[sdiy] More new Alfa chips - CEM3394

Neil Johnson neil.johnson71 at gmail.com
Thu Apr 4 14:14:56 CEST 2019

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.


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