The tape has to move at a constant 4.76cm/s rate. Based off the protoype design it didn't include the tachometer to measure the linear speed.
Im going to list 3 ways it may play correctly
- No tachometer, uses a microcomputer for reading the cassette tape and use software to stabilize the changing play speed
- Add a tachometer to read the 4,76cm/rate but the design becomes bulkier with extra parts.
- We discover a new way to play a cassette tape at the right speed somehow.