After the MIDI 1.0 standard was finalized in the early 1980's, numerous musical instruments with MIDI jacks appeared upon the market. Musicians started to attach these instruments via their MIDI ports, and quickly discovered that the MIDI 1.0 specification had overlooked some important concerns.

One typical scenario may have been as follows:

A musician attaches his Roland D-10 to his Yamaha DX-7, because he prefers the front panel of the D-10, but prefers the sound of the DX-7, and he wants to use the D-10 to "play" the DX-7. He selects the patch labeled "Piano" on the D-10, and he plays the D-10 keyboard, and on the DX-7 he hears... a trumpet? How did this happen? Well, it happened because MIDI sends a program change message that contains only a patch number -- not the actual name of the patch. So if patch #1 on the DX-7 is a trumpet sound, then that's what he gets on the DX-7, despite the fact that selecting patch #1 on the D-10 yields a piano sound on the D-10. The MIDI 1.0 specification did not require that particular sounds be assigned to particular patch numbers, so every manufacturer used his own discretion as to "patch mapping".

But the real problem was with MIDI files that the musician made. MIDI files contain only MIDI messages. So, any program change event in a MIDI file refers only to a patch number as well -- not the actual patch name. So, this musician creates a MIDI file using his D-10. He has a piano track, so he puts a program change event at the track's beginning to select patch #1, which happens to be a Piano sound on his D-10. He takes that MIDI file to a friend's house. The friend has a DX-7. They play the MIDI file on that DX-7, and suddenly, the piano part is playing with a trumpet sound. Well, that's because patch #1 on the DX-7 is not a piano -- it's a trumpet sound. To "fix" the MIDI file, now the musician with the DX-7 has to edit the MIDI tracks and change every MIDI Program Change event so that it refers to the correct patch number on his DX-7. This deviance among MIDI sound modules made it very difficult for musicians to create MIDI arrangements that played properly upon various MIDI sound modules.

There were also some other deviances among early MIDI modules that made it more difficult to use them together via MIDI. To address these concerns, Roland proposed an addendum to the MIDI 1.0 specification in the late 1980's. This new addendum was called "General MIDI" (GM). It added some new requirements to the base MIDI 1.0 specification (but does not supplant any parts of the 1.0 specification -- the 1.0 specification is still the base level to which all MIDI devices should adhere). GM has now been adopted as part of the MIDI 2.0 specification.


General MIDI Patches

So to make MIDI Program Change messages of more practical use, Roland found it necessary to adopt a standard "patch bank". In other words, what was needed was to assign specific instrument sounds to specific patch numbers. For example, it was decided that patch number 1 upon all sound modules should be the sound of an Acoustic Grand Piano. In this way, no matter what MIDI sound module you use, when you select patch number 1, you always hear some sort of Acoustic Grand Piano sound. A standard was set for 128 patches which must appear in a specific order, and this standard is called General MIDI (GM). For example, patch number 25 upon a GM module must be a Nylon String Guitar. The chart, GM Patches, shows you the names of all GM Patches, and their respective Program Change numbers.

Nowadays, most modules (including the built-in sound modules of computer sound cards) ship with a GM bank (of 128 patches) so that it is easy to play MIDI files upon any MIDI module, without needing to edit all of the Program Change events in the file.


General MIDI Multi-Timbral requirement

Another burgeoning technology in the late 1980's was the multi-timbral module. Typically, there were deviances in the way that various manufacturers implemented this, since the 1.0 specification did not specifically address such devices. For example, some early multi-timbral modules supported only a limited set of the 16 MIDI channels simultaneously, so if you had a MIDI file with tracks upon unsupported MIDI channels, you wouldn't hear those tracks play back. You may not have even realized that those parts weren't being played.

So, one requirement of a GM-compliant module is that it must be fully multi-timbral, meaning that it can play MIDI messages upon all 16 channels simultaneously, with a different GM Patch sounding for each channel.


General MIDI Note Number assignments

There were also deviances in regards to Note Number mapping. For example, some manufacturers mapped middle 'C' to MIDI Note Number 60. Others mapped it to Note Numbers 72 or 48. Some modules even had middle C mapped to various places in different patches, depending upon the instrument. For example, a bass guitar patch may have middle C mapped to the highest C on the keyboard (since the most useful range on a bass guitar is below middle C). A flute patch may have middle C mapped to the lowest C on the keyboard.

