RGB Gamut Errors Part 1: A Broadcast Colorist’s Nemesis

May 17, 2015

In this Insight, learn what RGB Gamut errors are and how to identify them before a broadcaster catches them for you!


Series

An Annoying Spec that Broadcast Colorists Must Adhere To

Imagine this:

You’ve color graded an hour of programing for a big name network or production company.

You’ve worked your butt off making fabulous grades and of course making sure that the video signal – since this is a broadcast show – was perfectly legal.

As you graded you made sure the signal wasn’t too bright or dark, and that saturation was in check.

Maybe you even used a ‘broadcast safe’ filter or effect somewhere in your pipeline to just catch any stray illegal pixels.

You did fantastic work, and the client is exceedingly happy!

You render the show, ship it off for delivery and go have a pint rewarding yourself for a job well done.

The following week your client calls (you’re thinking victory lap!) and tells you that there have been some things flagged in QC.

Not one to worry, you ask them to send the QC report over as you’re confident most of the issues are probably audio problems or something else that’s not really your concern.

As you scan down the report there they are – they almost jump off the page – RGB GAMUT ERROR (HIGH, RED) 01:13:23:18, there is another one RGB GAMUT ERROR (LOW,BLUE) 01:14:36:18!  Dang!!!

Has this ever happened to you?

We’ve talked quite a bit in MailBag episodes and at conferences Team Mixing Light has spoken at about RGB gamut errors.

These pesky QC flags can be infuriating, and ultimately feared, as they can be hard to clear up if you’re not sure exactly what they are and how to go about fixing them.

The first thing to understand about RGB Gamut Errors?

Every broadcast colorist has been there! You’re not a bad colorist!

Trust me, depending on the network and QC engineer, RGB gamut errors might not even be anything the eye can see – but only an automated alarm on a set of scopes can!

In this Insight, which is Part 1 of a two part series on RGB gamut errors,  I want to take a closer look at what these errors are, how you can monitor for them and some broad strategies on how you can fix them.

Part 1 is an article, but in Part 2 with the background knowledge in place, I’ll show you in a video how to use the techniques I outline below for dealing with pesky RGB gamut errors in your own projects.

Let’s jump in.

So What Is An RGB Gamut Error or Excursion?

I’m sure you understand basic video signal concepts like luma levels and saturation, but what exactly is an RGB gamut error?

First, these errors sometimes go by a different name depending who you are talking to and how geeky they are.

Another phrase you’ll hear all the time is RGB Gamut Excursion – this phrase is referring to essentially the same phenomenon, but some folks prefer it as excursion implies the state of signal as it ‘passes’ legal limits.

Use whichever you prefer – I generally use RGB gamut error.

Now that we’ve got the semantics out of the way, lets get back what an RGB gamut error is.

Many of us grade with formats that use Y’CbCr encoding but ultimately the video monitors and TVs that audiences use display that signal as R’G’B’.

In that conversion from Y’CbCr to R’G’B’ it’s possible to create pixels that are legal in Y’CbCr but illegal within the specified R’G’B’ signal limits of a broadcaster.

In other words, an RGB gamut error (you can also have luma gamut errors) is a math problem in the color space conversions that have to happen from Y’CbCR to R’G’B’ and sometimes back again.

Will an RGB gamut error blow up a transmitter or a viewers TV – ummm, no!

Chances are you can’t tell the difference visually between a shot with a RGB gamut error and one without.

canyoutell
Can you tell visually if this shot has any RGB gamut errors or excursions? Probably not!

 

Originally, like many QC tests, the idea was to protect transmission equipment from signal voltage anomalies and signal degradation that could occur.

These days RGB gamut errors are still flagged for a variety or reasons including composite transmission, possible signal degradation and yes because networks like to be hard asses about deliverables!

Networks that measure for RGB Gamut problems will typically specify the signal levels that they’re looking for with 0mV-700mV being the most common specification on an RGB waveform.

They might also specify signal be legal as seen on a dedicated gamut display (more on that in a moment).

Values above 700mV are referred to as high or upper errors and values below 0mV are referred to as under or lower errors.

But there is one more important thing I want to mention.  Gamut errors can be transient or persistent.

Many broadcasters (and QC equipment) don’t care or filter out the transient errors.

It’s the persistent ones that cause the majority of the QC flags you’ll encounter.

Check with the broadcaster you’ll be working with for what constitutes a persistent RGB gamut error.

If you’d like to dive deeper on the definitional side of RGB gamut errors check out these links from Tektronix and EyeHeight (makers of a legalizers):

Color Gamut Monitoring (Tek)

Preventing Illegal Colors (Tek)

What’s A Gamut Warning (EyeHeight)

Now that we have a basic understanding of RGB gamut errors, let’s talk about how to monitor for them and then fix them!

Member Content

Sorry... the rest of this content is for members only. You'll need to login or sign up to continue (we hope you do!).

Membership options
Member Login

Comments

7 thoughts on “RGB Gamut Errors Part 1: A Broadcast Colorist’s Nemesis”

  1. ”One thing I’m big fan of is setting a bit of soft clipping in Resolve at the timeline level. This my ‘first line of defense’ when it comes to RGB gamut excursions.

    Just be aware that working at the timeline level affects EVERYTHING below or on the clip level.

    This can be an issue if you have graphics, or text that have to have 100% black or white, or have to maintain an exact saturation level.”

    A good tip to get around this is too group all shots on the timeline minus graphics etc and apply your global adjustment as a post-clip grade

  2. Very informative insight ! I just have a question about the new lumetri Scopes found in premiere 2015, I’ve tried playing with it and found that with the “Waveform YC” you can see what goes beyond 0-100, you still though can’t see it in Luma neither in RGB waveform. However, another interesting feature is that when you do some adjustments in premiere you can have the numbers stretch and shrink while the actual signal remains still, it’s quite interesting as it let you see how the footage looks like when something goes beyond the limits, however, if the footage itself was clipped already, the only way to view it is through “waveform YC” .

    I wonder how this is useful for a resolve colorist who uses both applications ?

    Moreover, It would be interesting to have an insight explaining why resolve always uses video levels when exporting to DNxHD, even though full data range is selected; It looks a bit brighter or with lower contrast when it’s imported into premiere. I found that exporting to uncompressed solves the issue, or just adding back some contrast in premiere as a workaround.

    Thanks in advance !
    Ebrahim

  3. I know this is an old discussion, but I found it when googling RGB gamut. It may be worth noting that the EBU have recently updated their recommendations about QC of video. https://tech.ebu.ch/docs/r/r103.pdf gives the updated guidelines. It acknowledges that small brief excursions often occur, and are not actually a problem. I have passed this document on to the makers of ScopeBox, so that they can look into adding the recommended filtering to their software.

    It’s also interesting that the document says ‘Experience has shown that colour gamut “legalisers” should be used with caution as they may create artefacts in the picture that are more disturbing than the gamut errors they are attempting to correct.’

    1. This is terrific news. Now we need the QC device manufacturers to update their boxes to also do this filtering and stop rejecting programming with these meaningless excursions. The truth is, as long as those boxes keep analyzing for these types of excursions and as long as those boxes keep rejecting programming – we have no choice but to solve the problem.

Leave a Reply

Hundreds of Free Tutorials

Get full access to our entire library of 900+ color tutorials for an entire week!


Start Your Free Trial
Loading...