4ms Company Percussion Interface (PI) User manual

Percussion Interface (PI) & PI Expander
4ms Company
User Manual 1.0 – August 3, 2020
The Percussion Interface (PI) and PI Expander are a pair of Eurorack modules that generate gates
and envelopes from acoustic drums, samplers, drum machines and virtually any audio source.
Converting microphone, instrument, line and modular signals into CV and clock/gate signals, the PI and
PI Expander are suited for synchronizing sequencers or other modules to the rhythm of an audio track
or live percussion. The modules can simultaneously be used as a pre-amp for microphones or line-level
signals, while generating inverted envelopes useful for side-chaining. The PI module contains all the
core functionality and can be used without the Expander to conserve space. The Expander requires
the main PI module and adds three gain stage options, a modular-level audio output jack, a
1/4” (6.35mm) input jack, and envelope attenuation capabilities.
The PI Expander connects to the PI with an included ribbon cable and does not require a separate
power header.
The two modules are only available together as a pair.
Percussion Interface and PI Expander features:
• Sensitivity knob, Low/Medium/High gain switch, and clipping LED
• Sustain control to prevent misfires, increase gate length, and control the envelope
• Envelope decay control
• Non-inverted and inverted envelope outputs, useful for side chaining
• Four envelope outputs: two attenuated with independent attenuators, and two non-attenuated
• Envelope following or generation modes
• Gate output for clocking sequencers or any unit with a clock or trigger input
• Compatibility with drums, samplers, microphones, drum machines, etc.
• 1/4" input jack (6.35mm)
Env. Level
Inv. Level
Env. Out
Inv. Out
Audio Out
Input
PI EXPANDER
PERCU SS ION
INTERFACE
IN
+
–
Sensitivity
Sustain
Env. Decay
Follow Gen.
Gate
Env.
Out

Table of Contents
Setting up your Percussion Interface and PI Expander!3".........................................
Controls and Jacks: Percussion Interface!4".............................................................
Controls and Jacks: PI Expander!5"..........................................................................
Setting Input Gain!6"..................................................................................................
Using the PI with Various Sources!7".........................................................................
Envelopes in Gen Mode!8".........................................................................................
Envelopes in Follow Mode!9".....................................................................................
Velocity Sensing!10"...................................................................................................
Preventing Misfires and Undesired Gates!12"...........................................................
Calibrating Lockout Time for Gate Out!13"................................................................
Sidechaining with the PI!14"......................................................................................
Electrical and Mechanical Specifications!15"............................................................
My Patch Notes!16...................................................................................................
Page of2 16

Setting up your Percussion Interface and PI Expander
1. Power off your Eurorack system.
2. On the back of the Percussion Interface you will see two headers: a 10-pin header and an 8-pin
header. The 10-pin header connects to a Eurorack power header using the included power cable.
Connect the 16-pin end of the power cable to a 16-pin Eurorack power header on your power
supply distribution board and the 10-pin end to the PI with the red stripe on the power cable
oriented towards the bottom of the module.
3. On the back of the PI Expander you will see one 8-pin header. Using the provided 8-pin ribbon
cable, plug one end of the cable to the PI Expander and the other end to the 8-pin header on the
back of the Percussion Interface. The red stripe on the cable should be orientated towards the
bottom of both modules.
4. Using the included screws, securely attach the Percussion Interface and PI Expander to the rails
of your case.
5. Power on your Eurorack system.
Note: The Percussion Interface is reverse-polarity protected, but incorrectly connecting any module in
any system can damage other modules on the power bus.
Page of3 16

