FCP 6: poor export quality in mp4

Right. Red herring. Case closed. It's 720 not 768 or 788.

Incredibly difficult image to compress. I'm guessing these people aren't standing still much. High contrast. Lots of deep shadow area creating gradients in the blacks. You have to use a high data rate with this material. No way around that.

No! No, no. 768x576 is what you should be exporting to.

The problem is pixel aspect ratio. To re-iterate what we found earlier: 720x576 with rectangular pixels is what your video actually is, but your computer is displaying it as 768x576, since computers display things using square pixels. It's basically showing you a square-pixel version (because a computer display can't of course change the shape of it's pixels), but it's not actually changing it to square pixels. Just displaying it that way.

Now for the web, you always want to have things in square pixels, so you actually DO want to CONVERT your finished video to square pixels. So in Compressor, you want to indicate 768x576, with a Pixel Aspect of 1.0. If Compressor isn't letting you do this, try clicking Save As and saving it as a new custom setting, then try again.

If you think the resizing is lowering the quality, go to the Frame Controls tab, click the gear, turn them on, and change the Resize Filter to Better or Best.

Okay, you're not listening to me: you HAVE to resize the movie if you're going to the web, because you HAVE to convert it to square pixels.

Now if you turn on frame controls, and change the resize filter, you won't lose any quality from resizing.

Make sure you have the right setting selected when you're trying to change dimensions. When you saved it, it will have appeared as a custom setting. You should now be using that setting.