[sdiy] Interesting blog with Roland Alpha Juno details

[rburnett at richieburnett.co.uk]

I think the sample-rate might be switched with the "footage" setting of 
the DCO in the same way that the other Juno's switched the clock to the 
programmable interval timers.  Like 12MHz, 6MHz, and 3MHz.  Switching 
the sample-rate in octaves saves a few bits in the adders of the NCOs.

You can just about see the artefacts from the digital synthesis in the 
output of the synth with the VCF wide open, but you can see the 
high-frequency crap much more clearly by recording the output of the DCO 
chip directly with an audio interface at a decent sample-rate like 
192kHz.

-Richie,


On 2024-11-01 17:24, Tom Wiltshire wrote:
> So...can you have a guess what resolution the sawtooth output is?
> 
> The clock to the DCO chip is the same 12MHz clock used for the 8032 
> microprocessor. So if it's 12MHz sample rate, it's not half bad! It's 
> like an early version of the Novation Peak!
> 
> If we could get a nice little FPGA on a 24-pin DIP, it'd be nice to 
> make a modern version of something like this. Maybe with SPI.
> 
> Thanks,
> Tom
> 
>> On 1 Nov 2024, at 16:45, rburnett at richieburnett.co.uk wrote:
>> 
>> The DCO is entirely digital.  There are no integrator capacitors 
>> around the chip, and you can see the stepping in the sawtooth output 
>> on a spectrum analyser when generating really low pitch notes.  You 
>> can also see aliasing in the spectrum when you generate really high 
>> notes, but it's quite a long way down in amplitude due to the 
>> multi-MHz sample rate at play.
>> 
>> -Richie,
>> 
>> 
>> 
>> On 2024-11-01 16:35, John via Synth-diy wrote:
>>> Oh, very neat. I've long wondered about the stranger waveforms on the
>>> Alpha Juno; I'd suspected they might be an indication that the "DCO"
>>> was actually pure digital and the sawtooth constructed from weighted
>>> binary outputs (as on the Poly-800,) with the weird stuff created by
>>> selectively ANDing some of the bits, but looking at his scope outputs
>>> there's a definite smooth slope to the sawtooth even on the 
>>> chopped-up
>>> versions, so it appears that's not the case. Interesting!
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