Controls and Jacks: Percussion Interface
IN Jack
The IN jack accepts a wide range of signals from microphones directed at acoustic
sounds such as drums, electronic drum machines, samplers, guitars, synthesizers,
line-level equipment, or any other audio source. With the exception of microphones
that require power (phantom-powered and electret microphones), any microphone or
piezo transducer will work. The IN jack is mono, but will work with a stereo or
balanced cable.
Sensitivity Knob and Light
The Sensitivity knob controls how much the input signal is boosted. At maximum
gain settings, the smallest input signal that will trigger a gate output is roughly 5–
10mV. The Sensitivity range is determined by the position of the L|M|H switch on
the PI Expander (see Setting Input Gain section on page 6 for more details). The
light will show blue when a signal is detected and will gradually turn red as the unit
begins to clip at the input. Clipping may result in undesired gates or envelopes, also
known as misfires. See Preventing Misfires and Undesired Gates on page 12.
Sustain Knob
The Sustain knob controls the pulse width of the gate output as well as the sustain
time of the envelope while in Gen. mode (see Envelopes in Gen Mode on page 8).
When the gate output is high no additional triggers will be generated, thus misfires
can be reduced. See Preventing Misfires and Undesired Gates on page 12. The
minimum pulse width is 5ms, and the maximum is 0.5s.
Envelope Decay Knob
The Env. Decay knob sets the decay time of the envelope. The envelope has an
exponential curve.
Follow/Gen. Switch
The Follow/Gen. switch selects between two envelope modes. In Gen. mode, the
envelope has a sharp attack, followed by a sustain period and an exponential decay.
The attack and sustain period are synchronized with the gate output, that is, the
envelope remains high as long as the gate is high. In Follow mode, the envelope
responds to the peaks and valleys of the input signal and ignores the gate. See
Envelopes in Gen. Mode on page 8 and Envelopes in Follow Mode on page 9.
Gate Out Jack and LED Indicator
The Gate jack outputs a pulse which is generated from the input signal. It can be
used to clock sequencers and trigger other modules. When high, the Gate jack
outputs 8V, indicated by the white light above the jack.
Envelope Output Jacks
The top left jack, marked by the + symbol,
outputs a 0V to 9V envelope when
triggered by the input signal. The blue light
above the jack indicates the envelope’s
level.
The jack at the bottom right, indicated by
the – symbol, outputs an inverted copy of
the envelope. When the envelope is at rest,
this jack outputs 9V and the green light
above the jack is fully bright. When the
envelope is triggered, it falls to a minimum
of 0V and the green light will dim. This
envelope is useful for side-chaining (see
Sidechaining with the PI on page 14).
Page of4 16
+
–
Gate
Env.
Out
IN
Gate
Env. Decay
Sustain
Follow Gen.
Sensitivity
+ Env (blue) rests at 0V and sustains at +9V.!
-Env (green) rests at +9V and sustains at 0V.

Controls and Jacks: PI Expander
Env. Level Knob
The Env. Level knob attenuates the signal of the Env Out jack on the PI Expander.
When the knob is turned all the way down, the jack will output 0V (no signal). As the
knob is turned up, the signal will increase in amplitude to a maximum of 9V.
Inv. Level Knob
The Inv. Level knob attenuates the inverted envelope signal of the Inv. Out jack.
When the knob is turned all the way down, the jack will output 0V (no signal). As the
knob is turned up, the signal will increase in amplitude to a maximum of 9V. For
example, if the knob is at 50%, the Inv. Out jack will output a steady 4.5V until an
envelope is triggered, at which point it will fall to 0V and then decay back to 4.5V.
Env. Out Jack
The Env. Out jack on the PI Expander outputs an attenuated envelope signal. This
jack functions like the + Env. Out jack located on the main Percussion Interface
module, except that it can be attenuated with the Env. Level knob.
Inv. Out Jack
The Inv. Out jack outputs an attenuated inverted envelope signal. This jack functions
like the - Env. Out jack located on the Percussion Interface module, except that it
can be attenuated with the Inv. Level knob.
Audio Out Jack
The Audio Out jack outputs an amplified version of the input signal. It can be used
as a pre-amp for microphones or other low-level signals to boost them to modular
level. The gain/boost amount is determined by the position of the Sensitivity Knob
on the Percussion Interface and the L|M|H switch on the PI Expander.
L|M|H Switch
The L|M|H Switch selects the gain range of the input signal. The L setting
corresponds to low gain, the Mto medium gain, and the Hsetting to high gain. See
Setting Input Gain section on page 6 for more information.
1/4" Input Jack (6.35mm)
The 1/4” Input jack allows for the use of a mono 1/4’’ (6.35mm) instrument cable.
The 1/4” Input jack can only be used when the IN jack on the main PI module is
unpatched; patching into the main IN jack will disable the 1/4” Input jack. This jack is
mono (TS), so only the left channel of a stereo TRS cable, or the + signal of a
balanced cable, will be used.
Page of5 16
Input
Env. Level
Inv. Level
Env. Out
Inv. Out
Audio Out

