Testing presentation for low-resolution device

I prepare my presentations in my laptop (MINIMAL resolution 1024×768). The actual presentation goes through a converter that outputs only a 640×480 resolution (it’s an analog TV device). I canNOT change the output device.

My main problem is that often — although slides look fine in my display — especially the text looks awful in the output device.

What I want is a way to test my slides in my laptop in a way that simulates the 640×480 device. That way, I can play with the fonts and sizes to achieve a usable result in the final presentation device.

I have found some “resolution testers”, but all they do is to adjust the SIZE of the slides. Everything continues to look just fine. That is NOT what I need. Apparently they should adjust the number of pixels per inch, but don’t. In other words, they are useless for my purpose.

I understand that some versions of powerpoint (and perhaps libreoffice/openoffice) allow the user to set a resolution for the slide show. I suppose that would do it, but I haven’t been able to achieve that (only change the aspect ratio).

Perhaps there is an extension or website that can do what I need but I haven’t yet found it.

Thanks in advance!

Virgil