Sunday, September 11, 2011

SoundFonts want to be free

A "SoundFont" file  (suffix .SF2) is a definition of one or several musical instruments, which can be used with synthesizers (hardware or software) to render, or convert musical notes (eg MIDI files, suffix. MID) into digital sound, which can be used by an audio interface and speakers to play music. Another file format for the same purpose is DownLoadableSounds (suffix. DLS). Both include sound samples that can be entirely synthetic or digitized from real instruments.

General MIDI is a very popular specification, that among other things define a palette of instruments. The instrument #1 is a piano, #41 a violin, #57 trumpet, #74 flute... GM SoundFonts offer 128 instruments arranged in that particular order. GS and XG are extensions of this specification.

Linux needs GM soundfonts that could be distributed together with GPL programs, similar to the need of typographic fonts for text rendering applications. Many Linux distributions incorporate the FluidR3 soundfont in their repositories. It is free and produce good quality sound, but is not small: more than 140 megabytes. MuseScore distributes "TimGM6mb" soundfont, which "only" weights 5.8 megabytes.

There are several software synthesizers using SoundFonts. Well known are FluidSynth and TiMidity++, both with free licenses. One lesser known, but no less interesting, is Gervill. It is part of OpenJDK, and therefore GPLv2 licensed. It is implemented in Java, of course. I am not very fond of Java, and I do not usually use it except for commercial projects when the customer requires their use, but this time I will do it only for fun.

Gervill can use SoundFonts, DLS, or WAV files. A very interesting feature is the so-called "Emergency Soundbank", used when no other external SoundFont is available. Definitions of this SoundFont instruments are fully synthetic and follow the GM standard.

This EmergencySoundbank is a Java class, and does not reside on a file, therefore it can not be used on another synthesizer. However, nothing prevents us from creating a Java program that instantiates the class, and stores the instrument definitions on a disk file. How complicated may this program be? Let's see:

$ cat
public class MakeEmergencySoundfont {
        public static void main(String[] args) throws Exception {
                SF2Soundbank sf2 = EmergencySoundbank.createSoundbank();

7 lines is not much after all. Let's compile it:

$ javac warning: is internal proprietary API and may be removed in a future release
                SF2Soundbank sf2 = EmergencySoundbank.createSoundbank();
                ^ warning: is internal proprietary API and may be removed in a future release
                SF2Soundbank sf2 = EmergencySoundbank.createSoundbank();
2 warnings

We have earned two warnings for the naughty boys, because it is ugly to directly use classes in the namespace "*", and if the Oracle finds out our prank, he could lock the pantry. Here, take another cookie, Neo...

To compile and use this program you only need OpenJDK6 (runtime and compiler). For older Java versions you can get a "gervill.jar" from the project website.

Running the program produces a SoundFont file:

$ java MakeEmergencySoundfont
$ ls GervillEmergencySoundbank.sf2
-rw------- 1 pedro users 1.8M 2011-09-11 12:50 GervillEmergencySoundbank.sf2

The result weights less than 2 megabytes. Of course the quality of the SoundFont is not high, but it is better than installing nothing at all by default, leaving users wondering what do they need to hear something in their programs that depend on software synthesizers.

It is interesting that a similar technique, also very simple, can be used to produce other SoundFonts based on samples of arbitrary sounds. For more details, see the MakeSoundFont example in Gervill's repository.

Finally, a question remains about what license we can use to distribute the file "GervillEmergencySoundbank.sf2" generated by our program. As a general rule, the output of a GPL program has no restrictions. However, in this case the program output does not come from processing input data, but simply dumps the results of running the algorithms included in the EmergencySoundbank class. To play it safe, we should release it as GPL.