Setting Input Gain
The Percussion Interface boosts low level signals in order to generate gates and envelopes at
modular level. There are two controls for adjusting gain: the L|M|H switch sets the gain range, and the
Sensitivity pot adjusts within that range.
Setting the Gain Without a PI Expander
If you are using the Percussion Interface alone
(without the PI Expander) the gain range can be
set using a jumper on the back of the Percussion
Interface. Note that if the jumper is missing
completely, the default setting will be MEDIUM
GAIN.
Page of6 16
L|M|H switch position
Range
Use Cases
L (Low Gain)
0–2x
gain
Best for modular level and some professional line level
signals
M (Medium Gain)
0–20x
gain
Best for instrument, line, and some low impedance mic
signals
H (High Gain)
0–500x
gain
Best for high impedance mics, piezo discs (contact
mics), and other very low signals
LOW GAIN
MEDIUM GAIN
HIGH GAIN

Using the PI with Various Sources
The PI has three gain settings (LOW, MEDIUM, and HIGH) and a Sensitivity knob that can be used to
generate gates and envelopes from almost any sound source. The best setting for the L|M|H switch is
usually determined by the type of sound source, and the Sensitivity knob is usually adjusted until the
desired gate and envelope outputs are achieved. As always, if you’re not getting the results you wish,
experiment with other settings.
When using the PI to sense velocity (that is, in Follow mode), the widest range of dynamics can be
tracked by setting the gain to the lowest possible setting that still triggers with your softest input signal.
For example, hitting a drum very softly, with medium force, and then very hard should ideally produce a
small-, medium- and large-amplitude envelope, respectively. If the gain is turned up too high, the
medium and hard hits will produce similar envelopes, and thus some dynamic range is lost. However, at
higher gain settings, the PI can pick up very subtle variations of soft hits and quiet input signals. See
Velocity Sensing on page 10 for more details.
Piezo Discs / Contact Microphones
Piezo discs, often called contact microphones, are useful in isolating a drum from nearby drums or
sound sources. If using an acoustic mic generates false triggers from sounds in the vicinity of the drum,
a contact mic might be a better approach. The HIGH GAIN setting on the PI is typically the most useful
for contact mics. If you're experiencing noisy gates, try putting a soft material such as rubber or fabric
between the sound source and the contact mic. This may also help protect the piezo disc from being
damaged.
High-Impedance Microphones
Inexpensive microphones such as the types found in children’s toys and low cost karaoke systems are
typically high-impedance. These microphones are rarely used in a professional environment since they
are susceptible to external noise and audio degradation. If you find yourself using one of these
microphones, keep the cable as short as possible. The HIGH GAIN setting on the PI is typically the
best setting for high-impedance microphones.
Low-Impedance Microphones
Low-impedance microphones such as professional vocal or drum microphones typically require setting
your PI to MEDIUM GAIN. In some situations, LOW GAIN and HIGH GAIN settings will perform better,
so experiment to find the best setting.
Acoustic drums will often have a long resonance/decay period after being struck. This can cause the PI
to misfire by producing an unwanted gate. One way to prevent this is to dampen the drum head using
dampeners purchased from a music store, or a scrap of fabric taped to the drum head below the
microphone. If you want to prevent misfires without dampening your drums, refer to the Preventing
Misfires and Undesired Gates section on page 12.
Instrument Pickups
Pickups, such as the kind often found in electric guitars, can produce a wide range of signal levels
depending on the characteristics of the device. Setting the PI to MEDIUM GAIN is a good place to start
with any pickup. Some active pickups may perform better on LOW GAIN, so experiment to find the best
setting for your pickups and playing style. Since stringed instruments can be played percussively as
well as with continuous notes, adjusting the Decay knob and setting the PI to Follow mode can yield
interesting results.
Line Level Equipment (Keyboards, Drum Machines and Samplers)
Many line level instruments have a volume control. When using these types of signals with the PI, a
good practice is to center the volume control of the line level instrument and adjust the Sensitivity knob
on the PI until it triggers cleanly. Typically the PI works best with line level signals when set to either
LOW GAIN or MEDIUM GAIN. For more information on how to extract clean gates from drum machines
or samplers, refer to the Preventing Misfires and Undesired Gates section on page 12.
Modular Level Equipment
When using a modular level signal, set the PI to LOW GAIN. Modular synthesizers typically produce
signals which are very high in amplitude. For more detailed information on generating clean pulses
using an external sound source, see the Preventing Misfires and Undesired Gates section on page 12.
Page of7 16

