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Note: Transcribed and distributed with permission from the author, Jim Heckroth
of Crystal Semiconductor. This text refers to some diagrams, please refer
to the original application note #AN27REV3, in the Crystal Semiconductor
Databook, 1993. Please direct your inquiries to:
Crystal Semiconductor Corporation
4210 S. Industrial Dr.
Austin, Texas 78744
(512) 445-7222
Fax (512) 445-7581
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A Tutorial on MIDI and Wavetable Music Synthesis
by Jim Heckroth
Introduction
The Musical Instrument Digital Interface (MIDI) protocol has been
widely accepted and utilized by musicians and composers since its
conception in the 1982/1983 time frame. MIDI data is a very efficient
method of representing musical performance information, and this makes
MIDI an attractive protocol for computer applications which produce
sound, such as multimedia presentations or computer games. However,
the lack of standardization of synthesizer capabilities hindered applications
developers and presented MIDI users with a rather steep learning curve
to overcome. Fortunately, thanks to the publication of the General
MIDI System specification, wide acceptance of the most common PC/MIDI
interfaces, support for MIDI in Microsoft WINDOWS, and the evolution
of low-cost high-quality wavetable music synthesizers, the MIDI protocol
is now seeing widespread use in a growing number of applications. This
paper gives a brief overview of the standards and terminology associated
with the generation of sound using the MIDI protocol and wavetable
music synthesizers.
Use of MIDI in Multimedia Applications
Originally developed to allow musicians to connect synthesizers together,
the MIDI protocol is now finding widespread use in the generation
of sound for games and multimedia applications. There are several
advantages to generating sound with a MIDI synthesizer rather than
using sampled audio from disk or CD-ROM. The first advantage is storage
space. Data files used to store digitally sampled audio in PCM format
(such as .WAV files) tend to be quite large. This is especially true
for lengthy musical pieces captured in stereo using high sampling
rates. MIDI data files, on the other hand, are extremely small when
compared with sampled audio files. For instance, files containing
high quality stereo sampled audio require about 10 MBytes of data
per minute of sound, while a typical MIDI sequence might consume less
than 10 KBytes of data per minute of sound. This is because the MIDI
file does not contain the sampled audio data, it contains only the
instructions needed by a synthesizer to play the sounds. These instructions
are in the form of MIDI messages, which instruct the synthesizer which
sounds to use, which notes to play, and how loud to play each note. The
actual sounds are then generated by the synthesizer.
The smaller file size also means that less of the PCs bandwidth is
utilized in spooling this data out to the peripheral which is generating
sound. Other advantages of utilizing MIDI to generate sounds include
the ability to easily edit the music, and the ability to change the
playback speed and the pitch or key of the sounds independently. This
last point is particularly important in synthesis applications such
as karaoke equipment, where the musical key and tempo of a song may
be selected by the user.
MIDI Systems
The Musical Instrument Digital Interface (MIDI) protocol provides
a standardized and efficient means of conveying musical performance
information as electronic data. MIDI information is transmitted in
"MIDI messages", which can be thought of as instructions which tell
a music synthesizer how to play a piece of music. The Synthesizer
receiving the MIDI data must generate the actual sounds. The MIDI
1.0 Detailed Specification, published by the International MIDI Association,
provides a complete description of the MIDI protocol.
The MIDI data stream is a unidirectional asynchronous bit stream at
31.25 kbits/sec. with 10 bits transmitted per byte (a start bit, 8
data bits, and one stop bit). The MIDI interface on a MIDI instrument
will generally include three different MIDI connectors, labeled IN,
OUT, and THRU. The MIDI data stream is usually originated by a MIDI
controller, such as a musical instrument keyboard, or by a MIDI sequencer.
A MIDI controller is a device which is played as an instrument, and
it translates the performance into a MIDI data stream in real time
(as it is played). A MIDI sequencer is a device which allows MIDI
data sequences to be captured, stored, edited, combined, and replayed. The
MIDI data output from a MIDI controller or sequencer is transmitted
via the devices' MIDI OUT connector.
The recipient of this MIDI data stream is commonly a MIDI sound generator
or sound module, which will receive MIDI messages at its MIDI IN connector,
and respond to these messages by playing sounds. Figure 1 shows a
simple MIDI system, consisting of a MIDI keyboard controller and a
MIDI sound module. Note that many MIDI keyboard instruments include
both the keyboard controller and the MIDI sound module functions within
the same unit. In these units, there is an internal link between
the keyboard and the sound module which may be enabled or disabled
by setting the "local control" function of the instrument to ON or
OFF respectively.
The single physical MIDI channel is divided into 16 logical channels
by the inclusion of a 4 bit channel number within many of the MIDI
messages. A musical instrument keyboard can generally be set to transmit
on any one of the sixteen MIDI channels. A MIDI sound source, or
sound module, can be set to receive on specific MIDI channel(s). In
the system depicted in Figure 1, the sound module would have to be
set to receive the channel which the keyboard controller is transmitting
on in order to play sounds.
Information received on the MIDI IN connector of a MIDI device is
transmitted back out (repeated) at the devices' MIDI THRU connector. Several
MIDI sound modules can be daisy-chained by connecting the THRU output
of one device to the IN connector of the next device downstream in
the chain.
Figure 2 shows a more elaborate MIDI system. In this case, a MIDI
keyboard controller is used as an input device to a MIDI sequencer,
and there are several sound modules connected to the sequencer's MIDI
OUT port. A composer might utilize a system like this to write a
piece of music consisting of several different parts, where each part
is written for a different instrument. The composer would play the
individual parts on the keyboard one at a time, and these individual
parts would be captured by the sequencer. The sequencer would then
play the parts back together through the sound modules. Each part
would be played on a different MIDI channel, and the sound modules
would be set to receive different channels. For example, Sound module
number 1 might be set to play the part received on channel 1 using
a piano sound, while module 2 plays the information received on channel
5 using an acoustic bass sound, and the drum machine plays the percussion
part received on MIDI channel 10.
In the last example, a different sound module is used to play each
part. However, sound modules which are "multi-timbral" are capable
of playing several different parts simultaneously. A single multi-timbral
sound module might be configured to receive the piano part on channel
1, the bass part on channel 5, and the drum part on channel 10, and
would play all three parts simultaneously.
Figure 3 depicts a PC-based MIDI system. In this system, the PC is
equipped with an internal MIDI interface card which sends MIDI data
to an external multi-timbral MIDI synthesizer module. Application
software, such as Multimedia presentation packages, educational software,
or games, send information to the MIDI interface card over the PC
bus. The MIDI interface converts this information into MIDI messages
which are sent to the sound module. Since this is a multi-timbral
module, it can play many different musical parts, such as piano, bass
and drums, at the same time. Sophisticated MIDI sequencer software
packages are also available for the PC. With this software running
on the PC, a user could connect a MIDI keyboard controller to the
MIDI IN port of the MIDI interface card, and have the same music composition
capabilities discussed in the last paragraph.
