In the 30's we at Philips Research had the Philips Miller System. Look at
http://www.orbem.co.uk/tapes/pm.htm to read what it was and see some photographs of the system. It used black film, where a cutter removed a line in the black layer. The cutter moved vertically and thus a clear path with a variable width was cut. Reading was done optically.
As you can read, the BBC used it quite some time. It was practical for broadcast, as you can easilly edit the sound, just like film. During WW2 the Lab did experiments with stereophony, using simply two cutters next to each other. The sound quality was amazingly good, I heard that my former collegues made a recording of marching German soldiers over the street along the research lab, with two microphones in two open windows. That recording should still exist.
The advantage is / was that you cannot erase the sound, so there is more kept from the past.