Introduction

Xsynth-DSSI is a classic-analog (VCOs-VCF-VCA) style software synthesizer which operates as a plugin for the DSSI Soft Synth Interface.

Xsynth-DSSI began as a simple DSSI port of Steve Brooke's Xsynth, but has since grown considerably to include the following features:

• polyphonic operation,

• band-limited oscillators,

• a new, more stable filter mode, and

• velocity-sensitive envelopes.

You can hear some samples of Xsynth-DSSI at Brian Collins' excellent linuxsynths.com site.

Xsynth-DSSI is written by Sean Bolton, and licensed under the GNU General Public License, version 2 or later.

A Request

When developing open-source software for a niche market such as Linux audio, it can be hard to know how many people are using one's work, and to feel like it's worth the effort to maintain it.

If you find Xsynth-DSSI useful, please share your appreciation by sending me any or all of the following:

• an email stating where you're from and how you're using Xsynth-DSSI, sent to (sean at smbolton dot com)

• copies of or links to music you've created with Xsynth-DSSI (and let me know if I can share it here!)

• any patches you create for Xsynth-DSSI

• suggestions for improving Xsynth-DSSI

• stars for the Xsynth-DSSI github repo

• offers of paying work writing this sort of code!

Download

Most major distributions include an Xsynth-DSSI package:

• Arch

• Debian

• Fedora

• FreeBSD

• openSUSE

• PlanetCCRMA

• Ubuntu

Source tarballs: http://sourceforge.net/projects/dssi/files/xsynth-dssi/

Github repository: https://github.com/smbolton/xsynth-dssi

Requirements

Xsynth-DSSI requires the following:

• DSSI version 0.9 or greater.

• liblo version 0.12 or later (0.23 or later recommended), a library implementing the Open Sound Control (OSC) protocol.

• pkgconfig with PKG_CONFIG_PATH set appropriately to pick up DSSI and liblo.

• GTK+ version 1.2 or 2.x. (If GTK+ is not found, the plugin will be built without the GUI.)

• the LADSPA v1.x SDK.

• the ALSA headers (DSSI plugins use ALSA structures, but not the actual drivers, so you don't necessarily need the drivers installed.) Users of non-Linux systems can use libdssialsacompat.

• a working DSSI host. Xsynth-DSSI has been tested with jack-dssi-host, available in the DSSI distribution, and with ghostess.

• automake 1.7, autoconf 2.58, and libtool 1.4.3 or better if you wish to recreate the build files.

Installation

The generic installation instructions in the enclosed file INSTALL aren't particularly helpful, so try this instead:

1. Unpack the tar file.

2. Make sure PKG_CONFIG_PATH is set correctly to locate the dssi.pc and liblo.pc pkgconfig files. On many systems, this will be:

   $ PKG_CONFIG_PATH=/usr/local/lib/pkgconfig
   $ export PKG_CONFIG_PATH

3. 'cd' into the package directory and execute './configure'. configure will add '-Wall' and my favorite optimizations to CFLAGS for you if you don't include any '-Wall' or '-O' options. If you're using gcc and wish to provide your own optimization flags, you MUST at least use '-finline' and a non-zero '-O' flag to get decent results.

4. Enable debugging information if you desire: edit the file src/xsynth.h, and define XSYNTH_DEBUG as explained in the comments.

5. Do 'make'. Hopefully it should build without warnings (or errors.)

6. 'make install' will install the following:

   <prefix>/lib/dssi/xsynth-dssi.so
   <prefix>/lib/dssi/xsynth-dssi/Xsynth_gtk
   <prefix>/share/xsynth-dssi/factory_patches.Xsynth
   <prefix>/share/xsynth-dssi/version_0.1_patches.Xsynth

Feedback on your experiences building Xsynth-DSSI is appreciated.

Operation

To run the Xsynth-DSSI plugin under the jack-dssi-host provided in the DSSI distribution, I do the following:

1. Start JACK.

2. Start jack-dssi-host, supplying the correct plugin path and filename (substitute <prefix> as appropriate):

   $ DSSI_PATH=<prefix>/lib/dssi sudo jack-dssi-host xsynth-dssi.so

   jack-dssi-host should start, and moments later the Xsynth-DSSI graphic user interface should appear.

3. Use aconnect or kaconnect to connect jack-dssi-host to a MIDI source, such as vkeybd.

4. Begin playing sounds! If you get no response, try clicking the 'Send Test Note' button in the Xsynth-DSSI GUI. This sends a note directly via the host to the plugin, so if you hear sound now, look for a problem between the example host and your MIDI source. If you still don't hear sound, I'd look for a problem between the example host and your output device. If you continue having trouble, you might recompile with XSYNTH_DEBUG bit 2 set, which will cause the plugin to continuously output a quiet buzz to help debug your outgoing signal path.

