DOEPFER A-191 User manual

doepfer System A - 100 MCV 16 A-191
1
H Connecting the A-191 to the system bus
requires an additional special power sup-
ply (+5 V / 50 mA).
1. Introduction
Module A-191 is a combination of a MIDI-CV inter-
face and a Shepard Generator. Most of the controls,
indicators and in/outputs have a double function; there
is a switch which toggles between the two sets of
functions.
In use as a MIDI-CV interface, 13 of the 16 control
voltage outputs are dedicated to sending voltages
converted from a particular MIDI controller (such as
Mod Wheel, Volume, Pitch Wheel, Aftertouch, etc.).
Controller messages on your chosen MIDI channel
are converted into voltages in a range from 0 to 5 V.
The other three sockets output an internal LFO gene-
rated in sync with MIDI clock, in three waveforms:
sawtooth, triangle and rectangle.
In use as a Shepard Generator, the sockets output
eight different phases each of a triangle and sa-
wtooth wave. Using a variety of modules, the Shepard
Generator can create some interesting psycho-
acoustic effects.
All A-191 settings can be saved into non-volatile
memory.
MIDI
In
Frequ. +
MCV16
S0/
LFO
S90/
LFO
Frequ. –
Mode
MIDI /
SG
Reset/
Store
T0/
LFO
T315/
C16

A-191 MCV 16 System A - 100 doepfer
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2. MCV16 - Overview Controls and indicators
1LEDs : Indicators for triangle and/or sawtooth
waveform frequency
2Freq. + : Button to increase LFO frequency
3Freq. – : Button to decrease LFO frequency
4Mode : Shepard function switch
5, 6LEDs : Shepard function indicators
7Mode : Switch to select either Shepard Gene-
rator or MIDI interface
8LED : Indicator to show which out of the
Shepard Generator or MIDI interface
is selected
9Reset/Store : Button for saving set-ups or resetting
Shepard Function Generator
In- / Outputs
!sockets : CV outputs from the Shepard Genera-
tor and MIDI-CV interface (see text)
"MIDI In : MIDI input socket
MCV16
MIDI
In
S0 /LFO
MIDI-CV / SHEPARD
➀

➁
Freq. +
➂
➃
➄
➅
➇

T0 /LFO
S90 /LFO
T90 /AT
S180 /Pit
T180 /C1
S270 /C2
T270 /C4
S45 /C5
T45 /C6
S135 /C7
T135 / C8
S225 / C10
T225 / C11
S315 /C12
T315 /C16
Freq. –
Control
Control
Mode
MCV
Shepard
Fast
Slow
–
Learn
Reset/
Store
➈
➆

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3. Switching between the functions
Because the A-191 has two separate functions in it,
which share some of the controls and outputs, at any
one time it can be used either as a MIDI-CV interface
or as a Shepard Generator. To switch between the two
you simply press a button (see below).
H The A-191’s controls and in / outputs for
each function are explained in their
respective sections of this manual.
7 Mode button
To switch between functions, press the lowest mode
button 7
77
7.
8 LED
LED 8lights when the Shepard Generator is active;
otherwise it’s the MIDI-CV interface which is active.
H If the A-191 receives MIDI clock, LED 8
flashes (see chapter. 4.2, "MIDI In).
4. MIDI-CV interface
4.1 Basics
In its MIDI-CV mode, the A-191 provides a MIDI-CV
converter, and a MIDI-synchronized LFO.
Whenever the A-191 receives relevant MIDI control-
ler messages on the channel you’ve selected, it con-
verts them into control voltages (in a 0 V to +5 V
range), and sends these out on the corresponding CV
outputs. Table 1 on page 4 lists the MIDI controllers,
their corresponding CV output sockets, and their de-
fault values.
H After switching your A-100 system on, pres-
sing the Store/Reset button 9sets the
voltages at the CV outputs to the default
values (see table 1).
If you’re running your A-100 from a MIDI
sequencer, you’ll need to save the appro-
priate MIDI controller messages at the begin-
ning of a Song, to initialise the A-191’s CV
outputs correctly.

