AC-Coupled module inputs

[Martin Czech]

> I was pondering something last night about module design philosophy. Why
> would you put ac-coupling caps on module inputs, or have them be switchable?
> Wouldn't it make more sense to have all inputs uncoupled, and just make sure
> that any module that is supposed to produce or process audio ac signals
> doesn't have a dc offset on it's output?
> 
> ...and for external sounds have a separate ac-coupled input mixer /
> amplifier...
> 

What is ac , what is dc in a modular system?
This turns out to be a difficult question.

Having all audio path ONLY with DC rejection is limiting,
think of adding some offset to some audio in order
to achieve unsymmetrical clipping etc.

Having everything dc coupled seems the best way,
but some modules degrade in performance with the slightest
offset (e.g. carrier suppression in multipliers).

Sometimes it is difficult to avoid dc offset for outputs at all operation
points and over the whole temperature range. E.g. ota.

Sometimes a source controlls severall modules, some may need dc bias,
others won't like that. A ac/dc switch at all inputs would allow this
to be possible.

Sometimes it would be nice to have a dc and ac input option at the same
time. This would lead to no switch, but double input jack solutions.

Maybe, if you are having only one input for each parameter ((Moog style,
in contrast to Roland style) which I think is a resonable way to go),
this should be dc coupled (low drift op-amps).  In this case you will
need some voltage adders anyway, these can be equiped with ac/dc switches
for each input.

What should the frequency of the first order highpass be?
-lower end of audio (30Hz)?
At least. Even Lower ?

I don't know yet, there may be good reasons for 10Hz or 1Hz.

m.c.