How to Write LADSPA Plugins Using GDAM
Overview
The procedure has two steps:
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Create a flow graph graphically using GDAM (inside the mininetwork filter). Save it to XML.
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Convert the XML to a LADSPA plugin using ladspapluginmaker. Usually this will convert the xml to C code, then it runs the compiler.
Using the Mininetwork
The mininetwork has many types of "nodes" which you can create, delete, and connect.
Here is the example we will use throughout:
This actually sounds like a plucked string filter. (I'll describe it a bit more below).
You can see it contains a bunch of nodes which are connected. You can drag them around, and drag pipes from inputs to outputs. Here are exactly what the various mouse buttons do:
- left mouse button:
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If you clicked on a node's input or output, you may drag a pipe to another node's input or output. (you must connect inputs to outputs though!). If you clicked on a node, you may drag it, and it will be selected. When it is selected, a menu of adjustable parameters appears. (eg for the gain filter, a gain slider appears in that window.) If you clicked outside a node, a new unconnected node is formed.
- middle mouse button:
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If you clicked on a node, it is deleted, but its inputs are attached to its outputs. If you clicked on a pipe, a new node is formed in the middle of the pipe, properly connected.
- right mouse button:
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If you clicked on a node, it and all its pipes will be deleted. If you clicked on a pipe, it will be deleted.
The inputs to each node are on the top of the node -- each node has only one output, but you can connect an arbitrary number of things to that output.
Here are some common nodes:
- Mix
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This just adds all its inputs together. - Modulator
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This multiplies its inputs like a ring modulator. Or equivalently it is a variable gain. - Constant Gain
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This multiplies its single input by a constant value. [in the gui, you can change the constant, but it is constant once it is converted to a LADSPA plugin]. - Constant Delay
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This delays its input for a certain number of samples. [in the gui, you can change the constant, but it is constant once it is converted to a LADSPA plugin]. - Variable Delay
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This delays its input for a number of samples, which is computed from another nodes output.The actual delay at any time is given by:
delay = (base_delay) + (delay_fluctuation * input_value)
where the input_value is from the delay-input (which is the top-right connector on the variable delay).
- Input
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This represents where sound is input to the filter from the source above it. You cannot create new inputs or destroy the existing one. - Output
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The output from the filter. You cannot create new outputs or destroy the existing one.
Brief Digression into the Plucked String Filter
The plucked string is basically a feedback of the tuning frequency with a lowpass filter.
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In the upper-left corner, the input is revolumized (by a modulator) with a control node (at left). This implements the input gain slider.
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In the mix in the center is the mix which is collecting feedback.
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The right side is the feedback loop. It is revolumized to control how loud the reverb should be.
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The two lower-right nodes implement a very simple lowpass filter that just averages the last two samples.
Saving the New Filter
You just press the save button, at the left. It will prompt you for a filename -- you'll have to specify it in the next step.
Converting the Filter to a LADSPA Plugin
Suppose you saved the mininetwork as pluckedstring.xml. Then you would run the following command:
ladspapluginmaker --input pluckedstring.xml \
--output output_dir \
--class PluckedString \
--basename pluckedstring \
--description "Karplus-Strong Plucked String" \
--unique-id 10 \
--build
|
The --unique-id option deserves brief mention. Currently all plugins are centrally allocated by talking to Richard Furse, <>. He will assign you a list of UniqueIDs. In the interim, you can use ids less than 4000 for development purposes.
(TODO: put more of these options in the xml!)
This will create a directory output_dir with three files in it:
Loading LADSPA Filters with GDAM
GDAM automatically scans for LADSPA plugins when it starts up. It looks in /usr/lib/ladspa and /usr/local/lib/ladspa. It will write a summary file to ~/.gdam/ladspa.xml.
This is what the loaded plugin looks like:
