I'm really impressed by the ARP 1047

[Don Tillman]

   From: Haible Juergen <Juergen.Haible at nbgm.siemens.de>
   Date: Thu, 14 Dec 2000 16:16:30 +0100

   Don:
   >  It has a notch output and there's a balance control to tune the
   >  notch zero frequency with respect to the filter tuned frequency.

   I'm a little puzzled about this one. What is it good for ?

   Or is it the same as the variable mix of HP and LP as in the
   Oberheim SEM?  The off-center position (somewhere between HPF and
   Notch) is highly useful IMO, sort of resonant HPF without loosing
   all the bass.  But it's nothing I would describe as "tuning the
   Notch frequency". 

   So, is it the same thing on the ARP filter, or something else ?

[Checking the SEM schematic...]  Ahh, the SEM VCF feature is very
close, but there's a significant difference.

The SEM VCF has a pot that pans between the low-pass and high-pass
outputs of a state variable filter, followed by a buffer.  

(For those who aren't familiar with this, a state variable filter has
simultaneous low-pass, band-pass and high-pass outputs.  If you sum
the LP and HP outputs together you get a notch filter.  This doesn't
happen because there's a gap between the LP and the HP outputs, it
happens because of the phase relationship between those signals --
near the resonant frequency the HP leads by 90 degrees and the LP
trails by 90 degrees and this 180 degree difference creates a notch.)

So the SEM gives you a pan from LP to notch to HP functions.  Which is
very nice.

The ARP 1047 also has a pot that pans between the low-pass and
high-pass outputs of a state-variable filter, followed by a buffer.
But the ARP 1047 pans over a much smaller range, and the control is
calibrated on the front panel.  This control doesn't really function
as a pan pot, it adjusts the frequency of the notch relative to the
filter's resonant frequency over a range of -2 octaves to +2 octaves.
So if the notch is tuned to, say, +1 octave, you would have both a
resonance at the VCF's tuned frequency and a notch an octave above
that.

One nice feature is that you can have an interesting response curve.
Another nice feature is that the offset notch can be more musically
useful at higher Qs.

  -- Don

-- 
Don Tillman
Palo Alto, California, USA
don at till.com
http://www.till.com