[Nil[_2_]]

This was discussed here some time ago, I believe, and I thought I saved
the instructions, but I can't find them, so I hope for some advice...

I want to digitalize a mono LP. I record it to the computer as a stereo
WAV file. What's the best way to make it a mono WAV file? My usual
practice has been to pick the best-sounding/least-noisy channel and
eliminate the other. But I recall someone here recommending doing
something like inverting one channel and then summing them in order to
cancel out more noise. That's not it, but something like that.

Can someone please steer me in a good direction?