Trouble joining two mp4s (or two avis)

[nosillacast]

I read the forums for weeks before getting my courage and knowledge level up to phrase my question properly.

My goal in the end game is accept video files in multiple formats, and stitch another video onto the front of them, all in the command line in Linux. I'm using CentOS 4.5 for this as a Virtual Machine on OSX. I have created a pair of experimental files which are as identical as i can seem to get them (using mplayer -identify to compare them). The one thing that i can't get to "stick" is getting the kbps the same, so that might be the end issue. I have tried to join the files either as avis or as mp4s, the results are the same. using this:
mencoder -oac copy -ovc copy -o combined.avi file1.avi file2.avi
always returns the error
All video files must have identical fps, resolution, and codec for -ovc copy
using ffmpeg as follows:
ffmpeg -i file1.mp4 -i file2mp4 combined.mp4
it pretends to put the files together but the resulting file actually only includes the first of the two input files.

I tried using ffmpegX on OSX, using the two avi files as input to the join command, and they join perfectly. this leads me to believe I just have an error in my syntax of the mencoder commnad above, that in fact there is no problem with the file structures. If I could see what ffmpegX is doing at the command line, I could replicate it but I haven't found a way to do that.

I've been stuck for 2 weeks on this, sure hope someone can help!
Allison
NosillaCast at http://podfeet.com
A technology geek podcast with an EVER so slight Macintosh bias!

[Baldrick]

All video files must have identical fps, resolution, and codec for -ovc copy

Are your video identical? if not you must reconvert everything to same or it wont work. Maybe ffmpegX reconverts?

[nosillacast]

All video files must have identical fps, resolution, and codec for -ovc copy

Are your video identical? if not you must reconvert everything to same or it wont work. Maybe ffmpegX reconverts?

If ffmpegX can join them, then they must be identical enough to be joined, so is there something different I can add to the ffmpeg or mencoder commands to make them stick?

the one thing that is slightly off is the bit rate, but try as I might, I can't get the bit rates to match. for example running this command:

ffmpeg -i file1.avi -b 400k file1_400kbps.avi

sets file1 to 542.6kbps. running the identical command on file2 sets it to 559.6kbps. They started out vastly different, and now they're "close" but is that close enough? And why it doesn't actually go to 400 is a mystery to me.