converting stretched QuickTime video back to 4:3

Since this is an FCP forum, here's how simple it is (if it is indeed being saved as 16:9).

Take that into FCP, drop it into a 4:3 Project timeline.

Change the Spacial Conform to None.

Then change the Scale X to make it fit properly.

Export.

What is the clip codec? This looks like it may be an old dv content with nonsquare pixels.

In QuickTime Player, press Command-I and post a screenshot of the info

If you really don't want to deal with OBS:

In FCP, create a project that's 640x480 (or 1440 by 1080 if you want to upscale it). Add your stretched clips, select them, and set Transform > Scale > X to 75%. Set Spatial Conform to Fill.

Export. Or... Convert the resized clip to a compound clip and save it to an event*. This will keep from having to resave the video (and save a lot of room on your HD). Compound Clips are just "text notes" (XML) describing how the clip it to be used in projects. When you place the CC in an HD project, it will maintain its aspect ratio.

*If you have a lot of clips to convert, you might want to create a new Event and name it something like Resized Clips, or just CCs. You have the option, when creating a CC to send it to whatever Event you want.

Please open the file in Quicktime Player, press Command-I, disclose all the items and post a screenshot.

FWIW, when you say the source "has bars on the side", what is probably the case is that the bars show because the aspect ratio of the screen (typically 16:10 on a computer, or 16:9 on a tv) is wider than the aspect ratio of the movie. The bars are not actually part of the movie.

What's wrong with OBS?

I have it on all my computers and nothing bad has occurred . . . what's that smell of burning?

You could create a 4:3 project, insert your stretched video and then export it.

You never said what the dimensions of the "stretched" movie are like, nor whether it has square or rectangular pixels. We are discussing applications without detailed information.

I have nothing against OBS, but we are lacking a proper diagnostic of what is happening.

Maybe the already digitized video can quickly be "fixed".

Just curious but why were you worried about OBS?