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I want the best HD video export settings for TV broadcast, one that won't consume space on my hard disk and also exports very fast. I'm using an iMac running on Mojave OS
These sorts of things are difficult to answer, because:
a) What you think is acceptable quality or file size may not be to someone else.
b) In general, the smaller the file size, the lower the quality - you can't easily have both.
c) In general, the faster the export, the lower the quality - so adding this requirement makes it even harder.
I would suggest that you try the public beta version of MixEmergency 3.3.0. The export system in this version should be much faster than in all others. I would suggest exporting some small sections of a mix at different bitrates and figuring out which is acceptable to you in terms of speed, quality, and file size.