Cassette Tape Went out of Sync (In an Interesting Way)

I recorded my modular synth, both in a direct digital way, and using the onboard mic of the RTM B-1000EW Cassette tape player. In the first half of this video, is a sequence that has been recorded by both methods and when I went to edit it together I realised that the mic/cassette doesn't sync up with the direct digital recording. and slowly goes more out of sync. But does so in an interesting way. The second part of this video is the same modular sequence, but is just the digital recording on its own.

[@TottenPoncho]

Da fuq iz diz.

[@jembailey8757]

What. Exactly. Happened. Here? FM synthesis or analog delay? I'm out nerded, and puzzled. I see no cassette.

[@jembailey8757]

Oh, top right. Out of sync with what?