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Waveform "ringing" issue in downconvert

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Michael Klinger
Waveform "ringing" issue in downconvert
on Oct 21, 2007 at 6:23:10 am

Hi,

We've been chasing our tail on something that might not even be an issue, but it has caught our eye and its driving us crazy.

We noticed in doing down-conversion dubs from our HDCAM SR deck that sometimes we see what I would call "ringing" or "ghosting" in the highest luminance peaks in color bars on a an SD waveform monitor. Sometimes it would be there, sometimes it wouldn't. After tearing our hair out trying to nail down the variable (swapping out decks, waveform monitors, cabling, down-convert work-flow, inboard down-conversion, outboard down-conversion), I finally nailed the issue down to one variable. The variable is whether or not the down-convert is set to letterbox or anamorphic. If it is letterbox, the bars on the the WFM look fine, but in anamorphic (or crop) the high points of the bars seem to ring or have a ghost.

Now that I have sleuthed out the variable: What is causing this? Is it a problem? Is there a way to adjust for it? Should we stop worrying about it?

Within the deck and when down-converting inboard the only menu item that seems to have an impact on the ghosting is a "vertical interpolation mode." Default is mode 0. This menu when set to something else, changes the look of the ghost, but does not eliminate it. Also the Sony manual, doesn't explain what the menu option does. So, unless someone can explain to me what it is doing, I plan to leave it in default mode 0. especially since it doesn't "solve" the problem. Our outboard Teranex down-converter, has no menu options for down-convert control other than crop, letterbox or anamoprhic, so nothing to adjust there.

Any ideas?




Mike Klinger
Tree Falls
(323) 851-0299
mike@tfsound.com
http://www.tfsound.com


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glenn chan
Re: Waveform "ringing" issue in downconvert
on Nov 8, 2007 at 9:28:01 am

It might be scaling artifacts... the higher-quality filters can introduce overshoot + undershoot / ringing. Maybe this is the cause?

The Teranex device might give you control over the filtering... extremely soft filtering shouldn't give you ringing. Although that's not really what you want... but it might be a way of checking to see that it's the filtering that's causing your "problem". But it's not really a problem... it's generally considered the correct way of making high quality video (because the alternatives are blurriness and aliasing, which are more objectionable).

2- You can always insert edit over your color bars. Though if QC is not dinging you for it then that may not be necessary.


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