The Usenet MIDI Primer

Bob McQueer

Purpose

It seems as though many people in the USENET community have an interest in the Musical Instrument Digital Interface (MIDI), but for one reason or another have only obtained word of mouth or fragmentary descriptions of the specification. Basic questions such as “what’s the baud rate?”, “is it EIA?” and the like seem to keep surfacing in about half a dozen newsgroups. This article is an attempt to provide the basic data to the readers of the net.

Reference

The major written reference for this article is version 1.0 of the MIDI specification, published by the International MIDI Association, copyright 1983. There exists an expanded document. This document, which I have not seen, is simply an expansion of the 1.0 spec. to contain more explanatory material, and fill in some areas of hazy explanation. There are no radical departures from 1.0 in it. I have also heard of a “2.0″ spec., but the IMA claims no such animal exists. In any event, backwardscompatibility with the information I am presenting here should be maintained.

Conventions

I will give constants in C syntax, ie. 0x for hexadecimal. If I refer to bits by number, I number them starting with 0 for the low order (1′s place) bit. The following notation:

text                            

will be used to delimit commentary which is not part of the “bare-bones” specification. A sentence or paragraph marked with a question mark in column 1 is a point I would kind of like to hear something about myself.OK, let’s give it a shot.

Physical connector specs

The standard connectors used for MIDI are 5 pin DIN. Separate sockets are used for input and output, clearly marked on a given device. The spec. gives 50 feet as the maximum cable length. Cables are to be shielded twisted pair, with the shield connecting pin 2 at both ends. The pair is pins 4 and 5, pins 1 and 3 being unconnected:

                              2
                          5       4
                        3           1

 A device may also be equipped with a “MIDI-thru” socket which is used to pass the input of one device directly to output.

I think this arrangement shows some of the original conceptionof MIDI more as a way of allowing keyboardists to control multiple boxes than an instrument to computer interface. The”daisy-chain” arrangement probably has advantages for a performing musician who wants to play “stacked” synthesizers for a desired sound, and has to be able to set things up on the road.                          

Electrical Specification

Asynchronous serial interface. The baud rate is 31.25 Kbaud (+/- 1%). There are 8 data bits, with 1 start bit and 1 stop bit, for 320 microseconds per serial byte. MIDI is current loop, 5 mA. Logic 0 is current ON. The specification states that input is to be opto-isolated, and points out that SharpPC-900 and HP 6N138 optoisolators are satisfactory devices. Rise and fall time for the optoisolator should be less than 2 microseconds. The specification shows a little circuit diagram for the connections to a UART. I am not going to reproduce it here. There’s not much to it – I think the important thing it shows is +5 volt connection to pin 4 of the MIDI out with pin 5 going to the UART, through 220ohm load resistors. It also shows that you’re supposed to connect to the “in” side of the UART through an optoisolator, and to the MIDI-thru on the UART side of the isolator.

I’m not much of a hardware person, and don’t really know whatI’m talking about in paragraphs like the three above. I do recognize that this is a “non-standard” specification, which won’t work over serial ports intended for anything else. People who do know about such things seem to either have giggling or gagging fits when they see it, depending on their dispositions, saying things like “I haven’t seen current loop since the days of the old teletypes”. I also know the fast 31.25Kbaud rate pushes the edge for clocking commonly available UART’s.                        

Data Format

For standard MIDI messages, there is a clear concept that one device is a “transmitter” or “master”, and the other a “receiver” or “slave”. Messages take the form of opcode bytes, followed by data bytes. Opcode bytes are commonly called “status” bytes, so we shall use this term.

This is very similar to handling a terminal via escape sequences. There aren’t ACK’s or other handshaking mechanisms in the protocol.                       

Status bytes are marked by bit 7 being 1. All data bytes must contain a 0 in bit 7, and thus lie in the range 0 – 127. MIDI has a logical channel concept. There are 16 logical channels, encoded into bits 0 – 3 of the status bytes of messages for which a channel number is significant. Since bit 7 is taken over for marking the status byte, this leaves 3 opcode bits for message types with a logical channel. 7 of the possible 8 opcodes are used in this fashion, reserving the status bytes containing all 1′s in the high nibble for “system” messages which don’t have a channel number. The low order nibble in these remaining messages is really further opcode.

