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The dark and forgotten art of GENMIDI

Doom’s music consists (effectively) of MIDI music files. MIDI is an odd beast. The General MIDI standard defines a set of 128 common instruments supported by MIDI playback systems. For example, instrument #10 is “Glockenspiel”. You can think of them like the instrument presets on an electronic keyboard. You can find a full list of instruments on Wikipedia.

Modern synthesizers, keyboards and operating systems with MIDI playback capability use recorded samples of real musical instruments, so when the MIDI file specifies “Glockenspiel” as the instrument, you hear notes from a real glockenspiel playing the song. But Doom is from an older, more primitive age.

Computers in the early '90s (at least, the ones that had sound cards) often had a SoundBlaster-compatible sound card. As well as the digital playback functionality that provides the sound effects in Doom, these cards also had a YM3812 (OPL2) or YMF262 (OPL3) chip that was used for music playback. These are FM synthesis chips designed and made by Yamaha for their range of electronic keyboards.

Back then, music playback with samples was difficult (or at least expensive / CPU intensive), so FM synthesis provided a more primitive alternative. By generating two waves and modulating one with the other, it’s possible to create noises that at least vaguely resemble real musical instruments.

The OPL2 could generate 9 simultaneous voices simultaneously. You can think of each of these voices as being like a noise-making box with a bunch of knobs on the front to control the sound. Turn the knobs to one set of settings and the noise that comes out sounds like a Glockenspiel, change the settings and it sounds like a bass guitar or a snare drum instead.

These “knob settings” are commonly stored in .sbi instrument files. Looking online you can find huge collections of .sbi files created back when people still actually used them.

To play back MIDI music using an OPL chip, you need a bank of instrument data - effectively, instrument data for each instrument in the General MIDI set. Doom uses an IWAD lump named GENMIDI that contains exactly this - effectively 175 SBI files merged together (128 main instruments + 47 percussion effects).

It turns out that Doom’s instrument data is a particularly good set. Freedoom is using a set of instruments that comes from OpenBSD’s kernel, which isn’t so good. So this directory has a script that builds a GENMIDI lump out of individual SBI files, the idea being that they can individually be replaced with better ones to improve it.

Editing instruments

First of all, make sure you follow the same rules as any other Freedoom submissions. DO NOT USE SBI FILES OFF WEBSITES. Everything has to be completely original content.

Lots of editors exist for OPL instrument data. Probably the most powerful one out there is AdlibTracker2. AT2 is actually a tracker for making songs, but it has an instrument editor built-in. I’ll give some instructions for how to use this.

First of all, modern computers don’t have an OPL chip, so you’ll need to use DOSbox, which can emulate one. You need to do this even if you’re using Windows. Download a copy of AT2 off the website and a copy of DOSbox, and get the one running in the other.

Once you’ve opened up AT2, press Ctrl-I to bring up the instrument control panel. You should see a list of instruments, which will currently be empty. It’s probably best to start by loading an existing instrument file and editing it, so press Ctrl-L to load an instrument into the first slot. Find one of the existing SBI files that you want to improve and press enter to select it; you should see the instrument appear next to “iNS_01” in the first slot.

To hear what the instrument currently sounds like, you can use the keys on your keyboard like a piano:

 S D   G H J    2 3   5 6 7
Z X C V B N M  Q W E R T Y U

To improve the current instrument settings, press tab to bring up the instrument editor. You’ll get a panel with a wide range of different settings to play with, and you can still use the “piano keys” to hear what effect your changes are having. Press enter to switch between editing the carrier and the modulator - they both affect the sound in different ways.

One thing to be aware of when editing instruments is that you should only use the first four waveform types (Sine, Half-Sine, Abs-Sine, Pulse-Sine). Do NOT use the last four waveform types (Sine (EPO), Abs-Sine (EPO), Square, Derived square). The reason is that the full eight are only supported by the OPL3 chip - the OPL2 only suported the first four. You will get a warning message later if you use these.

When it comes to saving your changes, press Ctrl-S and type a filename for your new file. There’s a slight catch here in that although AT2 can load SBI format instrument files, it can only save in its own “a2i” instrument format. Fortunately we can convert back into SBI format later.

Generating a new GENMIDI

Now you’ve made your snazzy new instrument, you’ll want to hook it into the GENMIDI lump and try it out in-game. Put your .a2i file into the lumps/genmidi/instruments directory, then open up in your favorite text editor.

You’ll see two lists of instrument definitions, for the main instruments and for percussion. There are some instructions at the top of the file for how to write these definitions, so I won’t repeat them here. But if your file is named eg. snazzy.a2i, you’ll want to write something like:


If you’re editing a percussion instrument, make sure that you remember to specify a note you want to play. For example, for 'C#' in Octave -3:

Instrument("snazzy.a2i", note=On3.Cs)

Once you’re done, save your changes and type this to build a new GENMIDI for testing:

python3 mkgenmidi genmidi.lmp

To try it out, get a source port with OPL emulation support (eg. Chocolate Doom) and add genmidi.lmp on the command line, eg.

chocolate-doom -file genmidi.lmp

Converting back to SBI

Once you have your finished instrument, it’s probably best if you convert from the rather ugly .a2i format into the more common .sbi format. More tools support .sbi and some people might prefer to use other instrument editors instead of AT2. There’s a useful script included that will do exactly this. Specify the name of the a2i file and the name of the desired output file, as follows:

python3 a2i-to-sbi snazzy.a2i

Don’t forget to update to use the .sbi version of the file instead of the .a2i version!