Doepfer A-127 Triple VC Resonance Filter - Manual (Page 4)

A-127 Triple VC Resonance Filter System A - 100 doepfer
4
3. Basic function
The A-127 is a triple resonant filter containing three
band-pass filters each with controllable filter frequency
f
M
, resonance and amplitude. What a band-pass filter
does is to attenuate signals in frequency ranges both
above and below the filter frequency - referred to here
as the middle frequency - resulting in a bell-shaped
response curve (see fig. 1). In this way, a particular
part of the audio spectrum can be singled out.
fig. 1: Frequency response of a band-pass filter
with lower (on the left) and higher resonance
For each filter you use the frequency control to deter-
mine the exact position along the frequency axis, the
resonance control to set the width of the frequency
band let through, and the Audio Level control to set the
volume (see fig. 1).
To make it easier to understand how the A-127 is
configured, fig. 2 shows a block diagram of the
internal components.
fig. 2: Internal construction of the A-127 (from 1999
output § is not affected by the level control 7)
LFO
In Level
1
ext. CV
VCF
Mix
Audio
1
2
3
CV Ampl.
3
Frq.
5
Res.
6
Original
8
Frq.
2
Audio Out
§
Mix Out
$
Level
7
Audio In
!
ext. CV
"
Freq. f
M
Out
Freq. f
M
Out
VCF Frq. / CV VCF Frq. / CV