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