MIDI Basics, Part 1

Exploration

Published in SOS August 1995
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Technique : Theory + Technical

PAUL WHITE kicks off a new series for newcomers to MIDI.

 

You might imagine that most SOS readers already have a pretty firm grasp of MIDI, but new readers are joining us every month. Furthermore, there are those amongst our existing readership who mainly record using traditional multitrack methods, and they too could benefit from a refresher course in MIDI practices. One of the problems is that many of the musicians who could reap the benefits of MIDI are frightened off by the jargon -- and there's also the underlying suspicion that MIDI has something to do with computerising and dehumanising music. Furthermore, it's not always clear what MIDI can actually help you achieve. But before looking at all the great things you can do with MIDI, is it true that MIDI is complicated?

Technically, MIDI is quite complicated -- but then the same is true of TV, telephones, cars, and the insides of your hi-fi system. Even so, most of us take these things for granted and use them without giving a second thought to what really makes them tick. The ease of use of something doesn't necessarily relate to the complexity of the technology that makes it work, and that's certainly true of MIDI, because although it does provide the scope for you to do complicated things if you wish to, you can choose to approach it on your own terms and make use only of the facilities that you need.

WHAT IS MIDI?

MIDI is essentially a communications protocol or common language that enables any MIDI-equipped electronic instruments to be linked together in a musically useful way. The data that makes this possible is in digital form, hence the acronym MIDI (Musical Instrument Digital Interface). Don't worry if you don't know how digital data works -- it doesn't know how you work either, but that doesn't mean you can't work together!

MIDI compatible instruments and other MIDI devices are connected to each other via standard MIDI cables, with 5-pin DIN plugs on either end. There are a few simple rules determining what should be plugged where, but what would really help at this stage would be to talk more about this mysterious 'musically useful' information that MIDI instruments send to each other. In reality, there are many types of MIDI message, and to try to grasp them all at once would probably give you a headache, so what I'm going to do is cover the essentials first (and if I have to bend the truth occasionally to keep things simple, it won't do you any lasting damage!).

Electronic keyboard instruments are, by definition, electronic, which means that the sound is created by circuitry, not by something being hit, bowed, or blown. Whereas a piano key activates a mechanism which hits the string, the keys on an electronic MIDI instrument generate electronic signals to 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 transmitted, and when the key is released, a Note Off message is sent. The actually key that you depress dictates which musical note will be played, and 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' is one piece of MIDI jargon that crops up quite regularly.

To recap so far, the main parameters of a musical note played from a keyboard are: which note it is, when it starts, when it stops, and how loud it is. There are other things that you can do to a note, such as bending the pitch or adding vibrato, but they'll keep for now...

If pitch, Note On, Note Off and Velocity information all exists in the form of electronic signals, it must be possible to send these signals along a piece of wire and use them to control the sound generating circuitry in another electronic instrument, and it's precisely that concept which is at the heart of MIDI. (It might occur to you at this stage that you could send the same signals directly from a computer and cut out the middle man -- but that avenue of exploration comes later, when we look at sequencing.) The main point to get across to new users is that MIDI is not a means of transmitting audible sounds -- it is a means of transmitting instructions or messages. A good analogy might be to compare MIDI data with a written musical score; the score only tells the performer what to play, it doesn't have any influence over the sound of the instrument. What's more, you could read a score written for violin and choose to play it on a piano. MIDI allows us to play any piece of music using any sound at our disposal.

If we plug the MIDI Out of the keyboard we are physically playing (the Master keyboard) into the MIDI In socket of another MIDI instrument (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.

Great -- but why would I want to do that? Well, when playing live, the ability to link a second instrument via MIDI means that the sounds of both instruments can be played without changing keyboards. Not so flash as wearing a gold cape and standing in front of tiered banks of Moog synths, perhaps, but far more practical. Indeed, only the master synth needs to have a keyboard at all -- the other MIDI devices can simply be sound modules, which certainly saves on space if you have to drive to a gig in a Metro, and it saves money.

