Thanks ame, for your reply. I saw it only this morning...
Last year I tried many programs for mac but a lot were trial versions despite the words 'free'. It really ment: 'free trial'
Anyway, I already have the VLC media player. It does only that: play media. I cannot get streams to play. It has a very different interface from the Windows version. It cannot make clips. We discussed this last year in the technical 'help' pages.
AVC I also have but there are problems with synchronising sound and vision. There was also another thing about it but I don't remember what it was.
The only program I found (thanks to aita) that seemed to work is HandBrake. It can convert and make clips at the same time. You can adjust many settings, for instance, the framerate. So sound and vision are synchronised!
The only disadvantage of this program is that there is no screen. You can't see the footage you are making a clip from. You have to calculate the seconds: how many seconds from the beginning of the video your clip starts and how many seconds the clip lasts. Quite elaborate.
The last one you mention, NCHdebut seems to do one of the two things I want: make a video (clip) from screen from a stream. I made a few trials and it does seem to werk well! Some things I still have to work out. The information on the settings is not very helpful, it does not explain why you should or should not tick a box so I probably have to spend some time to find the right information. But it works!
One thing that didn't work so well is making a clip from an MKV video. The result is not very 'fluent', it looks as if many frames are missing. The birds are moving like robots on the nest, if you know what I mean. That has probably something to do with bitrate or framerate. If you or anyone has an idea to adjust this I would be delighted!
When I have tried some things and have other questions I will ask them here.