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Legal chroma levels for going out to broadcast
Hi,
my first post in this forum. I understand that when going out to broadcast video you should activate the button that says “normally scalled legal video”. It is supposed to legalize the video levels. And it works for the luma levels. But it seems not to work for chroma. I get illegal levels which exit their respective points in the vectorscope. Is there an automatic way to legalize these levels or do you have to do these manually?
Thanks for any response.
Greetings from Germany
Ingo -
13 Replies
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“Legal” broadcast chrominance is generally calculated in a Baseband “composite” mode, which in today’s Y’CbCr slash RGB world means it is being simulated. Composite in this sense means luminance+chrominance with the presence of subcarrier. At least in NTSC, we are allowed Y+C= between -20 and +120 IRE. In fact as you can see, there are legal Baseband levels that cannot be generated in RGB, and do not even translate, since there are no negative values. (Sort of heads in the direction of explaining 64-940) If you are looking at the 2% color bar targets as an outside envelope, that is incorrect as a guide, as those only correspond to a 75% standard saturation. Color bar yellow is a good example of “over” 100 IRE, and color bar blue and red both have negative IRE excursions in BASEBAND composite.
The only way to truly estimate legal boundaries is with an external scope with either an “IRIS” or “DIAMOND”-type display. Everything else is “best guess”.
jPo
You mean “Old Ben”? Ben Kenobi?
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Thank you for your insightful answer, which is very helpful. Just to clarify things for me: so it is possible to get an illegal signal out of DaVinci even though I am using the “Normally scaled legal video”? And the only way to avoid that is to use an external scope or better even an external legalizer? Because after the roundtrip to FCP7 I get an illegal chroma level warning which I can then fix by applying the FCP-legalize filter. Or is FCP giving me an incorrect reading?
Thanks again for an answer.
Ingo -
[Ingo Schulten] ” “Normally scaled legal video”?”
My bet is that that option simply defines an operating regime, and likely isn’t a “legalizer” per se.
FCP’s “legalizer” is a bit of a bad joke, since it only looks at overshoots, and completely ignores what is going on at “black levels” — and the same goes for the first and last lines of active video…. untouched.
Also, if there are “ringing” and high frequency modulation cross talk in the Y+C encoder, that is something that all the internal software scopes don’t even see. There are also going to be RGB/Y’CbCr transcoding issues (math problems) that will alter the actual values migrating between applications.
jPo
You mean “Old Ben”? Ben Kenobi?
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“Normally scaled legal video” means SMPTE scaling (16-235) as opposed to full range (0-255). It has nothing to do with broadcast legal limits. It is intended to send “proper” SDI output to a monitor or VTR.
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Ok, but is there a way to legalize the signal in DaVinci other then doing it by hand? Like in color where you can activate a broadcast-legal filter. There is no button in DaVinci as far as I know. Or is there a way to do that with a lut?
And if I am not mistaken a clip rendered with the “Normally scaled video” activated DaVinci at least does clip the luma level as far as I could see after the render in FCP.
Thanks for any clarfication
Ingo -
You cant render with normally scaled video, because its the video output, not the render output.
Easiest thing is to build yourself a LUT that does some scaling from the bottom and the top to the values given in the last post.
Thats what we do if its a file based delivery.
Technically its not really correct, and if you deliver that to a station which defaults on scaling you might get double scaling.
I suggest to ask your client to provide you with an example of what they expect, a color bar treated by their own method and then examine that and recreate its nature.A slice of color…
DaVinci 8.0.1 OSX 10.7
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Hi,
I did some additional tests. And for some reason however I render a clip out of DaVinci it always clips the luminance at 100% (no LUTs applied). I imported a clip with super-white did no grading to it at all. After rendering and importing back into FCP all values above 100 where cut. And there was no difference between either “Normally scaled legal video” or “Unscaled full range data”. How about that?
Greetings
Ingo -
Jumping in late to this thread,
as I understand it, “broadcast legal” levels are:
Y = digital 16 to 235
U and V = digital 16 to 240
Do I have this correct? Is there more to it than that?
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[Chris Clementson] “Do I have this correct? Is there more to it than that?”
There is more than that to it, because it’s possible to have very slight out-of-gamut issues that are technically correct according to conventional scopes.One tool that can help is the Gamut Mapping plug-in, which will automatically force any incoming signal to whatever colorspace you set it to, like Rec709 (for Luma and Chroma). I generally will grab this whenever I have a hot chroma signal that needs a little helping hand. High Soft clips can help as well. BorisFX has a new “Broadcast Safe” OFX plug-in that looks promising.
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“it’s possible to have very slight out-of-gamut issues that are technically correct according to conventional scopes.”
Can you give an example of this using 8-bit values?
Interesting that they have two flavors of NTSC (with and without setup) and two flavors of EBU, but no ATSC and no filter specifically for BT.709.
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