A-191 MCV 16 System A - 100 doepfer
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Table 1: Valid MIDI controllers, their corresponding CV
outputs, and default values.
The A-191’s internal MIDI-synchronised LFO has three
waveforms - sawtooth, triangle, and rectangle.
This LFO is only active when a MIDI clock is being
sent to the MIDI IN, from a START or CONTINUE
message until a STOP message. LED 8 indicates
that it’s active.
At a STOP message, the voltage at the CV output is
held; at a CONTINUE message, the LFO starts from
this value; and at a START message, the waveform
begins again, starting from 0 V.
The LFO rate can be slowed down by dividing the
clock rate. You set the divisor either with MIDI
controller #92 (Tremolo Depth) or manually, with
buttons 2and/or 3.
The note length N of a full cycle of the waveform
generated by the LFO follows this formula :
Nth Note = 1/16 x controller value (see Table 2)
So, for instance, in 4/4 time, with a controller value of
8, the note length of a full LFO cycle would be half of a
whole note: that is, the LFO would cycle twice for each
beat in the bar (see Fig. 1).
Output corresponding MIDI controller default [V]
AT After Touch 0
Pit Pitch Wheel 2.5
C1 CTRL. #01 - Modulation Wheel 0
C2 CTRL. #02 - Breath Controller 0
C4 CTRL. #04 - Foot Controller 0
C5 CTRL. #05 - Portamento 0
C6 CTRL. #06 - Data Slider 0
C7 CTRL. #07 - Volume 5
C8 CTRL. #08 - Balance 2.5
C10 CTRL. #10 - Pan 2.5
C11 CTRL. #11 - Expression 0
C12 CTRL. #12 0
C16 CTRL. #16 0

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Fig. 1: The sawtooth (1/2 note cycle) generated by
a MIDI controller value of 8.
Table 2: Typical values for controller #92 and the
resultant length of the LFO cycle.
H Because the A-191’s LFO is purely a soft-
ware device, the following considerations
apply, which it’s well worth remembering.
The A-191 uses an 8-bit D/A converter, whose maxi-
mum resolution is 256 steps per 0.02 V. Conse-
quently, the sawtooth and triangle waves can’t be as
smooth as, for instance, those of the A-145 LFO. The
waveforms on the A-191 LFO are digital ("stepped").
If you use it to control, eg, a VCO, the result is less of
a smooth continuous pitch sweep, and much more
akin to a glissando. How audible these steps are
depends on the MIDI-Clock tempo, and the divisor
you’ve chosen.
P If the steps are audible (and unwanted), you
can use a Slew Limiter (A-171) to smooth
out the waveforms.
Erratic or sudden changes of tempo or the divisor
will take a whole note to register, before the LFO
changes to this new frequency.
Controller value LFO cycle
0LFOoff
11/16
2 2/16 = 1/8
33/16
4 4/16 = 1/4
8 8/16 = 1/2
16 16/16 = 1 whole note
32 32/16 = 2 whole notes
64 64/16 = 4 whole notes
4
4

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4.2 Indicators and controls
1 LEDs
The LEDs 1indicate the frequency of the internal
LFOs. They indicate the state of the sawtooth signal
at output S0 and/or the triangle wave at output T0 .
2 Freq. + • 3Freq. –
Buttons 2and 3have a double function in the MIDI-
CV interface:
• Setting LFO frequency
Pressing buttons 2or 3increases and/or de-
creases the frequency of the internal LFOs.
While buttons 2or 3are pressed, LEDs 5and 6
blink to show the increase or decrease in the LFO
frequency. If you reach the upper or lower limit of
the frequency, the LED stops blinking.
HIt’s easier and definitely more precise to
alter the LFO frequency by MIDI controller
#92.
Setting the MIDI input channel
If you simultaneously hold down buttons 2and 3,
"learning mode" is activated. This is signalled by
LEDs 5, 6and 8 all flashing at once.
You can now set the MIDI channel you want the
MCV16 to respond to. To do this, send a valid MIDI
controller (see table 1) to the A-191. This automati-
cally ends learning mode, the LEDs go out, and the
MIDI channel of whatever controller was used beco-
mes the input channel for all the MIDI data sent after
this.
LFO frequency and MIDI- input channel are only tem-
porarily stored, unless you save them by pressing
button 9(see below).
H The labels on LEDs 5and 6("Fast", "Slow",
"--") don’t have a function yet in MIDI-CV
mode, but are hoped to have in a future
update.
9 Reset / Store
To save the settings for MIDI input channel and LFO
frequency press button 9.
LEDs 5, 6and 8 light for about a second to confirm
the save procedure has succeeded. At the same time,
all CV outputs are set to their default settings (see
table 1).

