5 Jun
2014
5 Jun
'14
12:58 p.m.
the mac and lisa 400k drives did the same thing. what i've heared is
that one of the sound channels was used for this. the other being the
mono sound out port.
Rougthly, yes. The sound and disk speed data were stored in unused
locaitons in the vdieo memory area (I think one was in odd bytes, the
otehr in even bytes)(. At the end of every scan line (IIRC), a pair of
byters were loaded into counters that produced a pair of PWM signals. One
was used fro the sound port (via a custom Apple IC which appears to be a
volume cotnrol and nto a lot else, maybe some filtering), the other was
the disk speed PWM signal.
The 800K drive does nto use this PWM signal, it's still variable speed
(IIRC), but changes the speed automatically based on the current head
position.
-tony