[sdiy] Interesting blog with Roland Alpha Juno details

[Benjamin Tremblay]

I liked the waveforms just fine but the vcf circuit was disappointing. Real Junos had really tight pitch tracking for harmonized resonance. The Alpha Juno could not track pitch for beans.
Benjamin Tremblay

> On Nov 1, 2024, at 1:32 PM, Tom Wiltshire <tom at electricdruid.net> wrote:
> 
> So...can you have a guess what resolution the sawtooth output is?
> 
> The clock to the DCO chip is the same 12MHz clock used for the 8032 microprocessor. So if it's 12MHz sample rate, it's not half bad! It's like an early version of the Novation Peak!
> 
> If we could get a nice little FPGA on a 24-pin DIP, it'd be nice to make a modern version of something like this. Maybe with SPI.
> 
> Thanks,
> Tom
> 
>> On 1 Nov 2024, at 16:45, rburnett at richieburnett.co.uk wrote:
>> 
>> The DCO is entirely digital.  There are no integrator capacitors around the chip, and you can see the stepping in the sawtooth output on a spectrum analyser when generating really low pitch notes.  You can also see aliasing in the spectrum when you generate really high notes, but it's quite a long way down in amplitude due to the multi-MHz sample rate at play.
>> 
>> -Richie,
>> 
>> 
>> 
>>> On 2024-11-01 16:35, John via Synth-diy wrote:
>>> Oh, very neat. I've long wondered about the stranger waveforms on the
>>> Alpha Juno; I'd suspected they might be an indication that the "DCO"
>>> was actually pure digital and the sawtooth constructed from weighted
>>> binary outputs (as on the Poly-800,) with the weird stuff created by
>>> selectively ANDing some of the bits, but looking at his scope outputs
>>> there's a definite smooth slope to the sawtooth even on the chopped-up
>>> versions, so it appears that's not the case. Interesting!
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> 
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