Envelopes in Gen Mode
When the PI is in Gen. mode, it will generate a consistent envelope shape each time it’s triggered. In
this mode, the Gate will always fire at the same time as the envelope. When triggered, the envelope will
rise quickly with a very fast attack, and then sustain for a period of time set by the Sustain knob. The
sustain period is the same amount of time as the gate pulse width. After sustaining, the envelope will
decay back to 0V at a rate set by the Decay knob.
Figures 1 and 2illustrate this process. The input signal is
shown in red, the gate output in orange, and the envelope
output in blue.
In Figure 1, the Sustain pot is set to about 50% and the
envelope sustains as long as the gate is also high. In
Figure 2, the Sustain pot is turned fully counter-clockwise
and both the pulse width of the gate output and the sustain
period of the envelope become very short.
Gen. mode is useful for isolating a sound source. For
example, when using a microphone to sense the sound of
a snare drum, other parts of the drum set could be loud
enough to trigger the PI.In Gen. mode, it’s easy to adjust Sensitivity until unwanted
envelopes and gates are no longer generated.
Another use case for Gen. mode is to trigger a PI with one
part of a drum (say, the snare head), but not with another
part of the drum (say, the rim). By setting the L|M|H switch
and Sensitivity knob appropriately, and placing the
microphone in a location where it picks up the head louder
than the rim, you can isolate the two regions. This
technique can be extended to use two PI modules on a
single drum: the rim triggers one PI, and the head triggers
another PI.
If you don’t want the PI to trigger every time a drum is
struck, but only want it fire when the drum is hit with high
velocity, the Sensitivity knob can be set very low. Gates and envelopes will be generated
from loud signals while quiet signals will have no effect on the PI. This technique of setting
a high threshold is useful if you want to trigger events such as advancing a sequencer, or play periodic
sounds (such as a long sample) instead of tracking every drum hit. This technique can also be
extended when using multiple PI modules: each unit can be set to fire at a different velocity, thus
allowing control of complex sounds and events from a single drum.
Page of8 16
Figure 2
Figure 1

Envelopes in Follow Mode
Using the PI in Follow mode allows the envelope to directly
follow the amplitude variations of the input signal. The
velocity of the input signal can be imparted onto the output
envelopes. The envelope shape can be more complex than
the sustain-decay envelope of Gen. mode. In Follow mode,
the envelope and Gate outputs can fire at different times.
Figures 3 and 4 use a similar input signal as Figures 1 and
2in the previous section. Notice that the envelopes
(represented by the blue waveform on the bottom) are not
uniform. Also notice that the shape of the envelope
correlates more strongly to the contours of the input source
(represented by red waveform at the top) as opposed to
the gate (represented by the orange waveform in the
middle). The amplitude of the input source directly influences the envelope’s maximum
voltage, allowing for more organic envelope shapes and
introducing some variability into the CV output.
In Follow mode the Decay knob controls how quickly the
envelope decays when the input amplitude falls. The
envelope attack is always very rapid. Figure 5 illustrates
the envelope shape when Decay is set fully
counterclockwise. You’ll notice the envelope begins to
resemble the input source more closely. This can be used
as an audio output, as a frequency modulation source, or
in other creative ways.
In Follow mode, the PI envelopes are sensitive to the
velocity of the input source and therefore produce a greater
variety of envelope shapes. Because envelopes generated in Follow mode follow the
amplitude of the input signal, louder sounds result in higher
voltage envelopes, and quieter sounds result in lower
voltage envelopes. Figure 6 shows two waveforms: the
envelope output in blue, and the input signal in red. The
Decay knob is set very low in this example. The input
signal consists of a bass drum hit followed by a high-hat
hit. Notice the bass drum hit causes a relatively high
voltage envelope, while the high hat produces a lower
voltage envelope. See the Velocity Sensing section below
for more information.
Page of9 16
Figure 3
Figure 4
Figure 5
Figure 6

