[sdiy] Interesting blog with Roland Alpha Juno details

[ColinMuirDorward]

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

There's no reason why you couldn't. There's plenty space in the voice
board's ROM and RAM to add in a per-voice detune

——> Note that it’s the more complex envelopes available on the alphaJ which
are needed to make the hooverbass (for that stylin pitch swoop). Detune
only gets you part way there.

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On Sat, Nov 2, 2024 at 4:22 AM <rburnett at richieburnett.co.uk> wrote:

> The DCO sawtooth waveform definitely has 256 discrete levels...  I
> checked an old recording I had of a very low-pitched sawtooth waveform
> with the VCF wide open.  There's quite a lot of background noise as
> you'd expect from a digital/analogue hybrid of that era, but by
> averaging consecutive cycles and differentiating the recording you can
> clearly see 256 evenly spaced steps along the ramp's slope.  The
> frequency spectrum also clearly shows every 256th harmonic of the
> sawtooth waveform is missing, which is symptomatic of a sawtooth that's
> been quantised to 256 discrete levels.  (The quantisation "noise" or
> error is essentially a sawtooth waveform at 256x the note's fundamental
> frequency, but with opposite polarity, and with 1/256th of the
> amplitude, so every 256th harmonic gets cancelled out in the resulting
> sawtooth spectrum.)
>
> You have to play a note below 78.125Hz before this artefact falls below
> 20kHz though, and most bass patches have a filter that's closed to some
> degree so you probably wouldn't notice this artefact unless you
> specifically went looking for it :-)
>
> Proving what the sample-rate (clock rate) of the NCO is by looking for
> aliasing will take a bit more effort.  I know I've seen evidence of
> aliasing in the spectrum of very high-pitched notes on the Alpha-Juno 2
> before, but don't seem to have any recordings immediately at hand.
>
> -Richie,
>
>
>
> > On 2024-11-01 17:24, Tom Wiltshire 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!......
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