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4.3 In- / Outputs
! CV outputs
Sockets !are the MIDI-CV interface’s CV outputs:
LFO • LFO • LFO
These sockets are the outputs for the sawtooth,
triangle and rectangle waves produced by the inter-
nal LFO. LEDs 1and 2give an indication of the
voltages at the sawtooth and triangle wave outputs.
H Don’t forget that the LFO will only work if a
MIDI clock is being received on the selected
MIDI channel (see chapter 4.1, Basics).
AT • ... • C16
These 13 sockets output the control voltages conver-
ted from their respective MIDI controllers (see table
1). Their voltage range is from 0 V to +5 V.
" MIDI In
This MIDI input socket should be connected to the
MIDI keyboard or sequencer, etc., that you want to
control the System A-100, with a standard MIDI lead.
As well as converting MIDI controllers After Touch,
Pitch Wheel, #01, #02, #04 to #08, #10, #11, #12 and
#16, and turning them into voltages available at the
corresponding CV outputs (see table 1), the A-191
also responds to MIDI clock - and to MIDI controller
#92 ("Tremolo Depth") for controlling the internal
LFOs.

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4.4 User examples
Modulation-rich synthesizer patch
The example in Fig. 2 shows a ‘classic’ synthesizer
patch: 2 VCOs, VCF (A-122) and VCA (A-130). Modu-
les A-190 and A-191 act as the link to a MIDI key-
board, and are set to the same MIDI channel. The
THRU from the A-190 is connected to the MIDI IN of
the A-191.
This patch gives a vast range of modulation possi-
bilities accessible from velocity, mod wheel, pedals,
and aftertouch:
• VCO 2 level
VCO 2’s amplitude level (ie. volume) can be con-
trolled with a pedal. In this example, the A-190’s
second CV output (CV 2) has been previously
programmed to respond to MIDI controller #04, but
CV output C4 on the A-191 could have equally
well been used.
• Modulation of VCF resonance
Filter resonance is modulated by a random vol-
tage source (the A-118’s random output). The
intensity of this modulation is controlled by the
modulation wheel (output C1 on the A-191).
• Amplitude modulation
The intensity of the amplitude modulation created
by the LFO patched into the VCA (A-130) is control-
led by aftertouch (output AT on the A-191).
• Overall volume
The overall volume of the output signal sent to the
monitoring system is controlled by MIDI controller #7
(Volume) (output C7 on the A-191). This controller can
be assigned to a pedal, pitch ribbon, etc. on your
master keyboard.
H If you want to use a MIDI controller which the
A-191 doesn’t support - for instance a
sustain pedal - then you can always set the
A-190 to respond to it, and output it from its
CV2 socket.
MIDI-synchronised LFO
The internal LFO on the A-191 is synchronised to MIDI
clock. This is particularly useful when you’re using a
MIDI sequencer or arpeggiator with the A-100.
An example is shown in Fig. 3. In this patch, the
internal LFO modulates the VCF’s cut-off frequency.

doepfer System A - 100 MCV 16 A-191
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Fig. 2: Classic synthesizer patch with multiple modula-
tion possibilities.
VCO 1
A-132
VCO 2
A-138a
CV 1
MIDI In
MIDI
Out
A-190
CV 2
A-122
A-130
ADSR 1
A-131
ADSR 2
A-118
A-132
Gate
MIDI Thru
A-191
LFO
MIDI In
A-132
QCV FCV
C 1
AT
C 7
Foot
Mod. Wheel
After Toucn
Volume

A-191 MCV 16 System A - 100 doepfer
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Fig. 3: VCF cutoff control with a MIDI-synced LFO.
Set the LFO frequency so that it completes one cycle
every whole note.
During the first four 1/8th notes of the sequence, the
filter is opening; and in the last four 1/8th notes, it’s
closing.
Another example of MIDI-synchronised use of the
internal LFO is shown in Fig. 5. Two A-125 phasers
set to opposite phase are sent to two audio outputs
(OutL and OutR) to produce MIDI-synchronised spa-
tial effects.
CV 1
MIDI In
MIDI
Out
A-190
VCF
VCA
ADSR
Gate
MIDI Thru
A-191
MIDI In
LFO
MIDI -
Sequencer
VCO

