AC-Coupled module inputs

[List, Christopher]

> -----Original Message-----
> From:	Martin Czech [SMTP:martin.czech at intermetall.de]
> Sent:	Monday, April 26, 1999 12:14 PM
> To:	synth-diy at mailhost.bpa.nl; Chris.List at sc.siemens.com
> Subject:	Re: AC-Coupled module inputs
> 
> > 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...
> > 
> 
> 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).
> 
	Exactly my point, why bother with ac coupling of inputs or extra
switches!

> Sometimes it is difficult to avoid dc offset for outputs at all operation
> points and over the whole temperature range. E.g. ota.
> 
	These modules (e.g. a 3080 based state-var filter that I find always
gives me offset problems) should have ac coupling at the final output...

> 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.
> 
	Agreed, but I would think this would be a rare circumstance, and
even rarer is (for example) an lfo that allows the addition of a dc offset
*at the lfo*. Usually you add the offset in somewhere by summing a dc and ac
control signal - in that case you still have the original ac source
available by itself.

> 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.
> 
	I could see a switch allowing for easy comparison between the two
signals, or a quick change of sound where it really matters (e.g. a ring
modulator - as Paul S. reminded me via private mail), but I wonder if that's
worth the extra panel space / hardware just for that circumstance. More
appropriate would seem to be a "cv mixer with variable offset" type module -
you could run one of your vco's through this before sending it to the
ring-mod and have the same effect.

> What should the frequency of the first order highpass be?
> -lower end of audio (30Hz)?
> At least. Even Lower ?
> 
	Another good question for friendly debate! - Regardless of whether
you think the filter should be on the input of some modules or the output of
others! :) I'm definitely a fan of a lower frequency - 10Hz or so.

	- CList