[sdiy] AS3397 what is it?

[René Schmitz]

On 18.01.2025 00:21, Benjamin Tremblay via Synth-diy wrote:
> Hello,
> I’m intrigued by what the AS3397 chip appears to be. But exactly what does it do? Reading old threads about it and the CEM3396 leave me scratching my head.
>
> It’s a what… Wave-shaper that takes a computer-generated square wave and gives it personality?

The waveshaper works to shape a sawtooth into a triangle at a certain 
amplitude.

The amplitude is controlled by the ratio of the reference clock (this is 
a DCO) to the integration current (i.e. slope of the sawtooth). With 
this you can make all sorts of spaced sawtooth and triangle waveforms.

You must vary the integration current relatively precisely (though not 
nearly as precise as with a traditional VCO), in order to maintain a 
constant amplitude.


>
> What are the correct inputs? I’m assuming it has traditional voicing like a vcf/vca stage and a mixer.


The datasheet is a bit unclear about this, while claiming "easy" 5 Volt 
control voltages, actually some of the voltages require a negative 
voltage as well.

(I ended up using opamps to level shift, negating the datasheets claimed 
advantage of "High Z inputs, so no buffers needed", if your DAC + S&H 
can provide negative voltages, then you can realize this. )

Feeding this thing requires a lot of CVs, and there are sublte 
differences to the CEM3396, namely there is only linear control of the 
VCAs.

And a mystery "Trim" input, which is in reality connected to the 
internal reference voltage.


>
> Do I need to code up one or more computer waves to send into it, and what is the relationship between the input and the output?

You need to add some transistors that periodically reset the integration 
cap, the chip provides a current mirror for charging, and you need to 
provide a control voltage to that as well.



Best,

  René


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