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Fig. 4: MIDI-synchronised "stereo" phasing.
CV 1
MIDI In
MIDI
Out
A-190
Gate
MIDI Thru
MIDI -
Sequencer
A-191
MIDI In
LFO
A-175
Out
L
Out
R
VCP 1
CV
VCP 2
CV
VCF
VCA
ADSR
VCO

A-191 MCV 16 System A - 100 doepfer
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5. Shepard generator
5.1 Basics
A Shepard generator is a modulation oscillator,
which produces various phase inversions of triangle
and sawtooth waveforms at its outputs.
The way the Shepard generator is set up produces
various psycho-acoustic phenomena, such as the
‘barber-pole’ effect - seemingly never-ending upward
or downward spirals of pitch, filter settings, phasing,
and stereo or quadraphonic sound-stage panning (see
chapter 5.3, User examples).
The triangle wave signal is sent via outputs T 0 to T
315, and the sawtooth wave signal is sent via out-
puts S 0 to S 315, (where the numbers refer to the
phase displacement for each output in degrees).
Fig. 5 shows this, but with only every other output
included, for the sake of clarity. TLFO is the time the
internal LFOs take to complete one cycle.
The Shepard mode - the direction of the sawtooth
waveform generated - is selectable. Fig. 5 shows the
mode with a sawtooth waveform. Fig. 5: Shepard generator output signals (half shown)
T
S 0
S 90
S 180
S 270
T 0
T 90
T 180
T 270
LFO
T
LFO

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5.2 Indicators and controls
1 LEDs
The LEDs 1give you a read-out of the frequency of
the Shepard generators. They show the state of the
sawtooth wave at output S0 and/or triangle wave at
output T0.
2 Freq. + • 3Freq. –
Pressing buttons 2or 3raises and/or lowers the
frequency of the Shepard generator. These settings
are only temporary: if you want to save them, press
button 9(see below).
4 Upper mode button
Button 4selects the Shepard mode, which determi-
nes the direction of the sawtooth signal generated by
the Shepard generator (see table 3). The mode cho-
sen is indicated by LEDs 5and 6.
These settings are only temporary: if you want to save
them, press button 9(see below).
5 LED • 6LED
LEDs 5and 6show the Shepard mode selected
(see table 3).
Table 3: Shepard modes and LED indicators.
9 Reset / Store
Pressing button 9saves the settings for frequency
and Shepard mode. Confirmation of the save proce-
dure is given by LEDs 5, 6and 8lighting up for about
a second.
The instant the Shepard generator is reset, the sa-
wtooth and triangle waves at outputs S0 and/or T0
start at their zero point.
Mode Effect LED 5LED 6
8 rising sawtooth waves on off
8 falling sawtooth waves off on
4 rising sawtooth waves at outputs
S0, S90, S180, S270;
4 falling sawtooth waves at out-
puts S45, S135, S225, S315
on on

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5.2 In- / Outputs
! CV outputs
The CV output sockets !send out sawtooth and
triangle waveforms. The number on each output
refers to the amount of phase displacement, in de-
grees:
S0 • S45 • S90 • S135 • S180 • S225 • S270 • S315
Sockets S0 to S315 output the sawtooth waveforms.
T0 • T45 • T90 • T135 • T180 • T225 • T270 • T315
Sockets T0 to T315 output triangle waveforms.
5.3 User examples
Generating "Shepard Tones"
The Shepard Tone is a psycho-acoustic phenome-
non, that gives the impression of a continuously rising
or falling tone.
Fig. 6 shows the sort of patch needed. The Shepard
generator’s sawtooth waveform controls the pitch of
eight VCOs, all with identical settings, while the tri-
angle outputs control 8 VCAs - one for each VCO. To
hear the effect properly, the outputs from the VCAs
must be patched to two mixers (2 x A-138).
The Shepard effect is pretty mind-blowing, because it
seems to be producing the impossible - a never-
ending upward (or downward) sweep of the note.
Although it seems almost miraculous, there’s nothing
mysterious about how it works. The Shepard tone
contains a large amount of octave-related harmonics
across the whole audio spectrum, all of which rise (or
fall) together. The harmonics towards the low and high
ends of the spectrum are gradually attenuated the
closer they get to the ends, while those in the middle
have maximum amplification.

