In the ten years or so since its inception, MIDI has become an integral part of music composition, performance and recording, yet many potential users are put off by its apparent complexity. Paul White introduces the subject on a 'need to know' basis.

Musicians who work daily with MIDI must occasionally wonder how they ever managed without it, but a far greater number of musicians who could reap the benefits of this new technology are frightened off by the jargon, the apparent complexity and the change in working methods required to use it effectively. They're also unsure as to what MIDI can actually help them achieve. So, what is the truth? Is MIDI complicated — and if not, is it actually useful?

On a technical level, MIDI is incredibly complicated — but the same can be said of the international telephone network, television and video recorders, yet we all take these things for granted and use them without giving a second thought to the technology behind them. On second thoughts, scrub video recorders from the list — my wife insists I'm still incapable of recording the right programme at the right time on the right day — with or without the manual! What I'm trying to put over here is that using MIDI can be relatively simple and bears no relationship to the complexity of the technology that makes it work. Indeed, most of the confusion surrounding MIDI seems to have been engendered by books that try to explain its inner workings in far too much depth.

MIDI is simply a standard communication system that enables MIDI-equipped electronic instruments to be linked together in a musically useful way, regardless of the model or manufacturer. Like computers, the data is in a digital form, the letters of the word MIDI standing for Musical Instrument Digital Interface.

Linking is accomplished by means of standard MIDI cables with five-pin DIN plugs on either end and the method of connection, as we shall see shortly, is quite straightforward. But what is important at this early stage is to appreciate what 'musically useful' information can be passed from one MIDI instrument or device to another. This following description covers the most important and basic aspects of MIDI but is by no means comprehensive. In order to keep the learning curve to an acceptable gradient, I'll save the more detailed stuff until later.

Electronic keyboard instruments are not like acoustic pianos — the keys don't trigger the sound directly, but generate electronic signals which tell the internal circuitry what note to play and how loud to play it. When a key is depressed, a signal known as a Note On message is sent, and when the key is released, a Note Off message is sent. The loudness of the note depends on how hard the key is hit, which is really the same thing as saying how fast the key is pushed down. This speed, or velocity, is read by circuitry within the keyboard and used to control the volume of the sound being played. The term 'velocity' should be committed to memory, as it is one piece of MIDI jargon that will crop up time and time again when we're referring to how loud a note is played or how hard a key is struck. The pitch of the note is determined by which key is pressed.

It takes only a small mental leap to realise that if pitch, Note On, Note Off and velocity information all exists in the form of electronic signals, it should be possible to send these signals along a piece of wire and use them to control the sound generating circuitry in another electronic instrument. At its most basic, this is exactly what MIDI allows us to do, and if we plug the MIDI Out of the keyboard we are currently playing (which we call the Master keyboard) into the MIDI In socket of another MIDI instrument (which we call the Slave), then the slave is able to play the notes as performed on the master keyboard. This simple arrangement is shown in Figure 1.

So far so good, but what's the real advantage? When playing live, the ability to link a second instrument via MIDI means that the sounds of both instruments can be used without changing keyboards — convenient, but hardly worth making a fuss over. However, a little further thought reveals that the second instrument doesn't actually need a keyboard of its own because it never gets used. Instead we can use a so-called synthesizer module as our second instrument, a module simply being the electronics of the instrument packaged in a rather more compact and generally less expensive box. Now we have two very real advantages — we save money and we save space. Furthermore, we're not limited to controlling just one module from our master keyboard, we can control several — but to appreciate how this is possible, we need to know about MIDI channels.

MIDI Channels

In a typical MIDI system, the way in which the instruments are linked means that they are all connected together and all receive the same MIDI information. In order to allow the Master instrument to communicate with the slaves on a more selective basis, the MIDI channel system was devised. There are 16 MIDI channels which are, logically enough, numbered 1 to 16 and their concept isn't that different to that of television channels. Staying with the TV analogy for a while, all the different TV broadcasts arrive at the same aerial and reach the set down the same piece of wire. Which one we actually watch depends on which TV channel we select on the set. All the programmes are available simultaneously, but the channel system allows us to pick them out one at a time.

It's just the same with MIDI and the information sent down the MIDI lead can be sent on any one of 16 channels as selected on the master keyboard. Likewise, the modules may be set to receive on any of the 16 channels, so if we, for example, set the master keyboard to MIDI channel 1 and connect three different MIDI modules set to receive on channels 1, 2 and 3, only the module set to channel 1 will respond. The others still receive the information, but the MIDI data tells them that the information is not on their channel so they politely ignore it. By switching channels at the master keyboard end, up to 16 different modules can be addressed individually, even though they are all wired into the same system.

Before leaving this area, there is a factor that it pays to be aware of — most MIDI instruments can be set to receive on any of the 16 MIDI channels, but there is also a setting which we call Omni mode, which allows the unit to respond to all incoming data, regardless of its channel. This has few practical uses, though there are some, as we will see later. Some more cynical users claim it was only included to enable inexperienced music shop staff to get MIDI instruments to communicate on a basic level without setting the right channels! I don't know how much truth there is behind this, but I do know that some MIDI equipment, especially older models, tends to default to Omni mode whenever it is switched on, which means that you have to set up the correct channel before you can do any work. Of course, I always forget and end up with the offending module trying to play every musical part at once, usually with quite inappropriate sounds!