There are a number of different configurations of PC-based MIDI systems
possible. For instance, the MIDI interface and the MIDI sound module
might be combined on the PC add-in card. In fact, the Microsoft Multimedia
PC (MPC) Specification states that a PC add-in sound card must have
an on-board synthesizer in order to be MPC compliant. Until recently,
most MPC compliant sound cards included FM synthesizers with limited
capabilities and marginal sound quality. With these systems, an external
wavetable synthesizer module might be added to get better sound quality. Recently,
more advanced sound cards have been appearing which include high quality
wavetable music synthesizers on-board, or as daughter-card options. With
the increasing use of the MIDI protocol in PC applications, this trend
is sure to continue.
MIDI Messages
A MIDI message is made up of an eight bit status byte which is generally
followed by one or two data bytes. There are a number of different
types of MIDI messages. At the highest level, MIDI messages are classified
as being either Channel Messages or System Messages. Channel messages
are those which apply to a specific channel, and the channel number
is included in the status byte for these messages. System messages
are not channel specific, and no channel number is indicated in their
status bytes. Channel Messages may be further classified as being
either Channel Voice Messages, or Mode Messages. Channel Voice Messages
carry musical performance data, and these messages comprise most of
the traffic in a typical MIDI data stream. Channel Mode messages
affect the way a receiving instrument will respond to the Channel
Voice messages. MIDI System Messages are classified as being System
Common Messages, System Real Time Messages, or System Exclusive Messages. System
Common messages are intended for all receivers in the system. System
Real Time messages are used for synchronization between clock-based
MIDI components. System Exclusive messages include a Manufacturer's
Identification (ID) code, and are used to transfer any number of data
bytes in a format specified by the referenced manufacturer. The various
classes of MIDI messages are discussed in more detail in the following
paragraphs.
Channel Voice Messages
Channel Voice Messages are used to send musical performance information. The
messages in this category are the Note On, Note Off, Polyphonic Key
Pressure, Channel Pressure, Pitch Bend Change, Program Change, and
the Control Change message.
In MIDI systems, the activation of a particular note and the release
of the same note are considered as two separate events. When a key
is pressed on a MIDI keyboard instrument or MIDI keyboard controller,
the keyboard sends a Note On message on the MIDI OUT port. The keyboard
may be set to transmit on any one of the sixteen logical MIDI channels,
and the status byte for the Note On message will indicate the selected
channel number. The Note On status byte is followed by two data bytes,
which specify key number (indicating which key was pressed) and velocity
(how hard the key was pressed). The key number is used in the receiving
synthesizer to select which note should be played, and the velocity
is normally used to control the amplitude of the note. When the key
is released, the keyboard instrument or controller will send a Note
Off message. The Note Off message also includes data bytes for the
key number and for the velocity with which the key was released. The
Note Off velocity information is normally ignored.
Some MIDI keyboard instruments have the ability to sense the amount
of pressure which is being applied to the keys while they are depressed. This
pressure information, commonly called "aftertouch", may be used to
control some aspects of the sound produced by the synthesizer (vibrato,
for example). If the keyboard has a pressure sensor for each key,
then the resulting "polyphonic aftertouch" information would be sent
in the form of Polyphonic Key Pressure messages. These messages include
separate data bytes for key number and pressure amount. It is currently
more common for keyboard instruments to sense only a single pressure
level for the entire keyboard. This "channel aftertouch" information
is sent using the Channel Pressure message, which needs only one data
byte to specify the pressure value.
The Pitch Bend Change message is normally sent from a keyboard instrument
in response to changes in position of the pitch bend wheel. The pitch
bend information is used to modify the pitch of sounds being played
on a given channel. The Pitch Bend message includes two data bytes
to specify the pitch bend value. Two bytes are required to allow
fine enough resolution to make pitch changes resulting from movement
of the pitch bend wheel seem to occur in a continuous manner rather
than in steps.
The Program Change message is used to specify the type of instrument
which should be used to play sounds on a given channel. This message
needs only one data byte which specifies the new program number.
MIDI Control Change messages are used to control a wide variety of
functions in a synthesizer. Control Change messages, like other MIDI
channel messages, should only affect the channel number indicated
in the status byte. The control change status byte is followed by
one data byte indicating the "controller number", and a second byte
which specifies the "control value". The controller number identifies
which function of the synthesizer is to be controlled by the message.
Controller Numbers 0 - 31 are generally used for sending data from
switches, wheels, faders, or pedals on a MIDI controller device such
as a musical instrument keyboard. Control numbers 32 - 63 are used
to send an optional Least Significant Byte (LSB) for control numbers
0 through 31, respectively. Some examples of synthesizer functions
which may be controlled are modulation (controller number 1), volume
(controller number 7), and pan (controller number 10). Controller
numbers 64 through 67 are used for switched functions. these are the sustain/damper
pedal (controller number 64), portamento (controller number 65), sostenuto
pedal (controller number 66), and soft pedal (controller number 67). Controller
numbers 16-19 and 80-83 are defined to be general purpose controllers,
and controller numbers 48-51 may be used to send an optional LSB for
controller numbers 16-19. Several of the MIDI controllers merit more
detailed descriptions, and these controllers are described in the
following paragraphs.
Controller number zero is defined as the bank select. The bank select
function is used in some synthesizers in conjunction with the MIDI
Program Change message to expand the number of different instrument
sounds which may be specified (the Program Change message alone allows
selection of one of 128 possible program numbers). The additional
sounds are commonly organized as "variations" of the 128 addressed
by the Program Change message. Variations are selected by preceding
the Program Change message with a Control Change message which specifies
a new value for controller zero (see the Roland General Synthesizer
Standard topic covered later in this paper).
Controller numbers 91 through 95 may be used to control the depth
or level of special effects, such as reverb or chorus, in synthesizers
which have these capabilities.
Controller number 6 (Data Entry), in conjunction with Controller numbers
96 (Data Increment), 97 (Data Decrement), 98 (Registered Parameter
Number LSB), 99 (Registered Parameter Number MSB), 100 (Non-Registered
Parameter Number LSB), and 101 (Non-Registered Parameter Number MSB),
may be used to send parameter data to a synthesizer in order to edit
sound patches. Registered parameters are those which have been assigned
some particular function by the MIDI Manufacturers Association (MMA)
and the Japan MIDI Standards Committee (JMSC). For example, there
are Registered Parameter numbers assigned to control pitch bend sensitivity
and master tuning for a synthesizer. Non-Registered parameters have
not been assigned specific functions, and may be used for different
functions by different manufacturers. Parameter data is transferred
by first selecting the parameter number to be edited using controllers
98 and 99 or 100 and 101, and then adjusting the data value for that
parameter using controller number 6, 96, or 97.
Controller Numbers 121 through 127 are used to implement the MIDI
"Channel Mode Messages". These messages are covered in the next section.
Channel Mode Messages
Channel Mode messages (MIDI controller numbers 121 through 127) affect
the way a synthesizer responds to MIDI data. Controller number 121
is used to reset all controllers. Controller number 122 is used to
enable or disable Local Control (In a MIDI synthesizer which has it's
own keyboard, the functions of the keyboard controller and the synthesizer
can be isolated by turning Local Control off). Controller numbers
124 through 127 are used to select between Omni Mode On or Off, and
to select between the Mono Mode or Poly Mode of operation.
