[sdiy] Roland Alpha Juno DCOs

[Adam Inglis]

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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