GusSoundfont

From the ALSA wiki

Jump to: navigation, search

Contents

GUS patches vs. AWE Soundfonts

by Greg Lee

GUS patches are more convenient to work with, I find, than big monolithic Soundfonts. You don't need a special utility to reconfigure your instrument banks or to tune up individual instruments in minor ways, for instance. And there are some midi players that can read GUS patches but can't deal with Soundfonts. On the other hand, the GUS patch world is moribund, and instruments in Soundfonts can have more features than are implemented in GUS patch files. E.g., velocity layers, stereo instruments, parameters to control an extra LFO, and so on.

There's a way to have the advantages of both GUS patches and Soundfonts. We need a tool to break up Soundfonts into sets of GUS patches and, to keep the musical advantages of Soundfont instruments, a way to extend the GUS patch format to comprehend the richer characterizations of the instruments in Soundfonts.

A subsidiary point concerns how config files describe instruments and banks of instruments for GUS patches and Soundfonts. For midi players that understand both Soundfonts and GUS patches, it would be convenient to have a unified config file syntax that characterizes intruments and banks in the same way for GUS and Sf2.

Extracting GUS patches from Soundfonts

George Foot (gfoot at users dot sourceforge dot net) already wrote a program to extract GUS patch files from a Soundfont. It's part of a tool distributed with the Allegro game programming library and is called pat2dat. It didn't quite suit my purposes, so I rewrote parts and called the new version unsf. The source code is in the package of demo programs described below. unsf finds each instrument in a given Soundfont, writes out a GUS style config file, creates a directory for each melodic and percussion bank in the Soundfont, then creates GUS patches for every instrument and puts the files into the bank directories it made.

Instruments and percussion banks are named in Soundfonts, but the names often have troublesome characters in them, so unsf fixes up the names before using them as names of files and directories. The patch files are in the extended GUS format described two sections below.

This utility program and all of the programs mentioned below are for Linux. Whether they can be used on other systems, I don't know. unsf may be useable on bigendian systems, but this is untested.

If someone knows of a Soundfont whose samples are really freely distributable, it's possible that unsf could make some contribution to the mission of freepats.

Demonstration software

Source code for the just described tool unsf and other related software can be found in this ftp archive. There is:

There are some example 6 channel ogg files derived from midis using gt and patches extracted from a large soundfont by unsf in gt-demo

In addition, the current CVS version of SDL and SDL_mixer includes a midi player, derived from timidity, which I've modified to read extended GUS patches (and also to do surround sound on Linux).

Extended GUS patches

It might not be 100% true that the GUS patch format is insufficient to describe Soundfont instruments. The are fields in the GUS patch header characterized as layers and instruments that could potentially be made use of. But for practical reasons, extra Soundfont information has to be hidden inside the patch, so as to be compatible with programs that know only about ordinary GUS patches. For instance, Timidity's patch file reader will not even try to load a patch file which says it has more than one layer or more than one instrument.

Luckily, the GUS format provides several unused, reserved areas, and extra information can be tucked away there, so that programs unaware of the sf2 extensions to GUS patches can still read and use the patches. There are two 40 byte reserved areas in the GUS patch area which I propose to use for velocity and stereo layer information, and there is a 36 byte reserved area in the GUS sample header which I want to use for sf2 parameters such as modulation envelope points.

There is room in the reserved areas of GUS patch headers to keep information about 19 velocity layers (by which I mean sets of sound samples tuned to timbres produced by making louder or softer notes). I hope that's enough. The highest number of layers I've encountered so far in an sf2 instrument is 12. An sf2 velocity layer can have a single set of samples (each sample for a different key range), or two, in the case of a stereo instrument. An unaware midi player or synth will see only the left channel of the first velocity layer, but there will be additional samples after the first one, and aware programs can find out about them by examining the hidden information in the patch header.

I've tried out Timidity++, which knows practically nothing about velocity layers, on the extended GUS patches produced by unsf, and it plays them just fine.

Here are details about the extended GUS format: detailed format

Config file syntax

There is no need to extend the syntax of cfg files to accommodate extended GUS patches, except, I suppose one might want add some options for sf2 parameters. However, the config syntax worked out by Takashi Iwai for Soundfonts, which is used in Timidity++, leaves something to be desired. So far as I know (I confess to never actually having used it), it doesn't allow choosing or rejecting individual patches within Soundfonts, or making changes like volume adjustments to specific patches. The config file reader of my demo version of Timidity, on the other hand, works for sound fonts almost the same as for GUS patches. Two differences of detail are:

In the case of GUS patches, of course the names given for patches have to be pathnames of actually existing files, but for Soundfonts, the names you give for patches or banks can be anything you make up. The Soundfont reader digs out the samples by bank and instrument number; the specific names used don't matter.

The downside to this way of configuring Soundfonts for use is that you have to declare in the config file the banks and instruments you want, otherwise they won't be loaded. Using Iwai's method is much simpler in this respect. And how do you know just what banks and instruments are there inside the Soundfont so that you can prepare a config file? Since that's not straightforward, I provided the utility sf2cfg, which examines a Soundfont and automatically constructs a config file for it. You can then edit the config file as convenient.

Comments

This is an excellent proposal, thank you Greg!

Very nice. Perhaps you might include the JACK sound driver...

Very nice. Perhaps you might include your thoughts on the origins of the universe

Retrieved from "http://alsa.opensrc.org/GusSoundfont"

Category: MIDI

GITHUB | EDIT