This information underpins much of the As and A2

This information underpins much of the As and A2
This information underpins much of the As and A2 courses, but will be of
special interest to A2 students, because there is an exam on controlling
and interpreting MIDI information - I will be builiding up this part of the
site over the next few weeks. I've started with an overview of MIDI: for
part of the final exam students need to show that they are familiar with
MIDI controllers and data, and that they can discuss and edit a standard
MIDI file.
What is MIDI? MIDI was invented in 1983 as a way for
musicians to link up their synthesisers and for them to send
information to each other. This is very basically put, so why
would a musician want to link up two or more synthesisers,
anyway? The roots lie in the fact that many synths in the
1970s simply didn't produce sounds which were 'fat' enough, so
musicians wanted to link up their synths so that two (or more) could be
playing the same notes, thereby making the sound much more beefy. Of
course they could not link up their synths unless they were made by the
same manufacturers - not good then, unless you wanted to own two or
three of the same instrument!
MIDI was developed, then, as a way of linking different musical
instruments via MIDI cables (like the one in the above photo). MIDI is an
acronym for Musical Instrument Digital Interface - it is digital system,
controlling digital synths (rather than the analogue synths that preceded
them.) In some ways it was miraculous, because it relied on the major
synth manufacturers (Korg, Roland, Sequential Circuits etc.) co-operating
to produce a system that could link up all their synths.
One of the most important things to know about MIDI is that it DOES NOT
TRANSMIT MUSIC!! It is a digital language which transmits information
telling instruments what to play, rather than playing the music itself.
MIDI instruments are connected via two MIDI cables, one to send MIDI
information from the MIDI OUT port, and one to receive MIDI information
through the MIDI IN port.
MIDI files
If you're a student of music technology, the chances are you will have
played, and have regular access to, MIDI instruments - the vast majority
of synths produced today are MIDI compatible. Also, the chances are you
will be regularly using a computer sequencing package where you can
produce MIDI sequences (like Cubase, Logic or Digital Performer). As
well as playing the standard files for these various packages, these
sequencers also play MIDI files (often called standard MIDI files, or SMF).
Put simply, a MIDI file is a way of storing MIDI messages on a computer
or disk. Check out THIS LINK for more about MIDI files.
MIDI files contain two basic elements:
tempo information for the sequence, and
MIDI messages, with information about when they should be
played.
This shows you
the simplest
type of MIDI
set-up: two
devices linked
via MIDI IN and
MIDI OUT.
Notice that there
are two cables,
because MIDI
information can only travel in one direction through the MIDI cable!
This diagram
shows a slightly
more
complex MIDI
set-up that you
might find in a
small studio. A
computer
(running MIDI
sequencing software) is connected to a sound module (a synth with no
keyboard), which in turn is connected to a MIDI sampler (a kind of digital
recording machine).
The links which connect the three pieces of equipment are common to
virtually all MIDI instruments, and this is what each means:
MIDI OUT - this sends MIDI data from a device to other devices.
MIDI IN - this receives MIDI data from other devices.
MIDI THRU - this sends messages arriving at the MIDI in port so
that other devices can be joined together (called 'daisy-chaining'). This
means that two or more sound modules can be played by the same
keyboard.
MIDI messages
As I said earlier, MIDI transmits messages to instruments telling them
what to play - but what form do these messages take?
Fundamentally, there are two basic sorts of messages - channel messages
and system messages. Channel messages are the simplest type, so let's
look at these first.
Channel messages: each MIDI port provides 16 channels along
which MIDI information can travel. In a MIDI sequence, you might have
channel 1 for piano, channel 2 for trumpet, channel 10 for drums, etc.
Information for each instrumental sound (or PROGRAM) in a sequence is
sent along a specific channel. There are 7 channel messages: here are
some, with short descriptions:
Note on - the most common MIDI message: it is sent whenever a
key on the instrument is pressed, and tells the instrument to make a
sound. Each 'note on' message carries other messages (usually about
other qualities in the sound, such as velocity, channel and pitch).
Note off - this message turns off a note, and has the same values as
a note on message (velocity, channel, pitch).
Pitch bend - if you move the pitch bend wheel on your synth, it
moves the pitch up or down: MIDI encodes the movement of this wheel in
to a message.
Program change - this is used for selecting different sounds
(programs) on your synth.
Control change - this is the most complex channel message, because
it is not a single message, but a collection of messages. Basically, control
change messages send controller data: channel, controller type and
controller value. Control change messages are useful because they take
into account the different types of MIDI controllers which exist
(keyboards, MIDI drums, MIDI wind instruments etc.).
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