To understand how the control of multiple modules is possible without them all playing at once, all of the time, the concept of MIDI channels has to be introduced.

MIDI CHANNELS

In a basic MIDI system, the way the instruments are linked means they 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 available, numbered 1 to 16, and they work in a very similar way to TV channels. Most people in the UK receive four TV channels (forget Sky just for now), yet all four channels 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.

With MIDI, the information sent down the MIDI cable can be transmitted on any one of 16 channels selected on the master keyboard; similarly, the sound modules may be set to receive on any of the 16 channels. If we, for example, set the master keyboard to transmit on MIDI channel 1 and connect up three different MIDI modules set to receive on channels 1, 2, and 3, only the first 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 and so they ignore it. Of course, you can set all your modules on the same MIDI channel and have them all playing at once, if you want to.

Putting it briefly, by switching channels at the master keyboard end, up to 16 different modules (set to the 16 different MIDI channels) can be addressed/played individually, even though they are all wired into the same system.

MULTITIMBRAL APPROACH

We've already discovered that a MIDI sound module is essentially a MIDI instrument without a keyboard, but many current MIDI modules actually contain the equivalent of several MIDI instruments, each of which can be addressed on a different MIDI channel. These are known as multitimbral modules, but the instruments inside are not usually quite as independent as they appear; for example, some parameters may affect all the voices globally, or the sounds may all be mixed to a single stereo pair of audio output sockets. Even so, it is always possible to change the relative volume levels of the different instrument voices and to change their left/right pan positions.

Why should you want a multitimbral module, after all, you only have one pair of hands? If you're playing live, then you probably can't take full advantage of multitimbral modules (though you could use them to assign different sounds to different regions of your keyboard), but if you want to add a sequencer to your setup to allow you to make multitrack MIDI recordings, just one multitimbral module can provide you with a complete backing band or orchestra, including the drums. Before multitimbral sound modules appeared, you needed a different MIDI instrument for each of the parts you wanted to sequence.

On top of that, all MIDI sound modules have what is known as a 'maximum polyphony' -- the maximum number of notes that they can play at any one time. This being the case, if some of the MIDI channels are already playing very busy parts, you might find that trying to play yet another part on top causes some of the notes to drop out or be cut short. The bottom line here is that the more polyphony you have (64-note polyphony is typically the maximum for modern modules), the better -- especially if you're in the habit of writing complex pieces of music where lots of sustained notes overlap.

Drum machines may also be used as MIDI modules, even though they have their own built-in rhythm sequencers. It is possible to access their sounds externally over MIDI, each drum sound being 'mapped' to a different note on the keyboard. Some MIDI drum modules, such as the Alesis D4, are specifically designed with no internal sequencing capability, just sounds.

MIDI CONNECTIONS

Most MIDI instruments have three MIDI sockets, labelled In, Out, and Thru, though some older models may not have all three. The master instrument always sends information from its MIDI 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 successive instrument. After passing through three or four instruments, the MIDI messages may start to become unreliable, resulting in notes which stick on or refuse to play at all.

A better way to interconnect multiple instruments, in anything other than the smallest MIDI system, is to use a so-called MIDI Thru box. This takes the Out from the master keyboard and splits it into several Thru connections, which then feed the individual modules directly. Figure 2 shows the standard method of daisy-chaining, followed by the same system using a MIDI Thru box instead. In practice, many people use a combination of MIDI Thru boxes and short daisy-chains of instruments.

The MIDI Outs of the slave units are normally unused during performance, but they are useful when you want to hook up your keyboard to a MIDI sound editor or librarian program, running on a MIDI-equipped computer.

PROGRAM CHANGE

So far, I've explained 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 there's a lot more useful information that you can send over MIDI.