If you are interested in receiving MIDI input, look over the system messages even if you wish to ignore them. Especially the”system exclusive” and “real time” messages. The real time messages may be legally inserted in the middle of other data, and you should be aware of them, even though many devices won’t use them.                       

Voice Messages

I will cover the messages with channel numbers first. The opcode determines the number of data bytes for a single message (see “running status byte”, below). The specification divides these into “voice” and “mode” messages. The “mode” messages are for control of the logical channels, and the control opcodes are piggybacked onto the data bytes for the “parameter” message. I will go into this after describing the “voice messages”. These messages are:

status byte meaning data bytes
0×80-0x8f note off 2 – 1 byte pitch, followed by 1 byte velocity
0×90-0x9f note on 2 – 1 byte pitch, followed by 1 byte velocity
0xa0-0xaf key pressure 2 – 1 byte pitch, 1 byte pressure (after-touch)
0xb0-0xbf parameter 2 – 1 byte parameter number, 1 byte setting
0xc0-0xcf program 1 byte program selected
0xd0-0xdf chan. pressure 1 byte channel pressure (after-touch)
0xe0-0xef pitch wheel 2 bytes gives a 14 bit value, least significant 7 bits first

Many explanations are necessary here: For all of these messages, a convention called the “running status byte” may be used. If the transmitter wishes to send another message of the same type on the same channel, thus the same status byte, the status byte need not be resent. Also, a “note on” message with a velocity of zero is to be synonymous with a “note off”. Combined with the previous feature, this is intended to allow long strings of notes to be sent without repeating status bytes.

From what I’ve seen, the “zero velocity note on” feature is very heavily used. My Sequential Six-Trak sends these, even though it sends status bytes on every note anyway. Roland stuff uses it.                      

The pitch bytes of notes are simply number of half-steps, with middle C = 60.

On keyboard synthesizers, this usually simply means which physical key corresponds, since the patch selection will change the actual pitch range of the keyboard. Most keyboards have one C key which is unmistakably in the middle of the keyboard. This is probably note 60.                      

The velocity bytes for velocity sensing keyboards are supposed to represent a logarithmic scale – “advisable” in the words of the spec. Non-velocity sensing devices are supposed to send velocity 64. The pitch wheel value is an absolute setting, 0 – 0x3FFF. The 1.0 spec. says that the increment is determined by the receiver. 0×2000 is to correspond to a centered pitch wheel (unmodified notes).

I believe standard scale steps are one of the things discussed in expansions. The Six-Trak pitch wheel is up/down about a third. I believe several makers have used this value, but I may be wrong. The “pressure” messages are for keyboards which sense the amountof pressure placed on an already depressed key, as opposed to velocity, which is how fast it is depressed or released.? I’m not really certain of how “channel” pressure works. Yamaha is one maker that uses these messages, I know.            

Now, about those parameter messages.Instruments are so fundamentally different in the various controls they have that no attempt was made to define a standard set, like say 9 for “Filter Resonance”. Instead, it was simply assumed that these messages allow you to set “controller” dials, whose purposes are left to the given device, except as noted below. The first data bytes correspond to these “controllers” as follows:

data byte  
0 – 31 continuous controllers 0 – 31, most significant byte
32 – 63 continuous controllers 0 – 31, least significant byte
64 – 95 on / off switches
96 – 121 unspecified, reserved for future
122 – 127 the “channel mode” messages I alluded to above. See below.

The second data byte contains the seven bit setting for the controller. The switches have data byte 0 = OFF, 127 = ON with 1 – 126 undefined. If a controller only needs seven bits of resolution, it is supposed to use the most significant byte. If both are needed, the order is specified as most significant followed by least significant. With a 14 bit controller, it is to be legal to send only the least significant byte if the most significant doesn’t need to be changed.

This may of, course, wind up stretched a bit by a given manufacturer. The Six-Trak, for instance, uses only single byte values (left justified within the 7 bits at that), and recognizes >32 parameters.                    

Controller number 1 IS standardized to be the modulation wheel.

?Are there any other standardizations which are being followed by most manufacturers?                     

