7.2 Signal classes

Every patch jack names the class of signal it carries, or unclassified (port list). EIS has seven classes: AUDIO, CV, and VIDEO, the signals a patch carries, with PITCH for calibrated pitch and GATE, TRIG, and CLOCK for timing. A module mixes them freely: a video module may take AUDIO, and an audio module may give VIDEO.

A port shall meet the limits of the class it names, at the range it names: an output keeps its full-scale signal within the range's ceiling, and an input accepts the range's full swing without clipping (SIG-005). The limits are in the class table.

Every patch jack keeps its safety (7.1), so a patch from one class into another damages neither jack; it only meets a level the input was not built for.

The shared interface

Every class but VIDEO has one interface: a source impedance of 1 kΩ typical and at most 2 kΩ, into a load of 100 kΩ typical and at least 47 kΩ. The worst conforming pair, a 2 kΩ source into a 47 kΩ load, loses 4 %. VIDEO has its own (7.3).

Each class has:

• a nominal level, what an output puts on the port;
• a full scale, the swing its range allows;
• a class ceiling and a damage limit (Terms);
• for gate, trigger, and clock, thresholds: what an input reads.

In the class table, a cell marked typical or recommended is a guide; every other figure is a limit (SIG-005). The levels are EIS figures; the gate thresholds follow A-100 practice (Scope 1.4).

Audio

AUDIO is Eurorack audio on a patch jack.

• Level: nominally ±5 V on the patch tip; full scale ±10 V peak at the default range B10, 7.07 V RMS for a full-scale sine. A port may claim B5 or B1 for a quieter signal.
• Coupling: AC or DC. An AC-coupled input passes 20 Hz within 1 dB; a DC-coupled output states its offset.

CV

CV is control voltage that is not calibrated for pitch: envelopes, LFOs, offsets, ramps, and the like. Its level is the range the port claims, bipolar or unipolar at 1, 5, or 10 V full scale. The default is B5, −5 to +5 V, which is what most control inputs and outputs are built for. An envelope from 0 to +8 V is CV at U10; a control on the VIDEO scale, 0 to +1 V, is CV at U1. A control port with a stated scaling other than 1 V/octave, such as Hz/V, is CV and names the scaling in its port list.

Pitch

PITCH is precision pitch CV: 1.00 V per octave within ±0.3 % of scale (3.6 cents per octave), over the span the port's range names. The default is B10; a 0 to +10 V sequencer or MIDI-to-CV output is U10, and a narrower span B5 or U5. The range states the span; the scale stays 1.00 V per octave.

EIS recommends:

• that 0 V correspond to C0 (16.35 Hz, MIDI note 12); every oscillator has a tune control, so the absolute pitch at 0 V is a calibration figure, not a requirement;
• that the manual publish the tracking error of a pitch output;
• that pitch ports be labeled 1 V/oct, so that they are not mistaken for video ports.

Video

VIDEO is video-rate signal on a patch jack, 0 to +1 V: pictures, ramps, keys, and control at the rate of a picture. Its interface, its paths, and its bandwidth are in 7.3.

Gate, trigger, and clock

GATE, TRIG, and CLOCK share one contract:

• An output drives high at +4 V to +12 V into 100 kΩ, and low at +0.5 V or less.
• An input reads +3 V or more as high and +1 V or less as low. EIS recommends at least 0.5 V of hysteresis between the two.

They differ in timing:

• GATE is a sustained high or low level. A run, reset, or start signal is a GATE.
• TRIG is a pulse at least 1 ms high (5 ms recommended). A TRIG input catches a 1 ms pulse.
• CLOCK is a repeating wave, high and low each at least 1 ms, at any duty. The product states the rate it accepts or produces and, for a tempo clock, the pulses per quarter note.

The gate line on the 16-pin header is BUS-GATE (6.6).

Ranges

A range is the full-scale swing a port is built for, named in the port list beside the class. The class table says which ranges a class takes and which is the default.

• A port that names no range has its class's default.
• A unipolar ceiling allows 20 % above full scale and 10 % below 0 V; a bipolar ceiling allows 20 % beyond full scale either way.
• An input accepts a swing when its gain at full scale is within 0.5 dB of its small-signal gain.
• Ranges do not relax patch safety.

EIS recommends that an output claim the smallest range that contains its full-scale signal, that an input claim the largest range whose full swing it accepts, and that a range other than the default be printed on the panel next to the jack.

Class table

The table is generated from data/signals.json. Every class is on a 3.5 mm mono jack. An impedance cell gives the typical figure and the limit; VIDEO's is a limit alone (SIG-009, SIG-010).