[LV2] plugin bypass

Robin Gareus robin at gareus.org
Tue May 6 12:50:07 PDT 2014

Hi David, LV2-devs,

This follows a discussion Fons (CCed) and I (and some others) had at the


At the moment it's up to the plugin-host to handle bypassing of LV2
plugins. In all LV2 hosts that I know this is currently implemented by
literally bypassing audio and midi around the plugin which is problematic:

Take a simple example: An amplifier, if that amp does not have a
zero-gain, en/disabling the bypass will cause a jump in the
signal-level. While in some cases x-fading on en/disable can work, it
can also introduce artifacts - particularly in cases where a plugin
introduces latency.


Only the plugin itself "knows" what to do when it is to be bypassed.

[Proposed Solution]

Therefore I propose to introduce a means to handle the situation:

1) a port-property intended for marking a lv2:toggled input port:

2) a host-provided feature (lv2:requiredFeature, lv2:optionalFeature):

(1) a host should not display this port but rather use it instead of the
host-provided bypass toggle mechanism/button.

(2) It allows to plugin to handle cases depending on host support. It
also provides "pro" plugins with a means to refuse to load in a host
that does not support (1).

A host supporting 'bypassFeature' must always add any plugin which does
have a port tagged 'bypass' in bypassed mode and un-bypass it after the
first process callback. That will allow for clickless insertion of the
plugin. (same for removal, first bypass, then remove.)

Thoughts and comments are very welcome.


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