[sdiy] GRP Genesi

[Michael E Caloroso]

"...the first read only memories ...64 bits"

The ProSoloist used the 7488 OTP PROM - made by TI - for instant patch
changes.  The 7488 had a capacity of 256 bits (not KB or MB!) in an 8x32
matrix - each PROM had 8 control lines for the 30 ProSoloist/ProDGX presets
(two addresses were unused "off"), and there were eight of these PROMs in
the PS.  Access time was a snappy 26ns (compared to 2-300 millisecond
access of later 27xxx EPROMs).  Jeremy exploited the open collector output
feature of those PROMs to enable or disable ProSoloist circuits by shorting
control lines to ground through the PROM output when the OC output was a
logic '1'.

I don't think any programmer had a profile for those old PROMs.  They
last appeared in a 1977 databook, yet ARP kept using them to make Pro/DGXs
into 1981...!

That novel feature was covered in the patent.  Other competitors - Moog,
Roland, Yamaha, etc - had their preset synths but none lasted as long as
the PS, and I don't think any of them could use PROMs for preset control
without infringing the patent.  I know that the Moog Satellite used a
matrix of fixed resistors for preset configuration, which ARP had abandoned
with the unreliable Soloist.

Woe for the 1970s repair tech who had to fix a malfunctioning ProSoloist
VCO... few if any understood what made it tick.  I asked a tech from those
days if he had fixed ARPs - "Woah... Evil..." (sign of the cross) :D

MC

On Mon, Jul 6, 2026 at 6:35 PM Tom Wiltshire <tom at electricdruid.net> wrote:

> Isn't it just the typical adding-up-a-load-of-octave-squarewaves to make a
> staircase "ramp" waveform? 6 bits in this case, iirc.
>
> Ah, yes, here it is, from the service manual:
>
>
>
> On 6 Jul 2026, at 13:01, Mike Bryant <mbryant at futurehorizons.com> wrote:
>
> Actually one of the cleverest bits is probably converting the down-scaled
> squarewaves to sawtooths whilst keeping the levels consistent across the
> whole frequency range.  No mention of how they did they - presumably by
> programming a constant current source with the key and octave number ?
> ------------------------------
> *From:* Synth-diy <synth-diy-bounces at synth-diy.org> on behalf of Roman
> Sowa via Synth-diy <synth-diy at synth-diy.org>
> *Sent:* 06 July 2026 12:06
> *To:* synth-diy at synth-diy.org <synth-diy at synth-diy.org>
> *Subject:* Re: [sdiy] GRP Genesi
>
> At first glimpse, just looking at block diagram, it looks like typical
> PLL, but instead of phase comparator it uses F-to-V conversion and
> voltage comparator. Same divider ratio is used in the feedback as in the
> vaweform output, so the VCO works only within 1 octave while overall
> output spans over many octaves.
>
> Roman
>
> W dniu 2026-07-05 o 11:26, Neil Johnson via Synth-diy pisze:
> > Michael E Caloroso wrote:
> >
> >     US 3,930,429
> >
> >
> > Don did a write-up of it here:
> > https://till.com/articles/arp/patents.html#US03930429
> > <https://till.com/articles/arp/patents.html#US03930429>
> > Very weird architecture indeed!
> >
> > Neil
> >
> > ________________________________________________________
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