Severe Pixelation every 15 Frames

[Dave Perry]

Hi Gang.

I've been going nuts over a problem that is causing us more than likely the loss of a client and network broadcast opportunity.

We've been working on a pilot submission to The Outdoor Channel. We are having problems when encoding the DVD to submit. We use the latest versions of everything Apple FCP Studio 2. The problem is that in certain points of the show there is a pulsating that occurs as the result of severely pixelated frames every 15 frames. It'll last for a few seconds then go away then start again in a different section.

I'm attaching 2 screen shots of the problem, a good frame, then the bad frame directly following it. This happens every 15 frames so you see a pulsating effect.

I've tried all sorts of things. I've tried saving the masters that were used for encoding to DVD as:

XDCAM, ProRes 422, DV, 10 uncompressed SD

I've tried working in XDCAM timeline set to render in XDCAM then tried timeline set to render as prores. Printed the whole show to tape then recaptured into FCP as various formats. Nothing seems to work.

I've tried high bit rates and low bit rates. I've tried a demo version of BitVice and found some slight improvements but no solution.

This problem has only started to appear in this project and it's driving me NUTS!

Any help would be greatly appreciated.

[Pete Cofrancesco]

I'd lean towards the camera/footage or how its being imported (ie frame rate/interlace/format). What's camera/format it was shot in?

I'd import a small portion of the footage in another app like iMovie or on a different computer. You could even post a 10 second clip of the footage online and have someone test it.

[Bob Hart]

I am totally useless for anything FCP however here's a suggestion for experimentation until better advice comes along.

Repetition of the problem cyclicly every 15 frames suggests a GOP issue. What camera did you shoot the problem footage on?

The shot is a wide shot with lots of textural detail in background, forest leaves mainly. These may be moving subtly in breeze but not enough to register as a movement between each frame, or represent collectively too much of a variation between frames and the movement is sacrificed and a reset of the image occurs with each new camera keyframe.

Try playing back the original tape at a slow rate if possble in the camera, the same footage and look for a detail "jump" about every half-second.

If it is on the original tape, I guess there is not much to be done in post, except maybe to pretend there is a shallower depth-of-field with After-effects and blur out the textural detail enough that the detail jump is less evident.

A more depserate measure might be to take a single frame, cut that frame as a background plate and insert it behind the human subject and the water foreground with a soft edge around the human subject. This hopefully will eliminate some of the detail load and the rest of the image will not juimp at keyframes in the final export to DVD.

If the detail sacrifice on the next keyframe is occurring in the post-processing, maybe experiment with more frequent keyframes on the DVD export.

Another thing which might be going on is a frame blend at the detail jump at the new keyframe in an attempt to disguise it which will cause a resolution loss in that and maybe some succeeding frames.

My ignorance is obvious as I re-read the above text. Please be gentle. Hopefully there may be a hint of something useful there to suggest some experiments.

[Dave Perry]

The cameras used in this were a Sony F300 XDCam and Canon XL H1. The editing timeline was in 1080p30 XDCam 35mbps. Thanks for the replies. I'm going to do some more testing this AM.

[Mike Gunter]

Hi

"XDCAM, ProRes 422, DV, 10 uncompressed SD" I just bumped into the forum - I don't know anything about Macs, nor the CODEC, but DV isn't uncompressed, nor is is 10-bit (if that is what that number means).

If the CODEC is a deliverable, it likely is a MPEG2 which would make some sense, but then it wouldn't be a "DV". If it is an archive, then it might be an uncompressed (hence the 4:2:2 in the quote, plus 10-bit - but then it wouldn't be DV - 5x compression and 4:1:1 [in the USA]).

It might suggest that some of the settings in the compressor menu need attention. I freely admit my ignorance. ;-)

My best and good luck!

Mike

[Dave Perry]

Yes Mike, all of these codecs are compressed, hence the term CODEC (COmpression, DECompression). The list of codecs used was to give folks an idea of what I've tried and the results. 10 bit uncompressed is not compressed for all intents and purposes but that's not the issue.

Anyway, some suggestions regarding tweaking the compression to MPEG2 would be helpful. I'm experimenting with different GOPs now. By default Apple compressor uses closed 15 frame GOP. I'm trying closed 7 frame GOP right now.

Anyone know why one would be better than the other and in what situations I would need to change the GOP size and structure?

[Aric Mannion]

I can't find from your post where the problem starts exactly? Is it in Final Cut? Are you saying you have authored a DVD and during playback this is happening? Is it in quicktime from your capture scratch that you are noticing this?

[Dave Perry]

Hi Aric. This is happening ONLY when encoded to MPEG2 for DVD. The source footage and timeline playback look great and we've even tried recapturing some of the footage.

I'm having some success however in adjusting the GOP size from 15 to 7 and raising the bit rate average from 5 to 6 and max staying at 7 mbps. I had one edit in the timeline I needed to correct, otherwise the last encoding I did would have been a good one.

If anyone could let me know more about GOP size and structure particulars or where to find more info on that I'd appreciate it.

[Pete Cofrancesco]

You might have better luck asking this question in the appropriate camera section. Most ppl here shoot in a prosumer format 1080i/60f or 720p 60/24f in HDV so we won't know any of the particularities associated with 1080p30 XDCam format.

[Dave Perry]

I have finally gotten some better results by changing the GOP size and raising the bit rate slightly. I changed the GOP size from 15 to 7 as I stated in a previous post. I raised the average bit rate from 5.2 mbps to 6 and the max bit rate stays at 7. The results are much better.

What I think I've found is not so much a problem with the encoding settings I've used in the past or the format of the source footage, but rather in the subject matter and composition of the shots. The fine details of leafy green outdoor backgrounds can often be hard for many digital cameras to capture cleanly. Both of the cameras used in this shoot are capable of it but this is the first project I've had to encode that has SO much of it that I didn't realize how tricky it can be when encoding for DVD.

Since this is the DVD and Web Video Delivery dorum, I didn't think it necessary to post in a camera specific forum or an Apple specific forum since at this point in the game I'm working with standardized, non-proprietary variables.

I'm still interested if anyone knows why and when changing GOP size and structure is beneficial or necessary in a given project. I've obviously found one situation but more experience from others having to do this would be helpful.

Thanks.

[Ervin Farkas]

Dave, that shot does not look all that difficult, especially when imported at such a high bitrate (Prores). There must be something else going on here.

I would try exporting first both in the project native format (if higher than DV) and DV. See if a QT movie (all I-frame) looks good - you need to isolate your problem either to the editing or the compressing side of your workflow.

[Dave Perry]

Hi Ervin,

If you look at the original post, you'll see the various formats I've exported masters to. All of them look fine, even DV looks good. I've isolated the problem to the encoding and I've found a solution.

I'm still interested in when and why I would change from 15 frame GOP to 7 frame GOP and when it's necessary to change the GOP structure though. I assumed, and experience bore this out in the past, that the default settings in Apple Compressor's highest quality 2 pass encoding would be fine. But this was a unique project where that setting needed to be adjusted and tweaked further.

[Roberto Bellini]

Hi,
I know exactly what you are talking about, and the same exact thing happened to me.
A few sections of a 45 min project had this pulsating bad encode at every 15 or so frames.
Like you, this only shows up on mp2 encode in compressor.
I even tried compression markers in the problem areas and nothing...
Will try the GOP thing now...

[Mark Shea]

Had the same annoying problem, will try changing GOP settings

I authored the same project in idvd and had no pulsing, but project started pixelating 10 minutes from end, so at the moment can't seem to win!!