[sdiy] Interesting blog with Roland Alpha Juno details

[Peter Pearson]

Sure, but you can't make the Hoover bass sound with "real Junos".

On Fri, Nov 1, 2024 at 9:31 PM Benjamin Tremblay via Synth-diy <
synth-diy at synth-diy.org> wrote:

> 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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