Delia Derbyshire again. This is how she worked, this is how the Doctor Who theme was created.
The more I watch it, the more incredible it gets. I used to work with tape machines at Uni (leftovers from decades ago), and no two would run at the same speed because of the different wear on belts etc. This is what she does on the third machine, adjusting the speed and restarting it when it gets to the right beat. And this is just with three or four machines for a few seconds. Imagine doing this for the length of a whole piece.