A2 Eurorack Vocabulary

VCO/LFO - Voltage controlled oscillator

    Most analog or digital oscillators will have an input for 1v/oct or Pitch, PWM or pulse width modulation and FM or frequency modulation. Some with have inputs for gate to trigger or strike an internal envelope. Most Analog oscillator will have individual waveform outs. The most common are sine, triangle, saw/sawtooth/inverse sawtooth, and square or pulse wave output, with control over the pulse width. Some oscillators will have a shape output and a knob and cv control over the shape. Turning the knob will morph between the different waveforms allowing new waveforms to emerge in between. VCOs operate at audio rate, meaning they are in the frequency range of human hearing. LFOs operate below audio rate, and are often used as control voltage for controlling parameters of other modules. LFOs will have all the same ins and outs, the frequency range is much slower.


    Alternatives to a VCO:

Filters

            Some filters can self oscillate when the Q or resonance it raised up. At this point the filter is feeding back on itself and this signal can be used as an oscillator. The waveform of a self oscillating filter is similar to a sine or triangle wave. The cutoff frequency now acts as the frequency or pitch of the oscillator. If the input for the cutoff frequency doesn’t specify 1v/oct it will most often not track 1v/oct pitch.

VCA

    A VCA is a voltage controlled amplifier or voltage controlled attenuator. A simple VCA consists of an input and an output, a knob to control the volume of the input that you want to let through to the output, and a cv input to control the volume knob. When cv is patched into the cv input, the knob acts as an offset to that CV signal. When the knob is turned all the way up or clockwise, all the of cv signal is let thru to control that knob. If the knob is half way turned up, it will only let the cv control the volume up until that point. If the knob is turned all the way down, the CV is not let in to control the knob, and the volume will no longer change based on the CV input.

CV - Control Voltage

    Control voltage can take many forms, and many sources can be used as control voltage. Control voltage refers to any voltage or signal you are using to control the parameter of another module. The more cv inputs a module has, the more options you have to control various parameters of the module. Say you have an oscillator going into your mixer and it’s always loud all the time, but you want the volume to go up and down slowly instead. You could turn the volume knob up and down slowly to get that effect or you could patch your oscillator into a VCA and send a sine wave LFO into the CV control over the amplitude of the oscillator. The volume of the oscillator will now slowly rise and fall following the shape and frequency of the LFO. Really anything can be used as CV so experiment.

1v/oct - one volt per octave pitch

    Each 1 V octave is divided linearly into 12 semi-tones. So the pitch produced by a voltage of 3 V is one octave lower than that produced by a voltage of 4 V.

FM - Frequency Modulation

    Frequency modulation refers to changing the frequency of an oscillator with a control voltage source, which is often another audio rate oscillator. This results in unique wave-shaping abilities that change the harmonic content of a waveform, often adding harmonics to the signal. Specific frequency modulation algorithms are used on more advanced digital synths to replicate or model physical timbres such as a marimba or Kalimba, but can also be used to create clean glassy synthetic tones at high frequencies or crazy intergalactic spaceship noises at lower ones.

Thru zero fm

VCF - Voltage Controlled Filter



    A voltage controlled filter is a slope put on a signal to filter out certain frequencies or harmonic content from a signal.

A low pass filter will cut out all of the high frequency content out of the incoming signal leaving only the lower harmonics left when the filter is fully swept downwards.

A band pass filter will only allow a small band of harmonics to pass based on where the cutoff frequency of the filter is. A notch filter does the opposite, it will let in all of the harmonic content except for where the cutoff frequency is placed.

A high pass filter will cut out all low frequency content as the cutoff frequency is swept upwards, only allowing the highest frequecy content to pass when the filter is fully swept upwards.

Q or resonance on a filter will amplify the signal at the cutoff frquecy, adding harmonics and emphisizing the specific frequecy of the filter cutoff.



Attenuator/Attenuverter

    An attenuator is essentially a volume knob for both audio and voltage.

An attenuverter’s volume knob is closed in the middle position, as you open it up clockwise, it lets in the audio or voltage the way it came in. As you turn it counter-clockwise it will invert the polarity of the incoming signal (all positive voltages will now be negative, phases will be inverted or shifted by 180 degrees).

One fun use for an attenuverter is to create a ring modulator. Take your signal and go into one channel of a crossfader. Copy that signal and go into the attenuverter to invert the signal, then take the inverted signal and patch it into the second channel of the crossfader. This is your input signal for the ring modulator. The cv input on your crossfader will be your modulator signal for the ring modulator.


AM and Ring Modulation - Amplitude modulation and Ring modulation

    Amplitude modulation refers to changing the volume or amplitude of a voltage or audio signal. Often it’s when the amplitude of one audio rate signal is controlled by another audio rate signal. If you are using a signal bipolar bipolar (meaning it is in both the positive and negative range, most oscillator signals are), and your VCA is completely closed, when the signal goes into the negative range it the volume will stay at zero until the signal goes back into the positive range, or it will simply cut out the signal until the controlling signal goes back into the positive range, unless there is an offset to the VCA or attenuator.  




Ring modulation is similar to amplitude modulation, however if the signal you are using to control the amplitude of the other signal is bipolar, when the controlling voltage or oscillator goes into the negative range it will let out the signal phase shifted by 180 degrees, or mirrored.



Phase shifting and modulation

  Shifting the phase of a module is often associated with audio rate signals. If you layer a signal with the same signal with a shifted phase of 180 degrees, the signal will cancel out completely and you will hear nothing. Shifting the phase of a signal is not audible, bit when you layer a signal with multiple phase shifted signals, whose phase is modulated by a CV signal such as an LFO, you get phaser swishy sounds. This is because based off of the position of the phase shifted signals, its cancelling out certain portions of the signal.



Wave folding

  Wave folders literally fold the waveform in on itself a specific amount of times depending on stage. A four stage wave folder will fold the wave in on itself 4 times. On a sine wave signal, this will also increase the frequency of the signal. Folding a sine wave once will make the frequency one octave higher. Wave folders will often have an input and an output, an input for cv over the folding of the waveform, a switch for the amount of stages, and a symmetry input.



Turing the symmetry knob clockwise will add a positive offset, or push the signal more into the positive realm, and turning the symmetry knob counter clockwise will add a negative offset to the input signal. the signal will still fold at 5v (this threshold may vary from module to module), so adding a positive offset will fold the waveform more on the positive peaks of the waveform.




Additive vs subtractive synthesis

Additive synthesis refers to adding harmonics to a signal with various techniques such as audio rate amplitude modulation, frequency modulation, ring modulation, distortion, wave folding etc.

Subtractive synthesis refers to subtracting harmonics from a signal by using filters, equalizers, slower rate amplitude modulation, envelopes etc.

Slew

Slew refers to interpolating a signal. For instance if you have a random stepped voltage source that steps to a new random voltage every time it’s triggered, adding slew will interpolate the signal from its previous value to its new value according to the slew time. This is how you would create a smooth random voltage from a stepped random voltage. The shape of this interpolation can be logarithmic, linear or exponential, but is often linear. These various shapes will change the perceived timing of the interpolation.