A music tracker (short version tracker ) is a type of music sequencer software for creating music. The music is represented as discrete musical notes positioned in several channels at discrete chronological positions on a vertical timeline. A music tracker's user interface is usually number based. Notes, parameter changes, effects and other commands are entered with the keyboard into a grid of fixed time slots as codes consisting of letters, numbers and hexadecimal digits.  Separate patterns have independent timelines; a complete song consists of a master list of repeated patterns.
Later trackers departed from solely using module files, adding other options both to the sound synthesis (hosting generic synthesizers and effects or MIDI output) and to the sequencing (MIDI input and recording), effectively becoming general purpose sequencers with a different user interface.
Music trackers like DefleMask and FamiTracker are commonly used to create chiptunes.
1987: origins on the Amiga
The term tracker derives from Ultimate Soundtracker ; the first tracker software.  Ultimate Soundtracker was written by Karsten Obarski and released in 1987 by EAS Computer Technik for the Commodore Amiga. Ultimate Soundtracker was a commercial product, but soon shareware clones such as NoiseTracker appeared as well. The general concept of step-sequencing samples numerically, as used in trackers, is also found in the Fairlight CMI sampling workstation of the early 1980s. Some early tracker-like programs appeared for the MSX ( Yamaha CX5M) and Commodore 64, before 1987, such as Sound Monitor , but these did not feature sample playback, instead playing notes on the computer's internal synthesizer. Later, in Rock Monitor 3 and 4 they implemented sample player, usually with short drum samples loaded in RAM memory.
The first trackers supported four pitch and volume modulated channels of 8-bit PCM samples, a limitation derived from the Amiga's Paula audio chipset and the commonplace 8SVX format used to store sampled sound. However, since the notes were samples, the limitation was less important than those of synthesizing music chips.
1990s: MS-DOS PC versions
During the 1990s, tracker musicians gravitated to the PC as software production in general switched from the Amiga platform to the PC. Although the IBM and compatibles initially lacked the hardware sound processing capabilities of the Amiga, with the advent of the Sound Blaster line from Creative, PC audio slowly began to approach CD Quality ( 44.1 kHz/16 bit/Stereo) with the release of the SoundBlaster 16.
Another sound card popular on the PC tracker scene was the Gravis Ultrasound, which continued the hardware mixing tradition, with 32 internal channels and onboard memory for sample storage. For a time, it offered unparalleled sound quality and became the choice of discerning tracker musicians. Understanding that the support of tracker music would benefit sales, Gravis gave away some 6000 GUS cards to participants. Coupled with excellent developer documentation, this gesture quickly prompted the GUS to become an integral component of many tracking programs and software. Inevitably, the balance was largely redressed with the introduction of the Sound Blaster AWE32 and its successors, which also featured on-board RAM and wavetable (or sample table) mixing.
The responsibility for audio mixing passed from hardware to software (the main CPU), which gradually enabled the use of more and more channels. From the typical 4 MOD channels of the Amiga, the limit had moved to 7 with TFMX players and 8, first with Oktalyzer and later with the vastly more popular OctaMED (all Amiga programs), then 32 with ScreamTracker 3 and FastTracker 2 on the PC and on to 64 with Impulse Tracker (PC) and MED SoundStudio (Amiga and later PC). An Amiga tracker called Symphonie Pro even supported 256 channels.
As such, hardware mixing did not last. As processors got faster and acquired special multimedia processing abilities (e.g. MMX) and companies began to push Hardware Abstraction Layers, like DirectX, the AWE and GUS range became obsolete. DirectX, WDM and, now more commonly, ASIO, deliver high-quality sampled audio irrespective of hardware brand.
There was also a split off from the sample based trackers taking advantage of the OPL2/OPL3 chips of the Sound Blaster series. Adlib Tracker II and many others survive to this day. All Sound Tracker was able to combine both the FM synthesis of the OPL chips and the sample based synthesis of the EMU-8000 chips in the Sound Blaster AWE series of cards as well as MIDI output to any additional hardware of choice.
2000s: Multiple platforms
Tracker music could be found in computer games of the late 1990s and early 2000s, such as the Unreal series, Deus Ex , Crusader: No Remorse , Jazz Jackrabbit and Hitman: Codename 47 . Tracker software continues to develop. Some of the early Amiga trackers such as ProTracker and OctaMED have received various updates, mostly for porting to other platforms. ProTracker having resumed development in 2004, with plans for releasing version 5 to Windows and AmigaOS, but only version 4.0 beta 2 for AmigaOS has been released. Other cross-platform trackers include Renoise, MilkyTracker and SunVox.