When Omni mode is On, the synthesizer will respond to incoming MIDI
data on all channels. When Omni mode is Off, the synthesizer will
only respond to MIDI messages on one channel. When Poly mode is selected,
incoming Note On messages are played polyphonically. This means that
when multiple Note On messages are received, each note is assigned
its own voice (subject to the number of voices available in the synthesizer). The
result is that multiple notes are played at the same time. When Mono
mode is selected, a single voice is assigned per MIDI channel. This
means that only one note can be played on a given channel at a given
time. Most modern MIDI synthesizers will default to Omni On/Poly
mode of operation. In this mode, the synthesizer will play note messages
received on any MIDI channel, and notes received on each channel are
played polyphonically. In the Omni Off/Poly mode of operation,
the synthesizer will receive on a single channel and play the notes
received on this channel polyphonically. This mode is useful when
several synthesizers are daisy-chained using MIDI THRU. In this
case each synthesizer in the chain can be set to play one part (the
MIDI data on one channel), and ignore the information related to the
other parts.
Note that a MIDI instrument has one MIDI channel which is designated
as its "Basic Channel". The Basic Channel assignment may be hard-wired,
or it may be selectable. Mode messages can only be received by an
instrument on the Basic Channel.
System Common Messages
The System Common Messages which are currently defined include MTC
Quarter Frame, Song Select, Song Position Pointer, Tune Request, and
End Of Exclusive (EOX). The MTC Quarter Frame message is part of
the MIDI Time Code information used for synchronization of MIDI equipment
and other equipment, such as audio or video tape machines.
The Song Select message is used with MIDI equipment, such as sequencers
or drum machines, which can store and recall a number of different
songs. The Song Position Pointer is used to set a sequencer to start
playback of a song at some point other than at the beginning. The
Song Position Pointer value is related to the number of MIDI clocks
which would have elapsed between the beginning of the song and the
desired point in the song. This message can only be used with equipment
which recognizes MIDI System Real Time Messages (MIDI Sync).
The Tune Request message is generally used to request an analog synthesizer
to retune its' internal oscillators. This message is generally not
needed with digital synthesizers.
The EOX message is used to flag the end of a System Exclusive message,
which can include a variable number of data bytes.
System Real Time Messages
The MIDI System Real Time messages are used to synchronize all of
the MIDI clock-based equipment within a system, such as sequencers
and drum machines. Most of the System Real Time messages are normally
ignored by keyboard instruments and synthesizers. To help ensure
accurate timing, System Real Time messages are given priority over
other messages, and these single-byte messages may occur anywhere
in the data stream (a Real Time message may appear between the status
byte and data byte of some other MIDI message). The System Real Time
messages are the Timing Clock, Start, Continue, Stop, Active Sensing,
and the System Reset message. The Timing Clock message is the master
clock which sets the tempo for playback of a sequence. The Timing
Clock message is sent 24 times per quarter note. The Start, Continue,
and Stop messages are used to control playback of the sequence.
The Active Sensing signal is used to help eliminate "stuck notes"
which may occur if a MIDI cable is disconnected during playback of
a MIDI sequence. Without Active Sensing, if a cable is disconnected
during playback, then some notes may be left playing indefinitely
because they have been activated by a Note On message, but will never
receive the Note Off. In transmitters which utilize Active Sensing,
the Active Sensing message is sent once every 300 ms by the transmitting
device when this device has no other MIDI data to send. If a receiver
who is monitoring Active Sensing does not receive any type of MIDI
messages for a period of time exceeding 300 ms, the receiver may assume
that the MIDI cable has been disconnected, and it should therefore
turn off all of its' active notes. Use of Active Sensing in MIDI
transmitters and receivers is optional.
The System Reset message, as the name implies, is used to reset and
initialize any equipment which receives the message. This message
is generally not sent automatically by transmitting devices, and must
be initiated manually by a user.
System Exclusive Messages
System Exclusive messages may be used to send data such as patch parameters
or sample data between MIDI devices. Manufacturers of MIDI equipment
may define their own formats for System Exclusive data. Manufacturers
are granted unique identification (ID) numbers by the MMA or the JMSC,
and the manufacturer ID number is included as the second byte of the
System Exclusive message. The manufacturers ID byte is followed by
any number of data bytes, and the data transmission is terminated
with the EOX message. Manufacturers are required to publish the details
of their System Exclusive data formats, and other manufacturers may
freely utilize these formats, provided that they do not alter or utilize
the format in a way which conflicts with the original manufacturers
specifications.
There is also a MIDI Sample Dump Standard, which is a System Exclusive
data format defined in the MIDI specification for the transmission
of sample data between MIDI devices.
Running Status
MIDI data is transmitted serially. Musical events which originally
occurred at the same time must be sent one at a time in the MIDI data
stream, and therefore these events will not actually be played at
exactly the same time. However, the resulting delays are generally
short enough that the events are perceived as having occurred simultaneously. The
MIDI data transmission rate is 31.35 kbit/s with 10 bits transmitted
per byte of MIDI data. Thus, a 3 byte Note On or Note Off message
takes about 1 ms to be sent. For a person playing a MIDI instrument
keyboard, the time skew between playback of notes when 10 keys are
pressed simultaneously should not exceed 10 ms, and this would not
be perceptible. However, MIDI data being sent from a sequencer can
include a number of different parts. On a given beat, there may be
a large number of musical events which should occur simultaneously,
and the delays introduced by serialization of this information might
be noticeable.
To help reduce the amount of data transmitted in the MIDI data stream,
a technique called "running status" may be employed. It is very common
for a string of consecutive messages to be of the same message type. For
instance, when a chord is played on a keyboard, 10 successive Note
On messages may be generated, followed by 10 Note Off messages. When
running status is used, a status byte is sent for a message only when
the message is not of the same type as the last message sent on the
same channel. The status byte for subsequent messages of the same
type may be omitted (only the data bytes are sent for these subsequent
messages). The effectiveness of running status can be enhanced by
sending Note On messages with a velocity of zero in place of Note
Off messages. In this case, long strings of Note On messages will
often occur. Changes in some of the the MIDI controllers or movement
of the pitch bend wheel on a musical instrument can produce a staggering
number of MIDI channel voice messages, and running status can also
help a great deal in these instances.
MIDI Sequencers and Standard MIDI files
MIDI messages are received and processed by a MIDI synthesizer in
real time. When the synthesizer receives a MIDI "note on" message
it plays the appropriate sound. When the corresponding "note off"
message is received, the synthesizer turns the note off. If the source
of the MIDI data is a musical instrument keyboard, then this data
is being generated in real time. When a key is pressed on the keyboard,
a "note on" message is generated in real time. In these real time
applications, there is no need for timing information to be sent along
with the MIDI messages. However, if the MIDI data is to be stored
as a data file, and/or edited using a sequencer, then some form of
"time-stamping" for the MIDI messages is required.
