mantaraya36's blog

Brain dump

More on LADSPA modes

After an IRC chat with vdust, he suggested the automatic modes might work most of the time, but that it might be good to allow more flexible connections for other type of streams like 5.1. Thinking about that later, I realized this might be very useful to do time delay compensation, and individual loudspeaker eq. I’ve thought about this, and I’ve come up with an idea, which could serve the purpose and builds on the work that’s already done.

Currently, the LADSPA host I wrote creates LADSPA plugin nodes which can actually hold several instances of a plugin (e.g. for MONO mode), but the plugin parameters are shared between all the instances, as well as any intermediate buffers. But for flexibility, it might be desirable to allow arbitrary routing of inputs and outputs, as well as independent control of plugin parameters. I think this can be accomplished in a sensible way through a new structure called stage that can wrap nodes and create complex chains.

Three separate configuration options have to be set. One is the list of plugins, one will determine routing and another will control the parameters.

Plugins and routing

To create a chain, you would pass a list like:

[ "", "" ]

This will create a chain that looks like:

input => => =>output

Notice that both the plugin library and the plugin name are given in the string, and they are separated by a colon.

On the routing config option, the inputs and outputs will be specified. Each element of the plugins list is a stage, so you need to declare the inputs to each of the stages and lastly the way the outputs from the last stage are mapped to the outputs, for example:

[ [1,2], [3, 4], [5,6] ]

Would mean that the first stage takes as input channels 1 and 2 (if channels don’t match the automatic modes for channel assignment are used), the second stage takes channels 3 and 4 from the previous stage, and finally outputs 5 and 6 from the second stage are passed to the two channels of the chain. This will give great flexibility in routing. One thing that is not supported is mixing channels from the config. A special LADSPA plugin downmixer would need to be used for this. Also note that to determine the number of channels for each stage, it must look both at input and output and use the largest number.

A more complex chain can look like:

[ ["",  ""], "" ]

The first stage will have plug1 and plug2 in parallel, which then feed plug3. You can specify routings like:

[ [ [1] , [2] ], [1, 2, 3, 4], [1, 4] ]

This means that the first plugin of stage 1 takes input channel 1, and the second takes 2. Assuming the plugins are mono to stereo, this means that although two channels enter the stage, 4 will come out. These will be numbered sequentially, for each consecutive plugin, so in this case, the second stage will take the output from plug1 on inputs 1 and 2 and the output of plug2 on inputs 3 and 4. Finally, the second stage will send outputs 1 and 4 out the xform chain.


A third config option deals with plugin parameters. These will use a JSON style schema notation, and will look like:

[ [  {"stage1param1":value , "stage1param2":value}, {"stage1param1":value, "stage1param2":value} ], {"stage2param1":value} ]

A dictionary will represent the parameters for each node within each stage. If a parameter name is not listed, it should take its default value.


When a single node is the whole stage, there is no need to put the node’s configuration options with an array of a single element. In this case just using the element directly is preferred.

When arrays are not used (i.e. only the pluginlib name is given not as part of an array), the LADSPA host will fall back to using a single node with automatic modes.

This proposal will wait implementation until the next stage of GSoC dealing with the implementation of schemas is done.

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