CMSCDHN1114/CMSCD1011 Introduction to Computer Audio

1
CMSCDHN1114/CMSCD1011 Introduction to Computer
Audio

• Lecture 6 The MIDI standard(Part I - Protocols)
• Dr David England
• School of Computing and Mathematical Sciences
• http//java.cms.livjm.ac.uk/homepage/staff/cmsd
  engl/ Teaching/cmscd1011.htm
• Email d.england_at_livjm.ac.uk

2
In this session...

• What is MIDI?
• The MIDI standard
• channels, events and messages
• General MIDI
• Note about Tutorials
• You must attempt the built-in tutorials for
  Cakewalk as advised by email (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

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
THRU
IN
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 (261.63 Hz)
• The velocity is how hard you hit the key and
  again is a 7-bit value (i.e. it has the range 0 -
  127)
• 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

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 1 - 9 and 11 - 16 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