Eduardo,

I don't intend to cut in front of Dave here and I still hope that he responds. I do expect that he has more video editing experience with Linux than I. But just to join the conversation:

VLC is pretty impressive in what it can do, more than just video playback. Still, it wouldn't be my first choice of tools for this task. I hope that someone can give you VLC settings to avoid the TS file production, but I don't know how to do that.

My first thought was that you didn't quite post enough information. Saying that a file is an AVI file really doesn't tell enough. AVI is a container that can hold video files encoded with a choice from a very large number of codecs. You really should state what codec the original file is encoded with. Also, is this the same codec that you want used for the 30 minute result? Once you cut to the TS file and then tried to go back to AVI you could have been re-encoding the new AVI with any codec, it doesn't seem likely to me that the re-encoding would have been with the original codec by chance.

My personal choice of tools to do the video edit would be Virtual Dub (VD). It takes a few minutes to learn to use and isn't the most obvious tool, but it is amazingly powerful. It can cut the 30 minute segment out of your video without ever re-encoding it to another codec (for retaining all of the quality of the original). Or, if you need the new clip encoded differently (for a different device that doesn't support the original codec, or to reduce the file size with a more efficient codec, or even change the size of the output video to fit a small portable device), VD can do that.

Full disclosure: my use of Virtual Dub has been exclusively under Windows. But a quick check showed me that it should be there on your Knoppix disc too. So I would definitely suggest that you look at understanding the Virtual Dub tool while you wait for a response from Dave. It is a little tricky to get started with, but the simple cut without re-encoding should be easy enough. Once you get past that you'll find that you can do things with it that you never even thought that you could do, although they may take a few minutes of setup and a lot of poking through the options.