Velocity Sensing
The envelope outputs of the PI can respond to the velocity of the input signal. Velocity is a term usually
used for percussive sounds when referencing how hard an instrument is hit (harder means more
velocity, and softer means less). When using non-percussive audio sources such as vocals, keyboards,
etc, you can think of velocity as a combination of the loudness of the notes (or perhaps the “attack” of
the notes). When using sound sources that don’t produce distinct notes, you can think of velocity as a
measurement of how loud the signal gets whenever there’s a transition from quiet to loud.
When sensing velocity, the PI must be in Follow mode. The Env. Out and Inv. Out jacks will produce
envelopes with an amplitude that depends on the velocity of the input signal. Harder hits will produce
envelopes with higher peak voltages, and softer hits will produce lower voltages.
The following discussion provides an example of one way to setup a PI for velocity sensing, using an
acoustic drum and a microphone as the input source. These concepts can be applied to most other
audio sources.
Setting the Velocity Response
While velocity-sensitive envelopes are generated whenever the PI is in Follow mode, setting
Sensitivity properly is important so that the entire range of velocity (very soft to very hard) produces a
wide range of envelopes. If Sensitivity is set too high, then medium and hard hits will not be
distinguished. On the other hand, if the knob is set too low, very soft hits will barely be detected.
A good rule-of-thumb is that if the gate output is firing only at your hardest hits (and not at medium or
soft hits), then the Env. Out jacks will be outputting velocity-sensitivity envelopes with plenty of
headroom and enough gain to respond to the dynamic range of a normal playing style.
A basic procedure for finding a good velocity-sensitivity range is as follows:
1. Flip the switch to Follow.
2. Turn Sensitivity all the way up.
3. While hitting the drum with maximum velocity, turn
Sensitivity down until the gate no longer triggers.
4. Nudge Sensitivity up until the PI gate begins to
fire again.
At this setting, the Env. Out and Inv. Out jacks will
dynamically respond to the velocity of the input signal: soft
hits will produce low-voltage envelopes, and hard hits will
produce higher voltage envelopes. This outcome is shown
in Figure 7. The red signal is the input, the blue line is the
Env. Out signal, and the orange pulse is the Gate output.
If you find that the envelope output level is too high, use
the Level knobs and attenuated envelope output jacks on
the PI Expander. While the Sensitivity knob can also be used to attenuate the output
level, adjusting it changes the velocity sensitivity range as well as the threshold at which the Gate
output fires. The Level knobs, on the other hand, only controls the output level on the PI Expander and
should be the preferred method for attenuating envelopes.
Troubleshooting and Suggested Applications
If the Sensitivity knob (or L|M|H gain range switch) is set
too high, the input signal will clip internally, indicated by
the input light flashing pink or red. This causes the PI to
output an envelope around the maximum voltage
regardless of the input velocity, illustrated in Figure 8.
Though this may be desired in some applications, it
restricts the dynamic range of the envelope output.
Turning down the Sensitivity or flipping the L|M|H switch
to a lower gain setting will solve this issue.
Another issue to consider is the Decay setting. If the
Decay knob is set too high, the envelope will fall so slowly
that it’ll never reach a low value before it’s triggered again. See Figure 9 for an example.
Page of10 16
Figure 7
Figure 8

When adjusting the PI, consider what type of microphone
or sensor is being used, how it is mounted, and what sort
of object it’s sensing. For example, piezo discs and
contact mics output a wide range of signal levels
depending on how firmly they’re attached to the object
being struck. A metal piezo disc directly attached to a
metal object will produce will likely output maximum
voltage regardless of the velocity at which the object is
being struck. To mitigate this effect, you can place some
rubber or soft material between the contact mic and the
object. This will improve the dynamic range of the sensor.
On the other hand, a contact mic loosely set onto a soft
object may result in very little signal when the object is
struct. Attaching it more firmly or to a more firm section of the object can help in this
situation.
Using a standard drum mic attached to a drum is a good way of introducing dynamics to your setup.
Drum mics are durable and typically easy to attach. Clipping a microphone to the rim of a drum will
enable you to use the rim as a trigger source more accurately, while picking up less of the signal
coming from the drum head.
If you have two PI modules, an interesting application includes using two microphones on the same
drum, with each mic running into a separate PI module. For example, you could place one contact mic
on the rim and send it to one PI, then place another contact mic on the drum head and send it to a
second PI. This will allow you to use a single object as two separate voltage / gate sources.
Page of11 16
Figure 9

