mantaraya36's blog

Brain dump

LADSPA host proof of concept

Proof of concept code for LADSPA hosting is now up in github. This code is still very inflexible, as you can’t even select the plugin you want to use (it uses a simple gain plugin amp.so, sorry oneman!), but it shows a working LADSPA host, which can take parameters from the server config. If you try it out, you will be able to modify the gain by using:

nyxmms2 server config ladspa.parameter0 1.0

Where the last number is the gain value for the LADSPA plugin.

This xform plugin accepts both S16 and float samples, and creates internal buffers to pass to the LADSPA plugin. If you have a look, there are still many TODOs, but most infrastructure is there, even to support multiple LADSPA plugins within the same xform.

I had done a LADSPA host before, so I tried first to write the host from a quick look at the LADSPA header. This turned out to be a bad idea, as there were many small details and problems which I could’ve avoided by reading some documentation like:

http://gdam.ffem.org/ladspa-doc/ladspa-4.html

In particular it is important in LADSPA to remember to allocate buffers for all ports (both audio and control), even the ones you are not going to use.

One of the challenges now is deciding how to handle different channel counts between the chain and plugins, and try to get input from the xmms2 devs regarding a way to define config properties from the init function rather than the setup function. Most xforms are OK with setting their properties in setup, but a LADSPA host definitely needs to register the config properties in the init function where the plugins are initialized. It should also be able to dynamically create and delete properties in case the plugin is changed in the middle of a song.

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