[sdiy] Roland Alpha Juno DCOs

[Tom Wiltshire]

Since the oscillators are NCO based, you could add a little linear offset to each one to fatten the sound up. The software might not allow this normally but there's nothing about the hardware design that makes it impossible (as Adam's pitch bend trick proves).

Russell, you said the NCO clock rate is divided down to get lower octaves. How did you determine this? And the /2048 factor suggests that the oscillator produces 11 octaves of notes, is that right? What output frequency is it producing by the time it's down to a 3KHz clock rate?

The use of division suggests that the phase accumulator isn't many bits, which means they have to reduce the clock rate rather than being able to down-shift the increment (since bits would get lost of the bottom, affecting the note frequency accuracy). I wonder if it's possible to work out the frequency accuracy (and thereby the NCO length) - are there detectable steps at any point?
I suppose we should be able to put some limits on it even without measurements. We know they have enough steps to do pitch bend and glide, but not enough to be able to cover the full range without division. That ought to give some clues. I'll have a think about it.

Sorry for so many questions but I'm very curious about stuff like this.

Thanks,
Tom 

On 1 May 2017, at 01:12, 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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