25 Jun
1998
25 Jun
'98
12:25 p.m.
Rather you than me. When you've got one big Asic,
with photodiodes (data
and focus error) going in at one end, and a bitstream for a DAC, and
servo drive signals at the other, then good luck in trying to figure out
how to tap off the data bitstream after the 1st error correction before
the digital filter stage, etc. And good luck in figuring out how to take
control from the servos.
Skip that stuff!
If you can get the raw signals off the photodiodes, and the
characteristics of the motors (focus, disk rotation, and seek), designing
a dumb control system would be trivial. You could even skip the focus
control, replacing it with a manual system that would need fiddling with
each time (although todays CDs are remarkably consistent). Just take
outputs of the photodiodes and stick them thru a very simple control
system (a PID system would be way overkill!) and feed it to the
servomotor. Yes, you might have to fiddle around with a bunch of
resistances to get the loop right, but that is not too bad. For a true
CLASSICOMP type, a Heathkit EC-1 could be used to get the control system
designed!
I am not thinking about having a random access device at all. If CD-ROM
drives are not available, go to the next medium. Read the old CD-ROMs
once and transfer, just like 7 track tapes.
William Donzelli
william(a)ans.net