Question about NTSC legal image

riredale wrote on 9/29/2002, 10:38 PM

Okay, here's another question for all you gurus out there...

I understand that VV3 has a filter that takes off the "top" and "bottom" luminance values in order to conform to NTSC standards. My question has to do with DVD production.

I note that in the CinemaCraft MPEG2 encoder there is an option to encode either from luminance values 16 to 235 or from 0 to 255. The manual says this is to make the signal conform to ITU-R BT 601.5. Am I correct in assuming that this does effectively the same thing? If so, then shouldn't I assume that I should NOT use the VV3 NTSC filter first?

Comments

mikkie wrote on 9/30/2002, 9:06 AM

Hard question -- hope I can help just a little...

"NTSC legal" has more to it, & strictly speacking (as far as I know etc.) involves a lot of stuff above and beyond the luminance values you're talking about. It can get pretty *interesting*... You might not even want to mess with it if you're not currently having problems.

If your source video stream came unaltered from a camera (DV or whatever), then it might be set to use 16 as absolute black, & 235 for absolute white. Or, it might be set to record super white (& black), allowing for more dynamic color range with the highlights and shadows -- in other words, it might use more of the full range, 0 - 255. Without going into all of the settings available on which DV cameras etc., where it can cause you problems is if you just go ahead and set your software to hard limit everything to the smaller luminance range. If you've got detail in that 235 - 255 range, this will wash it out, and of course you might lose some shadow detail as well, so I'd approach the setting in the mpeg2 encoder catiously.

In VV3 you can use the Clamp FX, which gives you some control & hopefully a smoother transition as the the entire luminance range of the image might have to be compressed to fit within the smaller range without visually noticable problems.

All that said, if your source video stream already conforms to the smaller luminance range, your PC will still show it in it's default 0 - 255 range that it uses for everything displayed on your monitor -- there just won't be anything in the 235 - 255 range if you look at a waveform or histogram. And in fact, that's where I'd start, checking to see if you even have a problem to worry about.

And finally, just because a video is not legal doesn't mean it won't play or look good on your TV. IF it aint broke... For more info search for stuff like I think it's 601 (what the luminance spec is referred to I think based on hardware), shelf values and so on (this stuff actually or originally refers to reference voltages, signal on a scope etc.). http://www.adamwilt.com/DV.html will tell you more then I ever could, and if you can find them at dv.com, there have been a few articles on this in issues of their mag going back a year or three.