Today's synthesizers are programmable, and they have memory banks full of sounds (often called 'patches') from which you can choose. MIDI provides direct access for up to 128 patches, sometimes numbered from 0 to 127 and sometimes from 1 to 128. The buttons that are used to select the patches on the master keyboard also enable Program Change 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 will play. These Program Change messages may also be used to switch to different effects patches on a MIDI effects unit that responds to MIDI Program Changes (most units do). In the case of a MIDI instrument that offers more than 128 sounds, the likelihood is that these sounds will be organised into banks, each bank containing no more than 128 sounds. The MIDI protocol now includes the facility to switch from one bank to another, though some older instruments have non-standard bank change systems which are usually explained in their respective operation manuals.

PERFORMANCE CONTROL

A typical MIDI synthesizer has two control wheels mounted to the left of the keyboard, and though these are often assignable to allow them to control various different effects, one is generally used to control pitch bend while the other controls vibrato depth. These controls work by generating electronic signals which, in turn, control the circuitry that creates the sound. And, like note information, control information may also be transmitted down a MIDI cable -- simply move the control wheel on the master, and the slave instrument will respond.

Time to introduce 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. Similarly when you're working with a sequencer, it makes sense to set up your instruments so that they all have the same pitch bend range.

Another useful MIDI controller is master volume -- most modern instruments respond to it while some older ones do not. On an instrument that transmits master volume information, turning up the master volume slider will send the appropriate control information over MIDI and the receiving slave instrument (providing it understands master volume) will respond to it. In fact, when you get into MIDI you'll find that there's a whole list of controllers that can be used to add expression to your performance, including sustain pedals, vector joysticks, sostenuto, and so on. You'll find a list of the controllers to which your MIDI instruments can respond in their respective manuals, and you'll notice that the controllers are divided into two types: switch controllers which are either on or off, and continuous controllers which allow something to be varied. For example, a sustain pedal is a simple on/off switch, but a volume control is a continuous controller. Because of the structure of MIDI data, you'll find that the maximum range of any MIDI parameter or controller is usually from 1 to 128. In other words, a continuous controller really provides you with 128 small (but separate) steps.

MORE ON MIDI

So far I've only touched on the basics of MIDI, and much of what MIDI can do has been left unsaid for the time being. Even so, with what you've learned so far, you should be able to start putting MIDI into practice. However, there is time to introduce just one more concept, and that's the idea of MIDI Clock.

Some MIDI instruments, like drum machines, have a built-in sequencer which allows drum patterns to be set up and played back at different tempos. Such instruments both send and receive MIDI Clock data, a series of electronic timing markers which go down the MIDI lead along with the other data. Think of it as the electronic equivalent of the sprocket holes at the edge of a cine film and you'll soon grasp the idea. MIDI Clock is very useful as it allows us to synchronise two or more MIDI devices together. For example, a drum machine could be slaved to a second drum machine so that both play together, allowing you to use sounds from both machines. And as we shall see later, MIDI Clock is what allows us to synchronise sequencers and drum machines together or to sync sequencers to tape recorders. Also associated with MIDI synchronisation are commands for starting and stopping things like drum machines and sequencers, and these are known as MIDI Real Time messages (see box).

There'll be more about MIDI Clock when we delve into the basics of sequencing next month."Like note information, control information may also be transmitted down a MIDI cable -- simply move the control wheel on the master, and the slave instrument will respond.""...when you get into MIDI you'll find that there's a whole list of controllers that can be used to add expression to your performance...""If you want to add a sequencer to your setup to allow you to make multitrack MIDI recordings, just one multitimbral module can provide you with a complete backing band or orchestra, including the drums."

 

MIDI MODES

Most MIDI instruments can be set to receive on any of the 16 MIDI channels, but there is also a setting called Omni mode, which allows the unit to respond to all incoming data, regardless of its channel. Some MIDI equipment, especially older models, tends to default to Omni mode when first switched on. Although this is a trifle tedious, it isn't really a problem so long as you remember to reset the instrument to the desired MIDI channel before you continue. If the instrument is set to receive on separate MIDI channels, then it is said to be in Omni Off mode.

