What Is M.I.D.I.?
Having asked the question its probably easier to start by looking at what M.I.D.I. isn't! The first possible surprise is that M.I.D.I. isn't a sound file format. It doesn't carry the same sort of information as a .WAV or .AIFF file for instance.

.WAV files contain digital recordings of real sounds, similar, in a way, to recordings stored on a CD. M.I.D.I. files don't contain this sort of information at all.

M.I.D.I. is actually a control system. A M.I.D.I. file contains instructions to perform particular commands.

A Bit Of History
The M.I.D.I. protocol was developed by musical instrument manufacturers in order to make the connection of different models and makes of instrument easier. Until this point it was difficult, for instance, to play one synthesizer from the keyboard of another. This meant that layering sounds was difficult, and that in order to have access to a wide variety of sounds the keyboard player had to be surrounded by synthesizers.

M.I.D.I. was devised as a Universal Control System, in other words, a way of one synthesizer controlling another, irrespective of the make and model of either. At its simplest level, when a note is pressed on the master keyboard in a M.I.D.I. setup, it will transmit an instruction to the instruments that are connected to it saying "play this note", and the receiving instrument(s) will respond.

So, a M.I.D.I. file simply contains the instructions for a receiving instrument. It transmits these instructions in the form of numbers, and in this way it allows musical instruments to talk to each other. It is an interface. This, of course, is where the term M.I.D.I. comes from, since it stands for:

Musical Instrument Digital Interface

One of the first results of the development of M.I.D.I. was the removal of keyboards from synthesizers. This began the proliferation of the M.I.D.I. Module. These usually come in the form of 19" rack mountable boxes (such as the Emu Vintage Keys, or Novation Bass Station) so that they can be easily integrated into music studios, but can actually take all sorts of shapes and sizes.

Is This Message For Me?
Let's suppose that the first receiving module in the chain is set to play a piano sound, and the second is set to play a brass sound. It would be helpful if the devices could distinguish between information which is targeted at each other, so that the piano sound could be played independently of the brass.

This is done through the use of M.I.D.I. Channels. Every instruction transmitted by the mother keyboard is identified as being on a particular M.I.D.I. Channel. Meanwhile the receiving modules are set to respond on their own M.I.D.I. Channels (these can be the same if desired). When an instruction arrives at the module, the device will only follow the instruction if the message's transmit channel (TX) corresponds to its receive channel (RX).

There are 16 M.I.D.I. channels within the basic protocol, but some hardware devices allow multiple streams of M.I.D.I. data, and hundreds of channels can be achieved.

So Where Does The Computer Fit Into All This Then?
Although the original idea was to allow one keyboard to control a number of sound sources (in the form of other synthesizers or M.I.D.I. modules) it wasn't long before computers started to be incorporated into the M.I.D.I. chain.

It was bound to happen, since M.I.D.I. involves instruments communicating by using a string of binary digits, and the one thing that computers can do is store and manipulate binary digits.

Although there are all sorts of computer programs designed to store and manipulate M.I.D.I. data, the most common is the Sequencer. This is a program which records and manipulates M.I.D.I. instructions, commonly called Events. Note that a M.I.D.I. sequencer doesn't record sounds (although the issue is getting somewhat confusing with the arrival of programs which integrate M.I.D.I. sequencing with Hard Disk Recording).

The most popular sequencer program across the P.C., Atari and Macintosh computer platforms is Cubase, published by Steinberg. However, I choose to use Logic, published by Emagic.

Ok. so, remembering that the sequencer only records the instruction emanating from a M.I.D.I. instrument will enable you to avoid one of the most common difficulties afflicting M.I.D.I. newbies. Here's the scenario:

Fledgling M.I.D.I. musician Jon-Mitchell Jam-Jar buys himself a computer, complete with a M.I.D.I. sequencer. Having read up on some of the classic synthesizers of the past 20 years he equips himself with a Yamaha DX7. He hooks up the M.I.D.I. cables correctly, and boots up his sequencer. He's ready to go.

He starts by recording a hypnotic repetitive bass part which he's very happy with. He now wants to add a string part over the top. He changes the sound on the DX7 so that its now playing a lush approximation of the London Symphony Orchestra, but ...... Oh No! The bass part is now being played by violins. "I've recorded my bass part into the computer" he says, "but its not playing back properly".

Of course, now that you know what a M.I.D.I. sequencer actually is you'll realize the mistake he's making. The computer and sequencer are simply recording M.I.D.I. instructions and not the sounds produced by the keyboard. When the sequencer plays the recorded data back it retransmits the instructions, just as the original keyboard would when it was played in the first place.

