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Why do luma levels change after render?

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Jeremy SaloWhy do luma levels change after render?
by on Sep 2, 2008 at 2:26:19 am

I'm editing a program for broadcast. In order to keep the video levels within network specs, I added and adjusted a video limiter filter to Luma Max 100% and Chroma 120%. The levels looks good through the waveform before it’s rendered. As soon as it’s rendered the luma levels peak past 100% and up to about 110% .

I tried to change Project Setting...Video Rendering...changed to Maximum Bit Depth, but the Luma levels still rise after the render past 100 IRE.

Does anyone know why my video levels appear to change after I render?

FYI: This is a widescreen project of 720:480 (1.2 aspect) originally shot on DVCAM.

I also tried the Broadcast Colors filter, but that cuts the luma and chroma down to 100% and we loose too much detail then in the color highlights. Either way, after the render the luma level also goes back to 110.

Any suggestions?


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Jeff BrownRe: Why do luma levels change after render?
by on Sep 3, 2008 at 12:44:35 pm

What are you rendering to (format/codec) ?


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Jeremy SaloRe: Why do luma levels change after render?
by on Sep 3, 2008 at 3:56:18 pm

I am rendering to the default; DV NTSC, Millions of colors



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Jeff BrownRe: Why do luma levels change after render?
by on Sep 5, 2008 at 2:35:16 pm

Hmmm-- I thought it might be a codec issue, but probably not. I'm stumped; sorry I can't help more. Unless your preview render is different than your "working" render.

-jeff


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Tim KolbRe: Why do luma levels change after render?
by on Sep 8, 2008 at 8:35:36 pm

The preview is generated with the display card...the render happens through the codec. Sometimes certain display cards have their gamma set differently for overlay preview than the desktop...you may want to check your display card settings...




TimK,
Director, Consultant
Kolb Productions,

CPO, Digieffects


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Jeremy SaloRe: Why do luma levels change after render?
by on Sep 8, 2008 at 9:50:48 pm

Yes but how do I get Premiere to actually render with the levels it's effects are set to?

Regardless of how it previews on any of the monitors I have set-up, the video limiter effect is set to a numeric value of 100 IRE maximum. That and all the other filters I tried to lower the luminance values numerically in preview mode raise past those settings when rendered. They still bleed past 100 IRE, some up to 110 IRE.

This problem isn't about how it looks on my screens or a graphics problem but how the default NTSC Premiere render is boosting certain luma values past broadcast legal specs of 100 IRE.



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Tim KolbRe: Why do luma levels change after render?
by on Sep 8, 2008 at 10:11:43 pm

Can you give some detail on what you're looking at to get a measurement before and after rendering...whether or not the pedestal (black) pops up with the bright areas, and what happens to the mid grayscales...

Something is weird here...are you using an SDI card or analog video card with an external scope?




TimK,
Director, Consultant
Kolb Productions,

CPO, Digieffects


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Jeremy SaloRe: Why do luma levels change after render?
by on Sep 8, 2008 at 10:30:40 pm

Sure:

I'm just using the built in Waveform Monitor in Premiere. I'm looking at the actual representation in the middle of the scope as well as the green bar graphic representation on the right side of the scope.

Ah yes, the black levels also pop/expand downwards after the render. It's hard to judge the mid levels but they seem to be the same. The black jumps from 7.5 to 0 after the render.

I do not own an external waveform monitor. For this episode I had to send it out to a post house to run it though a broadcast legalizer. I really need get Premiere to render legally, possibly by having it render with a different codec.



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Tim KolbRe: Why do luma levels change after render?
by on Sep 9, 2008 at 1:51:05 pm

What kind of file are you exporting to?

Most RGB DV codecs inside your computer do not lift black to 7.5 (The USA is one of very few places where NTSC is used with the raised pedestal...NTSC in Japan has black at 0 for instance).

That black level is typically controlled by the deck/video device...it's simply a scaling operation.



TimK,
Director, Consultant
Kolb Productions,

CPO, Digieffects


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Jeremy SaloRe: Why do luma levels change after render?
by on Sep 9, 2008 at 5:17:48 pm

I'm exporting to DVCAM.

They haven't tested the final tape at a post house yet so don't blame to deck. Once I render, before any export of any kind the levels expand on the Premiere waveform monitor.

The black was less of a concern. The main concern is the luminance being over 100.

Is there a plug-in or way to change to a different rendering codec in Premiere that will actual render with the setting you adjust it to?



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Tim KolbRe: Why do luma levels change after render?
by on Sep 9, 2008 at 5:52:22 pm

i'm not "blaming the deck"...I'm saying that I'd like to see the measurement on an external wfm after the video is exported to tape from PPro. You can go to an uncompressed Y'CbCr 4:2:2 codec...but you won't be able to export it to DVcam through the firewire port.

maximum bit depth won't affect DV...it's 8 bits max...

If you captured this video via firewire and edited it without cranking it up with color correction, DV has a hard clip anyway...you shouldn't have an issue.

These software scopes are ok to use as a guide, but I'd hook up an external wfm to get a real measurement. There are a lot of internal influences in the computer that could affect and edit>render pipe and may affect the way the wfm is interpreting what you're looking at.




TimK,
Director, Consultant
Kolb Productions,

CPO, Digieffects


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