Non-linear distortion due to the different shapes of the cutting and reproducing styli.
"Let me just say how deliriously happy I am with the job you did transferring the albums. I've played them all multiple times because I'm so amazed at how good they sound. How can I put this...? They are AWESOME, man!!! I couldn't imagine they'd be this good. I can't even hear the needle on the vinyl! How do you DO that?! Honestly, I don't think ..."