Midi explained, part one

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A basic explanation of what midi is and how it works. Part one of three.

What is midi?

MIDI (Musical Instrument Digital Interface) is concidered a protocol which defines a standard for generating,transmiting and storing a musical performance and instrument control information. It is also a specification that sets the rules that manufacturers must follow to make MIDI work.

By the early 1980s keyboardists wanted to be able to play on one keyboard and trigger sounds on another. In response, some manufacturers made their own innstruments "talk" to each other, but players wanted to see keyboards from differant manufacturers become compatible. US and Japanese manufacturers soon come out with the MIDI spc, and soon MIDI was used around the world for much more than the initial intent of triggering several sounds from one keyboard.

MIDI events:

A MIDI-enabled keyboard, percusion pad, or wind controller, etc. can generate information based on exactly how an action was performed--which key or pad you played,when you played it, how hard or fast you hit is, how long you held it, and so on. Each of these actions is called a MIDI event. Most MIDI instrumentsalso generate data from things like modulation wheels,pedals, knobs, sliders, patch selectors, and so on.

Devices called MIDI sequancers were developed that could record a stream of MIDI events and play it back at will. Editing capabilites were added that allowed users to manipulateMIDI events, and even create new ones. More MIDI events were defined in order to store samples, start and stop recorders,turn on and change stage lights, and much more, all via MIDI.

For the begginer:

One thing you need to be clear on, MIDI sequancers do not record music. They record MIDI data about events that may result in music (when the data is sent to an instrument or, on a computer, to an internal MIDI sound engine. MIDI data is similar to sheet music that cant be heard untill someone performs the printed notes on an instrument. Think of a player piano reading the holes punched in the rool of paper. MIDI data is a record of what an how you played, not the actual sound of the instrument.

Most MIDI sequancer programs these days will also let you record digital audio as well, and the programs let you edit both MIDI and digital audio with similar tools and screens, but they are definatly not the same thing.

Space savings:

Digital audio takes up a lot of space in memory and on your hard disk, while MIDI data does not. A MIDI note is only 3 bytes long, no matter how long you hold the note. Compare that to 5 MB/minute for a single track of 16-bit/44.1 kHz audio.

MIDI can save you hard disk or tape space and leave you more tracks for recording sounds. Instead of recording the sounds you simply record the data, which is played back and sent to the instruments which can be routed to your mixer, were you will eventually mix all the sounds down to a stereo track. You can think of these tracks as virtual tracks.

With MIDI you can also play and record a part at a slower tempo or even step record one note at a time, (which is great for writing drum tracks). Then you can play the part back at normal speed. You might also need to transpose the part to a differant key, or change the instrument sound you have choosen, or raise or lower the volume of maybe just some of the notes. With MIDI it is simply just a mouse click or two and you have it done.

Software now can sync both MID and audio data to one clock internally. You can also sync an external tape machine or drum machine to the software sequancer in many differant ways.

Connecting it all:

There are 3 basic MIDI connections, In, Out , and Thru. Not all devices have all 3, some only come with In and Out. these plugs are known as Din connections, they are special 5 pin connections commonly used in Europe for Hi Fi audio gear in place of RCA connections. But the cables used for audio gear cannot be used to connect MIDI gear since they use all 5 pins and MIDI gear only uses 3 of the 5 pins.

The simple MIDI hookup would be with only 2 devices with the MIDI out port of device 1 hooked to the MIDI in port of device 2. This settup will allow one way communication from device 1 to device 2. If you connect the out port of device 2 to the in port of device 1, you can transmit data both directions. An example would be a synth keyboard to a PC to record MIDI data and the PC to the synth keyboard to trigger the sounds on the synth. This does require 2 cables as MIDI data only travels in one direction.

Using the Thru port:

MIDI devices may also be connected ina chain, using the third connection Thru. Example, device 1 out to device 2 in, device 2 thru to device 3 in, and so on. This will pass the MIDI data generated by device 1 thru device 2 to device 3 and so on. The differance between the out and thru ports is that data coming from the out port has been generated on the device containing the out port. Data comeoout of a devices thru port is and exact duplicate of data recieved at the in port. Note that some gear has the ability to merge the data together to one port, but this does require the use of all 5 pins or some tricky rigging on the users part.

Daisy chaining synths like this is a common practice as it allows a fairly simple setup to be used to control several synnths from one common keyboard. But there are limitations to what you can do and how much you can do.

Limitations of the daisy chain of serial rig:

The major disadvantage to the daisy chain setup is the abilty to pass data quickly. As with most chained devices in a serial chain, the last device in the chain is of course the last to recieve data. This means that in a chain that is too long, the data is delayed as it passed from each synth to the next. This delay means that a note reaching the farthest out synths in the chain are played behind the beat and this is not favorable when recording. The reason this happens is that MIDI data is transmitted serially, meaning one bit after the other at a fixed rate of 31,250 bits per second. That is slower then your typical dial-up modem. A midi connection can choke if the data stream gets too heavy. There are solutions to this as you will see in the next section.

Multi Port interfaces or patch bay:

A multi port MIDI interface allows the data to be transmitted to many synths or other pieces of gear at once without delay. It also allows each port to transmit on all 16 channels at once. (more on MIDI channels in the next article) The interface is fed data from either a controller or the sequancer and distributed to all the synths hooked to the out ports of the device. It also can recieve data from all the devices transmitting back to the sequancer. (again more on this in the next article) Interfaces come in several sizes, from the simple one in one out (common on most Sound Blaster type sound cards) up to the eight in eight out of the bigger pro level interface. These larger interfaces can be programmed to serve many differant functions and also generally have onboard sync and the ability to generate their own MIDI clock or SMPTE timing.

A MIDI patcher or routing system is a little bit differant but will help avoid the daisy chain. this device can be as simple as one in and two out ports, and go up to the four in eight out which is the most common and also eight in eight out. The differance between the pathcer and an interface is the pathcer is only capable in the single controller method of recieving and transmitting the original 16 channels of the port it is fed data from. It essentially sends data to several devices kooked to its seperate out ports to the devices without the delay in a chain type setup. In the case of the multiple input patchers, they can have data fed toe the inputs from several output devices, which can all be sending 16 channels of data giving you a bit more channels to work with. but remember they will only pass on what is being sent to them. And most of these patchers are again programable to route data in many differant ways.

We will get a little more into the details of how MIDI works and the intriquite details of the types of data involved in the next article. So check back soon.

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