[linux-audio-dev] LADSPA needs & wishes

Lars Luthman lars.luthman at gmail.com
Sat Jan 27 06:09:04 UTC 2007

On Sat, 2007-01-27 at 16:05 +1100, Fraser wrote:
> Hi All,
> I've been converting my old VST plugins over to LADSPA and have come
> across something in the api which I really miss - the inability separate
> the algorithmic to the displayed value of a parameter.
> I'm finding this inability is leading to non-ideal programming habits.

There is the LADSPA_HINT_LOGARITHMIC port hint, which does part of what
you want - it tells the host that any knob or slider should use a
logarithmic mapping, but it doesn't give any display values. In LV2 this
will be done with the :scalePoint property, which you can use to label
the real value 1 as "0dB", 0.5 as "-3dB" and so on.

> Another example - presets
> I have a control that allows an operator to chose one of fifty presets
> (say a reverb with small room, medium room, large room, hall etc). I don't
> have a choice this time. Internally using an integer to represent the
> different presets is fine, it's exactly all I need.
> However even though I know what the preset is, I cannot display it's name
> back to the user, so our user is left with a set of meaningless numbers
> which they must resolve into names by some other means (print the doco out
> and stick it on the wall?)

Presets are done in LADSPA using the additional library liblrdf, which a
number of hosts (though not all) support. "Internal" presets could be
done in LV2 using an integer port and scalePoint labels.

> Now for a wish.
> GUI - under OSX or windows this isn't such a big drama, there's only one
> GUI environment to deal with. under Linux it's a different matter.
> I sometimes think the best thing to do is to provide enough hints to the
> host so it can render a more comprehensive gui, if it desires, rather than
> the plugin drawing the gui as is traditionally done. This would entail a
> few things.

There have been some suggestions for GUI "markup" standards for LADSPA,
I don't know if any real hosts have supported them. It's not a bad idea
as such. Another approach is DSSI (http://dssi.sf.net, an "instrument"
extension of the LADSPA spec) which does GUIs using standalone programs
distributed with the plugin that communicates with the host using OSC.


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