Mode Messages

These are messages with status bytes 0xb0 through 0xbf, and leading data bytes 122 – 127. In reality, these data bytes function as further opcode data for a group of messages which control the combination of voices and channels to be accepted by a receiver. An important point is that there is an implicit “basic” channel over which a given device is to receive these messages. The receiver is to ignore mode messages over any other channels, no matter what mode it might be in. The basic channel for a given device may be fixed or set in some manner outside the scope of the MIDI standard. The meaning of the values 122 through 127 is as follows:

first data byte meaning second data byte
122 local control 0 = local control off, 127 = on
123 all notes off 0
124 omni mode off 0
125 omni mode on 0
126 monophonic mode number of monophonic channels, or 0 for a number equal to receivers voices
127 polyphonic mode 0

124 – 127 also turn all notes off. Local control refers to whether or not notes played on an instruments keyboard play on the instrument or not. With local control off, the host is still supposed to be able to read input data if desired, as well as sending notes to the instrument, very much like “local echo” on a terminal, or “half duplex” vs. “full duplex”. The mode setting messages control what channels and how many voices the receiver recognizes. The “basic channel” must be kept in mind. “Omni” refers to the ability to receive voice messages on all channels. “Mono” and “Poly” refer to whether multiple voices are allowed. The rub is that the omni on/off state and the mono/poly state interact with each other. We will go over each of the four possible settings, called “modes” and given numbers in the specification:

mode 1: Omni on, Poly
voice messages received on all channels andassigned polyphonically. Basically, any notes it gets, itplays, up to the number of voices it’s capable of.
mode 2: Omni on, Mono
monophonic instrument which will receivenotes to play in one voice on all channels.
mode 3: Omni off, Poly
polyphonic instrument which will receivevoice messages on only the basic channel.
mode 4: Omni off, Mono
A useful mode, but “mono” is a misnomer.To operate in this mode a receiver is supposed to receiveone voice per channel. The number channels recognized will begiven by the second data byte, or the maximum number of possiblevoices if this byte is zero. The set of channels thus definedis a sequential set, starting with the basic channel.

The spec. states that a receiver may ignore any mode that it cannot honor, or switch to an alternate – “usually” mode 1. Receivers are supposed to default to mode 1 on power up. It is also stated that power up conditions are supposed to place a receiver in a state where it will only respond to note on and note off messages, requiring a setting of some sort to enable the other message types.

I think this shows the desire to “daisy-chain” devices for performance from a single master again. We can set a series of instruments to different basic channels, tie ‘em together, and let them pass through the stuff they’re not supposed to play to someone down the line.This suffers greatly from lack of acknowledgment concerning modes and usable channels by a receiver. You basically have to know your device, what it can do, and what channels it can do it on.I think most makers have used the “system exclusive” message (see below) to handle channels in a more sophisticated manner, as well as changing “basic channel” and enabling receipt of different message types under host control rather than by adjustment on the device alone.The “parameters” may also be usurped by a manufacturer for mode control, since their purposes are undefined.Another huge problem with the “daisy-chain” mental set of MIDI is that most devices always shovel whatever they play to their MIDI outs, whether they got it from the keyboard or MIDI in. This means that you have to cope with the instrument echoing input back at you if you’re trying to do an interactive session with the synthesizer. There is a drastic need for some MIDI flag which specifically means that only locally-generated data is to go to MIDI out. From device to device there are ways of coping with this, none of them good.                   

System Messages

The status bytes 0×80 – 0x8f do not have channel numbers in the lower nibble. These bytes are used as follows:

byte purpose data bytes
0xf0 system exclusive variable length
0xf1 undefined
0xf2 song position 2 – 14 bit value, least significant byte first
0xf3 song select 1 – song number
0xf4 undefined
0xf5 undefined
0xf6 tune request 0
0xf7 EOX (terminator) 0

The status bytes 0xf8 – 0xff are the so-called “real-time” messages. I will discuss these after the accumulated notes concerning the first bunch. Song position and song select are for control of sequencers. The song position is in beats, which are to be interpreted as every 6 MIDI clock pulses. These messages determine what is to be played upon receipt of a “start” real-time message (see below). The “tune request” is a command to analog synthesizers to tune their oscillators. The “system exclusive” message is intended for manufacturers to use to insert any specific messages they want to which apply to their own product. The following data bytes are all to be “data” bytes: that is, they are all to be in the range 0 – 127. The system exclusive is to be terminated by the 0xf7 terminator byte. The first data byte is also supposed to be a “manufacturer’s id”, assigned by a MIDI standards committee. The terminator byte is optional: a system exclusive may also be “terminated” by the status byte of the next message.