Xsynth-DSSI starts up with a small set of sound presets, or 'patches', that you can select either by selecting the GUI 'Patches' tab and clicking on the patch name, or by sending a MIDI program change from your MIDI source with the appropriate program number.

Voice Architecture

Xsynth-DSSI models a simple analog synthesizer, where two oscillators (VCO1 and VCO2) are mixed (MIXER) and fed to a low pass filter (VCF), which then feeds an amplifier (VCA) controlling the output volume:

Modulation of these elements is possible using two envelope generators (EG1 and EG2), a low-frequency oscillator (LFO), and the MIDI key and velocity.

MIDI Controller Mapping

For DSSI hosts that support MIDI controller mapping, Xsynth-DSSI requests that they map two MIDI controllers:

• MIDI Control Change 5 "Portamento time" is mapped to the PORTAMENTO 'glide' control, although in a somewhat backward way: higher CC values map to shorter glide times, and lower CC values to longer glide times.

• MIDI Control Change 8 "Balance" is mapped to the MIXER balance control.

Xsynth-DSSI itself interprets several other MIDI control messages:

• MIDI Control Change 7 "Volume" controls the output level, without affecting the VOLUME control.

• MIDI Control Change 1 "Modulation wheel" reduces the VCF cutoff.

• MIDI channel pressure and key pressure are combined to boost the VCF resonance.

Questions That Might Be Frequently Asked

Q1. Woah! Where'd that nasty sound come from?

A1. A certain grittiness has always been part of Xsynth's charm for me, but if you're getting really nasty sound from a patch, check for the following:

• Volume too high: especially when using high resonance, your signal may be so hot it's clipping.

• Resonance too high: yep, the filter will go to self-oscillation with high resonance settings. Try backing the resonance off.

• Resonance too low: Xsynth's filter is unstable near zero resonance when used with high cutoffs. Try setting the resonance to 0.2.

• Resonance getting boosted by keyboard or channel pressure: try playing softly or turning your keyboard's pressure sensitivity off. I know, this should be configurable....

• XSYNTH_DEBUG XDB_AUDIO set: if you've got a ~600Hz buzz in the output even when you're not playing anything, your plugin was probably compiled with the XDB_AUDIO debug bit set. Fix that and recompile.

• Pitch too high: even with the band-limited oscillators, it is possible to get audible aliasing on very high notes. This is especially true when using oscillator sync while VCO2 is producing a sine wave, since the band-limiting technique doesn't deal as well with waveforms having continuously varying slope.

Q2. Help! I just installed Xsynth-DSSI 0.9.x, and now my sessions that used the older Xsynth-DSSI 0.1.x don't sound right. What should I do?

A2. Xsynth-DSSI 0.1.3 is the last 0.1.x-compatible version, but it does include the anti-aliased oscillators and MVCLPF-3 filter. If you just want to use your existing sessions, you'll have to use version 0.1.3. If you want to convert them to 0.9.x, you'll need to save any custom patches or patch banks using a 0.1.x version, then install 0.9.x, load the custom patches, then tweak the mix and glide settings as needed. (Note that the version 0.1.x default patch set is installed by later versions as <prefix>/share/xsynth-dssi/version_0.1_patches.Xsynth)

Q3. Why aren't the GUI slider values and the corresponding LADSPA port values the same?

A3. The LADSPA port values are the actual numeric values used by the synthesis code, while the GUI slider values are all the 0-to-10 values used in Steve Brookes' original Xsynth interface.

Q4. Why aren't my patch edits being saved between sessions?

A4. When you edit and save patches, the patch data (which is really just the settings for the DSSI plugin ports) can exist in four places, only two of which may persist between runs. Twiddling the knobs in the Xsynth-DSSI Patch Editor, or in your DSSI host's port editor, changes the values the plugin is currently using to make sound (represented by the leftmost box in the diagram below.) If you want to save your edits as a one of the patch ‘Presets’ (the middle box), use the ‘Save Changes’ button on the Patch Editor. Note that the middle box only exists in the plugin's working memory, and if you want to permanently save your changes, you need to get them to one of the boxes on the right.

One option is to use ‘Save Patch Bank...’ from the ‘File’ menu, so save the patch bank to a file (the bottom right box) which you can then reload later using ‘Open Patch Bank...’.

The other option (the top right box) is to use a DSSI host which will save the session data for you, and restore it upon reopening the session (not all hosts do this). Until you fully understand how your host works, it's best to save a file, too.

Attribution and License

Xsynth-DSSI is written and copyright © 2017 by Sean Bolton, and licensed under the GNU General Public License, version 2 or later.

It includes contributions and bits of borrowed code from many others, all under various copyleft or non-copyleft open-source licenses. See the file AUTHORS for details.

In particular, if you're interested in Steve Brooke's original Xsynth, the code can still be found here.