The International MIDI Association publishes a Standard MIDI Files
specification, which provides a standardized method for handling time-stamped
MIDI data. This standardized file format for time-stamped MIDI data
allows different applications, such as sequencers, scoring packages,
and multimedia presentation software, to share MIDI data files.
The specification for Standard MIDI Files defines three formats for
MIDI files. MIDI sequencers can generally manage multiple MIDI data
streams, or "tracks". MIDI files having Format 0 must store all of
the MIDI sequence data on a single track. This is generally useful
only for simple "single track" devices. Format 1 files, which are
the most commonly used, store data as a collection of tracks. Format
2 files can store several independent patterns.
Synthesizer Polyphony and Timbres
The polyphony of a sound generator refers to its ability to play more
than one note at a time. Polyphony is generally measured or specified
as a number of notes or voices. Most of the early music synthesizers
were monophonic, meaning that they could only play one note at a time. If
you pressed five keys simultaneously on the keyboard of a monophonic
synthesizer, you would only hear one note. Pressing five keys on
the keyboard of a synthesizer which was polyphonic with four voices
of polyphony would, in general, produce four notes. If the keyboard
had more voices (many modern sound modules have 16, 24, or 32 note
polyphony), then you would hear all five of the notes.
The different sounds that a synthesizer or sound generator can produce
are often referred to as "patches", "programs", "algorithms", sounds,
or "timbres". Modern synthesizers commonly use program numbers to
represent different sounds they produce. Sounds may then be selected
by specifying the program numbers (or patch numbers) for the desired
sound. For instance, a sound module might use patch number 1 for
its acoustic piano sound, and patch number 36 for its fretless bass
sound. The association of patch numbers to sounds is often referred
to as a patch map. A MIDI Program Change message is used to tell
a device receiving on a given channel to change the instrument sound
being used. For example, a sequencer could set up devices on channel
4 to play fretless bass sounds by sending a Program Change message
for channel four with a data byte value of 36 (this is the General
MIDI program number for the fretless bass patch).
A synthesizer or sound generator is said to be multi-timbral if it
is capable of producing two or more different instrument sounds simultaneously. Again,
if a synthesizer can play five notes simultaneously, then it is polyphonic. If
it can produce a piano sound and an acoustic bass sound at the same
time, then it is also multi-timbral. A synthesizer or sound module
which has 24 notes of polyphony and which is 6 part multi-timbral
(capable of producing 6 different timbres simultaneously) could synthesize
the sound of a 6 piece band or orchestra. A sequencer could send
MIDI messages for a piano part on channel 1, bass on channel 2, saxophone
on channel 3, drums on channel 10, etc. A 16 part multi-timbral synthesizer
could receive a different part on each of MIDI's 16 logical channels.
The polyphony of a multi-timbral synthesizer is usually allocated
dynamically among the different parts (timbres) being used. In our
example, at a given instant five voices might be used for the piano
part, two voices for the bass, one for the saxophone, and 6 voices
for the drums, leaving 10 voices free. Note that some sounds utilize
more than one voice, so the number of notes which may be produced
simultaneously may be less than the stated polyphony of the synthesizer,
depending on which sounds are being utilized.
The General MIDI (GM) System
At the beginning of a MIDI sequence, a Program Change message is usually
sent on each channel used in the piece in order to set up the appropriate
instrument sound for each part. The Program Change message tells
the synthesizer which patch number should be used for a particular
MIDI channel. If the synthesizer receiving the MIDI sequence uses
the same patch map (the assignment of patch numbers to sounds) that
was used in the composition of the sequence, then the sounds will
be assigned as intended. Unfortunately, prior to General MIDI, there
was no standard for the relationship of patch numbers to specific
sounds for synthesizers. Thus, a MIDI sequence might produce different
sounds when played on different synthesizers, even though the synthesizers
had comparable types of sounds. For example, if the composer had
selected patch number 5 for channel 1, intending this to be an electric
piano sound, but the synthesizer playing the MIDI data had a tuba
sound mapped at patch number 5, then the notes intended for the piano
would be played on the tuba when using this synthesizer (even though
this synthesizer may have a fine electric piano sound available at
some other patch number).
The General MIDI (GM) Specification, published by the International
MIDI Association, defines a set of general capabilities for General
MIDI Instruments. The General MIDI Specification includes the definition
of a General MIDI Sound Set (a patch map), a General MIDI Percussion
map (mapping of percussion sounds to note numbers), and a set of General
MIDI Performance capabilities (number of voices, types of MIDI messages
recognized, etc.). A MIDI sequence which has been generated for use
on a General MIDI Instrument should play correctly on any General
MIDI synthesizer or sound module.
The General MIDI system utilizes MIDI channels 1-9 and 11-16 for chromatic
instrument sounds, while channel number 10 is utilized for "key-based"
percussion sounds. The General MIDI Sound set for channels 1-9 and
11-16 is given in table 1. These instrument sounds are grouped into
"sets" of related sounds. For example, program numbers 1-8 are piano
sounds, 6-16 are chromatic percussion sounds, 17-24 are organ sounds,
25-32 are guitar sounds, etc.
For the instrument sounds on channels 1-9 and 11-16, the note number
in a Note On message is used to select the pitch of the sound which
will be played. For example if the Vibraphone instrument (program
number 12) has been selected on channel 3, then playing note number
60 on channel 3 would play the middle C note (this would be the default
note to pitch assignment on most instruments), and note number 59
on channel 3 would play B below middle C. Both notes would be played
using the Vibraphone sound.
The General MIDI percussion map used for channel 10 is given in table
2. For these "key-based" sounds, the note number data in a Note On
message is used differently. Note numbers on channel 10 are used
to select which drum sound will be played. For example, a Note On
message on channel 10 with note number 60 will play a Hi Bongo drum
sound. Note number 59 on channel 10 will play the Ride Cymbal 2 sound.
It should be noted that the General MIDI system specifies sounds using
program numbers 1 through 128. The MIDI Program Change message used
to select these sounds uses an 8-bit byte, which corresponds to decimal
numbering from 0 through 127, to specify the desired program number. Thus,
to select GM sound number 10, the Glockenspiel, the Program Change
message will have a data byte with the decimal value 9.
The General MIDI system specifies which instrument or sound corresponds
with each program/patch number, but General MIDI does not specify
how these sounds are produced. Thus, program number 1 should select
the Acoustic Grand Piano sound on any General MIDI instrument. However,
the Acoustic Grand Piano sound on two General MIDI synthesizers which
use different synthesis techniques may sound quite different.