Yamaha, in particular, caused problems by not sending terminator bytes. As I understand it, the DX-7 sends a system exclusive at something like 80 msec. intervals when it has nothing better to do, just so you know it’s still there, I guess. The messages aren’t explicitly terminated, so if you want to handle the protocol (esp. in hardware), you should be aware that a DX-7 will leave you in “waiting for EOX” state a lot, and be sending data even when it isn’t doing anything. This is all word of mouth, since I’ve never personally played with a DX-7.                    

Some MIDI ID’s:

Sequential Circuits 0×01 Bon Tempi 0×20 Kawai 0×40
Big Briar 0×02 S.I.E.L. 0×21 Roland 0×41
Octave/Plateau 0×03   Korg 0×42
Moog 0×04 SyntheAxe 0×23 Yamaha 0×43
Passport Designs 0×05
Lexicon 0×06
PAIA 0×11 
Simmons 0×12 
Gentle Electric 0×13 
Fairlight 0×14

Note the USA/Europe/Japan grouping of codes. Also note that Sequential Circuits snarfed id number 1 – Sequential Circuits was one of the earliest participators in MIDI; some people claim its originator. Two large makers missing from the original lineup were Casio and Oberheim. I know Oberheim is on the bandwagon now, and Casio also, I believe. Oberheim had their own protocol previous to MIDI, and when MIDI first came out they were reluctant to go along with it. I wonder what we’d be looking at if Oberheim had pushed their ideas and made them the standard. From what I understand they thought theirs was better, and kind of sulked for a while until the market forced them to go MIDI.

? Nobody seems to care much about these ID numbers. I can only imagine them becoming useful if additions to the standard message set are placed into system exclusives, with the ID byte to let you know what added protocol is being used. Are any groups of manufacturers considering consolidating their efforts in a standard extension set via system exclusives?

Real Time Messages

This is the final group of status bytes, 0xf8 – 0xff. These bytes are reserved for messages which are called “real-time” messages because they are allowed to be sent at any place and any time. This includes in between data bytes of other messages. A receiver is supposed to be able to receive and process (or ignore) these messages and resume collection of the remaining data bytes for the message which was in progress. Realtime messages do not affect the “running status byte” which might be in effect.

? Do any devices really insert these things in the middle of other messages?                  

All of these messages have no data bytes following (or they could get interrupted themselves, obviously). The messages:

0xf8 timing clock
0xf9 undefined
0xfa start
0xfb continue
0xfc stop
0xfd undefined
0xfe active sensing
0xff system reset

The timing clock message is to be sent at the rate of 24 clocks per quarter note, and is used to sync. devices, especially drum machines. Start, continue, and stop are for control of sequencers and drum machines. The continue message causes a device to pick up at the next clock mark.

These things are also designed for performance, allowing control of sequencers and drum machines from a “master” unit which sends the messages down the line when its buttons are pushed. I can’t tell you much about the trials and tribulations of drum machines. Other folks can, I am sure.                   

The active sensing byte is to be sent every 300 ms. or more often, if it is used. Its purpose is to implement a timeout mechanism for a receiver to revert to a default state. A receiver is to operate normally if it never gets one of these, activating the timeout mechanism from the receipt of the first one.

My impression is that active sensing is largely unused.                   

The system reset initializes to power up conditions. The spec. says that it should be used “sparingly” and in particular not sent automatically on power up.

