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FluidSynth LADSPA Interface

The LADSPA (Linux Audio Developer's Simple Plugin API) binding can be used to route the FluidSynth audio output through any number of LADSPA plugins. Please note that even though the "L" in LADSPA stands for "Linux", it can also be used on different platforms, for example Windows or MacOS. Check the "LADSPA on other Platforms" section at the end of this guide for more information.

Configuration

To configure and compile FluidSynth with LADSPA support, make sure you have the LADSPA SDK installed (or at least the ladspa.h header file available in an include path). Then compile FluidSynth in the usual way. You should see LADSPA support: yes in the cmake output.

To enable the LADSPA engine, use the synth.ladspa.active setting when starting FluidSynth:

fluidsynth -o synth.ladspa.active=1 ...

Quickstart Tutorial

The following walks you through the process of adding a LADSPA plugin into your FluidSynth configuration. It assumes that you are running FluidSynth on Linux, that you have some experience with running Linux shell commands and that you know how to start FluidSynth from the command line and use it to play a MIDI file.

Introduction to LADSPA

You don't need to to have detailed knowledge of LADSPA to use effects with FluidSynth, but knowing some of it's concepts will help if you want to make the best use of it.

If you have the LADSPA SDK installed you should be able to use the listplugins Linux command to list all plugins installed in your LADSPA path. And to show more details about a particular plugin library, you can use the analyseplugin Linux command. Here is an example showing the details of the delay.so plugin from the LADSPA SDK:

user@host:$ analyseplugin /usr/lib/ladspa/delay.so

Plugin Name: "Simple Delay Line"
Plugin Label: "delay_5s"
Plugin Unique ID: 1043
Maker: "Richard Furse (LADSPA example plugins)"
Copyright: "None"
Must Run Real-Time: No
Has activate() Function: Yes
Has deactivate() Function: No
Has run_adding() Function: No
Environment: Normal or Hard Real-Time
Ports:	"Delay (Seconds)" input, control, 0 to 5, default 1
	"Dry/Wet Balance" input, control, 0 to 1, default 0.5
	"Input" input, audio
	"Output" output, audio

This output tells you that the delay.so library contains only a single plugin called "Simple Delay Line". Most importantly it lists the input and output ports, which can be used to set plugin parameters and connect the audio input and output to FluidSynth.

"Delay (Seconds)" and "Dry/Wet Balance" are input controls. They are the parameters that a user can set to affect the way the plugin works. They control how long the delay should be and how the dry and wet signals should be mixed before writing them to the output.

"Input" and "Output" are audio ports which carry samples into the plugin and out again after it has run. Mono plugins usually provide one set of input and output audio ports, stereo plugins usually provide two sets. But there are even plugins that only have a single output port and no input at all (think of noise generators...)

Also note the line Has run_adding() Function: No. This specifies that this plugin can not mix it's audio output into an output buffer, but will always replace anything that is already there. This will become important again later on.

FluidSynth Host Ports

Just as LADSPA plugins have input and output ports, FluidSynth provides it's own audio ports that can be connected to plugins. On a standard stereo setup, the following four ports are automatically created:

  • Main:L
  • Main:R
  • Reverb:Send
  • Chorus:Send

The "Main:L" and "Main:R" ports can be connected to effect input and output ports. They carry the main audio signals into the LADSPA effects and the modified signals back into FluidSynth.

"Reverb:Send" and "Chorus:Send" can be used as effect inputs. They carry the mono effect send signals (as determined by the reverb and chorus send generators for each voice) into the LADSPA effects.

Please note that if you run FluidSynth with the internal reverb and chorus effects active (which is the default), then those effects are already mixed into the Main:L and Main:R channels. Fore more details, please see the "Signal Flow" section below.

For host port setups in multi-channel configurations, please see the "Multi-Channel Output" section below.

