Understanding and Using MIDI - General Concepts and Setting Up Your MIDI System

Understanding and Using MIDI - Part 1
General Concepts and Setting Up Your MIDI System

For those of us who are not keyboard players, MIDI can look like quite an enigma. Even for those who are keyboard players, MIDI can be intimidating enough that the players don't want to dive into it at all. The simple truth is that MIDI holds unlimited potential to help you create fantastic pieces of music, and make a live musician's job easier. While it is true that MIDI and its commands seem pretty foreign, most of what is involved in understanding them is conceptual. This is my first of two articles on MIDI, and the purpose of this article is to run through the basics so that you can be confident enough to explore this versatile new world on your own.

MIDI is simply a protocol (like a digital language) created back in the early 1980s for electronic musical instruments to communicate with each other. MIDI stands for Musical Instrument Digital Interface, and is used for many different jobs, including musical sequencing, composition, and is even used in live shows to run lights and begin the intro of songs.

MIDI uses a 5 pin DIN cable of which only 3 pins are used, one for data, and the other two for ground and shield (The other two pins were left free to allow MIDI capabilities to grow in the future. At the time of MIDIs design, there were a lot of facets added in to allow for future expansion that they could not conceive at the time. This is part of what makes it so versatile.) These cables are used to inter-connect keyboards, sequencers, computers, tone generators, drum machines, effects processors, and numerous other pieces of equipment. However, don't confuse MIDI data with audio information. These cables do not send audio information. The audio information is sent from the audio outputs of the keyboard or tone generator to a mixing board as it does normally. The various musical instruments actually generate the audio, while the MIDI information is just how the parts are played, like what 'patches' are to be played, for how long, and how loudly.

Let's start out with describing the features of MIDI. The protocol (language) is a binary bit stream created by a master device and sent serially (one bit at a time) at a speed of 31.25k baud (number of bits per second) through the cables to the other joined units. Bits are just ones and zeros that signify number values. The bits used in MIDI are grouped into packets of 8 (a byte) that are read as a whole by the receiving device, which translates them into specific commands depending on their order. The master sends the information out, and the unit that receives the info is called the slave. Slave devices only receive information; they do not initiate it. This transfer of information is done through 3 different ports called in, out and thru. The 'out' port is where the master sends its info out. The slave receives the info on its 'in' port and sends it to be internally processed. The 'thru' port is a direct copy of the 'in' port information and is used to transfer it to the next slave or back to the master.

There are two ways of wiring up your MIDI system. You can do a simple daisy chain, where the 'out' of the master is connected to the 'in' of the slave, then the 'thru' of the slave goes out to the 'in' of another slave, and so forth. This is typically used when there are three or fewer devices to hook up. The rule here is not to have more than three slaves hooked up past the master. This rule prevents possible timing errors and possible MIDI data errors due to generation loss. Although these problems may not come up, there is potentional for either of them in circumstances where several slaves are being fed from one master




FIGURE 1 - DAISY CHAIN MIDI HOOKUP
(The drawing above is courtesy of The Recording Institute of Detroit)

The other wiring method is called a star configuration, where both the 'in' and 'out' of each unit is hooked up to a central thru-box device. You can think of this device like a patchbay, where the incoming signals from the master are routed to the 'out' jacks of the thru-box also going to each unit. This way, each unit receives the others' information and may process it, depending on its internal settings. In this type of configuration, the 'thru' jacks on each device are not used. This is a more versatile setup because you can attach numerous devices to a central unit and not have to worry about time delay or MIDI data error caused by generation loss. It is also used most often in keyboard/computer setups. However, with either setup, the maximum suggested MIDI cable length is 50 feet. Try to follow this rule whenever possible.



FIGURE 2 - STAR CONFIGURATION HOOKUP
(The drawing above is courtesy of The Recording Institute of Detroit)

MIDI is a system whose core is based around using 16 different channels to transmit its data. Each channel may be used to send out different information to devices set to receive that channel. For example, if you had a sequencer with a pre recorded MIDI track, you could have one slave device playing your bass line, another playing the drums, and yet another playing the background piano part, provided that each part was set to be transmitted on a different channel. Each of these different sounds are called voices. A voice can be a single sound, such as a basic piano sound, or a layer, such as a piano sound with a string sound mixed in with it. You can also create layers with two keyboards, each playing a different voice but the slave receiving the note data of the master. Most machines have a set of industry standard voices called general MIDI voices. These sound the same on each device and are a convenient tool to use for composition when you are transferring the piece between two different brands of keyboards.

