Introduction
The Erthenvar Patch Chord is a unique processing device for Eurorack modular systems. The
functionality is best described as somewhere between an oscillator and a waveshaper. It can be
used to create chords or pseudo-polyphony in a modular synthesizer.
The Patch Chord continuously tracks the duration and duty cycle of an input waveform. From
these values, a total of 25 independent pulse oscillators with continuous pulse width and
frequency values are derived. The first oscillator has the same frequency as the input signal, 12
have frequencies above the input signal, and 12 have frequencies below the input signal.
In the default mode, all outputs have a semitone relationship. For example, if the input
frequency was 440 Hz, the output immediately above the unison output would have a frequency
one semitone up (466.16 Hz) and the output immediately below the unison output would have
a frequency one semitone down (415.30 Hz). Each following output has a semitone relationship
to the adjacent output until a full octave up (880 Hz) and full octave down (220 Hz) are reached.
The duty cycle of all of the outputs is the same as the input waveform.
Modulations
There are four modulations in the Patch Chord, each with a mode switch and ACV (audio-rate
control voltage) input. The mode switch has three modes: OFF, CV, and ON. The ACV jack is
normalled to GND, so the mode switch is “off” when the switch is set to CV and there is no cable
plugged in. All inputs have a comparator front-end with a threshold around 600 mV, so almost
any signal can toggle the input.
The modulation types are: SQR, INT, ZERO, and HOLD.
The SQR modulation forces the input to be tracked as if it had a perfect 50/50 duty cycle.
This means all of the outputs will also have a 50% duty cycle. Square waves can be used to
generate sounds reminiscent of vintage video games or certain genres of electronic music.
The INT modulation changes the relationship of all of the outputs from semitone to integer
or harmonic relationships. In above example where a frequency of 440 Hz was tracked,
the outputs adjacent to the unison output would also have a frequency of 440 Hz, since the
first harmonic is actually the fundamental. The next output in the up direction would have a
frequency of 880 Hz, followed by 1,760 Hz, all the way up to 5,280 Hz (440*12). The next output
in the down direction would have a frequency of 220 Hz, followed by 110 Hz, all the way down
to 36.67 Hz (440/12). The harmonic relationships offer a different palette compared to semitone
mode and can be used to create both musical and dissonant sounds.
The ZERO modulation resets all of the oscillators for as long as the switch or ACV signal
enables the ZERO function. The functionality is somewhat similar to sync on a traditional
oscillator. The input responds well to ACV along with varying pulse widths. It can be used to
Patch Chord | 2012.04.13 | p.2
