Physical

The physical specification is based on the Eurocard standard of:

• 3U (5.06 inches or 128.5 mm), where height "U" is measured in rack units, rounded for a lip (nb: 3U in standard rack units would be 5.25 inches or 133.3 mm).
• 1HP (0.2 inches or 5.08 mm), where width "HP" is measured in horizontal pitch units. A card width is generally integer multiples of 1HP, although some manufactures work in multiples of 0.5HP.^[21]

Eurorack modules may be further characterized by depth: shallow modules (2.5 cm to 4 cm ) can fit into "skiff" cases and are casually referred to as "skiff friendly".

Electrical

The Eurorack electrical specification^[22] defines a common bipolar 12 V DC power bus (+12V , 0 V, −12 V). This can be distributed by one of two connectors with a standard 2.54 mm (0.1 in) pitch:

• 10-pin ribbon cable, with +12 V, ground, and −12 V pins
• 16-pin ribbon cable, with Gate, CV, +5 V, +12 V, ground, and −12 V pins

Audio and control signals are exchanged between modules via 3.5 mm mono jack cables. The electrical characteristics of signals are split into three loosely defined categories:

• Audio signals are typically a maximum of 10 V peak-to-peak (i.e. between −5 V and +5 V).
• Control voltages can either be unipolar or bipolar. Bipolar control voltages are typically 5 V peak-to-peak (i.e. from −2.5 V to +2.5 V), unipolar voltages between 0 V and 8 V. The V/Octave scale is used for pitch information.
• Trigger, Gate or Clock signals are digital 0 V to 5 V pulses typically used for timing and event signalling.

1U modules

Several manufacturers offer Eurorack-compatible modules in a smaller 1U tall format, sometimes referred to as "tiles". As of 2018, there are two competing standards for 1U modules, differing mainly in their height. 1U modules manufactured by Intellijel are 39.65 mm high,^[23] whereas 1U modules manufactured by Pulp Logic and other manufacturers are 43.2 mm high.^[24] Pulp Logic also proposes a more compact power connector for 1U modules, consisting of only three pins.

Voltage-controlled oscillators (VCOs)

The most common source of sound in any modular synthesizer is a voltage-controlled oscillator.^[25] They depend on a control voltage, a lot of times routed from external hardware (for example, an analog synthesizer with a CV output, or MIDI signals processed on a MIDI-to-CV converter), to both control pitch, and output different waveforms.

There are various control-voltage standards for determining the voltage/pitch relationship. The most popular ones are "volts-per-octave", where 1 V equals one octave, and "hertz-per-volt", where each octave equals doubling or halving the voltage.^[26]

Noise source

Under the category of source modules,^[27] these modules are responsible for producing different “types” of noises (or colors). They can output (1) white noise — where all frequencies in the spectrum are equally powered, (2) pink noise — where there is more power to the lower end, due to its logarithmic nature, (3) brown noise — similar to pink noise, but steeper slope, (4) blue noise — oversimplified, the opposite of the pink noise, with more power concentrated on the higher frequencies, among others.

Modulators

Under the category of processors,^[28] modulators modify an incoming signal.^[29] The effects produced are widely varied and a lot of times, modules are built for a specific function, for example:

• Ring modulators multiply two signals to produce sum and difference harmonics
• Voltage controlled amplifiers multiply signals with a control waveform to vary its amplitude

Filters

Another kind of processor,^[30] filters are modules shaping the sound by attenuating specific frequency ranges. These modules contain all or a selection of the following: (1) a high-pass filter (where anything above a certain frequency can “pass”), (2) a low-pass filter (anything below a given frequency can pass), (3) a band-pass filter (where anything “in between” two frequencies can pass), and (4) a notch filter (where one cuts a specific range of frequencies out allowing everything else to pass).^[31]

Sequencers

Essentially, modules that can both operate as a source or a processor of musical content in the form of CV or MIDI messages. The most common kind are step-sequencers, where each individual musical event is triggered in a “step” of a bigger sequence (or loop).

Utilities

Utility modules are the ones responsible for expanding certain capabilities of a specific setup. They can be used to combine, split, divide, multiply, quantize, or offset a signal.^[32] One example of a utility module is a multiplier, that allow one to send any CV output to many other inputs. Another example are the attenuators responsible for scaling the CV signal with a control knob (much like faders in a mixing console).

Effects

A lot similar to the concept in a guitar pedal, effects modules are used to change the sound of an incoming signal.^[33] They can be (1) dynamic processors, used to control the level of a signal (like compressors, or limiters), (2) equalizers, used to change the frequency characteristics of a given sound (sometimes in the form of a eurorack mixer module), or (3) special effects, like delays, reverbs, or choruses.