Preventing Misfires and Undesired Gates
When using the PI to generate gates from drum
machines with long decays or drums with lots of
resonance, you may experience “misfires”, or
undesired gates. This can usually be fixed by adjusting
the PI’s Sensitivity and Sustain knobs.
For this example, the input signal will be a techno track
that has a bass drum with about 150ms of decay. Our
goal is to have the PI generate a pulse whenever the
bass drum fires. In Figure 10, the green waveform on
top is the input signal, and the blue waveform on the
bottom is the Gate output of the PI. We see that three
gate pulses are generated when the drum fires, which
tells us the Sensitivity is set too high and the Sustain
is set too low. The second pulse is being generated by
the decay of the input source which the PI mistakenly
interprets as a second "hit”. The third pulse is being
generated because the gain setting is so high that the
slightest change in voltage is interpreted as another
“hit” by the PI.
Setting the Correct Gain
In Figure 11 we’ve reduced the gain by flipping the L|
M|H switch to L, removing one of the misfires. In order
to get rid of the second misfire, we need to adjust the
Sensitivity or Sustain.
Adjusting Sensitivity
In Figure 12, we’ve turned the Sensitivity knob down
until only one pulse is generated. This technique works
best when using samplers, drum machines or any
source with repetitive sounds and consistent
amplitudes.
Adjusting Sustain
With input sources of variable or inconsistent
amplitude, such as an acoustic drum or a live
recording, turning Sensitivity down might result in
missed triggers (producing less pulses than desired). In
these situations, a better technique is to adjust
Sustain.
When Sustain is turned up, the gate length will extend,
as seen in Figure 13. No additional gates will fire as
long as the gate is high. By setting Sustain high
enough that the gate is high for the entire bass drum
decay period, misfires will be prevented. The gate
length can be varied between 6ms and 560ms.
Properly adjusting Sustain reduces the likelihood that
changing Sensitivity will cause a misfire, allowing the
PI to work better with live drums, mic sources and
sounds with long decays. Keep in mind that if Sustain
is set too high, a rapid sequence of drum hits will miss
pulses, since the PI won’t re-trigger as long as the
Gate is high.
An advanced technique for preventing misfires is
discussed in the next section.
Page of12 16
Figure 10
Figure 11
Figure 12
Figure 13

Calibrating Lockout Time for Gate Out
On the back of the PI, there is a trim
pot labelled CAL LOCK-OUT. This trim
pot controls how quickly the PI is able
to re-trigger. It has been calibrated to
a period of 100ms, which is equivalent
to 10Hz, ten drum hits per second, or
600BPM. This is known as the “lock-
out period”.
Figure 14 shows a 10Hz square wave
in green being sent to the input of the
PI,and the Gate output of the PI in
blue. The Sustain knob is set to
minimum. Notice both signals are at
the same frequency of 10Hz.
In Figure 15 the frequency of the green
square wave has increased to around
12Hz. The PI Gate output in blue is
now half the frequency of the input.
This happens because the PI has been
calibrated to ignore frequencies higher
than 10Hz in order to prevent misfires
and undesired hits. The PI is still synchronized with the input
signal, but it is outputting half the tempo.
Depending on your preference, the lock-out period can be adjusted. For example, if you find the PI is
not responding to very rapid drum hits, you can turn the CAL LOCK-OUT trim pot counter-clockwise a
small amount. This will allow the PI to respond to faster drum hits. Keep in mind that this adjustment will
also increase the potential for extra undesired triggers on the gate output.
On the other hand, if you typically use an input signal with long
decays or less frequent hits, you can turn the trim pot clockwise
to increase the lock-out period and prevent undesired gates.
This is similar to adjusting the Sustain knob in order to clean up
drums with a long decay, but the trim pot doesn’t change the
pulse width of the Gate output. Setting the lock-out period with
the trim pot is useful in Gen.mode, as it enables you to adjust
the Sustain pot freely to control the envelope shape, without
worrying about undesired triggers.
The following procedure explains how to set a precise lock-out
period. You’ll need a frequency counter, oscilloscope, or some
way to detect frequency or fast BPMs. We’ll use five hits per
second, or 5Hz as our desired lock-out frequency.
1. Tune a square wave or clock signal to the desired frequency
and patch it into the PI’s input jack. In Figure 16 the input
signal is a 5Hz square wave in green.
2. Turn Sustain down fully, and adjust Sensitivity so that the
PI is firing pulses out the Gate jack.
3. Adjust the CAL LOCK-OUT trim pot until the PI gate output
is half the frequency of the input (2.5Hz). In Figure 16, the
blue signal is the Gate output.
4. Slowly turn the trim pot counter-clockwise until the
frequencies match, as shown in Figure 17.
At this point, the lock-out period will be 5Hz and no frequencies
faster than this will generate pulses. You can test to make sure
this was done correctly by increasing the frequency of the input signal and verifying that
the PI’s output frequency jumps down to half the input frequency.
Page of13 16
Figure 14
Figure 15
Figure 17
Figure 16
The trim pot is small
and delicate, so
adjust it gently using
the proper sized
screwdriver (2.0mm
flat-head or #00
Phillips screwdriver).

