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Legal Video Signals


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#1 Daniel Smith

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Posted 14 February 2010 - 07:57 PM

Hi,

I've been experimenting with legalising video signals, and hit a few stumps on the way that I'm hoping someone here can help me clear up.

I imported a 100% colour bars image into 'Premiere Pro' and looked at the YC waveform. As expected the signal used the full .7 volts and the colours corresponded to the correct positions on the vector scope. However, it was my understanding that any value higher than 235 or lower than 16 on the RGB scale was out of bounds for PAL transmission? On inspection, the black bar equalled 0:0:0 and the white bar equalled 255:255:255.

So, I edited the bars so that no value could succeed 235 or undermine 16, and re-imported the bars. However, as expected when looking at this on the YC waveform the white levels dropped below .7v and the blacks were raised above 0v. Which, to my understanding is incorrect? And, when looking at the vectorscope the colour points were way off.

I tried using the 'colour safe' effect within premiere, this disliked the original 100% bars and compressed the levels. It did however, like the newly edited bars where the RGB values were compressed and didn't touch them whatsoever.

However, it was also my understanding 100 IRE is the broadcast peak standard, not premiere's default 110. So when I pushed the IRE levels down to 100, it didn't like the newly edited bars so much and had and pushed the levels down further.


My guess is, premiere's scopes are not setup for PAL transmission, and are purely for legalising signals within an RGB scope?


Any advice is appreciated.
Thanks.
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#2 Daniel Smith

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Posted 14 February 2010 - 08:04 PM

Sorry, bit of a mistake, I know you can go below 16 on the RGB scale in PAL, I'm getting confused with NTSC where the IRE levels are set at 7.5 (RGB 16).
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#3 Jason Reimer

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Posted 16 February 2010 - 05:42 PM

I'd be curious to know how all of that works, as well. I recall a couple of years ago Walter Graff mentioning something about putting together some sort of primer on the subject of legal video signals, but I'll admit I never did go back and see if he did. You still out there Walter?
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#4 Adrian Sierkowski

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Posted 16 February 2010 - 05:45 PM

I think it also depends on whether it's an analog signal or a digital signal. If memory served, for something like DV video here in the NTSC world the blacks are @ 0IRE as opposed to 7.5. Just what I can recall though. It's been awhile since I've looked at the scopes for color bars in an NLE.
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#5 Daniel Smith

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Posted 24 February 2010 - 05:01 PM

My understanding was that camcorders shot at 0 IRE but the blacks were raised inside the NLE on ingest, and that using the 'broadcast safe' function on a lot of cameras was a big mistake because the NLE would just raise the blacks beyond 7.5 IRE.

I just find it a nuisance that theory and practice tend to contradict each other. Whilst 100% bars may show up perfectly on a vector scope and waveform monitor, it all gets changed when applying broadcast colour safe effects. And a lot of web sites tell me that the RGB levels can't go beyond 235, yet, the white RGB value on 100% bars IS 255 RGB.

Obtaining the best possible quality without making your video illegal seems to be a bit of a headache, and that's before going into flash pattern analysing...

Edited by Daniel Ashley-Smith, 24 February 2010 - 05:02 PM.

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#6 Rick Martinez

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Posted 06 March 2010 - 02:19 PM

Not clear about your specific system, but for NTSC video I've seen some equipment that automatically raises the black levels to 7.5 IRE to comply with the "setup" level because a setting that defines the black level (choosing between 0 or 7.5).

Using SMPTE bars the 100% pure white information should correspond to 100 IRE level, in the three levels of black (in the button right - small vertical bars on the "black" area), the left one correspond to 0 IRE or absolute black. These three levels are represented as a staircase in the waveform monitor.

100 IRE should correspond to 0.7 volts in luminance level and the video signal standard as a whole is one volt peak-to-peak, so there are 0.3 volts in the unseen sync.
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