Tiger / Compressor 1.2.1 and MPEG-2 output - How to avoid jumpy video.

Question

This post is to summarise my experience with compressor and mpeg2 transcoding in the hope that it will save someone else time and help them avoid similar problems.
I recently transcoded a series of mpeg2 files to a lower compression setting using compressor and derivations of the standard high quality presets. While the encodes were generally excellent, I noticed that in all cases, near the end, there was some jitter on large pan scenes.
After further examination, I found that in all cases, in every sequence of 4 frames, the first three frames were OK, but the fourth was a copy of the 3rd, hence the jitter in pan scenes.
I then rechecked the whole file for this behaviour, and found that it was only present at the end of the file. Each time it started at roughly the same point which was about 1.4Gb into the source file.
I reencoded the files using QT 7 and the same settings, and there was no problem.
Given that the 2*2.5 G5 doing the encoding has 1.5Gb of ram, I suspect that compressor has a memory issue, although I cannot confirm this.
I have also seen problems on small files encoded in a batch. These same files encoded alone did not show any problems.
I would therefore recommend that when transcoding mpeg2 to lower bitrates with compressor:
1. Free the maximum possible system RAM (reboot before job?)
2. Keep source files smaller than system physical ram
3. Dont run batches.
4. If you do use batches, and see problems with the second or 3rd job, rencode alone.
I may be way off of the mark, but there are many posts about similar problems and so I hope it may help.
BTW, these problems also seem to occur in compressor2 and may or may not be due to the same issue.
Perhaps someone from Apple would care to comment?
G5 2*2.5 1.5Gb   Mac OS X (10.4.4)  

Answer 1

What is the source framerate and size? What was the origianl source material that the original mpeg2 files were compressed from?
Sounds like a pulldown issue. This might happen if the original mpeg2 files were encoded at 23.98 and you transcoded them to 29.97. Although you should notice it throughout the entire clip, not just at the end.