Doepfer A-125 VC User Manual
doepfer
System A - 100
1. Introduction
VC Phaser
A-125
Level
CV
Audio
In
CV 2
CV
Audio
Out
A-125
VCP
Shift
Resonance
Mix
Module A-125 (VC Phaser) is a voltage controlled
phase shifter.
Phase shifting can be controlled either manually or by
voltage control.
Other parameters which can be controlled are reso-
nance (governing the depth of the comb filtering, and
tonal color - see page 3) and
original signal which is added to the phase-shifted
signal).
(the amount of the
mix
1
A-125
VC Phaser
System A - 100
doepfer
2. VC Phaser - Overview
A-125 VCP
VC PHASE SHIFTER
Audio In
0
0
CV
0
0
Audio Out
0
Level
10
Shift
10
10
Res.
10
Mix
10
CV
➀
➁
➂
➃
➄
Controls:
1 Level : Attenuator to control the level of the
signal at input
Shift : Control for manually setting the
2
!
amount of phase shift
: Attenuator for the phase shift voltage
CV
3
control signal at input
: Resonance control
Res.
4
Mix : Control for setting the amount of the
5
"
original signal added to the phaseshifted signal
In / Outputs:
Audio In : Audio input
!
" CV : Input for pitch-shift voltage control
Audio Out
§
: Audio output
2
doepfer
System A - 100
VC Phaser
A-125
3. Basic principles
The phasing process relies on dynamic comb filtering.
The comb filtering produces a series of gaps in the
audio spectrum (in Fig. 1, at 200 Hz, 1 kHz and 5
kHz), by the cancelling process which is created byhaving identical sounds 180° out of phase with each
other (or ‘inverted’).
10k
20k
ÖÖÖÖ
50
100
200
500
1k
2k
5k
Frequenz [Hz]
Fig. 1: The principle of phasing
These zero points are continuously swept through the
audio spectrum, cancelling out different frequencies,
and producing the characteristic phasing sound.
The principle can be explained by looking at the diagram (Fig. 2) of a phaser created by three band pass
filters. Here, audio is input to the three filters BP1 to
BP3 (notch filters also work), set to different middle
frequencies. A slow LFO modulates the frequencies.
The outputs of the band pass filters are then mixed
with the original signal. Because of the phase reversal
inherent in the filter design (most apparent close to the
middle frequency), different areas of the audio
spectrum are cancelled out.
Audio In
BP 1
f = x Hz
c
BP 2
f = y Hz
c
Mixer
Out
LFO
BP 3
f = z Hz
f ~ 3 Hz
: A phaser model using separate modules
Fig. 2
c
3
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