Root and Scale CV

ROOT voltages

ROOT is 1V/oct, with C at 0 V. Each semitone is 1/12 V, and the four-decimal figures below are rounded from those fractions. Key, Note and Arrange send a voltage from this table at their ROOT outputs, so those outputs never go below 0 V or above 0.9167 V.

A receiving module adds the voltage to its ROOT knob. Most modules then round the result to the nearest semitone, so any voltage within 1/24 V (0.0417 V) of a table value gives that root. On a module that wraps to one octave, 1 V gives C again and -1/12 V gives B. A module that does not wrap treats 1 V as a transposition up an octave. The table under ROOT and SCALE on each module shows which is which.

SCALE voltages

SCALE is 1 V per scale, counted from Chromatic at 0 V. A plain voltage is rounded to the nearest whole volt, so 1.6 V and 2.4 V both count as 2. The total of the knob and the voltage stops at index 0 at the bottom of the list and at index 18 at the top, or at index 19 on Key, which has one extra position. Every module uses the same list in the same order.

The channel count and degrees in this table are what a SCALE output sends as a scale bus, described in the next section. To convert a degree to semitones, multiply it by 12.

The harmonic series has twelve degrees that climb past the octave when it is chosen on a module's own knob. The bus requires every degree to be below the period, so a SCALE output folds the series into one octave and removes the repeats, which leaves the six degrees shown. Pelog and Slendro keep their fractional semitones on the bus, for example 1.2 and 2.7 semitones for the second and third degrees of Pelog.

The scale bus

A plain index can only select a scale from the list. The SCALE outputs of Key, Arrange and Note are polyphonic so that they can also send the scale itself. Rack returns channel 0 to anything that reads a single voltage, so channel 0 alone behaves as a plain index.

A bus holds at most fourteen degrees, because two of Rack's sixteen channels carry the index and the period. Key can load a Scala file with more degrees than that, and it sends the first fourteen. The fraction on channel 0 records how many were cut: a 31-note scale loses 17 degrees, which adds 17/128 V (0.1328 V). The count reads correctly for up to 63 cut degrees. At 64 or more the fraction reaches 0.5 V, so channel 0 rounds to the next index and the count reads as 0. When a scale is a custom note selection or a Scala file, channel 0 holds the list scale that shares the most pitch classes with it. A scale whose period is not an octave sets channel 1 to something other than 1 V: the Bohlen-Pierce scale repeats every 19.02 semitones, which is 1.585 V.

A receiving module accepts a cable as a bus only if all of these are true. Any other cable counts as a plain index, read from channel 0.

• The cable has at least three channels.
• Channel 1 is between 1/12 V and 4 V, which is a period of 1 to 48 semitones.
• Channel 2 is 0 V, to within 0.01 semitones.
• Each degree is higher than the one before it and lower than the period.

A polyphonic merge module can build a bus by hand. Five channels set to 0, 1, 0, 0.3333 and 0.5833 V send a major triad, with degrees at 0, 4 and 7 semitones above the root.

Plain index and scale bus

A plain index is added to the SCALE knob, and a bus replaces it. The difference shows when a module's knob starts on Major, which is index 1. Sending a plain 2 V to that module gives index 3, Pentatonic Major, and not index 2, Minor. To make a hand-made voltage match the table above, turn the knob to Chromatic first. A cable from Key, Arrange or Note carries a bus, so the module plays the sent scale wherever its knob is.

Key has one more knob position than the list has scales. Index 19 selects the loaded Scala file, and a plain voltage can reach that position when added to the knob. Note's SCALE output sends the scale in force after its own knob and input are combined, so a plain voltage into Note comes out as a bus.

The SCALE knobs on Loom and Slide have a Free tuning position below the list, so each of their other positions is one place higher than its index. With no cable in the jack, Free tuning leaves the strings unquantized. With a cable in, that position counts as index 0, so 0 V is Chromatic. Unplugging the cable is the only way to return to free tuning.

ROOT and SCALE on each module

The last column gives the scale from a plain 2 V with the SCALE knob at its default position. A scale bus gives the sent scale on every module with a SCALE jack. Arrange has no ROOT or SCALE inputs: it sends the root and scale of the playing phrase, so it does not appear here. Muse's ROOT is not rounded, so 0.05 V raises its output by 0.6 semitones.

Changes after version 2.23.1

This page describes the plugin as it stands after version 2.23.1. Four things work differently in 2.23.1 and earlier. Fugue and MetaFugue read ROOT at 1 V per semitone, so a cable from Key moved their root by 0 or 1 semitone. Arrange sent a plain index instead of a bus, so its scale was added to the knob of the receiving module. On Loom and Slide, 0 V at the SCALE jack meant free tuning. A SCALE output sent the harmonic series unfolded, so a receiving module rejected it as a bus and treated channel 0 as a plain index.