Hi,
Faust is a very modular language, and it's easy to reuse any Faust program
as a component of another, thanks to the `component()` construct. For
example: `component("freeverb.dsp")` lets you integrate a fully functional
reverb, with its user interface, inside another program.
But how to control the parameters of this reverb other than by hand via the
user interface? How to get the equivalent of a “voltage control”? How to
modulate one of the parameters with an LFO, for example? Well, if the
reverb designer didn't anticipate this from the beginning, it's simply
impossible!
The absence of such a capability in the Faust language limits the
reusability of existing code. A technique to fill this gap is to write the
code in such a way as to separate the audio algorithm itself from its user
interface. This approach makes it easy to reuse the audio algorithm in
another context or with another user interface. Such an approach is good
practice, but it's not very convenient.
The new language expression introduced in Faust 2.69.0: _widget
modulation_, addresses this very issue. It can be used on an existing
component, for example, to modulate a slider, replace one slider with
another, replace a slider with a constant, etc., without modifying the
component's source code!
Its simplest form is the following:
`["Wet" -> freeverb]`
This adds an input to the freeverb, that is used to modulate the "Wet"
slider. An LFO, for example, can be connected to this input:
`lfo(10, 0.5), _, _ : ["Wet" -> freeverb]`
The "Wet" label designates the slider to modulate. Of course, this
presupposes knowing the names of the sliders. But since the sliders appear
in the user interface, discovering their names is easy enough. If several
widgets have the same name, adding the names of the surrounding groups, for
example: `"h:group/h:subgroup/label"` can help distinguish them.
Multiple sliders can be indicated as in: `["Wet", "Damp", "RoomSize" ->
freeverb]`. In this case, three new inputs are added.
We haven't said how sliders are modulated. By default, when nothing is
specified, the modulation is a multiplication. The previous example is
equivalent to the explicit form `["Wet":*, "Damp":*, "RoomSize":* ->
freeverb]`. The multiplication can be replaced by any other circuit with
two inputs and one output.
For example, one could write `["Wet", "Damp", "RoomSize":+ -> freeverb]` to
indicate that the "RoomSize" parameter is modulated by addition.
The only constraint on the modulation circuit is that it must have only one
output and at most two inputs. We can therefore have `0->1`, `1->1`, or
`2->1` circuits. Only `2->1` circuits create additional inputs. Moreover,
`0->1` circuits lead to the elimination of the slider.
We can therefore rewrite `lfo(10, 0.5), _, _ : ["Wet" -> freeverb]` as
follows: `["Wet":*(lfo(10, 0.5)) -> freeverb]`. The latter form does not
lead to the creation of additional input, as the LFO is placed inside the
reverb. The form `["Wet":0.75 -> freeverb]` results in the deletion of the
"Wet" slider, replaced by the constant 0.75. Finally, the form
`["Wet":+(hslider("More Wet", 0, 0, 1, 0.1)) -> freeverb]` adds a second
slider to the freeverb interface.
Yann
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