Why use DSSI?

DSSI is an API for writing audio plugins with custom user interfaces. Although designed for synthesis plugins, it can also be used for effects. This document suggests a few reasons why you might want to consider using DSSI when writing synths or other plugins, or to consider adding DSSI support to a host application.

1. Synth authors

If you're thinking of writing a synth, here are some reasons you might want to make it a DSSI plugin.

Flexibility

DSSI plugins can accept MIDI and audio input, can have any number of controls and audio outputs, can support unlimited string-based configuration values as well as the numeric control ports, and can provide their own user interface using any GUI toolkit.

Using DSSI does not limit your synth to being used as a plugin in sequencers that support DSSI, because the DSSI distribution also includes a complete, efficient JACK and ALSA-sequencer based host application. This host will run a particular plugin automatically if invoked through a symbolic link -- so you can install your plugin in such a way as to be immediately operable as a standalone program as well, outwardly indistinguishable from any other JACK-based synth.
Structure

DSSI separates the plugin and user interface, using standard Open Sound Control (OSC) messages to communicate between them. This ensures that the plugin's controls are consistently externally available, provides a guarantee of automatability, and encourages clean plugin structure.

DSSI uses a parsed MIDI event structure to pass MIDI events to the plugin. This removes the responsibility of parsing MIDI in the plugin, a common source of problems in VST instruments.
Accessibility and control

A DSSI plugin can have any number of different user interfaces, which can be added at any time. It's also possible to use the same user interface code for more than one different plugin. DSSI user interfaces don't have to be graphical -- every DSSI plugin is naturally controllable, so can be used directly from external OSC control surfaces and other non-graphical interfaces.

DSSI also gives you a simple framework for basic requirements like mapping control ports onto MIDI controllers, so as to support MIDI controllers and MIDI automation as well as OSC and plugin port controls.
Simplicity

Writing the plugin is easy -- the only hard part is that troublesome synthesis...

Plugins can very easily be written incrementally, creating and testing the synthesis engine first before adding any advanced features or user interface. (Indeed there's absolutely no requirement to write a user interface at all.) DSSI is based on LADSPA, and the simplest DSSI synth plugins are just like LADSPA plugins with a single extra function to accept incoming note events.

Source code to a number of plugins is available to serve as examples or bases for new efforts. The DSSI distribution itself contains several plugins, including the simplest-possible monophonic synth, a straightforward polyphonic synth, and monotimbral samplers. Also available are several softsynths (Xsynth-DSSI, hexter, Sineshaper, WhySynth), a FluidSynth wrapper (FluidSynth-DSSI), a DSSI-to-VST bridge (dssi-vst), an X-Y controller GUI (xy-controller-dssi), a libconvolve wrapper (dssi_convolve), and an oscilloscope (ll-scope) which demonstrates shared memory use between plugin and UI. Most of these plugins include GUIs, with Qt and GTK examples available.
Clear host-neutral specification

DSSI has a written specification and an open, complete reference host. It was designed and tested independently of implementation in any existing host.

One of DSSI's designers is an author of a sequencer that is also a DSSI host, but none of the other people behind DSSI even use that host, and DSSI was largely designed and tested in its own test framework before being implemented in it. We're confident there are no hidden dependencies on any particular host.

The DSSI specifications also aim to reduce the amount of possible host dependency by specifying things like multithreading requirements, that usually go assumed in other APIs and end up causing portability problems.

2. Effects plugin authors

DSSI plugins do not have to be synths. DSSI is based on LADSPA 1.1, and is compatible with it. This makes for some interesting possibilities.

Dual LADSPA and DSSI plugins

A single loadable object file can provide both LADSPA and DSSI plugins through the separate LADSPA and DSSI discovery mechanisms. You can exploit this to make the same or related plugin code available via both APIs, for example to provide an effects plugin that supports true presets when queried via DSSI but uses an additional control input to mimic presets if queried via LADSPA (which does not support presets explicitly).
Custom GUIs for LADSPA plugins

The DSSI user interface standard consists of a set of guidelines for OSC message passing and user interface invocation. Nothing about it is necessarily specific to DSSI plugins, although the standard does assume that the plugin has at least the structure of a LADSPA plugin. You can take advantage of this to provide custom GUIs for standard LADSPA plugins. This is actually very easy to do, and the resulting GUIs will work already in any host that supports both DSSI and LADSPA.

3. Host authors

Here are some good reasons to consider adding DSSI support to your sequencer or other plugin host application.

Appropriateness for plugin authors

Any plugin API needs to have more plugins than hosts. The most important criterion for a host author is whether or not the API is a good one for writing plugins. DSSI is, for all the reasons given above.
Documented API and specification

DSSI is certainly more complex to implement for hosts than for plugins, and it's also more complex than some other synth APIs, but it's easier to get it right because almost everything is documented. Contrast this with something like VST: a VST host can be a simpler thing than a DSSI host, but writing a VST host involves so much guesswork that it's hard to be sure whether any given plugin will actually run in it.
Standard technology

DSSI's plugin interface is based on LADSPA and ALSA sequencer events, so any host that supports LADSPA, ALSA, and any form of synthesis will have most of the basics in place already. The GUI part is more unusual, but all it really consists of is an external control interface for plugins using the industry standard OSC -- which is a pretty good thing to have anyway.
Best chance as a workable standard

The options for plugin synth support in a Free Software MIDI application are limited, and DSSI is the only one we know of that even has a written specification. While there aren't very many DSSI plugins available at the time of writing -- hardly surprising, as it's still a new API -- DSSI is now supported by more hosts than any of the alternatives.
Free VST support

The dssi-vst bridge plugin gives any DSSI host free support for Windows VST instruments and effects.
Free LADSPA GUI support

Because DSSI is compatible with LADSPA, any DSSI implementation in an existing LADSPA host will get free support for custom LADSPA GUIs written to the DSSI GUI specification. (See the "Effects plugin authors" section above.)