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Masters "Rejected" for choma out of legal

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chris
Masters "Rejected" for choma out of legal
on Sep 16, 2007 at 4:29:14 pm

Hello folks,

I am running into a repeated problem. My SD Digibeta Masters are getting rejected in QC for having "Chroma out of spec". The SD Digi's are down-converts from a HDSR Master. We use a "Proc" to make it all Broadcast legal, but even still, stuff like red car lights at night can get through.

So my first question is; Does anyone know of any equipment to clamp the "rouge" highlights that sneak by the Proc. My quick fix has been to do tape to tape inserts and crank down the whole shot with the offending chroma, it's not really ideal.

My second question is; when I'm looking at the trace on the vectorscope it all falls well within the graticule... BUT, I also see a large faint ghosty glow around the trace. (the bloom?)

Some of this very faint glow is out of spec. Does this faint glow have to be entirely within the raticule to be in SMPTE spec?


Thanks,
Chris





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Glenn Chan
Re: Masters "Rejected" for choma out of legal
on Sep 16, 2007 at 8:25:28 pm

AFAIK... you should not be looking at the vectorscope to determine legal levels. Look at the waveform monitor with the lpass or low pass filter off. That will let you look at the composite signal (the luma and chroma modulated together).

There is a limit to how high the composite signal can get. Usually it is 115 IRE, but it can vary from broadcaster to broadcaster and some put the limit lower.

2- Some people just run their video through a video legalizer. If you have broadcasters that sets the limits differently then you can put different settings in the video legalizer.

The one thing you really have to watch out for is user error- make sure you patch the video through the legalizer (including when you do insert edits).

2b- Some NLEs also have legalizers built in. Though they may have bugs... e.g. Final Cut had a bug where the broadcast safe filter wouldn't render when you actually render the video. So it looks like you have legal video but you actually don't.
Software scopes may also be inaccurate (e.g. FCP doesn't seem to consistently show superblacks).

I don't have much experience with this (2b) approach.


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Mark Suszko
Re: Masters "Rejected" for choma out of legal
on Sep 26, 2007 at 9:35:43 pm

They must have fixed the FCP bug because the legal limit filter works fine in ours. However, that's not always the best way to "solve" a problem that likely occurred during the ingest stage of the project or even the shooting stage.

Also if you use the legalizer filter on the whole program timeline, it does a couple of things. One is it makes you lazy over time, lazy and sloppy, trusting the filter will "fix" everything, until the day it doesn't.

Secondly, the filter applied globally to the entire timeline is going to have some small but real effect on the video that was ok to start with. And you can create worse problems for yourself matching footage when compositing or laying-back into an existing master- NOW which footage was clamped and which was not?.


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Simon Kotowicz
Re: Masters "Rejected" for choma out of legal
on Oct 20, 2007 at 1:32:27 am

The best thing to do is grade each shot one at a time and watch the scope as you do it. Legalisers will just cut the signal off at the upper and lower limits. If you've got detail above and below those limits it will be lost. You should bring the video signal into those limits, thus saving all the detail. Set up your scope to display an alarm message on shots that are illegal. Correct those shots until the alarm disappears.

I read a really interesting article in IBE (International Broadcast Engineer) magazine. If a legaliser works in RGB it will first convert your YUV video signal into RGB, clamp it and then convert it back to YUV so it can be saved to tape. All colours within RGB gamut are legal. When it gets converted back to YUV 4:2:2 the colour signal is compressed (i.e 4:2:2 compression.) An artifact of this compression means colour values can diviate and end up being back within the realms of illegal, but only just. This might account for a few occassions of programme rejection.

Best thing to do is be on the safe side and correct the colours to a safe amount withing the legal limits. The difference to the naked human eye should be neglegable.

Simon Kotowicz
http://www.onlineeditor.co.uk


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