cards is that a lot of them seem to have crappy or badly calibrated ADC or AGC circuitry. I presume they tweak it to get bright whites, but the result is usually bright areas of the screen blooming to solid white or really ugly yellows. Some of these areas can be rescued if you capture in YCbCr and enable the "squish luma range" filter option, since YCbCr formats can capture superblacks and superwhites; these are levels that would be clamped in conversion to RGB and are shown as red in the video histogram. However, it usually isn't enough and the result is noticeable degredation in video quality. My WinTV and All-in-Wonder have this problem; my Phillips SAA7135 board, Pinnacle DC10/DC30+, and Plextor PX-M402U don't. If you have a Bt848/878 you can try the Bt8x8 Tweaker with the DScaler (formerly dTV) drivers and tweak the capture chip's white levels further to try to get the levels in range, which sometimes works. I didn't check whether this option still works in 1.6.3, but if it doesn't, drop me a note.

In version 1.6.4 it is possible to squish left and right side luma range separately. But when I squish luma range then it is a little bit too squished and it does not look good. So I tried the capture filter settings (the same settings that are available in ATI MMC settings video tab) without the virtualdub squish filter to adjust brightness and contrast and finally if was possible to squeeze the levels to fit inside the histogram without the red edges.

Is this adjustning really needed when converting to PAL DVD? I have read somewhere that PAL use a different black level than NTSC but I do not know for sure how it works. And should I adjust the brightness and contrast settings also when capturing directly to DVD format with ATI MMC 9.06? And is there a difference in the end result when I adjust brightness and contrast compared to the squishing method in VirtualDub, if I adjust them to look the same in the histogram?

I am thinking of replacing my capture card to a one with Phillips SA7134 because I have heard this circuit is better on PAL captures... I am thinking of buying the Terratec Cinergy 400 TV. Is this a good idea? I will use it in another computer and keep the All-in-Winder Radeon as reference, it may be better in some aspects.

Would be nice if someone know a good guide on using histogram in video capturing. I noticed that I got different results on different TV-channels and on different video tapes (probably recorded from different channels) so it seems each video source should be adjusted to its own settings before any capturing...

[Kunta Kinte]

This guide is dated, but has good info on how to tweak the histogram.

Click here.

[ronnylov]

That was the guide I used when capturing with earlier versions of virtualdub which did not have the red nd blue areas in virtualdub histogram.

I did a capture of a VHS tape yesterday evening by adjusting the histogram in virtualdub and noting the settings of brightness and contrast in the video filter when I got a setting that used the whole blue part of the histogram. Then I edited the contrast and brightness settings of ATI MMC with regedit (because MMC stores the settings in the register and the actual numbers were not shown in ATI MMC settings).

I captured to MPEG2 and the results looked good. The histogram is a great tool!

By the way I have ordered a Terratec Cinergy 400 just because I am curios to see if there are any quality differencies. This capture card was not too expensive but the quality should be fine.

[ronnylov]

I don't know if anybody cares, but I have now got my new Capture card the Terratec Cinergy 400 with Philips SA7134 chip and I think the quality is better than the ATI card. But it does the same with the levels at capturing so it still must be adjusted with the histogram. The Cinergy 400 card did not work in any version of virtualdub so I used Virtual VCR instead which worked fine. There is a histogram function in VirtualVCR too but it shows levels from 0 - 255 and there is no marks at black level 16 and white level 235.

I recorded to huffyuv and used the histogram function in avisynth and did the fine tuning this way. A good thing with virtual vcr is that I can save the settings to a settings file and the video capture filter settings is stored together with other settings so I can make adjustments for a given source and store them for later use.

To set the black level I adjust the brightness setting. Then I check the white level and if it's too low I must increase the contrast setting and adjust the black level again with brightness because the increasing the conrast setting made the black level lower and the white level higher at the same time.

To really fine tune it I compared a simultaneaous recording with my dvb card on the same TV-channel (I have some channels both in analog and digital form). But when I switch to another analogue channels I must fine tune the settings for that channel too because they are not completely the same. But the comparison gave me some methods to do it.

A completely black picture is best for setting the black level because it will give a spike in the histogram which should be set to the level 16 (assumed the original picture is supposed to be completely black). And a completely white picture is best to set the white level. Then check the histogram during playback of the video and see that most of the histogram curve fits within the 16-235 scale and look at the picture to see if it looks fine.

[adam]

To answer your earlier question, if the guide didn't already, it makes no difference whether you use PAL or NTSC as far as your luminance levels are concerned. PAL equipment does support a wider luminance scale than NTSC equipment does, at least by default, but this is an analogue measurement. Either way the digital source (DVD, SVCD, VCD, etc...) is still stored with its ranges constrained to 16-235.

[ronnylov]

Yes that is correct adam, I have found that also by my own experimenting. So that is why the histogram should be adjusted for black at 16 and white at 235 when capturing in YUY2. This is a part I have missed in the past when capturing. I have left the brightness and contrast settings at default and just captured...