I have a video which I have tried to deinterlace with QTGMC and the original format is has a Display aspect ratio of 16:9 (the original display aspect ratio 2.40:1).
As far as I'm aware I've not made any adjustments to aspect ratio, but the resulting video is expanded to letterbox.
Where am I going wrong?
Hybrid does not care about DAR (display aspect ratio) it only deals with PAR (pixel aspect ratio), since that is not reliant on crop.
So what happens is, that Hybrid analyses the video through mediainfo and whatever par that reports is passed through to the output.
Since you wrote, that you just deinterlaced the par set on the output is the PAR reported by the flags from the video headers.
If those are wrong, you can overwrite them under "Crop/Resize->Base->Pixel Aspect Ratio (PAR)->InputPAR".
If you need further help, you need to read the sticky and provide more details on what you are doing.
(a small sample of your source might also help)
Cu Selur