[sdiy] More new Alfa chips - CEM3394

paula at synth.net paula at synth.net
Thu Apr 4 20:51:57 CEST 2019

Unless you wanted to make it multi-timbral.


On 2019-04-04 19:29, Richie Burnett wrote:
> Very true.
> And Neil has a good point that not all CVs are unique to each synth
> voice in a polysynth, so they can be connected to a common analogue CV
> line. It would make little sense writing the same value multiple times
> to the same parameter register in each voice chip if everything was
> handled digitally over something like SPI.
> -Richie,
> Sent from my Xperia SP on O2
> ---- Tim Ressel wrote ----
>> you can get an 8-channel 8-bit DAC with SPI for about $2. no need to
>> muck about with muxes.
>> --timmers
>> On 4/4/2019 5:14 AM, Neil Johnson wrote:
>>> Hi Rich,
>>>> When I saw "Microprocessor controllable..." in the datasheets for 
>>>> these
>>>> CEM chips I was initially expecting to see an SPI interface! :-(
>>> Indeed not, as they say (AS3394E):
>>>> Thus interface to a microprocessor system may be accomplished simply 
>>>> with a single DAC, 4051-type CMOS multiplexer, and 8 hold 
>>>> capacitors.
>>> That said, for a polyphonic synth it is actually quite useful to have
>>> voltage control of these parameters, in EGs for example, where each
>>> parameter is common across all voices so you hang the controls off a
>>> voltage bus.  For example this is how the Siel Opera6 does things,
>>> with all the SSM EGs driven off common voltages for the A, D, S and 
>>> R.
>>> If you did this with SPI then you would either need some sort of
>>> addressing scheme (in that case use I2C), OR you end up with a huge
>>> mass of slave select lines!!  And you've now increased the cost of 
>>> the
>>> silicon by putting logic (another voltage rail) and some number of
>>> DACs plus reference on the die.
>>> Cheers,
>>> Neil
>> --
>> --Tim Ressel
>> Circuit Abbey
>> timr at circuitabbey.com
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