[LAD] ladspa: what does LADSPA_PROPERTY_REALTIME mean?

IOhannes m zmölnig zmoelnig at iem.at
Wed Sep 9 20:56:45 UTC 2015

hi richard, paul, stefan and fellow LADders,

the Debian folks currently are a bit at a loss of the proper
interpretation of the LADSPA_PROPERTY_REALTIME property.

ladspa.h says:
> /* Property LADSPA_PROPERTY_REALTIME indicates that the plugin has a
>    real-time dependency (e.g. listens to a MIDI device) and so its
>    output must not be cached or subject to significant latency. */

the discussion started about a year ago (see [760758]), and it seems
that there are two opposing interpretations of that property:

#1 setting the property indicates to the host, that the plugin must not
be used in a non-realtime environment.
this means that a plugin that has this property set, must not be used
for batch processing, e.g. because it uses wall-clocked input (like
MIDI-devices) that simply won't deliver proper input when running at a
speed that is decoupled from wall clock.


#2 setting the property indicates to the host, that the plugin can be
run at a lower (timing) priority; so the host can deliberately add
latency without compromising the usefulness of the full processing
chain. that would be mostly for plugins that do not do any input ->
output processing, but only take input (e.g. metering, recording).

that's just a quick summary. there's more arguments in the bugreport

personally i lean towards #1 (as i don't see much use cases for the
second interpretation), but given that someone as deeply involved into
linux audio as fons favours #2, i would like to ask the creators of
LADSPA for their canonical view.

thanks for your insights,


[760758] https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=760758

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