[sdiy] GRP Genesi

[Tom Wiltshire]

Missing the 64th, 128th, 192nd, 256th harmonic (etc) but otherwise identical. The 64th harmonic is -36dB down so it's probably audible but hardly a big deal. In my view, it's close enough for synthesis - this is raw material, not a pure test signal. Is it rich and does it provide plenty to work with? Yep!
The Roland Alpha Juno "DCO" chip (NCO really) does the same thing, but manages to produce an 8-bit staircase, which reduces the errors even further. No-one ever complained the "Hoover" synth wasn't producing a rich enough waveform!


> On 6 Jul 2026, at 23:27, Mike Bryant <mbryant at futurehorizons.com> wrote:
> 
> Ah right.  Not really a proper sawtooth then.  Thanks
> 
> From: Tom Wiltshire <tom at electricdruid.net>
> Sent: 06 July 2026 23:25
> To: Mike Bryant <mbryant at futurehorizons.com>
> Cc: synth-diy at synth-diy.org <synth-diy at synth-diy.org>; Roman Sowa <modular at go2.pl>
> Subject: Re: [sdiy] GRP Genesi
>  
> 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:
> 
> <PastedGraphic-8.png>
> 
>> On 6 Jul 2026, at 13:01, Mike Bryant <mbryant at futurehorizons.com <mailto: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 <mailto:synth-diy-bounces at synth-diy.org>> on behalf of Roman Sowa via Synth-diy <synth-diy at synth-diy.org <mailto:synth-diy at synth-diy.org>>
>> Sent: 06 July 2026 12:06
>> To: synth-diy at synth-diy.org <mailto:synth-diy at synth-diy.org> <synth-diy at synth-diy.org <mailto: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> 
>> > <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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