Table 1: General MIDI Sound Set (All Channels Except 10)
Prog# Instrument Name
===== ========================
1 Acoustic Grand Piano
2 Bright Acoustic Piano
3 Electric Grand Piano
4 Honky-tonk Piano
5 Electric Piano 1
6 Electric Piano 2
7 Harpsichord
8 Clavi
9 Celesta
10 Glockenspiel
11 Music Box
12 Vibraphone
13 Marimba
14 Xylophone
15 Tubular Bells
16 Dulcimer
17 Drawbar Organ
18 Percussive Organ
19 Rock Organ
20 Church Organ
21 Reed Organ
22 Accordion
23 Harmonica
24 Tango Accordion
25 Acoustic Guitar (nylon)
26 Acoustic Guitar (steel)
27 Electric Guitar (jazz)
28 Electric Guitar (clean)
29 Electric Guitar (muted)
30 Overdriven Guitar
31 Distortion Guitar
32 Guitar harmonics
33 Acoustic Bass
34 Electric Bass (finger)
35 Electric Bass (pick)
36 Fretless Bass
37 Slap Bass 1
38 Slap Bass 2
39 Synth Bass 1
40 Synth Bass 2
41 Violin
42 Viola
43 Cello
44 Contrabass
45 Tremolo Strings
46 Pizzicato Strings
47 Orchestral Harp
48 Timpani
49 String Ensemble 1
50 String Ensemble 2
51 SynthStrings 1
52 SynthStrings 2
53 Choir Aahs
54 Voice Oohs
55 Synth Voice
56 Orchestra Hit
57 Trumpet
58 Trombone
59 Tuba
60 Muted Trumpet
61 French Horn
62 Brass Section
63 SynthBrass 1
64 SynthBrass 2
65 Soprano Sax
66 Alto Sax
67 Tenor Sax
68 Baritone Sax
69 Oboe
70 English Horn
71 Bassoon
72 Clarinet
73 Piccolo
74 Flute
75 Recorder
76 Pan Flute
77 Blown Bottle
78 Shakuhachi
79 Whistle
80 Ocarina
81 Lead 1 (square)
82 Lead 2 (sawtooth)
83 Lead 3 (calliope)
84 Lead 4 (chiff)
85 Lead 5 (charang)
86 Lead 6 (voice)
87 Lead 7 (fifths)
88 Lead 8 (bass + lead)
89 Pad 1 (new age)
90 Pad 2 (warm)
91 Pad 3 (polysynth)
92 Pad 4 (choir)
93 Pad 5 (bowed)
94 Pad 6 (metallic)
95 Pad 7 (halo)
96 Pad 8 (sweep)
97 FX 1 (rain)
98 FX 2 (soundtrack)
99 FX 3 (crystal)
100 FX 4 (atmosphere)
101 FX 5 (brightness)
102 FX 6 (goblins)
103 FX 7 (echoes)
104 FX 8 (sci-fi)
105 Sitar
106 Banjo
107 Shamisen
108 Koto
109 Kalimba
110 Bag pipe
111 Fiddle
112 Shanai
113 Tinkle Bell
114 Agogo
115 Steel Drums
116 Woodblock
117 Taiko Drum
118 Melodic Tom
119 Synth Drum
120 Reverse Cymbal
121 Guitar Fret Noise
122 Breath Noise
123 Seashore
124 Bird Tweet
125 Telephone Ring
126 Helicopter
127 Applause
128 Gunshot
Table 2: General Midi Percussion Map (Channel 10)
Note # Drum Sound
====== =====================
35 Acoustic Bass Drum
36 Bass Drum 1
37 Side Stick
38 Acoustic Snare
39 Hand Clap
40 Electric Snare
41 Low Floor Tom
42 Closed Hi-Hat
43 High Floor Tom
44 Pedal Hi-Hat
45 Low Tom
46 Open Hi-Hat
47 Low Mid Tom
48 Hi Mid Tom
49 Crash Cymbal 1
50 High Tom
51 Ride Cymbal 1
52 Chinese Cymbal
53 Ride Bell
54 Tambourine
55 Splash Cymbal
56 Cowbell
57 Crash Cymbal 2
58 Vibraslap
59 Ride Cymbal 2
60 Hi Bongo
61 Low Bongo
62 Mute Hi Conga
63 Open Hi Conga
64 Low Conga
65 High Timbale
66 Low Timbale
67 High Agogo
68 Low Agogo
69 Cabasa
70 Maracas
71 Short Whistle
72 Long Whistle
73 Short Guiro
74 Long Guiro
75 Claves
76 Hi Wood Block
77 Low Wood Block
78 Mute Cuica
79 Open Cuica
80 Mute Triangle
81 Open Triangle
The Roland General Synthesizer (GS) Standard
The Roland General Synthesizer (GS) functions are a superset of those
specified for General MIDI. The GS system includes all of the GM
sounds (which are referred to as "capital instrument" sounds), and
adds new sounds which are organized as variations of the capital instruments.
Variations are selected using the MIDI Control Change message in conjunction
with the Program Change message. The Control Change message is sent
first, and it is used to set controller number 0 to some specified
nonzero value indicating the desired variation (some capital sounds
have several different variations). The Control Change message is
followed by a MIDI Program Change message which indicates the program
number of the related capital instrument. For example, Capital instrument
number 25 is the Nylon String Guitar. The Ukulele is a variation of
this instrument. The Ukulele is selected by sending a Control Change
message which sets controller number 0 to a value of 8, followed by
a program change message on the same channel which selects program
number 25. Sending the Program change message alone would select
the capital instrument, the Nylon String Guitar. Note also that a
Control Change of controller number 0 to a value of 0 followed by
a Program Change message would also select the capital instrument.
The GS system also includes adjustable reverberation and chorus effects. The
effects depth for both reverb and chorus may be adjusted on an individual
MIDI channel basis using Control Change messages. The type of reverb
and chorus sounds employed may also be selected using System Exclusive
messages.
Synthesizer Implementations: FM vs. Wavetable
There are a number of different technologies or algorithms used to
create sounds in music synthesizers. Two widely used techniques are
Frequency Modulation (FM) synthesis and Wavetable synthesis. FM synthesis
techniques generally use one periodic signal (the modulator) to modulate
the frequency of another signal (the carrier). If the modulating
signal is in the audible range, then the result will be a significant
change in the timbre of the carrier signal. Each FM voice requires
a minimum of two signal generators. These generators are commonly
referred to as "operators", and different FM synthesis implementations
have varying degrees of control over the operator parameters. Sophisticated
FM systems may use 4 or 6 operators per voice, and the operators may
have adjustable envelopes which allow adjustment of the attack and
decay rates of the signal. Although FM systems were implemented in
the analog domain on early synthesizer keyboards, modern FM synthesis
implementations are done digitally.
FM synthesis techniques are very useful for creating expressive new
synthesized sounds. However, if the goal of the synthesis system
is to recreate the sound of some existing instrument, this can generally
be done more accurately with digital sample-based techniques. Digital
sampling systems store high quality sound samples digitally, and then
replay these sounds on demand. Digital sample-based synthesis systems
may employ a variety of special techniques, such as sample looping,
pitch shifting, mathematical interpolation, and polyphonic digital
filtering, in order to reduce the amount of memory required to store
the sound samples (or to get more types of sounds from a given amount
of memory). These sample-based synthesis systems are often called
"wavetable" synthesizers (the sample memory in these systems contains
a large number of sampled sound segments, and can be thought of as
a "table" of sound waveforms which may be looked up and utilized when
needed). A number of the special techniques employed in this type
of synthesis are discussed in the following paragraphs.