Buzz, OpenMPT, Renoise, Psycle, and others offer features undreamed-of back in the day (improved signal-to-noise ratios, automation, VST support, internal DSPs and multi-effects, multi I/O cards support etc.).
In 2005, FamiTracker, a tracker for the NES and Famicom, was officially released. As of version 0.5 beta, it is so far the only tracker for the system with full expansion audio support including the Sunsoft 5B.
2010s: Current state
In 2010, Modplug Tracker ( OpenMPT) and Renoise were two of the most popular and actively developed trackers.
Four new trackers, Aodix, Buzé, Radium, and SVArTracker, had extended the tracker concept with new features such as piano roll, tickless events, and graphical automation.
In 2011, DefleMask was released, a multi-system chipmusic tracker, supporting SEGA Genesis, SEGA Master System, Nintendo Game Boy, Yamaha's SMAF, among other systems.
and are two trackers running in the web browser.
There are several elements common to any tracker program: samples, notes, effects, tracks (or channels), patterns, and orders.
A note designates the frequency at which the sample is played back. By increasing or decreasing the playback speed of a digital sample, the pitch is raised or lowered, simulating instrumental notes (e.g. C, C#, D, etc.).
An effect is a special function applied to a particular note. These effects are then applied during playback through either hardware or software. Common tracker effects include volume, portamento, vibrato, retrigger, and arpeggio.
A track (or channel ) is a space where one sample is played back at a time. Whereas the original Amiga trackers only provided four tracks, the hardware limit, modern trackers can mix a virtually unlimited number of channels into one sound stream through software mixing. Tracks have a fixed number of "rows" on which notes and effects can be placed (most trackers lay out tracks in a vertical fashion). Tracks typically contain 64 rows and 16 beats, although the beats and tempo can be increased or decreased to the composer's taste.
A basic drum set could thus be arranged by putting a bass drum at rows 0, 4, 8, 12 etc. of one track and putting some hihat at rows 2, 6, 10, 14 etc. of a second track. Of course bass and hats could be interleaved on the same track, if the samples are short enough. If not, the previous sample is usually stopped when the next one begins. Some modern trackers simulate polyphony in a single track by setting the "new note action" of each instrument to cut, continue, fade out, or release, opening new mixing channels as necessary.
A pattern is a group of simultaneously played tracks that represents a full section of the song. A pattern usually represents an even number of measures of music composition.
An order is part of a sequence of patterns which defines the layout of a song. Patterns can be repeated across multiple orders to save tracking time and file space.
There are also some tracker-like programs that utilize tracker-style sequencing schemes, while using real-time sound synthesis instead of samples. Many of these programs are designed for creating music for a particular synthesizer chip such as the OPL chips of the Adlib and SoundBlaster sound cards, or the sound chips of classic home computers.
Tracker music is typically stored in module files where the song data and samples are encapsulated in a single file. Several module file formats are supported by popular music player programs such as Winamp or XMMS. Well-known formats include MOD, MED, S3M, XM and IT. Many of these formats can also be imported into existing trackers, allowing to view arrangement, instrumentation and the use of effect commands. This also makes the self-teaching of music composition using trackers easier and allows to extract instruments for later use in own songs, which was very common. 
Use in professional music
The hardcore techno scene has had many releases originally written using trackers and released on CD and vinyl.  Notable artists include Nasenbluten, Noisekick, Neophyte and Deadmau5.   
Also in video game soundtracks tracker music was used by notable artists as Andrew Sega, Alexander Brandon, Dan Gardopée and Peter Hajba. Video game developer Nicklas Nygren used Fast Tracker 2 in his early works (e.g. Knytt Stories  ) to compose the video game music. 
Selected list of music trackers
Windows, macOS, and Linux
|Name|| Latest |
|License||OS versions||File format support|| VST |
|2016-01||GPL / BSD||Yes||No||No||Load||Load||Load||Load||Yes||Yes||Yes|
|||2017-12||GPL / Commercial||Yes||Yes||Yes||Load||Load||Load||No||No||Yes||Yes|
These trackers run in virtual machines.
