[sdiy] Roland Alpha Juno DCOs

Mon May 1 14:34:03 CEST 2017

Hi Tom - measurements were taken with a digital scope right off the dco
chip.  At low and medium frequencies the steps in the signal were clear as
day on the scope.  At most frequencies I tested, there were some doubled or
tripled steps - i.e. the same voltage was output for two or three divide
clocks.  This to me is a dead giveaway that the accumulator based
architecture I mentioned is being used.

The other clue here comes from Richie - at high frequencies, aliasing
artifacts are present around a sampling rate of 6MHz.  Since aliasing is
present, we know the ramp is being generated by an accumulator counting at
a non divisor of the sample rate.

The reason I know there are at least 13 bits is because I once saw more
than 16 single steps between "doubled" steps.  This indicates that there
are at least 5 "hidden" bits under the DAC which was measured as 8 bits (by
counting output steps).  16 bits was just a guess.

It looks like the highest frequency it can do is 11kHz and the lowest is
~2.84Hz (pulse every 0.352ms).  I believe I was testing 10Hz when I
measured the 2048 divider (clock was 2929Hz - so I guess this is a 4096
divider of the 12MHz clock).  I wouldn't be surprised if I missed a divider
stage since I didn't realize the oscillators could go that slow until after
I had closed up the synth.

Thanks,
-Russell

On Mon, May 1, 2017 at 6:28 AM Tom Wiltshire <tom at electricdruid.net> wrote:

> 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
> >> _______________________________________________
> >> Synth-diy mailing list
> >> Synth-diy at synth-diy.org
> >> http://synth-diy.org/mailman/listinfo/synth-diy
> >>
> >
> >
> > _______________________________________________
> > Synth-diy mailing list
> > Synth-diy at synth-diy.org
> > http://synth-diy.org/mailman/listinfo/synth-diy
>
>
> _______________________________________________
> Synth-diy mailing list
> Synth-diy at synth-diy.org
> http://synth-diy.org/mailman/listinfo/synth-diy
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://synth-diy.org/pipermail/synth-diy/attachments/20170501/38220a4a/attachment.htm>