Doepfer

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.