The computer is doing the job of a mechanized keyboard player. In fact, in many ways it is similar to the old pianolas, or player-pianos, which played music according to holes punched in a roll of paper. The sequencer punches the computerized equivalent of holes in the paper. Without the sound producing mechanism the holes are just ....uhm... holes. If Jon wants a bass sound and a string sound playing at once he'll either need two modules/synthesizers, with one sound each, or a device that is capable of playing more than one sound at once, on more that one M.I.D.I. channel (more of this in a bit).

If you connect the master keyboard so that it can send M.I.D.I. data to the computer, but is no connection to its M.I.D.I. In means that the computer can't "play" the master keyboard. This is a common scenario since many master M.I.D.I. keyboards are in fact "dumb", featuring no sound generating hardware themselves, but rather being devoted solely to producing M.I.D.I. instructions. The alternative is to use a genuine synthesizer for both roles.

The second thing to note is that the computer only has two M.I.D.I. connections. One is the M.I.D.I. In which receives instructions from the master keyboard. The other is a M.I.D.I. Out/Thru, which covers both functions. This is because when the master keyboard is being played it is necessary to retransmit the instructions arriving at the M.I.D.I. In to the following modules, but when the M.I.D.I. data is being replayed from the sequencer the computer itself needs to originate the data.

Don't All Talk At Once!
The master keyboard with a cable attached to its M.I.D.I. In is required if the master keyboard is in fact a synthesizer with its own in built sounds rather than a dumb controller. With the cable attached to the M.I.D.I. In the sequencer will be able to send instructions back to the master keyboard in order for it to generate its own sounds in response.

This causes a problem though.
If the keyboard isn't connected to any other M.I.D.I. devices, and a key is pressed on its keyboard, we want the key to cause the sound generator in the synthesizer to play the corresponding note. If this didn't happen then it wouldn't actually be possible to play the synthesizer as a stand-alone instrument, which of course it is.

When the keyboard is connected to the computer, the sequencer program will retransmit the received information, which will eventually be received back at the synthesizers M.I.D.I. In.

In short, the synthesizer will be receiving instructions from two places at once. This can result in two identical notes being played using the same sound, or if the targets for the two instructions are different, then unwanted sounds may play inappropriate parts. Either result halves the polyphony.

There are is a solution to this problem. In fact there is more than one, but I'll only look at the best, which breaks the connection between the sound generating part of the synthesizer and the keyboard. This is achieved by turning Local Off on the synthesizer. This may be easy or difficult depending on the complexity of the synthesizer concerned. The Alesis Quadrasyth for instance has a reference to Local Off in its manual, but the controls used to achieve it aren't called by that name, and are rather confusing. On the other hand the Kawai KC20 has a simple page where Local can be turned on or off.

Of course, leaving Local Off makes things simpler when working with a sequencer, but you'll need to remember to turn it back on again when you use the synthesizer as a stand-alone instrument. If you find the keyboard doesn't generate any sound in this situation, its probably worth checking that Local has been turned back on.

Is There Anything Else You Need To Tell Me?
When a key is pressed on the master keyboard, and instruction is sent to any receiving device to play the corresponding note, but there are other sorts of information besides the note numbers that need to be transmitted and received. For instance, it is possible for Program Changes to be sent via M.I.D.I.

In addition, the speed, or velocity at which the key is pressed is used to convey how hard the note was struck, movement of the pitch bend and modulation wheels can be transmitted via M.I.D.I.

This sort of information comes under the heading of M.I.D.I. Controller Data, some of which is pre-defined as being volume data, or pan data, and some which can be defined by the user so that the receiving instrument may map the data to an unspecified parameter.

What the receiving instrument will do with the incoming controller data is determined by that instrument, not by the transmitting device, so that if you were playing a keyboard on which full downwards pitch bend only produced a shift of a semi-tone, you may find that the receiving instrument shifted its pitch by an octave, unless it was set up to respond in the same way.

When the transmitting device sends a M.I.D.I. message it begins by saying what sort of message it is. Each of the controller types is numbered, so that if the receiving instrument receives a message labeled as Controller 7 it knows that this will be intended to set the instrument's volume.

Most instruments should conform to the controller assignments shown here:

Control Number Function
0 Undefined
1 Modulation Wheel
2 Breath Controller
3 Undefined
4 Foot Pedal
5 Portamento Time
6 Data Entry MSB
7 Main Volume Control
8 Balance
9 Undefined
10 Pan
11 Expression Controller
12-15 Undefined
16-19 General Purpose Controls
20-31 Undefined
32-63 LSB For Controls 0 to 31
64 Sustain Pedal
65 Portamento
66 Sostenuto
67 Soft Pedal
68 Undefined
69 Hold 2
70-79 Undefined
80-83 General Purpose Controls
84-90 Undefined
91 External Effects Depth
92 Tremolo Depth
93 Chorus Depth
94 Celeste Depth
95 Phase Depth
96 Data Entry Increment
97 Data Entry Decrement
98 Non-Registered Parameter MSB
99 Non-Registered Parameter LSB
100 Registered Parameter MSB
101 Registered Parameter LSB
102-120 Undefined
121-127 Channel Mode Messages

M.I.D.I. Files, Schmiddy Files
Once people started creating sequencer files it became obvious that they would want to exchange them, either because they were working with other M.I.D.I. musicians, or because they simply wanted to play their work to other people.

