Chopping AVI files

[Shock]

Ok. I have been cutting and joining AVI file for years with vdub.
Every once in a while I get a file that wont let me cut it correctly.
Its "key frame" starts at frame 1 and when I hold in shift to drag
the slider over. I cant move it. I can move it without holding shift.
But it allways starts at frame 1 and not the frame I want it to start
at. Is there a way around this? And why are there key frame anyway?
Am I going to have to re-encode the whole film strip in another format?
I have been converting it into mpeg1 then splitting it in several file
and joining all the file and then converting it back into its original format.
Xvid or divx. And thats a long and painful process.
I just get tired of yelling at my monitor saying (Thats not what I did!!!)
I`ll owe you a beer if you can help me out.

[celtic_druid]

You only need to re-encode the section around where you are cutting. So if you want to cut from 100 onwards, but the first keyframe is at frame 250, then you only need to re-encode frames 100-250.

As for why there are key frames. The way MPEG codecs work is basically to store differences in movement. Only keyframes contain the whole image, so you can only cut on them. For more details on MPEG, frame types, etc. try somewhere like wikipedia.