The result was that, it became confusing to keep track of which key (ie, MIDI Note Number) played middle C for each patch. Also, when a MIDI track was played back upon certain modules, the part may play back an octave too high or low.

It therefore was decided that all patches must sound an A440 pitch when receiving a MIDI note number of 69. (ie, Note Number 69 plays the A above middle C, and therefore Note Number 60 is middle C).

There were deviances in regards to "drum machines" as well. Most drum machines (and drum units built into multi-timbral modules) play a different drum sound for each MIDI Note Number. But the 1.0 specification never spelled out which drum sounds were assigned to which MIDI note numbers. So, whereas note number 60 may play a snare upon one drum unit, upon another drum unit, it may play a crash cymbal. Again, this caused trouble with MIDI files, since sometimes a drum part would play back with the wrong drum sounds.

To address this discrepancy, the GM addendum contains a "drum map". This assigns about 48 common drum sounds to 48 specific MIDI Note Numbers. The assignments of drum sounds to MIDI notes is shown in the chart, GM Drum Sounds. Also, it was decided that a GM drum unit should default to using MIDI channel 10 to receive MIDI messages. Therefore, a composer of a GM MIDI file can safely assume that his drum part will play correctly if he uses the GM Drum note assignments and records the drum part upon MIDI channel 10.


General MIDI polyphony

Polyphony is how many notes a module can sound simultaneously. For example, perhaps a module can sound 32 notes simultaneously. Early MIDI modules typically had very limited polyphony. For example, the Prophet 5 could sound only 5 notes simultaneously.

This discrepancy in polyphony among MIDI modules made it difficult for arrangers to create MIDI files that played properly upon various modules. For example, if the arranger created too "busy" an arrangement, it could exceed the polyphony of a particular module, and therefore some of the notes may not be heard.

To address this discrepancy, the GM addendum stipulated that a GM module should be capable of sounding at least 24 notes simultaneously. (Ie, It must have 24 note polyphony). It could exceed 24 note polyphony, but it had to have at least that level of polyphony. In this way, if an arranger ensured that he never had more than 24 notes sounding simultaneously in his MIDI file, all notes of his arrangement would be heard upon any GM module.


Other General MIDI requirements

Finally, the GM addendum attempted to address some other discrepancies by spelling out a few more requirements.

A GM module should respond to velocity (ie, for note messages). This typically controls the VCA level (ie, volume) of each note, but the GM addendum unfortunately did not set a specific function for velocity. Some modules may allow velocity to affect other parameters on some patches.

The pitch wheel bend range should default to +/- 2 semitones. This allows an arranger to use pitch bend messages in his arrangement without worrying whether a bend that is supposed to be up 2 whole steps will instead jump up 2 octaves upon a certain sound module.

The module also should respond to Channel Pressure (often used to control VCA level or VCO level for vibrato depth). Again, the GM addendum unfortunately did not set a specific function for channel pressure, although typically it defaults to controlling the volume of a note while it is being held.

Finally, a GM module should also respond to the following MIDI controller messages: Modulation (1) (usually hard-wired to control LFO amount, ie, vibrato), Channel Volume (7), Pan (10), Expression (11), Sustain (64), Reset All Controllers (121), and All Notes Off (123). Additionally, the module should respond to these Registered Parameter Numbers: Pitch Wheel Bend Range (0), Fine Tuning (1), and Coarse Tuning (2).

There were also some default settings that a GM module should apply upon power up. Channel Volume should default to 90, with all other controllers and effects off (including pitch wheel offset of 0). Initial tuning should be standard, A440 reference.


General MIDI messages

The GM addendum did specify a couple System Exclusive messages to alter settings that are common to all GM units, but which were not addressed by the 1.0 specification.

One such message is for Master Volume -- not just the volume of a patch upon any one MIDI channel, but the master volume of the module itself.

There is also a System Exclusive message that can be used to turn a module's General MIDI mode on or off. This is useful for modules that also offer more expansive, non-GM playback modes or extra, programmable banks of patches beyond the GM set, but need to allow the musician to switch to GM mode when desired.


