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Legalizing Chroma for Broadcast

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Christian Stoehr
Legalizing Chroma for Broadcast
on Oct 23, 2012 at 2:55:59 pm

I figured out how to setup a LUT to keep the White and Black levels within legal specs automatically. Is there such a way to setup a limiter for chroma too, so I have a backup. And also, what are the standart Chroma limits for Broadcast, and whats the best way to check them, I`ve heard different versions via the Vectorscope.

Thanks.


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Joseph Owens
Re: Legalizing Chroma for Broadcast
on Oct 23, 2012 at 4:00:14 pm

Legal broadcast for luma/chroma is a bit of a can of worms when you are working in an RGB environment.

Tolerance is 2 IRE around 0/100 for luminance, ie -2 to 102 is the gamut limit for an iron-clad deliverable, and 20 IRE is the combined over and under for luma plus chroma ( that is -20 to +120) when you are measuring it in composite baseband. Which you probably can't do if you are depending on the Resolve scopes, since they don't even display luminance, let alone simulated composite.

Basically all you can do with RGB scopes is a wild guess, so building precision LUTs is a waste of time if not a deception. The final matrixed composite value cannot be estimated looking at an RGB parade. You can, sort of, set up an approximate envelop using the CbCr vectroscope representation... by not letting the chroma go outside the 75% saturation dots -- but -- this approach doesn't even qualify as crude, and a colorist will never be able to push a look with this kind of blinkers applied. Where the parade evaluation method really falls down is in the blacks. Saturated reds and blues near 0 IRE (police car markers are a good example) routinely are outside broadcast legal and are otherwise difficult to detect, not like bright yellows, particularly the values that graphics artists almost invariably choose. (100% R plus 100% G with 0% B would be the one that would guarantee trouble, even though it looks legal on the parade.)

The only way to accurately evaluate broadcast legal is with a display/monitor/scope system that actually does the math. Gamma-adjusted Y'CbCr is not a linear transform. You only have to mistakenly confuse the full-scale / linear video settings once to find that out.

Tektronix Diamond and Videotek Iris displays are good examples, and they can even be programmed to flag the timecodes and overages. In the absence of a dedicated hardware legalizer, that is making adjustments based on composite baseband, an operator's only option is to fix the OOGs (out-of-gamut) on the spot with grade qualifiers. And to massage those values until the scope stops squawking.

jPo

"I always pass on free advice -- its never of any use to me" Oscar Wilde.


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Dean Manion
Re: Legalizing Chroma for Broadcast
on Oct 23, 2012 at 4:17:51 pm

jPo is correct and thorough. You need a hardware limiter with correct settings based on your delivery specs, and maybe add a little soft clip to help out.

Correcting gamut errors in Resolve will limit your creativity and eat up your time. Spend the $$ on a Harris and sleep at night.

d


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Robbie Carman
Re: Legalizing Chroma for Broadcast
on Oct 23, 2012 at 7:56:47 pm

I of course agree with JPo and Dean but I should be noted that even the DL860 is not a perfect machine. Recently on super saturated, dark and grainy footage (recreation scenes) even the 860 is not fixing the out of gamut stuff (0-700mv on RGB Gamut). This stuff of course was 9 billion percent illegal so I didn't expect it to really fix it - you still from time to time give shots some manual love.

On shows like that I'm a big fan placing a track node and applying a little clipping at the top and bottom end of things for safety sake. I agree that legalizing manually RGB gamut issues can be a little tedious but some soft clipping goes a long way!

And as JPo said the only way you can really know for sure is by having a dedicated Gamut scope like the diamond or eye.

Robbie Carman
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Christian Stoehr
Re: Legalizing Chroma for Broadcast
on Oct 23, 2012 at 9:01:09 pm

Thanks guys, so without running out and buying a Hardware scope right now, are there any options software only, or do I have to play the (75% Vectorscope) crude game I guess.

I am confused that a software like DaVinci can`t integrate a limiter node or full fledged scope ?
I am waiting for the delivery of a Flanders Scientific monitor that has build in scopes, not sure if that will help ?