Sidechaining with the PI
The PI can be used for sidechaining and
ducking effects. Sidechaining is a technique
where one audio signal triggers an effect on
another audio signal. Ducking is a commonly
used form of sidechaining where the kick
drum of an audio track will cause the other
instruments to suddenly decrease in
loudness, or “duck”.
The following is a basic procedure for
creating a ducking effect:
1. Run an audio source such as an
oscillator or a musical track into the
audio input of a VCA.
2. Patch the Inv. Out from the PI Expander
to the CV input of the VCA.
3. Patch the kick drum signal into the IN on
the PI or the Input on the PI Expander.
4. Flip the switch to Gen. mode.
5. Adjust Sensitivity and the L|M|H switch
so that the white Gate light fires every
time the kick drum is hit. Adjust Sustain
as low as possible without causing mis-
fires.
6. Listen to the audio being “ducked” as
you adjust Inv. Level and Env. Decay to
set the amount of effect and “recovery”
speed.
When triggered, the Inv. Out signal will jump low, causing the VCA to drop the volume of the audio. As
the inverted envelope decays, it rises back to its resting voltage, and the audio source increases in
volume.
The Inv. Level knob controls the amount of decrease in the audio signal’s amplitude. Turning this knob
down might require you to turn the VCA’s offset or gain control up to compensate.
The Env. Decay knob controls the “recovery” time (the time it takes for the audio signal to return to
normal after a kick drum).
The Sustain knob can also help craft the sound by adding a delay after the kick drum and before the
envelope begins to rise back up to maximum voltage.
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PERCUSS I ON
INTERFACE
IN
+
–
Sensitivity
Sustain
Env. Decay
Follow Gen.
Gate
Env.
Out
Env. Level
Inv. Level
Env. Out
Inv. Out
Audio Out
Input
PI EXPANDER
KICK DRUM
To set the amount of
sidechain eect,
adjust Env. Decay
VCA
AUDIO IN
OSCILLATOR
CV IN

Electrical and Mechanical Specifications
• Percussion Interface
•4HP Eurorack format module
•0.98” (25mm) maximum depth (includes power cable)
•10-pin Eurorack power header
• PI Expander
•4HP Eurorack format module
•1.13” (28.7mm) maximum depth (includes expander cable)
•8-pin cable connects to PI
• Power consumption
•Maximum with PI Expander:
•+12V: 104mA
•-12V: 65mA
•Maximum without PI Expander:
•+12V: 69mA
•-12V: 56mA
•Audio IN jacks (1/8” [3.5mm] on PI main module, and 1/4” [6.35mm] on PI Expander)
•100k input impedance, AC-coupled
• Audio Out jack
•Voltage range: -10.4V to +10.8V, DC-coupled
•1k output impedance
• Gain characteristics
•Gain range: < -100dB to +54dB (500x)
•Clip light turns blue when signal (post-gain) exceeds 2.5Vpp
•Gate is triggered when signal (post-gain) exceeds 6.3Vpp
•Clip light flashes red when signal clips on Audio Out jack (exceeds +10.8V)
• Gate Out jack
•1k output impedance
•+8V gate/trigger signal
•6ms – 0.5s pulse width
•+ Env Out jack (jack on PI main module)
•1k output impedance
•+9V peak (sustain), -60mV resting (off)
•15ms attack time
•70ms – 6s exponential decay time (1% of peak voltage)
•– Env Out jack (jack on PI main module)
•1k output impedance
•+9V resting (off), -150mV peak (sustain)
•15ms attack time
•70ms – 6s exponential decay time (1% of peak voltage)
•Env. Out jack (attenuated jack on PI Expander)
•1k to 26k variable output impedance
•+9V maximum
•-60mV minimum
•Inv. Out jack (attenuated jack on PI Expander)
•1k to 26k variable output impedance
•+9V maximum
•-150mV minimum
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My Patch Notes
Page of16 16
Env. Level
Inv. Level
Env. Out
Inv. Out
Audio Out
Input
PI EXPANDER
PERCUSS I O N
INTERFACE
IN
+
–
Sensitivity
Sustain
Env. Decay
Follow Gen.
Gate
Env.
Out