Re: Ultima 3 "Mockingboard" version.

[mjmahon@aol.com (Michael J. Mahon)]

Rubywand wrote:

>Linards Ticmanis writes ...
>> 
>> Rubywand wrote:
>> 
>> >      Mockingboard uses off-the-shelf chips....
>> 
>> Cool. The 6522 should still be available I guess, since it's a really
>> common chip for any 65xx thing. As for the PSG, maybe they still make
>> one that's compatible, I'll see...
>> 
>> It sounds like a fun summer project. Don't know yet if I'll find the
>> time to do it. You don't happen to have any schematics, do you?
>>
>
>     No; have never come across such a diagram. Maybe I'll pull our
>Mockingboard "Sound II" and try diagraming the circuit.

Actually, it's simplicity itself.  Each PSG is controlled by a parallel
port of an associated 6522 VIA.  As Linards notes in a later post,
the PSG is pretty easy to control, and the Mockingboard does nothing
but interface it to the Device Select addresses on the Apple.

FWIW, there is a design omission in the Mockingboard which is
worth correcting:  Reset on the Apple is not propagated to the
PSG's which can leave them "sounding" after the machine is
reset.  A diode between the reset line and the PSG reset will
fix this.  (A wire is not a good idea, since the 6522's can reset
the PSG, too, and you don't want them resetting the Apple!)

(And, Ruby, although you are absolutely correct that software
can modulate the envelopes of the PSG, I know of no software
ever written for the Mockingboard that does this, while virtually
all Commodore SID software uses full ADSR control, since it
is trivial, and does not require constant housekeeping.  So in
this case, "Turing equivalent" is not nearly so useful.  ;-)

-michael

 Email:  mjmahon@aol.com
 Home page:  http://members.aol.com/MJMahon/