One of my old-time hobbies has been the open reel tape recorder. I'm a big fan!
Magnetic tape adds a certain "warmth" to the music. It seems this effect comes from the harmonic effect generated as the tape slides past the tape head. People like this effect so much, that modern-day digital music editors come with "tape effect plugins" for popular tape and decks.
So, I ran an experiment. I took the OLPC XO laptop and used Pippy Activity to generate a sine wave (6 beeps) and recorded it on tape. I should see one peak at 1000Hz.
Then, I played it back from tape, and looked at the signal on Measure Activity. We see a major peak at 1000Hz, but smaller peaks at 3000Hz and 5000Hz.
Very exciting! Definitely some harmonic effect going on here. Will have to investigate more to see what else happens on tape, and how it differs across brands and machines.