More About Modules

So far we have described modules as being MIDI instruments in boxes but without any keyboard. To a point this is true, but some modules actually contain several sound generating sections, each of which can be addressed on a different MIDI channel. These are known as multitimbral modules and may, in many respects, be considered as being several independent instruments sharing the same case. They are not truly independent, as some parameters may affect all the voices globally, while many output all their voices through a single stereo pair of sockets. Even so, it is usually possible to change the relative levels of the different voices and to change their left/right pan positions.

Drum machines may also be considered as MIDI modules, though they have their own inbuilt sequencers, allowing them to store and replay rhythms. It is possible to access their sounds externally over MIDI, each drum sound being 'mapped' to a different note on the keyboard.

MIDI Connections

Now seems an appropriate time to examine the way in which the MIDI sockets on the back of an instrument are used. Most MIDI instruments have three MIDI sockets, labelled In, Out and Thru, though some models may not have all three. The master instrument always sends information from its Out socket, which must be connected to the MIDI In socket of one of the slaves. The MIDI Thru of the slave is then connected to the MIDI In of the next slave and its Thru connected to the In of the next one and so on. What we end up with is a daisy chain and, in theory, this can be indefinitely long. Not so in practice, however, because the MIDI signal deteriorates slightly as it passes through each instrument, and after it has gone through three or four instruments, it starts to become unreliable and notes may stick or refuse to play at all. A better way to work with anything other than the smallest MIDI system is to use a so-called MIDI Thru box which takes the Out from the master keyboard and splits it into several Thru connections which feed the individual modules directly. Figure 2 shows the standard method of daisy chaining, followed by the same system using a MIDI Thru box (Figure 3). Of course the two systems can be used in combination; if an instrument is fed from a Thru box, its Thru socket may be linked to another module to form a short daisy chain, the only proviso being that these individual chains contain no more than two or three modules.

A Few More Secrets

I said at the outset that I would reveal information on a 'need to know' basis, because if you are confronted with it all at once, there's no way of knowing what is most important or indeed, why all the facilities exist. You now know that MIDI operates on 16 channels and can be used to send note information from a MIDI-compatible master instrument to a MIDI-compatible slave, but what I didn't tell you is that there's a lot more useful information goes down the wire too! Synthesizers tend to offer not just one but a large selection of sounds, and these are identified by their patch number.

The term 'patch' was coined because the very early electronic synthesizers used patch cables, rather like an old-fashioned telephone exchange, to connect the various sound generating building blocks. MIDI caters for up to 128 patches, sometimes numbered from 0 to 127 and sometimes from 1 to 128 — even standards aren't always all that standard! The buttons that are used to select the patches on the master keyboard also enable patch information to be transmitted to the slave synthesizer modules, so now we can play the modules remotely and we can select the sound or patch that they are set to. These patch change messages are known as Program change messages and may also be used to switch the effects patches on a MIDI effects unit that responds to MIDI Program Changes. And there's more — much more.

A typical MIDI synthesizer has two control wheels mounted to the left of the keyboard, and though these are sometimes assignable to allow them to control various different effects, they are generally used to control pitch bend and vibrato depth. These controllers also work by generating electronic signals which, in turn, control the circuitry that creates the sound. And, like note information, control information may also be sent over MIDI — simply move the control wheel on the master, and the slave will respond.

And now, a possible pitfall. MIDI instruments can often be 'scaled' so that, for example, the maximum travel of the pitch bend wheel might cause a pitch shift of as little as one semitone or as much as an octave. It is important to ensure that any instruments likely to play at the same time are set with the same scaling values, especially for pitch bend, otherwise when you try to bend a note on the master keyboard, the sound coming from the master keyboard might go up by a third and the sound from the slave by a fourth — clearly not desirable.

One other controller not mentioned so far is master volume — some instruments send it while others don't. On an instrument that does, turning up the master volume slider will send the appropriate control information over MIDI and the receiving synth will respond to it. Other commonly implemented MIDI controllers include the sustain pedal, which prevents the note envelopes from entering their release phase until the pedal is released.

That's probably enough MIDI brain ache for one month, and I'll finish off by saying that while MIDI is a standard, not all MIDI instruments have all the features. The newer General MIDI standard attempts to introduce a greater degree of conformity between MIDI instruments, but as this is a relatively new standard, don't expect all existing MIDI instruments to have every feature. It's pretty safe to assume that all will transfer note and program change (patch change) information but not all synthesizers are designed to respond to velocity information, especially older or cheaper models, and not all will respond to all the controller information sent. For example, a great many instruments don't respond to MIDI master volume control. More frustratingly, some very early MIDI instruments only work on MIDI channel 1 and can't be changed. Don't let these little differences put you off, though; usually they don't cause serious problems or can be worked around in some way. I only mention them so that you'll be aware of potential problem areas when you come to set up your own MIDI system. There are other types of MIDI information which we haven't covered yet, but don't worry — they'll be introduced as they're needed.

Next month I'll be looking at MIDI sequencing, probably the most useful and certainly the most exciting aspect of MIDI.