[sdiy] Re: [AH] CEM / SSM Chips used in non-synths? ~``~

[Sean Costello]

A few years ago, my coworker looked at the raw code for the Williams
software synth, and did a little reverse engineering. It is interesting to
view the ROM code in a waveform editor - you can see where the sine table
is! Anyway, it looks like a lot of the Williams code produces effects such
as pulse width modulation and oscillator sync. The noise algorithm was
interesting, as it produced a waveform of a constant slope, but randomly
switching direction.

The tricky part about the Williams code is that it looked like it could
directly control the sampling rate of the output DAC. This was a technique
used in the earliest samplers. It made for more expensive polyphony (1 DAC
per voice, as opposed to all voices being mixed digitally and sent to a
single DAC), but it allows you to generate arbitrary waveforms without
aliasing. Trying to get the Williams code to work at 44.1 KHz means that you
have to implement anti-aliasing tricks, assuming you are going for accuracy.
On the other hand, for low frequencies (i.e. bass drone PWM) aliasing is
less of a concern, and for special effects it probably won't be
objectionable. Does anyone know if MAME uses anti-aliasing?

http://www.slack.net/~ant/bl-synth/ addresses the issues involved, from the
perspective of game sound chip emulators (cool!), but the same techniques
can be applied to other band limited waveforms.

Sean Costello

----- Original Message ----- 
From: "Travis Shire" <tshire at charter.net>
To: "David Moylan" <dave at westphila.net>
Cc: <synth-diy at dropmix.xs4all.nl>
Sent: Thursday, December 15, 2005 9:51 PM
Subject: Re: [sdiy] Re: [AH] CEM / SSM Chips used in non-synths? ~``~


> I nearly forgot.....all the early Williams videos (Defender, Joust,
> Stargate, Robotron, etc) used a software synth complete with ring
> modulation. A similar system was used in the Williams and Bally Pinballs
as
> far back as 1978.
> Here's an idea......if anyone wants to reverse engineer the Williams soft
> synth soundboard and design some sort of interface to be controlled from
the
> outside world, I'll provide one with schematics. The taker gets to keep it
> in exchange for his trouble. Could be a great noisemaker. Here's the
schemo
> if anyone wants to check it out. Not all that much to it.
> http://www.cityofberwyn.com/schematics/williams/JOUsnd.tiff
>
> BTW, the Bally/Sente soundboard was actually OEM'd by Sequential. Says so
> right on the pcb ;)