[sdiy] Roland Alpha Juno DCOs

[W. James Meagher]

Ooooohhh . . . please report back!

On Sun, Apr 30, 2017 at 7:25 PM, Adam Inglis <21pointy at tpg.com.au> wrote:

> Actually, I just remembered, these synths can operate in midi mono mode,
> so theoretically you could acheive detune by applying slightly different
> amounts of pitchbend in each of the 6 midi channels. I’ll have to give this
> a try!
>
> > On 1 May 2017, at 10:12 AM, Adam Inglis <21pointy at tpg.com.au> wrote:
> >
> > Thanks for sharing this Russell.
> > I’ve always felt the Alpha Juno sound to be a bit too clean and sterile!
> I have the rack version. Unison mode is a bit disappointing - it thickens
> the sound, but doesn’t make it particularly rich or lush. The onboard
> chorus does help a little with this.
> >
> > From your description, it doesn’t sound like there would be a way of
> introducing some slop or detune between the 6 DCOs?
> >
> >> On 1 May 2017, at 8:58 AM, Russell McClellan <
> russell.mcclellan at gmail.com> wrote:
> >>
> >> After reading the very interesting conversation regarding the new
> >> novation peak synthesizer, I was made curious by something Richie
> Burnett
> >> mentioned in that thread regarding the Alpha Juno oscillators.  This
> >> led to some further investigation on my part and some may be
> >> interested in hearing about the results.
> >>
> >> I had always (incorrectly, it turns out) thought that the alpha juno
> >> oscillators followed the same basic topology of the Juno 6, 60, and
> >> 106 synths - which, for those unfamiliar, is a complicated
> >> digital/analog hybrid; basically an analog ramp wave hard-synced to a
> >> digitally generated pulse.
> >>
> >> However, it turns out that the Alpha Juno has a completely different
> >> design, with much less of an analog component.  There is a custom
> >> "DCO" chip which has 6 independent digital oscillators.  Each
> >> oscillator takes the 12MHz master clock and divides it down by a power
> >> of two based on the note being played.  For the highest notes the
> >> effective clock is 6MHz, and for the lowest notes this seems to be
> >> divided by 2048 to form an effective sample rate of 3kHz.  On each
> >> divided clock, an increment is added to a 16 bit accumulator (I'm sure
> >> the accumulator is at least 13 bits, but I'm not sure of the exact
> >> number of bits).  Then, the top 8 bits of the accumulator are sent to
> >> a digital waveshaper, and then to what looks like an R-2R based DAC
> >> on-chip.  This signal is then sent straight to the fully-analog
> >> filter.
> >>
> >> In general, the output is "pretty" clean - certainly there's no
> frequency
> >> drift since it's based on the crystal.  There are aliasing artifacts,
> >> but since the sampling rate is always so high compared to the note
> >> they are usually fairly quiet (I noticed some audible aliasing on the
> >> lowest notes).  Since the DAC is only 8-bits, quantization noise is
> >> also an issue.
> >>
> >> Anyways, I was really stunned to learn that the alpha juno had so much
> >> digital horsepower, and a bit surprised that I haven't seen this
> >> oscillator design before.  Does anyone know of any synths with a
> >> similar approach? (Other than the new novation, which seems to run at
> >> a fixed sampling-rate, but is otherwise similar)
> >>
> >> I think it would be a fun project to create a similar oscillator using
> >> an FPGA and an R-2R dac - perhaps with some additional capabilities
> >> like inter-oscillator FM and a sine lookup table shaper.
> >>
> >> Thanks to Richie for mentioning this in the other thread and for
> >> helping me off-list to investigate this.
> >>
> >> Thanks for your time,
> >> -Russell
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> >
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