Hi
> ...and this may or may not be related here, but I've come across AC
> coupling vs. DC coupling in the archives and am wondering what this
> means exactly for synthesizer CV and audio signals.
What does AC coupling or DC coupling mean?
In general this describes a way, how two circuits or modules are
connected electrically to each other. As prerequisite I assume, that
'module A' generates a voltage, 'module B' is controlled by a voltage.
If the connection is DC-coupled, this means: Each action of the output
voltage of module A is direct translated to the parameter value of
module B.
Example: module A is a joystick, module B is a VCO. If you move the
joystick that way that its output voltage is increased for one Volt,
then the pitch of the VCO will go up for one octave. If you leave the
joystick in the new position, the oscillator will stay one octave up.
If the connection is AC-coupled module B will recognize only changes
within a certain time. Voltages that do not change are handled as "no
voltage". In our example this means:
Raise the youstick fast for one volt and then leave it there. This will
cause the VCO to get for around one octave up, but the fall down again
to the original tuning.
Mathematically spoken the modulation is dependend on the delta of V0/t0
to V1/t1
Technically seen the AC coupling is simply realized by a capacitor in
series to the CV-flow.
A good example to watch the difference is:
Modulate a A-111 with an slow (maybe 1 cycle in 2 seconds or slower)
rectangle LFO through CV2. This will sound as two notes; the CV2 is
DC-coupled. Now remove the the LFOs output from CV2 and insert it to
'Lin. FM'. This will make the VCO jump short up or down (depending on
the rising or falling edge of the rectangle waveform of the LFO), and
the fall back to the original tune.
Where dow I want which coupling?
VCOs: Tuning control must be always DC-coupled; modulation at
audiofrequency ranges should be (but must not) AC coupled, because it
helps to separate tuning and modulation.
VCFs: For tuning and modulation same as with VCOs. For audio input:
usually AC-coupling is used. But DC coupling with some voltage offsets
can produce interesting types of distortion in the filter (where
distortion is a difficult term, because filtering itself is a kind of
distortion).
VCAs: For loudness control and modulation as with the VCO or VCF. For
signal path we have to differentiate: If we want to control the level of
an audio signal, we may use AC coupling. If we want to control the level
of another control voltage, we MUST use DC coupling.
(See the hint at http://www.doepfer.de/a130.htm: "Remark: The new
versions of A-130/131 (those with CA3080) are able to process even
slowly varying control voltages (DC coupled in/outputs). The old
versions of A-130/131 (those with CEM3381/CEM3382) are not able to
process slowly varying control voltages (AC coupled in/outputs).")
I hope this helped a littlebit. Additions are welcome.