And now, climbing to the pulpit …

 … from here on out.There are many deficiencies with MIDI, but it is a standard. As such, it will have to be grappled with.The electrical specification leaves me with only one question – Why? What was wanted was a serial interface, and a perfectly good RS232 specification was to be had. Why wasn’t it used? The baud rate is too fast to simply convert into something you can feed directly to your serial port via fairly dumb hardware, also. The “standard” baud rate step you would have to use would be 38.4Kbaud which very few hardware interfaces accept. The other alternative is to buffer messages and send them out a slower baud rate – in fact buffering of characters by some kind of I/O processor is very helpful. Hence units like the MPU-401, which does a lot of other stuff too, of course.The fast baud rate with MIDI was set for two reasons, I believe: to allow daisy-chaining of a few devices with no noticeable end-to-end lag, and to allow chords to be played by just sending all the notes down the pipe, the baud rate being fast enough that they will sound simultaneous.It doesn’t exactly work – I’ve heard gripes concerning end-to-end lag on three instrument chains. And consider chords – at two bytes (running status byte being used) per note, there will be a ten character lag between the trailing edges of the first and last notes of a six-note chord. That’s 3.2 ms., assuming no “dead air” between characters. It’s still pretty fast, but on large chords with voices possessing distinctive attack characteristics, you may hear separate note beginnings. I think MIDI could have used some means of packetizing chords, or having transaction markers. If a “chord” message were specified, you could easily break even on byte count with a few notes, given that we assume all notes of a chord at the same velocity. Transaction markers might be useful in any case, although I don’t know if it would be worth taking over the remaining system message space for them. I would say yes. I would see having “start” and “end” transaction bytes. On receipt of a “start”, a receiver buffers up but does not act on messages until receipt of the “end” byte. You could then do chords by sending the notes ahead of time, and precisely timing the “end” marker. Of course, the job of the hardware in the receiver has been complicated considerably.The protocol is very keyboard oriented – take a look at the use of two of the opcodes in the limited opcode space for “pressure” messages, and the inability to specify semitones or glissando effects except through the pitch wheel (which took up yet another of the opcodes). All keyboards I know of modify all playing notes when they receive pitch wheel data. Also, you have to use a continuous stream of pitch wheel messages to effect a slide; the pitch wheel step isn’t standardized; and on a slide of a large number of tones you will overrun the range of the wheel. 

? Some of these problems would be addressed by a device which allowed its pitch wheel to have selective control – say modifying only the notes playing on the channel the pitch wheel message is received in, for instance. The thing for a guitar synthesizer to do, then, would be to use mode 4, one channel per string, and bends would only affect the one note. You could play a chord on a voice with a lot of release, then bend a note and not have the entire still-sounding chord bend. Any such devices?                   

I think some of the deficiencies in MIDI might be addressed by different communities of interest developing a standard set of system exclusives which answer the problem. One perfect area for this, I think, is a standard set for representation of “non-keyboard/drum machine” instruments which have continuous pitch capabilities, like a pedal steel, for instance, or non-western intervals, like a sitar.There is a crying need to do something about the “loopback” problem. I would even vote for usurping a few more bytes in the mode messages to allow you to turn off input echo by the receiver. With the local control message, you could then at least deal with something that would act precisely like a half or full duplex terminal. Several patchwork solutions exist to this problem, but there ought to be a standard way of doing it within the protocol. Another thought is to allow data bytes of other than 0 or 127 to control echo on the existing local control message.The lack of acknowledgement is a problem. Another candidate for a standard system exclusive set would be a series of messages for mode setting with acknowledgement. This set could then also take care of the loopback problem.The complete lack of ability to specify standardized waveforms is probably another source of intense disappointment to many readers. Trouble is, the standard lingo used by the synthesizer industry and most working musicians is something which hails back to the first days of synthesizer design, deals with envelope generators and filters and VCO / LFO hardware parameters, and is very damn difficult to relate to Fourier series expressing the harmonic content or any other abstractions some people interested in doing computer composition would like. The parameter set used by the average synthesizer manufacturer isn’t anyplace close to orthogonal in any sense, and is bound to vary wildly in comparison to anybody elses. There are essentially no abstractions made by most of the industry from underlying hardware parameters. What standardization exists reflects only the similarity in hardware. This is one quagmire that we have a long way to go to get out of, I think. It might be possible, eventually, to come up with translation tables describing the best way to approximate a desired sound on a given device in terms of its parameter set, but the difficulties are enormous. MIDI has chosen to punt on this one, folks.Well, that’s about it. Good luck with talking to your synthesizer.Bob McQueer22 Bcy, 3151All rites reversed. Reprint what you like.