>In SergeModular@yahoogroups.com, "riluttante" <riluttante@...> wrote:
>I know what AC and DC stand for, but can someone explain the
>functional difference in the Serge system between these two?
AC coupled means there is a capacitor at the input which the signal
must go through to be further processed. The functional effect of this
is that DC will not be passed. Therefore if you place an AC signal at
this input, its DC component (its offset from gnd, above OR below, or
both!) will be removed. This can help in "converting" CV's into "audio"
or "signals". It may also be used to eliminate a CV "carrier" from an
audio signal.
Words like signal, CV and Audio are necessarily limiting in a
discussion like this. When does a fast changing CV become a signal?
When does a slow moving "audio" signal become a CV? best to simply know
that there is a capacitor there, and then seek to learn enough about
capacitors to understand what will happen to various types of inputs
applied there.
Other capacitor effects may be present at AC coupled inputs, such as
input resistance changes due to capacitive reactance. (A fancy way of
saying that a capacitor acts like a resistor to a greater and lesser
degree depending upon its size in uF and the relationship between that
size and the input frequency.) Usually the circuit designer in most
general synth audio paths will attempt to minimise these kinds of "side
effects" for the expected signal frequencies to be encountered, by
careful selection of the capacitor size chosen for AC coupling. Other
times a capacitor may be chosen to take advantage of this effect, and
instead of attempting to be "transparent" to the signal going through,
it is acting as a consciously applied filter of the incoming signal.
DC coupled means there is no capacitor. Just a direct wire. Therefore
the DC component of the input is allowed to pass through maintaining
its "height" above or below ground. AC signals are not affected in any
way. This can cause problems when an AC signal is not centered about
ground equally. Because then the DC components do not cancel out and we
may hear their potential differences expressed as nasty clicks, pops,
or other (usually!) undesirable artifacts in the sound output. (Which
is one reason why we are given the option of AC coupling!)
>I understand from the above description that a bipolar, -5v to +5v
>output may still be DC coupled?
Yes. DC or AC coupling does not limit the inputs you can use, it
affects what gets passed on to the next stage in sound or CV processing.
Put a DC CV into an AC coupled input and nothing gets through, unless
the CV is changing fast enough in voltage level to be considered as AC
by the capacitor at the input. Conversely, if you have summed(mixed) CV
into your audio, and you put this through a DC coupled input; further
processing may not behave as expected due to the fact that the audio is
no longer centered around ground.
So there are reasons to use these AC and DC coupled inputs as intended,
and also reasons to go against their originally intended functions.
Just know that there is a capacitor on the AC coupled inputs and
experiment from there!
>Thanks for the questions and answers, good to see some of the basics
>explained!
Yes, thank you as well.
Kind regards, Randal
> > Very early Serges sometimes used Green for sync and
> > white for CV, and I've seen white used for the coupler
> > on SSGs (which swings -10v to +10v).
> > Steve