1. CMSCDHN1114/CMSCD1011 Introduction to Computer Audio
Semester /2001
CMSCDHN1114/CMSCD1011 Introduction to Computer Audio
Lecture 6: The MIDI standard (Part I - Protocols)
Dr David England
School of Computing and Mathematical Sciences
Teaching/cmscd1011.htm
Lecture notes 6: Intro to MIDI Part 1

2. In this session... What is MIDI? The MIDI standard General MIDI
channels, events and messages
General MIDI
Note about Tutorials
You must attempt the built-in tutorials for Cakewalk as advised by (1st March)

3. What is MIDI?
MIDI is an acronym for Musical Instrument Digital Interface
It was created in 1983 as a means for the real-time control of music devices
MIDI is used to capture the performance aspects of music as opposed the actual digital audio
MIDI does not explicitly encode the sound timbre
The MIDI standard specifies a set of performance commands that can be sent to MIDI devices which then synthesise the original performance
The receiving device decides how to reproduce it

4. Uses for MIDI
The use of MIDI to supplement digital audio is now widespread in multimedia applications and computer games
Most games use MIDI for background music
The Sony PlayStation™ also uses MIDI for some music
This is thanks in part to affordable, good quality wavetable synthesisers in modern soundcards
MIDI files are extremely small (compared to raw digital audio) and therefore offer the multimedia developer additional benefits
Savings in storage space, network transmission time, etc.

5. The MIDI interface MIDI devices generally include three connectors
IN, OUT and THRU
To control one device (the SLAVE) from another (the MASTER) you would connect the OUT from the MASTER to the IN of the SLAVE
OUT IN
MASTER controller
SLAVE

6. MIDI channels
Although there is only one physical OUT connector, MIDI information can be transmitted on any of 16 logical channels
The master can usually transmit (TX) on any of the 16 channels while slaves are set to receive (RX) on one or more of these channels
A device that can receive and play on more than one channel is known as a multitimbral device
In the previous slide, if the keyboard is set to transmit on channel 10 and the drum machine is set to receive on channel 10, the keyboard can be used to control the drum machine

7. Connecting MIDI devices
THRU IN
OUT
MASTER controller IN
THRU IN
The THRU port can be used to pass all messages through the device. Any devices later in the chain will then also pick up those messages.

8. MIDI transmission
MIDI is a serial protocol (its messages are sent as a series of individual bits)
Information is sent in 10-bit packets (words) made up of:
A start bit (whose value is always zero)
8 bits of data (the contents)
A stop bit (whose value is always one)
One or more words comprise a MIDI message
The start and stop bits are only needed for communication purposes and are stripped off by the actual device

9. MIDI messages
At the highest level, MIDI messages are classified as being either:
Channel Messages (they apply to a specific channel)
System Messages (they apply to the system as a whole and not to a channel)
Channel messages can be further split into:
Channel Voice Messages (these carry musical performance data)
Channel Mode Messages (these affect the way a receiving instrument will respond to the channel voice messages)
Every MIDI message is preceded by a status byte to show what type it is

10. Channel voice messages
The channel voice messages are:
Note Events (Note On, Note Off)
Program Changes (changing the sound used to play a particular piece)
Controller Changes (various controllers such as Volume, Pan, Foot controller, Modulation, etc)
Pitch Bend (bending notes up or down)
Channel and Polyphonic Aftertouch (pressing further down on a key after it has been pressed)

11. Channel mode messages
Channel mode messages are used to tell an instrument how to respond to voice messages
These messages are:
Turn all sound off
Turn all notes off
Reset all controllers (sliders, etc)
Local control on and off
Omni modes:
This controls how the device responds to multichannel data

12. System messages
System messages allow you send system specific messages to connected devices
These include:
System Exclusive data (this allows you to send non-MIDI information to a device)
Sequencer commands
Song Select, Song Start, Song Stop, Song Continue
Active sensing
System reset

13. Note events
Every note event (both Note On and Note Off) specifies what note was played and what velocity it was played at
Note pitch values are stored in 7 bits, therefore there are 27 = 128 possible notes
C0 is note 0 (8.17 Hz)
C5 (known as middle C) is note 60 ( Hz)
The velocity is how hard you hit the key and again is a 7-bit value (i.e. it has the range )
Every note on should have a corresponding note off event

14. Status and data bytes
The stream of MIDI data is divided into two types of bytes: status bytes and data bytes
A status byte has a 1 in the leftmost bit
A data byte has a 0 in the leftmost bit
The status byte can be split into two nibbles
A nibble is four bits
Status byte
Note On Event
Channel 1
( = )
Key value 64
Velocity value 80
Data byte

15. Running status
Having to send 3 bytes for every note event can consume a lot of the MIDI bandwidth in very dense musical passages (more on this next week)
Running status is a technique that can be used to reduce this overhead
Under running status, if the next event is the same type as the previous event, it will not send the status byte again
This causes problems since every note on has a note off event which has a different status byte
This can be overcome by sending a note on with a velocity of zero (silence) instead of a note off thereby ensuring that the running status value is maintained

16. General MIDI
The General MIDI Specification (which was created well after the birth of MIDI) specifies the relationship between program numbers and actual sound timbres
A program change message can be sent on any channel to tell the MIDI device which instrument to use
It specifies:
Sound timbre groups, names and number
Channel allocations
Channels and are for chromatic sounds
Channel 10 is for drum and percussion sounds

17. Summary MIDI is used to capture musical performances
MIDI devices are responsible for generating the actual sound based on the MIDI instructions
This can lead to wide variations in the actual performance not unlike real musicians interpreting a musical score
The MIDI protocol specifies what messages are to be understood by MIDI devices and what form these messages should take
The handouts for this lecture contain much more information which you should read for next time!

18. Next lecture...
We will continue looking at the MIDI standard by discussing the various types of MIDI hardware that is available and seeing how to set up MIDI equipment and instruments