MUTU – MUTUAL VCO

A synthesizer with two oscillators in a mutual relationship with each other.

Goals

• flexible sound-generator
• simple and DIY-friendly
• eurorack-compatible (CV inputs for FM and VCA, size fits eurorack)

Non-Goals

• No perfect tonal tracking intended
• No features that raise the parts count too much

Topology

The Mutu consists of two nearly identical voices. Each of the voices has a VCO (Voltage Controlled Oscillator) and a VCA (Voltage Controlled Amplifier). You can use both voices summed (mixed) together by plugging a cable into the OUT B only OR you can use the voices separately by plugging a cable into both OUT A and OUT B.

VCOs

The Base Waveform

The VCOs of mutu have an oscillator core that can be morphed between triangle and sawtooth waveforms by turning the SHAPE knobs. For technical reasons the pitch of the oscillator will go slightly up when turning the knob towards the sawtooth waveform. The triangle wave has less overtones and is good for a low deep bass or organ-like tones. The saw wave has more overtones and is good to produce harsher more energetic sounds.

Frequency: The Right Vibrations

The pitch/frequency of the oscillator can be changed and modulated using three different ways (potentially all at once).

First and most simple by turning the FREQUENCY knob. Second way is to let the waveform of the other oscillator influence the waveform of the this one, by turning up the MODULATION knob. This is where the name of the the synth comes from: the structure of mutual influence between the two VCOs. The third way to modulate the frequency of an VCO is by using an external control signal on the FM CV INPUT. The amount of this external control signal can be adjusted using the FM CV ATTENUATOR.

Having both oscillators influence each others is a way to create more noisy sounds. Frequency modulation at high rates will also bend the waveform in weird and complex ways.

The signal present at the external CV input of VCO A is normalized ("copied") to the external CV input of VCO B if no cable is plugged there. This allows you to use one signal to modulate both VCOs in different amounts without using a Y-cable or a stack-cable.

Tracking: Controlling both VCOs at once

You can make VCO B follow the frequency of VCO B by turning on the Track switch. If this option is active. The external FM signal and frequency knob of VCO A will be added to whatever is set up at VCO B. The tracking is not perfect, but hey.

Syncing

VCO B can be hard-synced to VCO A by turning on the SYNCswitch. What is a hard sync? Wikipedia says:

As one oscillator finishes a cycle, it resets the period of another oscillator, forcing the latter to have the same base frequency. This can produce a harmonically rich sound, the timbre of which can be altered by varying the synced oscillator's frequency.

This means every time VCO A finishes it's cycle it restarts VCO B which adds another, very different sounding mode to the synth.

VCAs

The Voltage Controlled Amplifers in the VCA section allow you to change the output volume of each VCO. This section can be used in two ways:

1. Just turn the LEVEL knob of each voice and dial in the level you want
2. Plug an external control voltage into the VCA CV INPUT and the LEVEL knob becomes an attenuator for the incoming signal

A small LED will display the current level of each voice. Internally the VCA uses an electronic element called vactrol, these elements react well to short pulses of sound.

Interface

Panel draft with 16 HP (~80 mm wide, 119 mm tall)

Inputs

Name	Function	Normalization
CV-VCA-A	CV-Input for the first VCA. Positive voltages turn up the level of the first oscillator	Fixed positive voltage
CV-VCA-B	CV-Input for the second VCA. Positive voltages turn up the level of the second oscillator	Fixed positive voltage
CV-FM-A	Frequency modulation (FM) input for the first oscillator	-
CV-FM-B	Frequency modulation (FM) input for the second oscillator	Signal plugged to the first FM-Input (CV-FM-A)

Knobs

Name	Function
FREQ-A	Sets the base frequency (pitch) of the first oscillator
FREQ-B	Sets the base frequency (pitch) of the first oscillator. If Track is on, this gets added to the frequency set by FREQ-A
MOD-A	Changes the amount of frequency modulation (FM) the second oscillator applies to the first
MOD-B	Changes the amount of frequency modulation (FM) the first oscillator applies to the second
ATT-FM-A	Attenuates any incoming (external) modulation signal plugged into the input CV-FM-A
ATT-FM-B	Attenuates any incoming (external) modulation signal plugged into the input CV-FM-B. If no signal is plugged into CV-FM-B the signal of CV-FM-A is used here.
SHAPE-A	Morphs the waveform of the first oscillator between triangle and sawtooth. Might also detune the oscillator
SHAPE-B	Morphs the waveform of the second oscillator between triangle and sawtooth. Might also detune the oscillator
LVL-A	Sets the level of the first oscillator. If something is plugged into the CV-input CV-LVL-A this knob attenuates the level of the incoming CV-signal.
LVL-B	Sets the level of the second oscillator. If something is plugged into the CV-input CV-LVL-B this knob attenuates the level of the incoming signal.

Switches/Toggles

Name	Function
S_TRACK	Switches on or off the tracking function where the second oscillator follows the frequency changes of the first.
S_SYNC	Hardsyncs the second oscillator to the first one

Outputs

Name	Function
OUTPUT	Outputs the summed signal of both oscillators