Wavetable Synthesis Techniques
Looping and Envelope Generation
One of the primary techniques used in wavetable synthesizers to conserve
sample memory space is the looping of sampled sound segments. For
a large number of instrument sounds, the sound can be modeled as consisting
of two major sections, the attack section and the sustain section. The
attack section is the initial part of the sound, where the amplitude
and the spectral characteristics of the sound may be changing very
rapidly. The sustain section of the sound is that part of the sound
following the attack, where the characteristics of the sound are changing
less dynamically. Figure 4 shows a waveform with portions which could
be considered the attack and the sustain sections indicated. In this
example, the spectral characteristics of the waveform remain constant
throughout the sustain section, while the amplitude is decreasing
at a fairly constant rate. This is an exaggerated example, in most
natural instrument sounds, both the spectral characteristics and the
amplitude continue to change through the duration of the sound. The
sustain section, if one can be identified, is that section for which
the characteristics of the sound are relatively constant.
A great deal of memory can be saved in wave-table synthesis systems
by storing only a short segment of the sustain section of the waveform,
and then looping this segment during playback. Figure 5 shows a two
period segment of the sustain section from the waveform in Figure
4, which has been looped to create a steady state signal. If the
original sound had a fairly constant spectral content and amplitude
during the sustained section, then the sound resulting from this looping
operation should be a good approximation of the sustained section
of the original.
For many acoustic string instruments, the spectral characteristics
of the sound remain fairly constant during the sustain section, while
the amplitude of the signal decays. This can be simulated with a
looped segment by multiplying the looped samples by a decreasing gain
factor during playback to get the desired shape or envelope. The
amplitude envelope of a sound is commonly modeled as consisting of
some number of linear segments. An example is the commonly used four
part piecewise-linear Attack-Decay-Sustain-Release (ADSR) envelope
model. Figure 6 depicts a typical ADSR envelope shape, and Figure
7 shows the result of applying this envelope to the looped waveform
from Figure 5.
A typical wavetable synthesis system would store separate sample segments
for the attack section and the looped section of an instrument. These
sample segments might be referred to as the initial sound and the
loop sound. The initial sound is played once through, and then the
loop sound is played repetitively until the note ends. An envelope
generator function is used to create an envelope which is appropriate
for the particular instrument, and this envelope is applied to the
output samples during playback. Playback of the initial wave (with
the the Attack portion of the envelope applied) begins when a Note
On message is received. The length of the initial sound segment is
fixed by the number of samples in the segment, and the length of the
Attack and Decay sections of the envelope are generally also fixed
for a given instrument sound. The sustain section will continue to
repeat the loop samples while applying the Sustain envelope slope
(which decays slowly in our examples), until a Note Off message is
applied. The Note Off message triggers the beginning of the Release
portion of the envelope.
Loop Length
The loop length is measured as a number of samples, and the length
of the loop should be equal to an integral number of periods of the
fundamental pitch of the sound being played (if this is not true,
then an undesirable "pitch shift" will occur during playback when
the looping begins). Of course, the length of the pitch period of
a sampled instrument sound will generally not work out to be an integral
number of sample periods. Therefore, it is common to perform a "resampling"
process on the original sampled sound, to get new a new sound sample
for which the pitch period is an integral number of sample periods.<N>
In practice, the length of the loop segment for an acoustic instrument
sample may be many periods with respect to the fundamental pitch of
the sound. If the sound has a natural vibrato or chorus effect, then
it is generally desirable to have the loop segment length be an integral
multiple of the period of the vibrato or chorus.
One-Shot Sounds
The previous paragraphs discussed dividing a sampled sound into an
attack section and a sustain section, and then using looping techniques
to minimize the storage requirements for the sustain portion. However,
some sounds, particularly sounds of short duration or sounds whose
characteristics change dynamically throughout their duration, are
not suitable for looped playback techniques. Short drum sounds often
fit this description. These sounds are stored as a single sample
segment which is played once through with no looping. This class
of sounds are referred to as "one-shot" sounds.
Sample Editing and Processing
There are a number of sample editing and processing steps involved
in preparing sampled sounds for use in a wave-table synthesis system. The
requirements for editing the original sample data to identify and
extract the initial and loop segments, and for resampling the data
to get a pitch period length which is an integer multiple of the sampling
period, have already been mentioned.
Editing may also be required to make the endpoints of the loop segment
compatible. If the amplitude and the slope of the waveform at the
beginning of the loop segment do not match those at the end of the
loop, then a repetitive "glitch" will be heard during playback of
the looped section. Additional processing may be performed to "compress"
the dynamic range of the sound to improve the signal/quantizing noise
ratio or to conserve sample memory. This topic is addressed next.
When all of the sample processing has been completed, the resulting
sampled sound segments for the various instruments are tabulated to
form the sample memory for the synthesizer.
Sample Data Compression
The signal-to-quantizing noise ratio for a digitally sampled signal
is limited by sample word size (the number of bits per sample), and
by the amplitude of the digitized signal. Most acoustic instrument
sounds reach their peak amplitude very quickly, and the amplitude
then slowly decays from this peak. The ear's sensitivity dynamically
adjusts to signal level. Even in systems utilizing a relatively small
sample word size, the quantizing noise level is generally not perceptible
when the signal is near maximum amplitude. However, as the signal
level decays, the ear becomes more sensitive, and the noise level
will appear to increase. Of course, using a larger word size will
reduce the quantizing noise, but there is a considerable price penalty
paid if the number of samples is large.
Compression techniques may be used to improve the signal-to-quantizing
noise ratio for some sampled sounds. These techniques reduce the
dynamic range of the sound samples stored in the sample memory. The
sample data is decompressed during playback to restore the dynamic
range of the signal. This allows the use of sample memory with a
smaller word size (smaller dynamic range) than is utilized in the
rest of the system. There are a number of different compression techniques
which may be used to compress the dynamic range of a signal.
For signals which begin at a high amplitude and decay in a fairly
linear fashion, a simple compression technique can be effective. If
the slope of the decay envelope of the signal is estimated, then an
envelope with the complementary slope (the negative of the decay slope)
can be constructed and applied to the original sample data. The resulting
sample data, which now has a flat envelope, can be stored in the sample
memory, utilizing the full dynamic range of the memory. The decay
envelope can then be applied to the stored sample data during sound
playback to restore the envelope of the original sound.
Note that there is some compression effect inherent in the looping
techniques described earlier. If the loop segment is stored at an
amplitude level which makes full use of the dynamic range available
in the sample memory, and the processor and D/A converters used for
playback have a wider dynamic range than the sample memory, then the
application of a decay envelope during playback will have a decompression
effect similar to that described in the previous paragraph.
Pitch Shifting
In order to minimize sample memory requirements, wavetable synthesis
systems utilize pitch shifting, or pitch transposition techniques,
to generate a number of different notes from a single sound sample
of a given instrument. For example, if the sample memory contains
a sample of a middle C note on the acoustic piano, then this same
sample data could be used to generate the C# note or D note above
middle C using pitch shifting.
