black levels

zipwax wrote on 11/10/2003, 3:43 AM
I'm finally trying to do a "professional" encode of some DV footage. I've read all the vegas online documentation, I don't know if there's something better that explains the difference (how Vegas deals with) between "broadcast" black levels and "pc" black levels. I already know about how/why YUV luma shouldn't go outside of 16-235 levels (I'm not a total novice), and how "black" on a TV is 16, and "white" is 235. But I still have questions...

Question 1: Microsoft's DV codec, I think, decodes DV to YUV, but it clamps the YUV between 16 and 235. I think this makes it so that when the YUV is converted to RGB, the "headroom" and "toeroom" are lost. No more "superblack". Can anyone anyone confirm?

Question 2: I think one of the differences between MS's dv codec and the one Vegas uses is that Vegas doesn't throw out the non-clamped YUV values. True?

Question 3: Does the vegas DV decoder expand (decoded) YUV 16-235 to RGB 16-235, or to RGB 0-255?

Here's what I notice as a "bug", or maybe this is how it's supposed to work:

1. use 3rd party codecs is OFF. Use MS DV codec is OFF.
2. put gray color bars onto timeline.
3. open up histogram
4. notice that black is luma level 16, white is luma level 235.
5. render timeline to DV file.
6. open up any other video editing utility, have it decode to YUV flavors (not to RGB) and you see as expected, YUV varies from 16 to 235.
7. reimport DV file into vegas.
8. look in histogram
9. notice former "black" is now at YUV level 32, not 16. former "white" is now grayer.
10. What happened!?

God, I hope somebody can help me soon, I'm dying to know the answer!


farss wrote on 11/10/2003, 4:09 AM
I've done a bit of work with this and even found a few oddities in VV but as I don't have any other app to try putting the video through don't know if I can help much.

I do know that going backwards and forwards between RGB and YUV can be a bit of a nightmare. Certainly unless told otherwise VV will output video from 0 to 255. Even applying BC fx onto video buss will not guarantee that you don't end up with illegal levels.

Step 9 in your post I don't understand, as far as I know VV only works in RGB not YUV, maybe thats the problem, you've converted to a YUV space and bought that back into VV and it assumes it's RGB, so effectively you've gone RGB to YUV to (RGB) to YUV if you get what I mean. Also if you're in NTSC land is the 7.5 setup getting into the act as well.
AlistairLock wrote on 11/10/2003, 4:12 AM
I'm completely making this up, I'm not posting from a position of knowledge but:

Could there be further clamping going on, so the black level of 16 becomes 32, and the white level of 235 becomes 219?

The nearest analogy I thought of was audio compression, where a recording that has already been compressed gets mixed down and put on another track in the same project, and so gets compressed again.

So the question becomes, is this what is happening, and if so, why?

(the post above appeared in the time it took to type this one)
TheHappyFriar wrote on 11/10/2003, 7:41 AM
I read an article in DV magazine last night that may shed some light on the whole broadcast legal thing.

Aparently, most (not all) DV camera's don't clamp the black/white. So, when you're recording DV, you're getting MORE colors then what NTSC allows (and that's aparently NTSC in the US. Japan made 0-255 legal).

I'd say most NLE's/codec's that do you a "favor" by clamping it for you make it broadcast legal, but then on a computer it wouldn't look right. So, the SoFo guys, back when they made their codec, lets us use the full range of color. Then we can do work for the PC, TV, big screen, etc and we have the choice of output.

Also, check to se if your camera has a "clamp" feature. that would save you a headache in the future (if you plan to broadcast in NTSC in the US/Canada).
Bill Ravens wrote on 11/10/2003, 8:40 AM
you guys should read the article over at adam wilt's website. he makes the point that the DV standard doesn't define a "black level". Black level is something defined as soon as you import the bitstream into the NLE.
TheHappyFriar wrote on 12/1/2003, 7:52 PM
Hey, I installed Vegas at my work computer so I don't have to use premiere anymore. :) Well, I am working on a promo for a weekend group of shows. I ended up importing some video from a Paramount TV show (which shall remain nameless). Wouldn't yu know it, the whites are hot and the blacks are low! Both well out of the "safe" level zone.

so, zipwax, if the guys at Paramount can ignore the levels, we can to i guess. :)