Let's focus on the master device for a second. The master device always has to have a way to create MIDI information, such as keyboard keys, a drum machine pad, or a breath device (slaves do not need this, since they are used for their tone generation and are not creating MIDI data). When a key is pressed, multiple messages will go out signifying the type of message and also a specific value for that message in the form of a note number. There are 128 possible note numbers and these values are assigned to different things depending on the message being sent out. For example, initially pressing the key sends out a 'note on' message, and a note number is also sent to specify the correct note value. Note number 60 corresponds to a C note in the 4th octave of the piano. Note number 61 would thus be a C sharp in the 4th octave. Every 12 notes begins a new octave, so an octave above this C4 would be C5. Along with this 'note on' and note value information, the pressure with which you hit the key is also sent out in the form of a pressure message and a note number to signify volume. Again, you have 128 different values here.

Now, this note information is sent out to the 'out' jack on a specific channel that you need to specify. Of course, when you are playing, you need a way to hear the voice. This is as simple as using the 'Local On/Off' setting on the global section of your keyboard (global meaning it affects the entire keyboard performance). Turn this on when you want to hear the sound, and the MIDI notes will be routed to the keyboard's internal synthesizer so it can be heard, as well as transmitting the MIDI notes to the 'out' port. Turn it off if you are hooked up to a sequencer such as a computer (since you should really hear the sound coming back after it has reached the computer). You can also leave this off if you want to try out the sounds of an outboard tone generator but not hear the sounds on the keyboard. Just make sure that the master and slave are set to the same channels (this is explained more below).

Now we shift focus to the slave device. The information is being transmitted from the master via its 'out' port to the slave via the 'in' port. How the slave responds to this information depends on two things; The global receiving mode it is set to, and whether or not the same channel the master is transmitting on is on or off on the slave. There are 4 global receiving modes:

1) Omni-on Poly The slave will respond to all 16 channels transmitting and will play multiple notes.
2) Omni-on Mono The slave will respond to all 16 channels transmitting but will play only one note at a time.
3) Omni-off Poly The slave will respond to only the channel it is set to receive, but will play multiple notes at one time.
4) Omni-off Mono The slave will respond to only the channel it is set to receive on and will only play one note at a time.

The most commonly used mode is omni-off poly, and the least used is omni-on mono. Almost all electronic musical devices made today are multi-timbral; that is, able to receive multiple channels and perform the information separately. On top of that, most are polyphonic, which is the ability to play 2 or more notes at the same time. The exact number of notes that each device can play is called its polyphony.
Polyphony is a specific number that is spread out globally over all channels. For example, if your keyboard has 64-note polyphony, then it can play 64 notes at a time on one channel, or 32 on one channel and 32 on another, or any other combination of these over the different channels that adds up to 64. But it can't exceed 64 notes at one time. Granted, that is a lot of notes at one time to be listening to, but this becomes important when you get into sequencing because you may have many tracks and layers being played at one time, including drum voices, which can take up a lot of notes.

Going back to the slave device now, it is now receiving the master information, and let's assume that it is set to omni-off poly mode. Now it will receive only information on the channel that it is set to. To receive the information the master is sending out, the slave must be set to the same channel that the master is transmitting on. If it is sending out info on channel 3, then the slave must be set to receive info on channel 3. The messages received on the other 15 channels are ignored. You can usually set the receiving channel on the slave in the global settings.

Now, let's assume the slave is set to omni-on poly. This is a mode that is commonly used when your setup includes a dedicated sequencer that only does sequencing, or a non-dedicated sequencer, such as a computer. In this case, you will have the sequencer sending data on multiple channels out to one or many slaves, and the slaves have to receive this data and process it effectively. Since the slave will respond to all channels, you now need to use the individual on/off switches for each of the 16 channels on a slave. These are called channel enable/disable switches. For each channel being sent from the master that you want to hear on a particular slave, you would use these switches to enable the receiving channels on the slave. Even though each slave is receiving all 16 channels, you may want a certain slave to only play back channels 1-3, so you would enable those channels, and disable the others. On another slave, you might want to have it play channels 4-8. Again, enable those channels you want to play, and leave the others off. Not only does this keep you slave from playing unnecessary channels, it also helps to decrease the amount of notes the device is playing. Every unnecessary note that the slave is playing uses up polyphony that could be used on another channel. This is especially important with older keyboards where the amount of global polyphony is low. You may also have a keyboard that has an onboard sequencer. Again, each 'track' can be assigned a channel to be sent out on, and you must enable and disable specific tracks on your slave devices to make this work properly.

You should now have a decent understanding of how MIDI works and have enough information to set up your equipment and run it efficiently. In this article, I briefly went over the content of MIDI messages sent from the master, and in the next article, I want expand on this concept by describing all the messages, what they mean, when they are sent, and what they do. You'll really begin to appreciate the flexibility and power of MIDI. Until then, have fun setting up your system