Ideas for analog playground

I got two chips I want to use, BUT, as one may deduce from my last post regarding wire gauges or my first enclosure attempt … I’m not necessarily the most electronics or design savvy Shruthi owner on the forums.

I have a CEM3379 and SSM2300. I’m kinda starting to wrap my head around the conceptuals of CEM3379…but can someone explain what an SSM2300 does? Imagine you’ve been asked to describe your favorite hobby to a classroom of attentive 8 year-olds AND you happen to be very enthusiastic about the SSM2300. Anthropomorphizing audio signals and the like will be much appreciated.

The SSM2300 can be used to drive 8 VCF/VCA chips from a single CV source, with time multiplexing. Let’s say you have 8 VCFs. It would be a waste of resources to have 8 DACs to output the control voltages to them! Especially in the 80ies when digital or converters were expensive and finicky. Instead you write rapidly, in sequence, the 8 CV values to a SSM2300 and it extracts from the stream and hold each value to its 8 outputs. Sort of like an analog equivalent of a shift register.

8 year old metaphor is: how to water plenty of plants from a single garden hose. You need something that quickly, repetitively move a single stream of water to each plant.

Imagine this thing in a synth, routing from a single DAC control voltages to multiple VCF chips.

Take home message: this has nothing to do with filtering/processing an audio signal, but more some polysynth-specific plumbing.

Ok. Correct me if I’m wrong:

Two CEM3379s would both have 4 VCAs and 1 VCF, correct? So I can send CV to an SSM2300 and then use that to drive any combination of 8 of the available ten options of 8 VCAs and 2 VCFs.

And if I had four CEM3379s and 2 SSM2300s, I could send CV1 to one set with the above scenario and CV2 to the another.

Why would the sample and hold character be so useful vs the 4051?

2 CEM3379s contain 2 VCF (with cutoff and resonance CVs), 2 VCA (with gain CVs), and 2 panners (with pan CVs) = 8 CV in total ; so it would make sense to drive them from a single DAC and a 2300 if we lived in the 80s. I don’t know where you get those 4 VCA / 1 VCF (maybe from the chip diagram) - but what matters are the CV “exposed” to the outside of the chip (for example the panner works internally with 2 VCA, but it only has one CV).

You mention CV1 and CV2 and I am under the impression that you want to do a Shruthi-1 filter board with a SSM2300 and some CEM3379s. This makes very little sense. To be useful, the SSM2300 needs 8 time-multiplexed CVs as its input, and the matching data on its address bus. What the Shruthi outputs is one single, plain CV and no address data. I don’t know how to explain it - but in short you are trying to drive 8 different parameters from a signal containing only information for 1 parameter.

Let’s take an example… You have two CEM 3379 corresponding to two voices, and your patch and envelopes are such that at a given point in time you need to output the following CVs to the voices:

voice 1:
cutoff: 2.5V
reso: 0V
vca gain: 5V
pan: 2V

voice 2:
cutoff: 1.5V
reso: 0.5V
vca gain: 4V
pan: 3V

Instead of having 8 DAC outputting those voltages to the 2x4 CV inputs on the CEM3379 (the modern solution), you can have one DAC and 3 digital lines connected to a SSM2300 (the 80s solution), in this case your microcontroller needs to cyclically emit the following sequence

digital lines = 000 ; DAC = 2.5V
digital lines = 001 ; DAC = 0.0V
digital lines = 010 ; DAC = 5.0V
digital lines = 011 ; DAC = 2.0V
digital lines = 100 ; DAC = 1.5V
digital lines = 101 ; DAC = 0.5V
digital lines = 110 ; DAC = 4.0V
digital lines = 111 ; DAC = 3.0V

If you look at the CV signal on a scope, it looks like staircases which will be “demultiplexed” into the 8 CVs by the 2300: what you will get on the output 1 of the SSM2300 is a continuous 2.5V signal, on the output 2 a continuous 0V signal, etc… And this is why you need the sample & hold aspect : the voltage on the input of the SSM2300 will carry the voltage to output on its output 1 only 12.5% of the time, so the sample and hold “fills the gaps” while the input voltage successfully sets the other outputs.

If you connect the CV output of a Shruthi-1 to the input of a SSM2300, one of the following thing will happen: either you’ll get the CV signal on output 1 and crap on the other output ; or you’ll get the CV on 8 outputs. Both are equally useless. You need logic in the firmware to write the sequence of values, and 3 digital lines (which you don’t have) to write the address.

Enough said… the SSM2300 is a boring plumbing chip which made sense in synth architectures the Shruthi clearly doesn’t belong to. This chip is from an age when DACs were parallel and horribly expensive, and MCUs slow. Now for the price of its modern days equivalent (the AD SMP08) you can get 2 or 3 awesome octal SPI DACs from TI. Don’t be fooled by the rare status of the SSM2300: in a modern context it’s worthless.