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Aliasing in NTSC conversion


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#1 Jim Hyslop

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Posted 06 April 2009 - 06:16 PM

Hi,

I shot something in HD, and when outputting it to NTSC, I get a lot of jaggies. Is this normal for an NTSC downres?

I've uploaded snapshots to flickr. Here's the original: Posted Image


and here's the converted:Posted Image

The rails on the porch show it the best (worst?). Any suggestions on how to minimize the artifacts?
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#2 Phil Rhodes

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Posted 06 April 2009 - 07:59 PM

Hi,

What you're seeing isn't normal.

However, I could do with a bit more information - what hardware and software are you using? Your first still isn't at HD resolution, so I can't be absolutely sure what's going on.

That said, even given the low res preview stills, it's clear what's happening - it's being downsized to create a 720x480 anamorphic widescreen NTSC frame, and the algorithm that's doing the downsizing isn't making a very good job of it. This is like rescaling an image in Photoshop with the "nearest neighbor" resampling selected.

Somewhere in your software there ought to be an option for this, although frankly these days it's unusual for anything to be quite as bad as that is. If you're doing it in some piece of hardware, then, crikey, you have a cheap and nasty downscaler in it.

P
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#3 Jim Hyslop

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Posted 06 April 2009 - 11:04 PM

Hi,

What you're seeing isn't normal.

However, I could do with a bit more information - what hardware and software are you using? Your first still isn't at HD resolution, so I can't be absolutely sure what's going on.

Oh, sorry, I should have provided that info up front. The footage was captured on a Sony EX1, 1080-30p, edited at full resolution on a Macbook Pro using Final Cut 6.0.5. The NTSC version was created using FCP's export to Quicktime Movie, and selecting Custom settings: Aspect ratio: hdtv720p... and, uh, that'll be the problem, I bet.

Yep, I just re-exported using Quicktime Conversion and settings much closer to NTSC, and it's MUCH better now:
Posted Image

Thanks for forcing me to review the settings (which I should've done before I started this thread. I'll get me hat.)
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#4 Adrian Sierkowski

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Posted 06 April 2009 - 11:18 PM

Why wouldn't you just export uncompressed 8 bit for your SD? it would be the highest quality of anything. it's my normal EX1 work flow for when I go to strike DVDs etc.
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#5 Jim Hyslop

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Posted 07 April 2009 - 07:51 PM

Why wouldn't you just export uncompressed 8 bit for your SD? it would be the highest quality of anything. it's my normal EX1 work flow for when I go to strike DVDs etc.

See, that's why you get paid the big bucks! Thanks for the suggestion. I'll experiment with that.

--
Jim
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#6 Adrian Sierkowski

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Posted 07 April 2009 - 07:54 PM

What big bucks? :huh:

Good luck, and be careful, the file sizes will be substantially larger than normal.
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