Basically, it's the history of Music technology from Edison's wax recorder onwards. try wiki-ing:
Microphones, recording, electronic instruments, synthesis, amplification, broadcasting, effects & processoring, compare digital to analogue. Jonny Martin's Music Technology Study Guide has quite a bit of stuff but you're right - it's not as thorough as his AS guide. I think he's working on the A2 one. There's also one by Mortimer Rhind-Tutt (yes it's a real name - I've met him) but I haven't seen it.