Well they do have meaning when the file is played out through a broadcast distribution. 601/709 keeps values between 16-235 in the RGB channels. Head and bottom room allows for a safety margin without clipping. On a waveform of an HD or SD SDI signal the RGB prime values are 0 to 700 millivolts in spec. If you file goes over or under you will be delivering a signal blacker than black or whiter than white. This will lose detail either by an elegant form of legalizing or just clipping the signal a 0 and 700 millivolts. Now this probably doesn't cause the traditional problems on an HD tv but according to a Tek engineer at a seminar an HD delivery to a network will be view 80% of the time in NTSC either through a cable or satellite system that downconverts the HD downlink with an IRD, Integrated Receiver Decoder, that does the down convert with no user adjustable parameters. People watching OTA with a set top downconverter could experience the old chyron buzz with signals that are too hot. Also as has been mentioned on this list ABC is rejecting HD shows that have spikes in the horizontal interval and transient specular spikes over 100% because it is causing white lines on the top and bottom of the letter box Granny is watching from her set top box more than likely feeding the NTSC set over Ch. 3. My question is more of the nature of the file workflow and where might signal elements that are out of range get clipped. I have a show that shows severe RGB errors on my Tek scope with default alarm settings. This timeline was exported to a Prores file as far as I know. With all the level errors I see how did this file pass QC? How was it QC'd. The way I do it is play the file back through Avid and send the output to my scope. In these tests an ama link seems the most transparent but what happens when these overshoots hit a product like Tektronix Cerify. I believe they can correct the file but I bet it's a brute force clip. Just want to know what people do to insure their files are correct. I'm starting to doubt the veracity of those doing "file based QC."
--- In Avid-L2@yahoogroups.com, Nat Jencks <natjencks.lists@...> wrote:
>
> What do these standards mean in a file based / digital world?
>
> Each pixel has an R, G, and a B value which range from 0-1023 or from 65-940 (or their 8-bit equivalents), and thats about all there is to it right?
>
> As long as the the content is is making to to the air without having a mismatch of SMPTE vs full range signals, I dont know what possible values could cause "problems" in a digital environment. Why limit chroma or luma, or anything for that matter? Were not talking about overloading an analog circuit with a high voltage potentially damaging broadcast equipment or anything like that.
>
> Someone wiser than me please explain to me what the point of perpetuating all this is?
>
> -N
>
> On Aug 1, 2012, at 3:49 PM, John Moore wrote:
>
> > I've done a lot of looking at Cerify for QC of files. So far my best approach has been to export a QT same as source from Avid and then either import that file back into older versions of symphony or on newer version link to ama with the file and play it back through my Tek wfm 7020 running the error log set to Tek default settings, which according to Tek support engineers will mimic the traditional Tek error alarms through all their legacy products. I have a show done by someone else who used safe color limit with 422 checked but did not set the RGB limits to 16 and 235. As a result the outputs to tape going through an outboard legalizer are okay but show occasional RGB errors, our legalizer isn't as good as the Harris/Video Tek which would catch these errors. We also deliver a prores file of the show for DVD authoring. Well bypassing the legalizer to get an idea of what the safe color limit will do for the file export lights up like a Xmas
> > Tree as I would expect with no RGB limiting in the safe color effect. I'm told one of these shows already passed QC so that tells me they aren't too precise on their end. What will the RGB errors really do on a DVD encode? It seems nobody cares about Rec 709 spec or worries about these RGB errors. Sometimes I think I shouldn't care either but that's not right. Can anybody share any issues with bad RGB levels in the file based world?
> >
> > John Moore
> >
> > Barking Trout Productions
> >
> > Studio City, CA
> >
> > bigfish@...
> >
> > [Non-text portions of this message have been removed]
> >
> >
>
> ________________
> natjencks@...
> www.inrs.net
>
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> [Non-text portions of this message have been removed]
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