There were problems though. Firstly, a file recorded using one sequencer program couldn't be played by another sequencer program, and secondly, although it was possible to embed instructions to change programs within sequencer files, the sounds that were attached to various programs within different synthesizers didn't necessarily bare any relation to each other.

The first difficulty was overcome by the development of M.I.D.I. files. These are sometimes referred to as MIDIs, or .MID files. These allowed sequences to be transported between different software setups.

The second problem was more difficult though.
It was solved by the development of a standard by which sounds were assigned to the 128 available program numbers, so that, in short, when the original composer determined that a part should be played by a trumpet, it would be played by a trumpet when loaded into someone else's system.

The standard allocation of sounds is called General M.I.D.I., or GM.
This standard is somewhat frowned upon within the professional music making world, since it is seen partly as a way of dictating to M.I.D.I. musicians what sounds they should use, but also because it is lumped together as a close relative of the one finger auto-accompaniment found on many home keyboards.

Having said that, it is undoubtedly a very useful way of ensuring that M.I.D.I. Files sound close to the way they were intended. Of course, an Akai piano won't sound the same as an Ensoniq piano, but at least piano parts will be played by pianos when GM is used.

The standard sound allocation for GM sounds applies to all M.I.D.I. channels except M.I.D.I. Channel 10. More of this in a minute. On all other channels the sounds are allocated to program numbers as follows:

Prog. No. Instrument
1 Grand Piano
2 Bright Acoustic Piano
3 Electric Grand Piano
4 Honky Tonk Piano
5 Electric Piano 1
6 Electric Piano 2
7 Harpsichord
8 Clavi
9 Celesta
10 Glockenspiel
11 Music Box
12 Vibraphone
13 Marimba
14 Xylophone
15 Tubular Bells
16 Dulcimer
17 Drawbar Organ
18 Percussive Organ
19 Rock Organ
20 Church Organ
21 Reed Organ
22 Accordion
23 Harmonica
24 Tango Accordion
25 Acoustic Guitar (Nylon)
26 Acoustic Guitar (Steel)
27 Electric Guitar (Jazz)
28 Electric Guitar (Clean)
29 Electric Guitar (Muted)
30 Over driven Guitar
31 Distortion Guitar
32 Guitar Harmonics
33 Acoustic Bass
34 Electric Bass (Finger)
35 Electric Bass (Pick)
36 Fretless Bass
37 Slap Bass 1
38 Slap Bass 2
39 Synth Bass 1
40 Synth Bass 2
41 Violin
42 Viola
43 Cello
44 Contrabass
45 Tremolo Strings
46 Pizzicato Strings
47 Orchestral Harp
48 Timpani
49 String Ensemble 1
50 String Ensemble 2
51 Synth Strings 1
52 Synth Strings 2
53 Choir Aahs
54 Choir Oohs
55 Synth Voice
56 Orchestral Hit
57 Trumpet
58 Trombone
59 Tuba
60 Muted Trumpet
61 French Horn
62 Brass Section
63 Synth Brass 1
64 Synth Brass 2
65 Soprano Sax
66 Alto Sax
67 Tenor Sax
68 Baritone Sax
69 Oboe
70 English Horn
71 Bassoon
72 Clarinet
73 Piccolo
74 Flute
75 Recorder
76 Pan Flute
77 Blown Bottle
78 Shakuhachi
79 Whistle
80 Ocarina
81 Lead 1 (Square)
82 Lead 2 (Sawtooth)
83 Lead 3 (Calliope)
84 Lead 4 (Chiff)
85 Lead 5 (Charang)
86 Lead 6 (Voice)
87 Lead 7 (Fifths)
88 Lead 8 (Bass & Lead)
89 Pad 1 (New Age)
90 Pad 2 (Warm)
91 Pad 3 (Polysynth)
92 Pad 4 (Choir)
93 Pad 5 (Bowed)
94 Pad 6 (Metallic)
95 Pad 7 (Halo)
96 Pad 8 (Sweep)
97 FX 1 (Rain)
98 FX 2 (Soundtrack)
99 FX 3 (Crystal)
100 FX 4 (Atmosphere)
101 FX 5 (Brightness)
102 FX 6 (Goblins)
103 FX 7 (Echoes)
104 FX 8 (Sci-Fi)
105 Sitar
106 Banjo
107 Shamisen
108 Koto
109 Kalimba
110 Bagpipe
111 Fiddle
112 Shanai
113 Tinkle Bell
114 Agogo
115 Steel Drums
116 Woodblock
117 Taiko Drum
118 Melodic Tom
119 Synth Drum
120 Reverse Cymbal
121 Guitar Fret Noise
122 Breath Noise
123 Seashore
124 Bird Tweet
125 Telephone Ring
126 Helicopter
127 Applause
128 Gunshot

