[sdiy] Roland Alpha Juno DCOs

[James Grenert]

And different GLIDE RATES?!

On Sun, Apr 30, 2017 at 5:32 PM, W. James Meagher <w.james.meagher at gmail.com
> wrote:

> 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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