Pitch shifting is accomplished by accessing the stored sample data
at different rates during playback. For example, if a pointer is
used to address the sample memory for a sound, and the pointer is
incremented by one after each access, then the samples for this sound
would be accessed sequentially, resulting in some particular pitch.
If the pointer increment was two rather than one, then only every
second sample would be played, and the resulting pitch would be shifted
up by one octave (the frequency would be doubled).
Frequency Accuracy
In the previous example, the sample memory address pointer was incremented by
an integer number of samples. This allows only a limited set of pitch
shifts. In a more general case, the memory pointer would consist
of an integer part and a fractional part, and the increment value
could be a fractional number of samples. The integer part of the
address pointer is used to address the sample memory, the fractional
part is used to maintain frequency accuracy. For example if the increment
value was equivalent to 1/2, then the pitch would be shifted down
by one octave (the frequency would be halved). When non-integer increment
values are utilized, the frequency resolution for playback is determined
by the number of bits used to represent the fractional part of the
address pointer and the address increment parameter.
Interpolation
When the fractional part of the address pointer is non-zero, then
the "desired value" falls between available data samples. Figure
8 depicts a simplified addressing scheme wherein the Address Pointer
and the increment parameter each have a 4-bit integer part and a 4-bit
fractional part. In this case, the increment value is equal to 1
1/2 samples. Very simple systems might simply ignore the fractional
part of the address when determining the sample value to be sent to
the D/A converter. The data values sent to the D/A converter when
using this approach are indicated in the Figure 8, case I. A slightly
better approach would be to use the nearest available sample value. More
sophisticated systems would perform some type of mathematical interpolation
between available data points in order to get a value to be used for
playback. Values which might be sent to the D/A when interpolation
is employed are shown as case II. Note that the overall frequency
accuracy would be the same for both cases indicated, but the output
is severely distorted in the case where interpolation is not used.
There are a number of different algorithms used for interpolation
between sample values. The simplest is linear interpolation. With
linear interpolation, interpolated value is simply the weighted average
of the two nearest samples, with the fractional address used as a
weighting constant. For example, if the address pointer indicated
an address of (n+K), where n is the integer part of the address and
K is the fractional part, than the interpolated value can be calculated
as s(n+K) = (1-K)s(n) + (K)s(n+1), where s(n) is the sample data value
at address n. More sophisticated interpolation techniques can can
be utilized to further reduce distortion, but these techniques are
computationally expensive.
Oversampling
Oversampling of the sound samples may also be used to improve distortion
in wavetable synthesis systems. For example, if 4X oversampling were
utilized for a particular instrument sound sample, then an address
increment value of 4 would be used for playback with no pitch shift. The
data points chosen during playback will be closer to the "desired
values", on the average, than they would be if no oversampling were
utilized because of the increased number of data points used to represent
the waveform. Of course, oversampling has a high cost in terms of
sample memory requirements.
In many cases, the best approach may be to utilize linear interpolation
combined with varying degrees of oversampling where needed. The linear
interpolation technique provides reasonable accuracy for many sounds,
without the high penalty in terms of processing power required for
more sophisticated interpolation methods. For those sounds which
need better accuracy, oversampling is employed. With this approach,
the additional memory required for oversampling is only utilized where
it is most needed. The combined effect of linear interpolation and
selective oversampling can produce excellent results.
Splits
When the pitch of a sampled sound is changed during playback, the
timbre of the sound is changed somewhat also. For small changes in
pitch (up to a few semitones), the timbre change is generally not
noticed. However, if a large pitch shift is used, the resulting note
will sound unnatural. Thus, a particular sample of an instrument
sound will be useful for recreating a limited range of notes using
pitch shifting techniques. To get coverage of the entire instrument
range, a number of different samples of the instrument are used, and
each of these samples is used to synthesize a limited range of notes. This
technique can be thought of as splitting a musical instrument keyboard
into a number of ranges of notes, with a different sound sample used
for each range. Each of these ranges is referred to as a split, or
key split.
Velocity splits refer to the use of different samples for different
note velocities. Using velocity splits, one sample might be utilized
if a particular note is played softly, where a different sample would
be utilized for the same note of the same instrument when played with
a higher velocity.
Note that the explanations above refer to the use of key splits and
velocity splits in the sound synthesis process. In this case, the
different splits utilize different samples of the same instrument
sound. Key splitting and velocity splitting techniques are also utilized
in a performance context. In the performance context, different splits
generally produce different instrument sounds. For instance, a keyboard
performer might want to set up a key split which would play a fretless
bass sound from the lower octaves of his keyboard, while the upper
octaves play the vibraphone. Similarly, a velocity split might be
set up to play the acoustic piano sound when keys are played with
soft to moderate velocity, but an orchestral string sound plays when
the keys are pressed with higher velocity.
Aliasing Noise
The previous paragraph discussed the timbre changes which result from
pitch shifting. The resampling techniques used to shift the pitch
of a stored sound sample can also result in the introduction of aliasing
noise into an instrument sound. The generation of aliasing noise
can also limit the amount of pitch shifting which may be effectively
applied to a sound sample. Sounds which are rich in upper harmonic
content will generally have more of a problem with aliasing noise. Low-pass
filtering applied after interpolation can help eliminate the undesirable
effect of aliasing noise. The use of oversampling also helps eliminate
aliasing noise.
LFOs for vibrato and tremolo
Vibrato and tremolo are effects which are often produced by musicians
playing acoustic instruments. Vibrato is basically a low-frequency
modulation of the pitch of a note, while tremolo is modulation of
the amplitude of the sound. These effects are simulated in synthesizers
by implementing low-frequency oscillators (LFOs) which are used to
modulate the pitch or amplitude of the synthesized sound being produced. Natural
vibrato and tremolo effects tend to increase in strength as a note
is sustained. This is accomplished in synthesizers by applying an
envelope generator to the LFO. For example, a flute sound might have
a tremolo effect which begins at some point after the note has sounded,
and the tremolo effect gradually increases to some maximum level,
where it remains until the note stops sounding.
Layering
Layering refers to a technique in which multiple sounds are utilized
for each note played. This technique can be used to generate very
rich sounds, and may also be useful for increasing the number of instrument
patches which can be created from a limited sample set. Note that
layered sounds generally utilize more than one voice of polyphony
for each note played, and thus the number of voices available is effectively
reduced when these sounds are being used.
Polyphonic Digital Filtering for Timbre Enhancement
It was mentioned earlier that low-pass filtering may be used to help
eliminate noise which may be generated during the pitch shifting process. There
are also a number of ways in which digital filtering is used in the
timbre generation process to improve the resulting instrument sound. In
these applications, the digital filter implementation is polyphonic,
meaning that a separate filter is implemented for each voice being
generated, and the filter implementation should have dynamically adjustable
cutoff frequency and/or Q.
