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Jittery Image

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Russ Haskell
Jittery Image
on Dec 19, 2010 at 9:41:14 pm

I am working on an iMac i5, OSX 10.6, Compressor 3.5.3. Also on a MBP Core 2, OSX 10.5, Compressor 3.0.5. (FCS 3 and FCS 2, respectively.)

The problem I have is unfortunately repeatable - regardless of which machine I use in compressing this movie to MPEG-2. I'm using standard 90 minute best quality settings.

It's a 10 minute QT 720P movie, edited in Pro Res and exported as Pro Res. The clips were originally shot in 720P and 1080P H.264.

Everything seems good before running through Compressor. Afterwards, everything is good except for a 20-25 second section of scenes that is terribly "jittery". So the movie is essentially problem-free for about 7 minutes… then has a half minute of headache-inducing scenes - and concludes with a return to 2 minutes of goodness. These issues are all easily seen when viewing the MPEG file on the Mac's display and then when playing a disk on a TV.

That problem section contains both clips that were natively 720P and 1080P and there is nothing remarkable about either. For example, the jittery stuff starts with a wide shot of kids in a pumpkin patch and a lot of autumn foliage in the background. And it continues in several cuts of close ups taken in quite different lighting conditions (indoors, in fact).

I've looked very carefully at the QT movie and I can't see any signs of trouble.

Thanks for your suggestions in solving this.

Russ


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Russ Haskell
Re: Jittery Image
on Dec 21, 2010 at 10:57:46 pm

Perhaps this should have been posted in the DVDSP forum. If so, let me know.

I'll take brief stab at trying to better describe the problem that I called jittery. It's not that the whole frame is jumping about; nor is it flickering. Rather it's the sudden and intermittent appearance of artifacts in sizable parts of the image. The worst of it (the pumpkin scene was shot without any camera movement and without any zooming. The fall foliage was handled well by Compressor in other clips.

I just don't know enough about compression matters to know where to start. So any advice would be welcome.


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Chris Blair
Re: Jittery Image
on Dec 22, 2010 at 3:08:35 am

One thing I've seen cause what you describe is incorrect keyframe settings. If your settings offer keyframe options, set them to about 10 seconds or 300 frames. Some programs call them other things. I believe "scene change" is a replacement term in some encoding apps.

One high-end program we use (Carbon Coder) uses a really odd term for it (that I can't recall). Most encoding gurus suggest setting keyframe intervals at aforementioined 300 frames.

Another thing to check is if you're using VBR compression, don't set your low data rate value too close to your high value. The idea with variable compression is to allow the program to take the data rate really low in non-complex scenes (like during black sections of the video or really simple graphics, like a white background with one line of text), then take it fairly high in detailed scenes (like the fall foliage or bright orange pumpkin scenes you mention).

So most encoding folks suggest setting your highest bit rate at around 8000Kb/sec, your average bit rate at around 6000, and your lowest bit-rate at about 2000-4000. If you set them at something like 6000-low, 7000-average and 8000-high, you might see the weird artifacts you mention. It sounds counterintuitive, but I've seen this phenomenon with my own eyes so I know it's true. Another thing you could try is using CBR or constant bit rate, which should eliminate that issue altogether...but it will produce a larger final file since your bitrate is constant.

Hope that helps.

Chris Blair
Magnetic Image, Inc.
Evansville, IN
http://www.videomi.com
Read our blog http://www.videomi.com/blog


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Russ Haskell
Re: Jittery Image
on Dec 23, 2010 at 8:32:30 pm

Thanks for the suggestions, Chris. I will give them a try and let you know how I make out.

Russ


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Russ Haskell
Re: Jittery Image
on Dec 24, 2010 at 2:20:35 am

Chris,

Just to let you know that I bumped the max rate and lowered the average rate…which made for a dramatic
improvement. This first attempt turned a nearly unwatchable scene into one that is really decent.

I'll try more settings to see whether it can be fine tuned.

Thank you!

Russ


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Chris Blair
Re: Jittery Image
on Dec 25, 2010 at 4:24:10 am

Good to hear. Like I said, it's one of those things that is counterintuitive on the surface, but once you learn a little about how compression works, it starts to make sense (sort of).

Chris Blair
Magnetic Image, Inc.
Evansville, IN
http://www.videomi.com
Read our blog http://www.videomi.com/blog


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Russ Haskell
Re: Jittery Image
on Dec 27, 2010 at 2:41:48 am

Chris,

I encoded the disks with the file compressed @7.8 K/sec max and 5.8K/sec average bit rate respectively… and while the problems didn't go away completely, it looks very good overall. I tried some higher bit rates and while the quality was even better, the disk wouldn't play on one of my two players, which I took as a warning to back off.

Anyway, it was a good outcome (and a big relief) and I do appreciate your advice in making it work.

Thanks,

Russ


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