Creating a Configuration File

You can configure LADSPA effects using the FluidSynth shell, but writing the commands into a file and loading it at startup is much more comfortable. So let's create a file effects.txt with the following contents:

effects.txt

ladspa_effect e1 /usr/lib/ladspa/delay.so
ladspa_link e1 Input Main:L
ladspa_link e1 Output Main:L

ladspa_effect e2 /usr/lib/ladspa/delay.so delay_5s
ladspa_link e2 Input Main:R
ladspa_link e2 Output Main:R

ladspa_start

As the "Simple Delay Line" plugin only works on a mono signal, the configuration above creates two effects: the one we named "e1" reads from and writes to the left FluidSynth audio channel "Main:L", the "e2" effect reads from and writes to the right channel "Main:R".

Please note that we only specified the path to the library /usr/lib/ladspa/delay.so when creating the "e1" effect, but not which plugin from the library to use. This is possible because the delay.so library contains only a single plugin. If you want to use a library that contains more than one plugin, you would need to give the plugin name as well, as we've done when creating the "e2" effect. The string to use here is what is called "Plugin Label" in the analyseplugin output.

Using the Configuration File

Lets start FluidSynth with ALSA output and the standard SoundFont, enable LADSPA effects, load the effects.txt config file and give it a test MIDI file to play: (You will need to replace the test.mid with your own MIDI file and maybe change the paths to the effects.txt file and the SoundFont)

user@host:$ fluidsynth -a alsa -o synth.ladspa.active=1 -f effects.txt FluidR3_GM.sf2 test.mid

You should now hear the MIDI file played at a slightly lower volume with a one second delay effect added on both left and right channel. If not, please check the FluidSynth output for any error messages.

Changing Parameters

You probably noticed that we did not set any values for the "Delay (Seconds)" and "Dry/Wet Balance" control ports. The delay plugin specifies default values for these parameters: 1 second delay and a dry/wet balance of 0.5 (check the analyseplugin output above). So when you don't override them, the defaults are automatically used for rendering.

Let's set different values now and set the delay time on the left channel to half a second and to 1.5 seconds on the right channel:

ladspa_effect e1 /usr/lib/ladspa/delay.so
ladspa_link e1 Input Main:L
ladspa_link e1 Output Main:L
ladspa_set e1 Delay 0.5

ladspa_effect e2 /usr/lib/ladspa/delay.so
ladspa_link e2 Input Main:R
ladspa_link e2 Output Main:R
ladspa_set e2 Delay 1.5

ladspa_start

Start FluidSynth again and you should hear that the delay is shorter on the left channel, longer on the right. You can even change control parameters while FluidSynth is running. Just type the ladspa_set ... commands into the FluidSynth shell.

And to check the difference that the LADSPA effects have on the sound output, you can turn them off and on again during run-time. Just type in ladspa_stop and ladspa_start into the FluidSynth shell.

Port Name Matching

Plugin port names are sometimes very long, because the plugin writers want them to be self-documenting. But note that we didn't need to give the complete port name "Delay (Seconds)" in the ladspa_set commands, but chose to use a much shorter version: "Delay".

When specifying a port name for the ladspa_link and ladspa_set commands, the system will look for any port that starts with the name you gave it. If there is only one match, then that port is chosen. If there are multiple matches (meaning your port name is ambiguous), you will see an error asking you to be more specific. So the configuration for the "e1" effect could also have been written with much shorter port names:

ladspa_effect e1 /usr/lib/ladspa/delay.so
ladspa_link e1 In Main:L
ladspa_link e1 Out Main:L
ladspa_set e1 Del 0.5

Signal Flow

The LADSPA effects unit runs immediately after the internal reverb and chorus effects have been processed. When no effects have been configured, the LADSPA engine is dormant and uses no additional system resources.

When at least one effect is configured and the engine is activated, the rendered audio is passed into the LADSPA effects engine, the effects are run in the order that they were created and the resulting audio is passed back into FluidSynth (and from there to the sound card or other output).