Conclusion

GM Standard makes it easy for musicians to put Program Change messages in their MIDI (sequencer) song files, confident that those messages will select the correct instruments on all GM sound modules, and the song file would therefore play all of the correct instrumentation automatically. Furthermore, musicians need not worry about parts being played back in the wrong octave. Finally, musicians didn't have to worry that a snare drum part, for example, would be played back on a Cymbal. The GM Standard also spells out other minimum requirements that a GM module should meet, such as being able to respond to Pitch and Modulation Wheels, and also being able to play 24 notes simultaneously (with dynamic voice allocation between the 16 Parts). All of these standards help to ensure that MIDI Files play back properly upon setups of various equipment.

The GM standard is actually not encompassed in the MIDI specification proper (ie, it's an addendum), and there's no reason why someone can't set up the Patches in his sound module to be entirely different sounds than the GM set. After all, most MIDI sound modules offer such programmability. But, most have a GM option so that musicians can easily play the many MIDI files that expect a GM module.

NOTE: The GM Standard doesn't dictate how a module produces sound. For example, one module could use cheap FM synthesis to simulate the Acoustic Grand Piano patch. Another module could use 24 digital audio waveforms of various notes on a piano, mapped out across the MIDI note range, to create that one Piano patch. Obviously, the 2 patches won't sound exactly alike, but at least they will both be piano patches on the 2 modules. So too, GM doesn't dictate VCA envelopes for the various patches, so for example, the Sax patch upon one module may have a longer release time than the same patch upon another module.


GM Patches

This chart shows the names of all 128 GM Instruments, and the MIDI Program Change numbers which select those Instruments.

The patches are arranged into 16 "families" of instruments, with each family containing 8 instruments. For example, there is a Reed family. Among the 8 instruments within the Reed family, you will find Saxophone, Oboe, and Clarinet.


Prog#   Instrument            Prog#    Instrument
 
 PIANO                           CHROMATIC PERCUSSION
1    Acoustic Grand             9   Celesta
2    Bright Acoustic           10   Glockenspiel
3    Electric Grand            11   Music Box
4    Honky-Tonk                12   Vibraphone
5    Electric Piano 1          13   Marimba
6    Electric Piano 2          14   Xylophone
7    Harpsichord               15   Tubular Bells
8    Clavinet                  16   Dulcimer
 
  ORGAN                          GUITAR
17   Drawbar Organ             25   Nylon String Guitar
18   Percussive Organ          26   Steel String Guitar
19   Rock Organ                27   Electric Jazz Guitar
20   Church Organ              28   Electric Clean Guitar
21   Reed Organ                29   Electric Muted Guitar
22   Accoridan                 30   Overdriven Guitar
23   Harmonica                 31   Distortion Guitar
24   Tango Accordian           32   Guitar Harmonics
 
  BASS                           SOLO STRINGS
33   Acoustic Bass             41   Violin
34   Electric Bass(finger)     42   Viola
35   Electric Bass(pick)       43   Cello
36   Fretless Bass             44   Contrabass
37   Slap Bass 1               45   Tremolo Strings
38   Slap Bass 2               46   Pizzicato Strings
39   Synth Bass 1              47   Orchestral Strings
40   Synth Bass 2              48   Timpani

  ENSEMBLE                       BRASS
49   String Ensemble 1         57   Trumpet
50   String Ensemble 2         58   Trombone
51   SynthStrings 1            59   Tuba
52   SynthStrings 2            60   Muted Trumpet
53   Choir Aahs                61   French Horn
54   Voice Oohs                62   Brass Section
55   Synth Voice               63   SynthBrass 1
56   Orchestra Hit             64   SynthBrass 2

  REED                           PIPE
65   Soprano Sax               73   Piccolo
66   Alto Sax                  74   Flute
67   Tenor Sax                 75   Recorder
68   Baritone Sax              76   Pan Flute
69   Oboe                      77   Blown Bottle
70   English Horn              78   Skakuhachi
71   Bassoon                   79   Whistle
72   Clarinet                  80   Ocarina

  SYNTH LEAD                     SYNTH PAD
81   Lead 1 (square)           89   Pad 1 (new age)
82   Lead 2 (sawtooth)         90   Pad 2 (warm)
83   Lead 3 (calliope)         91   Pad 3 (polysynth)
84   Lead 4 (chiff)            92   Pad 4 (choir)
85   Lead 5 (charang)          93   Pad 5 (bowed)
86   Lead 6 (voice)            94   Pad 6 (metallic)
87   Lead 7 (fifths)           95   Pad 7 (halo)
88   Lead 8 (bass+lead)        96   Pad 8 (sweep)
 
