MOTM

On Friday 09 January 2004 20:21, Mark wrote:

> The output of an MOTM 800 is not an AC signal.  It is a
> positive voltage that never crosses zero.  AC stands for
> "alternating current" -- it "pushes" current one way when
> the voltage is positive, and "draws" current the other
> way when its voltage is negative.

Ah, I see.

>
> Most audio inputs are AC coupled.  Their inputs have
> "blocking capacitors" to remove DC offset.  Most CV
> inputs are DC coupled -- they can accept both AC and DC
> signals. 

Yes, that's what I just learned from Richards post. Never 
new this, but it's an eye opener for me.

>  There is a difference between AC and DC
> coupling when doing ring modulation, oscillator FM, etc.,
> but understanding this difference is very important when
> using patching filters and VCA's.  The modulation inputs
> are DC coupled, but the signal inputs are usually AC
> coupled.
>
> For example, one VCA in each of the MOTM dual VCA's can
> be used to process control voltages (the 110 has a switch
> allowing you to bypass the capacitors) so that they can
> be used to control modulation depth. The other VCA in
> each dual VCA is AC coupled, so it can only be used to
> process audio signals.  

Aha, that's why Paul recently said that only VCA #2 in the 
190 can be used for DC signals. That input must be DC 
coupled, while input #1 is AC coupled, I assume. So that 
means that input #2 of the 190 cannot handle DC shift and 
could cause pops in the sound. Correct?

> The 820 lag processor functions
> as DC-coupled low-pass filter.  It can be used to process
> control voltages and audio signals.  Afaik, all of the
> other MOTM filters have AC coupled inputs, so they cannot
> process control voltages.  If you try to ignore this,
> your patch won't "work".

I'm starting to understand this now.

I never understood the fact that I can apply an LFO to a DC 
coupled (CV) input. I always assumed that an LFO must be a 
DC signal, since it was used for DC coupled inputs. But an 
LFO is just a slow moving audio signal, very AC. I didn't 
understand. But now that I know that DC coupled inputs also 
accept AC signals, the whole LFO issue is finally 
understood. :)

Thanks,
Robert