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Fig. 7: Shepard tone patch
To help understand this weird science, just picture the
lowest harmonic of the tone (with the Shepard genera-
tor in its "Up" mode).
It starts inaudible, but as its pitch goes up, so does its
volume. By the time it gets to the middle of the audio
spectrum it is at its maximum amplitude, and will then
gradually get quieter again, until the highest frequency
is reached, and it’s inaudible again.
Meanwhile, other harmonics have been replacing it,
starting very quietly at the lowest frequency.
All eight of the VCOs in the Shepard tone produce
these ‘layered’ harmonics, re-starting at the bottom
once they’ve reached the highest frequency. The
result is, to our ears, a tone which is rising all the time,
but never gets any higher! The same applies in the
“down” mode, with the tone this time seeming to fall
continuously.
Shepard control of signal processing modules
The Shepard generator is useful not just for these
constantly rising or falling tones, but also to control
modules which are signal modifiers.
Fig. 8 shows the standard patch - this time, instead of
VCOs, using signal processing units, like phasers or
filters, and patching an audio signal into them to be
processed. Phasers, in particular, produce a wonder-
fully unique effect, far richer and more animated than a
standard phaser.
VCO 1 VCA 1
Out
1
S 0
T 0
VCO 2 VCA 2
S 45
T 45
VCO 8 VCA 8
S 315
T 315
A-191
Out
2
Out
8

A-191 MCV 16 System A - 100 doepfer
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Alright, we have to admit that to use the Shepard ge-
nerator to maximum effect does take a huge number
of modules. There aren’t going to be that many Sy-
stem A-100 owners (are there?) who have eight VCOs
or VCFs or VCPs and eight VCAs to use to produce a
perfect Shepard tone.
What’s much more important is to use the modules
you do have creatively.
Try all sorts of unlikely combinations of modules and
control voltages. For instance, two VCAs can produce
a very nice stereo panning effect, if you use two tri-
angle wave outputs which are 180° out of phase with
each other.
With four triangle waves which are 90° out of phase
with each other, and four VCAs, whose outputs are
fed to a quadraphonic sound system, you can produce
an interesting rotating effect (see Fig. 9).
It’s worth repeating: this is one of the modules where
experimentation is even more crucial than on some
others.
For instance: in the patch shown in Fig. 9, try using
triangle waves with different, unbalanced phase relati-
onships, for strange spatial lurches; or phasers for 3D
phasing ...... anything is worth trying.
Fig. 8: Patch for Shepard control of signal processing
VCA 1
Out
1
A-191
SPU 1
VCA 2SPU 2
VCA 8SPU 8
Audio In
S 0
T 0
S 45
T 45
S 315
T 315
Out
1
Out
1

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Fig. 9: Patch for quadraphonic spatial effects
Quad Space
VCA 1
T 0
VCA 2
T 90
VCA 4
T 270
A-191
VCA 3
T 180
Audio In

A-191 MCV 16 System A - 100 doepfer
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6. Patch-Sheet
The following diagrams of the module can help
you recall your own Patches. They’re designed so
that a complete 19” rack of modules will fit onto an
A4 sheet of paper.
Photocopy this page, and cut out the pictures of
this and your other modules. You can then stick
them onto another piece of paper, and create a
diagram of your own system.
Make multiple copies of your composite diagram,
and use them for remembering good patches and
set-ups.
P • Draw in patchleads with colored
pens.
MCV16
MIDI
In
S0 /LFO
MIDI-CV / SHEPARD
Freq. +
T0 /LFO
S90 /LFO
T90 /AT
S180 /Pit
T180 /C1
S270 /C2
T270 /C4
S45 /C5
T45 /C6
S135 /C7
T135 / C8
S225 / C10
T225 / C11
S315 /C12
T315 /C16
Freq. –
Control
Control
Mode
MCV
Shepard
Fast
Slow
–
Learn
Reset/
Store
MCV16
MIDI
In
S0 /LFO
MIDI-CV / SHEPARD
Freq. +
T0 /LFO
S90 /LFO
T90 /AT
S180 /Pit
T180 /C1
S270 /C2
T270 /C4
S45 /C5
T45 /C6
S135 /C7
T135 / C8
S225 / C10
T225 / C11
S315 /C12
T315 /C16
Freq. –
Control
Control
Mode
MCV
Shepard
Fast
Slow
–
Learn
Reset/
Store