   SYNTH EFFECTS                  ETHNIC
 97  FX 1 (rain)               105   Sitar
 98  FX 2 (soundtrack)         106   Banjo
 99  FX 3 (crystal)            107   Shamisen
100  FX 4 (atmosphere)         108   Koto
101  FX 5 (brightness)         109   Kalimba
102  FX 6 (goblins)            110   Bagpipe
103  FX 7 (echoes)             111   Fiddle
104  FX 8 (sci-fi)             112   Shanai

   PERCUSSIVE                     SOUND EFFECTS
113  Tinkle Bell               121   Guitar Fret Noise
114  Agogo                     122   Breath Noise
115  Steel Drums               123   Seashore
116  Woodblock                 124   Bird Tweet
117  Taiko Drum                125   Telephone Ring
118  Melodic Tom               126   Helicopter
119  Synth Drum                127   Applause
120  Reverse Cymbal            128   Gunshot

Prog# refers to the MIDI Program Change number that causes this Patch to be selected. These decimal numbers are what the user normally sees on his module's display (or in a sequencer's "Event List"), but note that MIDI modules count the first Patch as 0, not 1. So, the value that is sent in the Program Change message would actually be one less. For example, the Patch number for Reverse Cymbal is actually sent as 119 rather than 120. But, when entering that Patch number using sequencer software or your module's control panel, the software or module understands that humans normally start counting from 1, and so would expect that you'd count the Reverse Cymbal as Patch 120. Therefore, the software or module automatically does this subtraction when it generates the MIDI Program Change message.

So, sending a MIDI Program Change with a value of 120 (ie, actually 119) to a Part causes the Reverse Cymbal Patch to be selected for playing that Part's MIDI data.


GM Drum Sounds

This chart shows what drum sounds are assigned to each MIDI note for a GM module (ie, that has a drum part).

 
MIDI    Drum Sound          MIDI    Drum Sound
Note #                      Note #
 35   Acoustic Bass Drum     59   Ride Cymbal 2
 36   Bass Drum 1            60   Hi Bongo
 37   Side Stick             61   Low Bongo
 38   Acoustic Snare         62   Mute Hi Conga
 39   Hand Clap              63   Open Hi Conga
 40   Electric Snare         64   Low Conga
 41   Low Floor Tom          65   High Timbale
 42   Closed Hi-Hat          66   Low Timbale
 43   High Floor Tom         67   High Agogo
 44   Pedal Hi-Hat           68   Low Agogo
 45   Low Tom                69   Cabasa
 46   Open Hi-Hat            70   Maracas
 47   Low-Mid Tom            71   Short Whistle
 48   Hi-Mid Tom             72   Long Whistle
 49   Crash Cymbal 1         73   Short Guiro
 50   High Tom               74   Long Guiro
 51   Ride Cymbal 1          75   Claves
 52   Chinese Cymbal         76   Hi Wood Block
 53   Ride Bell              77   Low Wood Block
 54   Tambourine             78   Mute Cuica
 55   Splash Cymbal          79   Open Cuica
 56   Cowbell                80   Mute Triangle
 57   Crash Cymbal 2         81   Open Triangle
 58   Vibraslap
A note-on with note number 42 will trigger a Closed Hi-Hat. This should cut off any Open Hi-Hat or Pedal Hi-Hat sound that may be sustaining. So too, a Pedal Hi-Hat should cut off a sustaining Open Hi-Hat or Closed Hi-Hat. In other words, only one of these three drum sounds can be sounding at any given time.

Similiarly, a Short Whistle should cut off a Long Whistle. A Short Guiro should cut off a Long Guiro. An Mute Triangle should cut off an Open Triangle. A Mute Cuica should cut off an Open Cuica.

Normally, all the above drum sounds have a fixed duration. Regardless of the time between when a Note-On is received and when a matching Note-Off is received, the drum sound always plays for a given duration. For example, assume that a device has a "Crash Cymbal 1" sound that plays for 4 seconds. If a Note-On for note number 49 is received, that cymbal sound starts playing. If a Note-Off for note number 49 is received only 1 second later, that should not cut off the remaining 3 seconds of the sound. The exceptions may be Long Whistle and Long Guiro, which may use the duration between the Note-On and Note-off to determine how "long" the sound plays.

If a drum is still sounding when another one of its Note-Ons is received, typically, another voice "stacks" another instance of that sound playing.