[sdiy] Roland Alpha Juno DCOs

[Adam Inglis]

Works as expected!

You have to dial back the pitchbend response in the patch if you don’t want it to get too wobbly, say from 12 semitones to just one, and then program in for each channel just a few points of PB either side of zero for a rich detune effect.

The glide rate is fixed at the patch level I think, however each DCO can separately be bent by velocity via the EG, so it can get pretty wonky if that’s what you’re after.

I’m using a MIDIPAL (in Dispatch mode) so I can hear the effects in real time of playing a six note chord across 6 channels. Of course, as soon as I touch the pitch wheel, the detune is zeroed across all six channels, so I’ve programmed a little loop of six channel PB info into the sequencer to play along with me. 

I had forgotten but years ago I had messed around a lot with midi mono mode in synths such as the Akai AX-60 and Casio CZ-101. It allows you to do stuff like simulate velocity response (albeit monophonically) in synths that don’t have it. 

Sorry to de-rail the thread.

AI
> On 1 May 2017, at 10:32 AM, 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 <mailto: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 <mailto: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 <mailto: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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