It is also possible to tell an instrument whether to play polyphonically or monophonically, and while polyphonic operation is by far the most common requirement, mono operation has certain advantages, not least for guitar synth users. In Mono mode, a single polyphonic synth can be made to behave as several single-voice synths, each voice being on a different MIDI channel. If you have a MIDI guitar, it makes sense to set up the system so that each guitar string controls its own single synthesizer voice on its own MIDI channel. Not only does that make the note allocation mirror that of the guitar (where each string can only be played monophonically), but it also allows independent amounts of pitch bend to added to each string.

The four possible combinations of Omni On/Off and Poly/Mono operation form the four modes of MIDI operation and are defined as follows:

Mode 1: Omni On/Poly

Mode 2: Omni On/Mono

Mode 3: Omni Off/Poly

Mode 4: Omni Off/Mono

Most of the time, players using keyboards will use Mode 3, which is the default mode for the majority of MIDI instruments. In Mode 3, the instrument works polyphonically and responds only to notes sent on its chosen MIDI channel (or channels, in the case of a multitimbral instrument).

Mode 2 is the least useful mode -- indeed, I've never met anyone who's found any use for it at all! Stories abound that it crept into the MIDI specification as the result of a misunderstanding, so if your synth doesn't support Mode 2, don't feel you're missing out.

 

MIDI JARGON BUSTER

MIDI Musical Instrument Digital Interface.

MIDI Clock Series of tempo-related electronic timing markers embedded in the MIDI data stream.

Note On MIDI message sent when note is played (key pressed).

Note Off MIDI Message sent when key is released.

MIDI Module Sound generating device with no integral keyboard.

Multitimbral Module MIDI sound source capable of producing several different sounds at the same time, controlled on different MIDI channels.

MIDI Channels The 16 channels over which MIDI information can be sent.

MIDI Mode MIDI information can be interpreted by the receiving MIDI instrument in a number of ways, the most common being polyphonically on a single MIDI channel (Poly-Omni Off mode). Omni mode enables a MIDI instrument to play all incoming data regardless of channel setting.

MIDI Program Change Type of MIDI message used to change sound patches on a remote module or the effects patch on a MIDI effects unit.

MIDI Controller MIDI message sent in response to movement of certain physical controls on the master keyboard (or other master MIDI instrument).

MIDI Thru Box Device which splits the MIDI Out signal of a master instrument or sequencer to avoid daisy-chaining.

MIDI In Socket used to receive information from a master controller or from the MIDI Thru socket of a slave unit.

MIDI Out Socket on a master controller or sequencer used to send MIDI information to the slave units.

MIDI Thru Socket on a slave unit used to feed the MIDI In socket of the next unit in line.

 

REAL TIME MESSAGES

Before MIDI arrived on the scene in 1982/83, attempts were made to provide tempo-related clock systems to allow devices from different manufacturers to be synchronised together, but quite often they used different numbers of clocks per bar which meant some form of convertor box had to be employed. MIDI uses 96 clock pulses per 4-beat bar (or 'whole note', as the Americans like to call it) so any piece of MIDI gear that can send or read tempo information will sync to any other. If the tempo of the master machine is speeded up, its MIDI Clock rate speeds up accordingly, so the slave tempo is forced to follow.

Even when the master machine is not playing, it is still sending out MIDI Clock data at the current tempo, which means that any connected slave device knows exactly what tempo to take off at when it receives a Start command. The Stop command will cause both the master and slave machines to stop running, and a further command, Continue, allows the machines to continue playing from wherever in the song they were stopped. Start always causes the master and slave to start from the beginning of the song.

If you're wondering how the machines know whether they're supposed to be the master or a slave, it's because they can all be switched for internal sync (master) or external MIDI sync (slave) operation. Any machine switched to external MIDI sync can be used as a slave. As with MIDI note information, the MIDI connection runs from the master's MIDI Out to the slave's MIDI In.

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