LV2 Plugin Control Units in Ardour

LV2 has had a "units" extension since the beginning, which allows plugins to specify units like Hz or dB for their controls. To date this information has not been used very widely by hosts, so I've done some work in Ardour to use this information for better UI generation and intelligent plugin control.

Units can specify a format string which describes how to print a value in that unit. This is now used to draw the text on control sliders:

An Ardour control dialog for an LV2 plugin.

The same controls are used in automation lane headers. If the control is a note number, then right-clicking will present a menu with an option to pop up a note selector dialog where the value can be chosen on a piano keyboard:

The select note menu on a note number port.

The Ardour note selector dialog.

Similarly, numeric frequency controls have a context menu which can set the value to a specific number of beats in the current tempo, if the frequency range is low:

Setting a low frequency port in beats.

If the frequency range is high, then numeric frequency ports can be set with the note selector dialog just like note numbers:

Setting an audible frequency port by note.

In the future, it would be nice to have this idea applied more extensively so automation lanes can "pretend" a port is in the desired unit, for example allowing the user to automate an LFO frequency in beats, or a cutoff frequency in notes. Until then, being able to at least easily set controls to musically sensible values makes many production tasks easier, particularly in electronic music where it's often desirable to set plugin controls based on key or tempo.

Up next is setting time ports based on tempo, for cases like setting delay lines to a certain number of beats, but many plugins are missing the unit information required to make this possible. Hopefully better and more widespread host support will provide some incentive for plugin authors to specify the units of their ports. It is very simple to do so, see the LV2 units documentation for examples.