1.0 MIDI - gtimedia

1.0 MIDI - gtimedia
1.0 MIDI
Midi stands for Musical Instrument Digital Interface.
Midi is a digital communications language that enables musical instruments to transmit and receive
performance and control information. For example, every time a musician plays a note on a MIDI
keyboard, the keyboard is capable of generating performance information to indicate that a note has
been played. In addition to this, it is possible to start and stop the playback of tape machines and
computer programs like Logic using MIDI commands sent from a musical keyboard.
Midi Notes
Every Midi Keyboard has a certain range of notes available for playing. Each key on the keyboard
has a number, starting from 0 at (C-2) up to 84 (C5).
0
12
24
36
48
60
84
As mentioned previously, MIDI keyboards generate MIDI commands whenever a note is pressed on
the keyboard. This type of Command is called a “Note On” message. In the example below, the
player presses a note on the Keyboard. This creates a Note On message. However it is also
important to indicate which note has been turned on. This is identified using a Note Number, and in
the picture below the player has pressed the note C1, which is Note number 36.
MIDI keyboards also transmit the pressure that players apply when playing the note. This pressure
is called Velocity and can have a value between 0 and 127. So in the example below the Commands
sent from the Keyboard would include “Note On” + Note Number + Velocity.
Note On + Note Number + Velocity
Note On + 36 + 70
So we have seen what happens when a Note is pressed on a MDI keyboard, but what happens when
a note is released? “A Note Off” message is generated to indicate that the note should stop
sounding and the velocity is reset to zero.
Note Off + Note Number + Velocity
Note Off + 36 + 0
2.0 Midi Interfaces, Connections and Cables
If MIDI keyboards are capable of generating commands when played there must be some way of
connecting these keyboards to other MIDI devices such as other keyboards, computers, samplers
and drum machines. Sometimes it is possible to connect a keyboard to a computer directly with a
USB cable. However, many serious musicians will buy a dedicated Midi Interface unit such as the
one shown below. This unit connects to the the computer with USB and then allows the use of the
three MIDI ports to send and receive data to and from the computer.
USB carries midi inforamtion in and out of the computer.
Midi
Interface
Box
In
Midi Out Port
Out
Thru
Sends a copy of all data
received on the “In” port and
sends it somewhere else. For
example to another computer.
Midi In Port
Midi Out Port
This is used to transmit Midi Data from the computer to keyboards etc
Midi In Port
This is used to transmit Midi Data form a keyboard into the computer.
Midi Thru Port.
This port makes a copy of all information received at the in port and sends it out of the thru port. It
acts almost like a Midi splitter sending information to somewhere else like another computer.
Midi Cables
Keyboards are conected to the Midi Interface using standard MIDI cables such as the one shown
below. Midi cables are used to connect musical devices that are fitted with suitable MIDI ports or
sockets. The maximum recommended length of a MIDI cable is 50 meters before errors start to
occur. It is important to note that Midi data can only be transmitted in one direction in a single
cable.
Note: Many modern devices can transmit and
receive midi using USB cables in addition to
traditional midi connectors.
What exactly are Channel Numbers
Chanel numbers are use to address different instruments.
The maximum number of channels available on a single midi cable is 16.
Examine the Diagram below. There is one maser keyboard that is connected up to 4 other slave
keyboards. What this means is that when the user presses a key on the master keyboard it sends a
midi message to play a note to all of the other keyboards. However each slave keyboard will only
react to the message if the message is addressed to it.
Midi Message Structure & Decoding messages.
Whenever a key is played on a midi keyboard a Midi Message is sent out from the Out port on the
keyboard.
Midi Message
Out
The Structure of the Midi Message
Each Midi message is consists of 3 bytes of data. The first Byte is called a Status Byte and the next
two are data byes.
Status Byte
Data Byte
Data Byte
1MMM NNNN
0 ???????
0 ???????
A Single Midi Message
Lets examine this structure in more detail.
Status Byte
The status byte starts with a 1. The next 3 digits (MMM) are for Message Type.
After this the Next 4 digits (NNNN) are for Channel Number.
Here are some Message Types along with the Number that represents them.
Message: Note Off
Message: Note On
Message: Control Change
Message: Program Change
Number is: 8
Number is: 9
Number is 11
Number is:12
Data Bytes
The data bytes start with a 0. The next 7 digits (???????) are variables. This means that their
meaning changes depending on the type of message being sent. The only way of knowing what the
data bytes contain is by checking the table below for their meaning.
Message Type (Status)
Data 1
Data 2
Note Off (8)
Note Number
Velocity
Note On (9)
Note Number
Velocity
Control Change (11)
Controller Number
Data Value 0-127
Program Change (12)
Program Number
-Blank-
Exercise
So with this in mind here are some examples of midi messages. Start by finding out what the data
bytes mean and fill this in under the data byte.
Status Byte
Data Byte
Data Byte
1
8
16
0
48
0
0
1
9
08
0
36
0
70
1
8
08
0
36
0
0
Questions
Describe what happens when a note is pressed down as in the the picture below. Describe the
message that is generated.
Describe what happens when a note is pressed down as in the the picture below. Describe the
message that is generated.
What are the 3 Midi Ports. Explain what each does?
What is the maximum length of a midi cable before errors occur.
What are Midi Channels use for when there are several keyboards connected, explain with a
diagram
How many Midi Channels are available one a single Midi Cable
What is the difference between a Midi Out port and a Midi Thru port.
What does the following Midi messages mean
1
9
1
8
16
07
0
48
0
72
0
60
0
0
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