Effect Sends

Please note that SoundFont designers can specify how much signal each instrument should add to the reverb and chorus effect sends. When FluidSynth renders a block of audio, all currently sounding instruments are mixed into the Main output channels. In addition, all instruments add their signal to the effect send ports (Reverb:Send and Chorus:Send) according to the effect send amount specified in the SoundFont.

If you want to replace the internal reverb or chorus effects with a LADSPA plugin and you want to honour the decisions made by the SoundFont designer, you should use the Reverb:Send or Chorus:Send ports as effect input and Main:L and Main:R ports as effect outputs. (See the "Example Setups" section below for an example on how to replace the internal reverb with a LADSPA plugin.)

Please note that FluidSynth uses a mono signal for both effects, that is why there is only a single send port for reverb and chorus.

LADSPA Command Reference

The following is a description of all LADSPA-related commands that are available in the FluidSynth shell if it has been compiled with LADSPA support.

  • ladspa_effect: Create a new effect from a plugin library
  • ladspa_buffer: Create a new buffer
  • ladspa_link: Link an effect port to a host port or a buffer
  • ladspa_set: Set the value of an effect control
  • ladspa_check: Check the effect setup for any problems
  • ladspa_start: Start the effects unit
  • ladspa_stop: Stop the effects unit
  • ladspa_reset: Reset the effects unit

ladspa_effect

ladspa_effect <effect-name> <library-path> [plugin-name] [--mix [gain]]

Load the LADSPA plugin library given by <library-path> and create a new effect (i.e. an instance of a plugin). <effect-name> can be chosen by the user and must unique. <plugin-name> is optional if the library contains only one plugin.

If the optional --mix parameter is given, then the LADSPA engine will call the run_adding interface of the plugin. This will make the effect add it's output to the output buffers instead of replacing them. The --mix parameter takes an optional float value gain, which will be multiplied with each sample before adding to the output buffers.

Please note that there is no command to delete a single effect once created. To remove effects, please use ladspa_reset to clear everything start from scratch.

Can only be called when the effect unit is not active.

ladspa_buffer

ladspa_buffer <buffer-name>

Create a new audio buffer called <buffer-name>. The buffer is able to be used as mono output or mono input to an effect. Buffers can be used to connect plugins between each other without overwriting the host ports with temporary data.

Please note that there is no command to delete a buffer. To remove buffers, please use ladspa_reset to clear everything and start from scratch.

Can only be used when the effect unit is not active.

ladspa_link

ladspa_link <effect-name> <audio-port-name> <buffer-or-host-port-name>

Connects an effect input or output port with a buffer or a host port. This command can be called multiple times and will overwrite the previous connection made on that effect port.

Please note that there is no command to unlink an effect port. Use ladspa_reset to clear everything and start from scratch.

Can only be used when the effect unit is not active.

ladspa_set

ladspa_set <effect-name> <control-port-name> <float-value>

Sets a control port of an effect to a float value. Can be used at any time, even when the effect unit is active.

ladspa_check

ladspa_check

Checks the LADSPA effect configuration for errors. This command is also implicitly called when executing ladspa_start.

ladspa_start

ladspa_start

Activates the effects unit and inserts the configured effects into FluidSynth's audio rendering pipeline.

ladspa_stop

ladspa_stop

Deactivates the effects unit and removes the configured effects from FluidSynth's audio rendering pipeline. The configuration is left untouched, so it can be started again with ladspa_start.

ladspa_reset

ladspa_reset

Deactivates the effects unit if active and clears all configuration and loaded plugins.

Example Setups

All examples assume that your LADSPA_PATH environment variable points to the directory containing the plugin libraries (e.g. /usr/lib/ladspa).

Single Plugin

The following loads the delay.so plugin library from the LADSPA SDK and instantiates the delay effect under the name "e1". It connects the main left audio channel from FluidSynth with the plugin input and output and starts the effects engine.

ladspa_effect e1 delay.so
ladspa_link e1 Input Main:L
ladspa_link e1 Output Main:L
ladspa_start

The audible effect should be an untouched right channel and a slightly lower volume on the left with a delay effect of 1 second on top.

