I'm processing user-uploaded videos on a CentOS web server with ffmpeg. I need to convert each video to a standard size and format, then extract a 30-second sample clip from each video. I want to use the "-vcodec copy" flag in the extraction command to avoid encoding a second time.

This command works for my initial conversion:

ffmpeg -i uploaded.mov -f mp4 -vcodec libx264 -vpre medium -acodec libfaac -r 15 -b 360k -ab 48k -ar 22050 -s 480x320 formatted.mp4

And this sometimes works for the extraction:

ffmpeg -i formatted.mp4 -vcodec copy -acodec copy -ss 0 -t 30 formatted_sample.mp4

However, when I run the extraction command on some videos, the extracted sample clip starts with several seconds of blank video. The audio starts right away but the video doesn't start for 3-6 seconds.

To demonstrate the problem, I've uploaded two video clips and run the above commands on them. I created the first clip in Final Cut Express and encoded it with Handbrake before uploading to the web server:

1a) uploaded clip

1b) converted with first command

1c) extracted with second command, missing first six seconds

By comparison, this second clip comes from Apple's website and does not show the problem:

2a) uploaded clip

2b) converted with first command

2c) extracted with second command, no problem

Can anyone see what's different about the two source clips? And if so, is there anything I can do in my conversion command so that when the extraction command runs, the clip is set up to avoid the missing video?

By the way, I initially had the problem with ffmpeg 0.6.1 installed from yum, but I upgraded to the latest git version and the problem remains.