19 February 1997

The first computer with enhanced musical capabilities was the Amiga, a lesser known¹ system introduced in 1985. It had four digital channels of audio and a bonus co-processor to feed them. These advanced audio abilities did not go unnoticed by the many young programmers that owned them, and quickly a standard emerged for musical playback: the "Soundtracker Module", or MOD. The MOD is simply an audio format designed to take advantage of this four-channel setup.

The MOD format approaches music differently than you may be accustomed to, even if you know your computer sounds formats. Two other popular sounds formats that exist on computers are samples and MIDIs². By samples, I mean the general category of straight recordings of some sound. Often files have extensions like .wav, .au, and .aiff. Since a sample is merely a recording of a sound, it can be arbitrarily complex, but will often be very large. A typical three minute song would be several megabytes. Most newer music formats that a web user would be familiar with, including real audio and mpeg audio, are "sample" type sounds.

A MIDI is much more compact because it doesn't encode the actual sounds, but rather information on when to play a certain instrument. MIDI was developed for computer interaction with a device that knows how to make its own sound, most often a digital keyboard. A midi file says "play these notes on a piano, and these other notes on a trumpet." A MOD file most closely resembles a MIDI, with the addition of the samples, which are the instruments. A MOD file says "play these notes and use the recording of my dog as the instrument." Mods are usually much shorter than a similar piece of music recorded as a sample.

Each sample can be played back at a variety of frequencies to generate different notes on any of the four channels. A simple MOD might consist of a few drum samples and a piano. Then, perhaps two channels are used to lay down the drums and the other two channels for the piano sounds. In addition to playing different notes, the playing of a sample could be tweaked by select effects such as arpeggio and vibrato³.

Each channel has an associated 'track' which is merely a time sequence for what sample to play and how. Each track is assigned one of the digital channels for a total of four tracks. These are then organized into patterns, which have 64 rows of time steps in which to position notes. It looks something like a high-tech player piano roll. The mod player just looks at the first row, and simultaneously plays the samples (up to four) contained there. Then it waits a set amount of time and plays the samples on the next row. The patterns are played in the specified order to create the whole song.

This allowed for surprising freedom of composition, and MODs became hugely successful, with many trackers and players popping up. Even today, with Amigas no longer in production⁴, the MOD lives on. After DOS computers become aurally enhanced by the introduction of such sound cards as the Sound Blaster, people familiar with MODs ported the players to DOS⁵. More players and trackers appeared, and soon just as many MODs were being authored on DOS machines as on Amigas.

But on the DOS machines, the situation under the hood was quite different. The Amiga had four digital channels for outputting the sounds, but the newfangled Sound Blaster had only one. Fortunately, it is a fairly easy operation to mix waveforms. Each bit of each sample is added together, and the result is divided by four. It is simple wave interaction, with peaks cancelling troughs. The Amiga wasn't fast enough to mix samples this way, but the PC was, with the only disadvantage being a slight degradation of sound quality.

It didn't take long for the PC folk⁶ to ask themselves, "We can mix four sounds, why not eight?" The MOD file format was extended to accommodate MODs of eight channels. Pretty soon people got even fancier and added capabilites for up to thirty-two channels, as well as a wide array of new effects. These are enough removed from the original format to merit a new filename extension like .s3m or .xm, but there are essentially still based on MOD.

When I was first learning Java, I wanted to create a MOD player applet. However, I soon discovered that Java had only a very primitive and high-level sound support. Playback of sampled sounds is the only thing supported. A cool feature it did have is that you merely need to play back whatever samples you want, and the mixing of all currently playing samples is done automatically. That would certainly be a help if there was a way to play back the sounds at different frequencies.

It currently looks hopeless, as sound playback is only possible with java.applet.AudioClip, an opaque object that looks suspiciously like it was added to the language at the last minute⁷. A few people have written applets that play music in a very MOD-like fashion, but they download a different sample for each note, which destroys most of the coolness (and compactness) of MODs.

Recently, while tooling around with some of the private Sun classes to see how it all works, I realized it would be possible to create my own AudioClips with resampled versions of audio data. After some experimentation I came up with a sample that could be played at twice the speed! My code is not entirely up to spec: it relies on some Sun classes that are not a part of the standard JDK. They are usually behind the scenes for audio playback, but are present in all the Java supporting browsers that I know of. The classes work, but a serious applet or application should not depend on them.

MOD players are completely viable to program in Java. There is, however, a question of quality of the sound playback. Here, I have implemented a simple pattern player. I download the sample and resample it across a range of notes, and construct AudioClips for each of these notes. After this, all the hard work is done, and I just have to play back the right samples at the right times.

A bonus feature is the ability to draw upon a high quality source sample in creating my samples. Java only supports the playback of 8000 Hz samples, which as audio enthusiasts know is a very poor sound quality, about that used in the digital answering machines. The player I wrote can use a higher quality sample to construct the samples it needs for the music, resulting in better sound in the lower frequency samples.

I have coded up a simple tracker for you to use. Do some experimenting by creating your own small "song."

Oh no! You don't seem to have Java support! You can't see the cool tracker applet that I made just for you.

Instructions:

Since most of you aren't musicians anyway, I have provided a fun set of whimsical "instruments." Select an instrument and then click on the piano to test the note. Place the notes in the pattern by clicking on the cell of your choice. You can clear a cell by selecting the "clear sample" instrument. The speed setting is the number of milliseconds to wait between each row.

A complete java MOD player is a possibility save for effects, and if I have enough free time I would like to finish it. Certainly being "ported" to the Web will extend its life for years to come. From its humble beginnings on an alternative computer, the MOD has come a long way, yet has changed relatively little. Many of the original songs composed by young Amiga hackers are still quite enjoyable, and I recommend grabbing a player and visiting one of the brimming archives and listening to some of the classic pieces that have helped fuse technology and art.

-- Ray <ray@go2net.com> is a Dreamer of Dreams at go2net. If you told him he was gullible, he'd probably believe you.

Source code to the applet written for this article (archived):
tarred and gzipped or zipped.