Atari ST / STE / Falcon
- ST SoundTracker (Equinox, 1990)
- Audio Sculpture (Synchron Assembly, 1990)
- Protracker STE (Equinox, 1991)
- Octalyser (Code & Alan F, 1992)
- Megatracker (Simplet & Axel Follet, 1994)
- DBE-Tracker (ST-Ghost, 1995)
- Digital Tracker (Emmanuel Jaccard, 1996)
- Graoumf Tracker (Laurent de SORAS, 1996)
- Digital Home Studio (Emmanuel Jaccard, 1997)
- FlexTrax  (Thomas Bergström, 2000)
- ACE Tracker  (Thomas Bergström, 2002)
- maxYMiser (Gareth Morris / gwEm, 2005)
- Hextracker (Paulo Simoes, 2011)
Commodore 8 bit machines (C64 / Plus/4 / C16)
- JCH-editor by JCH - a well-established C64 tracker from the 90s still in use today by professional SID-composers
- SDI by 6r6 - resembles JCH-editor, it suppors some MIDI-interface types too
- X-SID by Jeff - has JCH-style interface as well, with many instrument settings
- Ninjatracker by Cadaver - a simple but powerful tracker with low CPU usage (a few rasterlines' time)
- SID-Wizard by Hermit - a full-featured C64 tracker with big UI and MIDI file & input support
- 1raster-tracker (Hermit) - produces music in the timeframe of a single rasterline (cca. 63 microseconds)
- TEDzakker (Hermit) - tracker for the TED chip of Plus/4 and Commodore 16 with innovative sound tricks
- Ultimate Soundtracker (Karsten Obarski, 1987)
- Protracker (Lars Hamre, Anders Hamre, Sven Vahsen, Rune Johnsrud, 1990)
- OctaMED (1989, Teijo Kinnunen)
- NoiseTracker (Pex Tufvesson, Anders Berkeman, 1989)
- Audio Sculpture
- Radium 
- Scream Tracker (Sami Tammilehto, 1990)
- Fast Tracker (Triton Demo-Crew, 1995)
- Impulse Tracker (Jeffrey Lim, 1995)
- Sound Club ( Priit Kasesalu, Ahti Heinla, Jaan Tallinn: Estonia, 1993) .SN .MOD .MIDI modules
- Digitrakker (Prodatron)
Other operating systems
Allows users to create music for one system on another. Usually uses emulation to produce sound, some of trackers also can use real sound chips connected to host system.
- ChibiTracker (2006, Juan Linietsky) – ChibiTracker has a GNU open-source license and runs on FreeBSD, BeOS and Nintendo DS. 
- HoustonTracker (2015, irrlicht project) – An open source tracker for the Texas Instruments TI-82, TI-83/82STATS, and TI-83+/84+/SE calculators. 
- LittleGPTracker (2009) – LittleGPTracker is aimed at the GP2X hand-held console, but also runs on PSP, Windows, macOS and Linux. 
- Little Sound Dj (Johan Kotlinski) – Game Boy.
- MilkyTracker (2008, pailes et al.) – MilkyTracker has a GPL open-source license and runs on Linux, FreeBSD, Windows, Windows CE, macOS, Xbox 360, AROS and AmigaOS 4.  An unofficial port for Android is available on Google Play.  It supports .MOD and .XM module files, and attempts to recreate the user experience of Fasttracker II .
- NitroTracker (Tobias Weyland, 2006, GPL) – Nintendo DS 
- Propulse Tracker (Joel Toivonen, 2016-2017, GPL) - Linux, macOS, Windows.
- Schism Tracker (2003, Mrs. Brisby, Storlek et al.) – GPL licensed for Windows, macOS, Linux, FreeBSD, Wii, Pandora (console) (and further platforms with GCC 4 and SDL support). Based on a highly modified version of the ModPlug engine as a reimplementation of Impulse Tracker for modern operating systems. 
- Soundtracker (Jarosław Burczyński, 1990, public domain) – Soundtracker sequenced the three channels of the AY-3-8910 audio chip on the 128K models of ZX Spectrum. Though the instruments were referred to as samples, they were chip-generated sounds. The user is able to modify each sample via use of amplitude envelopes, noise frequency envelopes and pitch modifications. When these "samples" were played in the tracker they could also be modified by arpeggios (referred to as ornaments). A second program, ST Compiler , allowed Soundtracker songs to be integrated into other code.
- SunVox (2011, Alex Zolotov) – SunVox is freeware for Windows, Linux, macOS, Palm OS and Windows Mobile ( Windows CE); and available commercially for the iPhone/iPod touch/iPad and Android.
- GoatTracker by Cadaver - targets the Commodore 64 SID chip, highly optimizes the exported files
- CheeseCutter by Abaddon - based on the format of JCH-editor for the Commodore 64
- Bassoon Tracker - A tracker running in the web browser.