Can you describe the soft clipping process for chroma, how do you set that up. I was able to do a LUT for the luminance that works great, since I can now crush the blacks as much as I want without having to worry.


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Joseph Owens
Re: Legalizing Chroma for Broadcast
on Oct 24, 2012 at 3:01:34 pm

[Christian Stoehr] "I am confused that a software like DaVinci can`t integrate a limiter node or full fledged scope ?
I am waiting for the delivery of a Flanders Scientific monitor that has build in scopes, not sure if that will help ? "


On item A, adding a fuller-figured scope to Resolve would eat a significant number of CPU cycles -- even BMD's Ultrascope needs to run on a separate machine not to get in each others' way. Observing Resolve's behaviour in Legal Video mode though, it is very difficult to generate values outside normal limits, although it can be done with extraordinary means. *Not saying that Resolve cannot generate out of gamut values.*

It would also add about $8-$10K to the price tag, considering the MSRP of a base-model TVM-900, for example.

The Flanders scopes are about the same, nobody really does YRGB parade plus gamut except the hardware scope manufacturers, with the exception of the on-screen area warnings that you can turn on in many models. However, having lurid, flashing pixels in your supposedly nicely behaving grade monitor can be at minimum annoying if not ultimately distracting.

I'm very happy having all the flashing lights and sirens going off somewhere other than my judgement view.

jPo

"I always pass on free advice -- its never of any use to me" Oscar Wilde.


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Frédéric Völker
Re: Legalizing Chroma for Broadcast
on Jul 5, 2013 at 5:25:12 pm

Hello all,

Please, considering that buying stuff is not an option, what is my best bet at observing rec. 709 using only premiere pro controls : video limiter and broadcast colors effects ?

Should I use both or only one at the same time and how should I set their values ?

I work in PAL values.

Thanks a lot. Any help greatly appreciated !

Fred


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Frédéric Völker
Re: Legalizing Chroma for Broadcast
on Jul 7, 2013 at 11:10:08 am

"Tolerance is 2 IRE around 0/100 for luminance, ie -2 to 102 is the gamut limit for an iron-clad deliverable, and 20 IRE is the combined over and under for luma plus chroma ( that is -20 to +120) when you are measuring it in composite baseband."

This would make me think that the premiere pro video limiter using the "smart limit" set at values -20 to +120 would do the trick.

I have all sorts of material (dslr, ex3, sometimes even mobile phones) mixed into the same film.

I am perfectly aware that this needs to be done with dedicated equipment to be 100% sure that the video is broadcast legal. However, I have by no means enough money to invest, and neither do I want, if possible, to stop making movies just for that reason.

I am making color corrections with the colorista 2 plugin in premiere. I deliver in Avi uncompressed, on hard drive.

Let's say, to use another example, I have to follow these specs :

Color space RVB
Electric level 700mV
Highs +1% 710mV
Lows -1% -10mV
"Tolérance spatiale" ("space tolerance" ?) 2% (as long as 2% of the pixels of the active image is not beyond this threshold)

Color space YUV
Electric level 700mV
Highs +3% 710mV
Lows -1% -10mV
"Tolérance spatiale" 1% (as long as 1% of the pixels of the active image is not beyond this threshold)

Thanks in advance again for any kind of help.


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David Hover
Re: Legalizing Chroma for Broadcast
on May 9, 2014 at 8:31:23 pm

My ten bits on this thread : with Resolve 10 you can choose to change the color mode of a node from RGB to HSL, and that changes the way the channel controls work. So in HSL, soft clip for green becomes soft clip for saturation. Setting the high soft to 100 effectively brings saturation back to what looks legal on the vectorscope.
This really has to be done in Track, not Clip. Any change in Soft clip clips the signal and voids the 32bit float, so whatever is outside the scopes (luma or chroma) at that point in the node tree can't be recovered downstream, so I only use it in the last node of a clip in any case.
I can't guarantee this is broadcast legal, it just behaves the way a saturation limiter should. I also haven't quite understood if switching to HSL in Resolve limits the RGB color space in any way, but I haven't seen anything noticeable yet.


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