Under the GM standard, M.I.D.I. Channel 10 is reserved for percussion sounds, and a system for allocating sounds to particular notes is specified too:

Note No.Note Name Sound
35 B1 Acoustic Bass Drum
36 C1 Bass Drum 1
37 C#1 Side Stick
38 D1 Acoustic Snare
39 D#1 Hand Clap
40 E1 Electric Snare
41 F1 Low Floor Tom
42 F#1 Closed Hi-Hat
43 G1 High Floor Tom
44 G#1 Pedal Hi-Hat
45 A2 Low Tom
46 A#2 Open Hi-Hat
47 B2 Low Mid-Tom
48 C2 High Mid-Tom
49 C#2 Crash Cymbal 1
50 D2 High Tom
51 D#2 Ride Cymbal 1
52 E2 China Cymbal
53 F2 Ride Bell
54 F#2 Tambourine
55 G2 Splash Cymbal
56 G#2 Cowbell
57 A3 Crash Cymbal 2
58 A#3 Vibraslap
59 B3 Ride Cymbal 2
60 C3 High Bongo
61 C#3 Low Bongo
62 D3 Mute Hi Conga
63 D#3 Open Hi Conga
64 E3 Low Conga
65 F3 Hi Timbale
66 F#3 Low Timbale
67 G3 High Agogo
68 G#3 Low Agogo 2
69 A4 Cabasa
70 A#4 Maracas
71 B4 Short Whistle
72 C4 Long Whistle
73 C#4 Short Guiro
74 D4 Long Guiro
75 D#4 Claves
76 E4 High Woodblock
77 F4 Low Woodblock
78 F#4 Mute Cuica
79 G4 Open Cuica
80 G#4 Mute Triangle
81 A5 Open Triangle

General M.I.D.I. also specifies that:

The receiving instrument should be 16 note polyphonic across M.I.D.I. channels 1 to 9, and 11 to 16.

The receiving instrument should be 8 note polyphonic on M.I.D.I. channel 10.

All 128 program numbers should be implemented.

Controllers 1, 7, 10, 11, 64, 121 and 123 must be implemented

Do I Need A Synth Then?
Because they simply contain instructions to play a musical note, rather than digitized recorded audio, M.I.D.I. files are relatively small, and therefore provide a relatively easy way of providing a soundtrack to the World Wide Web, or CD ROM based multimedia software.

Many web pages now include embedded M.I.D.I. files, and computer games provide a musical accompaniment through M.I.D.I.

Of course, not everyone who wants to listen to music in this way has a M.I.D.I. equipped keyboard at their disposal. Luckily they don't need one.

Although M.I.D.I. files which are played via an internet connection can be routed through a M.I.D.I. interface to an external device, it is also possible to have them played by a M.I.D.I. device housed inside the computer itself. The quality of the device and its method of operation will depend on the computer, along with the additions that have been made to it, but generally, M.I.D.I. based music is an accepted facility that computers now provide.

In the case of PCs (IBM compatibles/Windows and Intel based machines), the M.I.D.I. processing hardware is usually housed along with the audio hardware on the computer's soundcard. This may simply offer the sounds themselves along with an output port to enable connection to a stereo system, or to external speakers. Alternatively, the soundcard may offer M.I.D.I. Ins and Outs, enabling the connection of a keyboard and receiving devices.

Apple Macs are already equipped with sound generating hardware, and include a software addition called Quicktime Musical Instruments, as part of the system software. This is effectively a basic GM synthesizer which will not only play M.I.D.I. files from within other programs, but may also be accessed by a sequencer as an instrument.

If you visit a web page with an embedded M.I.D.I. file you'll be able to play it as you look at the page, as long as you have a Plug-In which can process the M.I.D.I. data and send it to a M.I.D.I. equipped soundcard or to Quicktime Musical Instruments.

If you download and save a M.I.D.I. file you'll be able to play it with a variety of software when you're off-line. There are many shareware M.I.D.I. file players available, my Mac favorite is Arnold's M.I.D.I. Player, or if you have a sequencer program you could load it onto there and play it on a GM synthesizer.

And Finally...
If you're a musician M.I.D.I. will provide you with a way of getting your ideas to your ears (and hopefully to other people's), and if you're a music consumer M.I.D.I. will provide you with a way of hearing other people's work. There will be times when you won't know whether you're listening to a M.I.D.I. or an audio file. That's the way it should be. Just enjoy the music. But at least you now know something about how it all works.