Replacing the FluidSynth Reverb Effect

If you would like a different reverb implementation than the one built-in to FluidSynth, you can use a LADSPA reverb plugin like the "TAP Reverb" from Tom's Audio Processing plugins.

Here is the analyseplugin output for the tap_reverb.so plugin:

user@host:$ analyseplugin /usr/lib/ladspa/tap_reverb.so

Plugin Name: "TAP Reverberator"
Plugin Label: "tap_reverb"
Plugin Unique ID: 2142
Maker: "Tom Szilagyi"
Copyright: "GPL"
Must Run Real-Time: No
Has activate() Function: Yes
Has deactivate() Function: No
Has run_adding() Function: Yes
Environment: Normal
Ports:	"Decay [ms]" input, control, 0 to 10000, default 2500
	"Dry Level [dB]" input, control, -70 to 10, default 0
	"Wet Level [dB]" input, control, -70 to 10, default 0
	"Comb Filters" input, control, toggled, default 1
	"Allpass Filters" input, control, toggled, default 1
	"Bandpass Filter" input, control, toggled, default 1
	"Enhanced Stereo" input, control, toggled, default 1
	"Reverb Type" input, control, 0 to 42.1, default 0, integer
	"Input Left" input, audio
	"Output Left" output, audio
	"Input Right" input, audio
	"Output Right" output, audio

Using this information we can create a LADSPA configuration:

effects.txt

ladspa_effect e1 /usr/lib/ladspa/tap_reverb.so
ladspa_link e1 "Input Left" Reverb:Send
ladspa_link e1 "Input Right" Reverb:Send
ladspa_link e1 "Output Left" Main:L
ladspa_link e1 "Output Right" Main:R
ladspa_start

Start FluidSynth with the internal reverb disabled. (You will need to replace the test.mid with your own MIDI file and maybe change the paths to the effects.txt file and the SoundFont)

user@host:$ fluidsynth -a alsa -R0 -o synth.ladspa.active=1 -f effects.txt FluidR3_GM.sf2 test.mid

You will hear the output with a reverb effect from the plugin. And you can change the reverb control ports with the ladspa_set command while the MIDI file is playing.

Multi-Channel Output

FluidSynth is capable of generating multi-channel output by specifying the synth.audio-groups and synth.audio-channels configuration settings. Explaining multi-channel output in detail is out of scope for this guide. But using multiple output channels has an effect on the host ports that are available to LADSPA plugins.

As soon as you configure more than one audio-channel, the main audio ports will not be called "Main:L" and "Main:R" anymore, but will have indices added to their name. So if you start FluidSynth with -o synth.audio-groups=2, then the following ports will be created:

  • Main:L1
  • Main:R1
  • Main:L2
  • Main:R2
  • Reverb:Send
  • Chorus:Send

If you want all main ports to act as outputs as well as inputs to the effects, then you also need to increase the synth.audio-channels setting.

LADSPA on other Platforms

LADSPA is a very simple plugin architecture and only requires the ladspa.h header file as compile-time dependency. To build FluidSynth on non-Linux platform with LADSPA support, download the ladspa.h file from https://www.ladspa.org and place it somewhere in your compiler include path. Then configure and build LADSPA as you normally would.

All information in the above documentation is valid for all other platforms as well. Just make sure you use the file path format specific to your platform in the ladspa_effect calls. For example, on Windows you should use

ladspa_effect c:\path\to\ladspa\plugin.dll

instead of

ladspa_effect /path/to/ladspa/plugin.so

Audacity provides a large number of precompiled LADSPA plugins for Windows and MacOS: https://www.audacityteam.org/download/plug-ins/

To get the analyseplugin and listplugins commands on Windows, you can either compile them yourself using the LADSPA-SDK source code from ladspa.org or install ladspa-sdk via Cygwin.