[sdiy] Roland Alpha Juno DCOs

[Adam Inglis]

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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