For many acoustic instruments, the character of the tone which is
produced changes dramatically as a function of the amplitude level
at which the instrument is played. For example, the tone of an acoustic
piano may be very bright when the instrument is played forcefully,
but much more mellow when it is played softly. Velocity splits, which
utilize different sample segments for different note velocities, can
be implemented to simulate this phenomena. Another very powerful
technique is to implement a digital low-pass filter for each note
with a cutoff frequency which varies as a function of the note velocity. This
polyphonic digital filter dynamically adjusts the output frequency
spectrum of the synthesized sound as a function of note velocity,
allowing a very effective recreation of the acoustic instrument timbre.
Another important application of polyphonic digital filtering is in
smoothing out the transitions between samples in key-based splits. At
the border between two splits, there will be two adjacent notes which
are based on different samples. Normally, one of these samples will
have been pitch shifted up to create the required note, while the
other will have been shifted down in pitch. As a result, the timbre
of these two adjacent notes may be significantly different, making
the split obvious. This problem may be alleviated by employing a
polyphonic digital filter which uses the note number to control the
filter characteristics. A table may be constructed containing the
filter characteristics for each note number of a given instrument. The
filter characteristics are chosen to compensate for the pitch shifting
associated with the key splits used for that instrument.
It is also common to control the characteristics of the digital filter
using an envelope generator or an LFO. The result is an instrument
timbre which has a spectrum which changes as a function of time. For
example, It is often desirable to generate a timbre which is very
bright at the onset, but which gradually becomes more mellow as the
note decays. This can easily be done using a polyphonic digital filter
which is controlled by an envelope generator.
The PC to MIDI Interface and the MPU-401
To use MIDI with a personal computer, a PC to MIDI interface product
is generally required (there are a few personal computers which come
equipped with built-in MIDI interfaces). There are a number of MIDI
interface products for PCs. The most common types of MIDI interfaces
for IBM compatibles are add-in cards which plug into an expansion
slot on the PC bus, but there are also serial port MIDI interfaces
(connects to a serial port on the PC) and parallel port MIDI interfaces
(connects to the PC printer port). The fundamental function of a
MIDI interface for the PC is to convert parallel data bytes from the
PC data bus into the serial MIDI data format and vice versa (a UART
function). However, "smart" MIDI interfaces may provide a number
of more sophisticated functions, such as generation of MIDI timing
data, MIDI data buffering, MIDI message filtering, synchronization
to external tape machines, and more.
The defacto standard for MIDI interface add-in cards for the PC is
the Roland MPU-401 interface. The MPU-401 is a smart MIDI interface,
which also supports a dumb mode of operation (often referred to as
"pass-through mode" or "UART mode"). There are a number of MPU-401
compatible MIDI interfaces on the market. In addition, many add-in
sound cards include built-in MIDI interfaces which implement the UART
mode functions of the MPU-401.
Compatibility Considerations for MIDI Applications on the
PC
There are two levels of compatibility which must be considered for
MIDI applications running on the PC. First is the compatibility of
the application with the MIDI interface being used. The second is
the compatibility of the application with the MIDI synthesizer. Compatibility
considerations under DOS and the Microsoft Windows operating system
are discussed in the following paragraphs.
DOS Applications
DOS applications which utilize MIDI synthesizers include MIDI sequencing
software, music scoring applications, and a variety of games. In terms
of MIDI interface compatibility, virtually all of these applications
support the MPU-401 interface, and most utilize only the UART mode. These
applications should work correctly if the PC is equipped with a MPU-401,
a full-featured MPU-401 compatible, or a sound card with a MPU-401
UART-mode capability. Other MIDI interfaces, such as serial port
or parallel port MIDI adapters, will only work if the application
provides support for that particular model of MIDI interface.
A particular application may provide support for a number of different
models of synthesizers or sound modules. Prior to the General MIDI
standard, there was no widely accepted standard patch set for synthesizers,
so applications generally needed to provide support for each of the
most popular synthesizers at the time. If the application did not
support the particular model of synthesizer or sound module that was
attached to the PC, then the sounds produced by the application might
not be the sounds which were intended. Modern applications can provide
support for a General MIDI (GM) synthesizer, and any GM-compatible
sound source should produce the correct sounds. Some other models
which are commonly supported are the Roland MT-32, the Roland LAPC-1,
and the Roland Sound Canvas. The Roland MT-32 was an external MIDI
sound module which utilized Roland's Linear Additive (LA) synthesis,
and the MT-32 combined with an MPU-401 interface became a popular
MIDI synthesis platform for the PC. The LAPC-1 was a PC add-in card
which combined the MT-32 synthesis function with the MPU-401 MIDI
interface. The Sound Canvas is Roland's General Synthesizer (GS)
sound module, and this unit has become an industry standard.
Microsoft Windows and the Multimedia PC (MPC)
The number of applications for high quality audio functions on the
PC (including music synthesis) grew explosively after the introduction
of Microsoft Windows 3.0 with Multimedia Extensions ("Windows with
Multimedia") in 1991. The Multimedia PC (MPC) specification, originally
published by Microsoft in 1991 and now published by the Multimedia
PC Marketing Council (a subsidiary of the Software Publishers Association),
specifies minimum requirements for multimedia-capable Personal Computers. A
system which meets these requirements will be able to take full advantage
of Windows with Multimedia. Note that many of the functions originally
included in the Multimedia Extensions have been incorporated into
the Windows 3.1 operating system.
The audio capabilities utilized by Windows 3.1 or Windows with Multimedia
include audio recording and playback (linear PCM sampling), music
synthesis, and audio mixing. In order to support the required music
synthesis functions, MPC-compliant audio adapter cards must have on-board
music synthesizers.
The MPC specification defines two types of synthesizers; a "Base Multitimbral
Synthesizer", and an "Extended Multitimbral Synthesizer". Both the
Base and the Extended synthesizer must support the General MIDI patch
set. The difference between the Base and the Extended synthesizer
requirements is in the minimum number of notes of polyphony, and the
minimum number of simultaneous timbres which can be produced. Base
Multitimbral Synthesizers must be capable of playing 6 "melodic notes"
and "2 percussive" notes simultaneously, using 3 "melodic timbres"
and 2 "percussive timbres". The formal requirements for an Extended
Multitimbral Synthesizer are only that it must have capabilities which
exceed those specified for a Base Multitimbral Synthesizer. However,
the "goals" for an Extended synthesizer include the ability to play
16 melodic notes and 8 percussive notes simultaneously, using 9 melodic
timbres and 8 percussive timbres.
The MPC specification also includes an authoring standard for MIDI
composition. This standard requires that each MIDI file contain two
arrangements of the same song, one for Base synthesizers and one for
Extended synthesizers. The MIDI data for the Base synthesizer arrangement
is sent on MIDI channels 13 - 16 (with the percussion track on channel
16), and the Extended synthesizer arrangement utilizes channels 1
- 10 (percussion is on channel 10). This technique allows a single
MIDI file to play on either type of synthesizer.
Windows applications generally address hardware devices such as MIDI
interfaces or synthesizers through the use of drivers. The drivers
provide applications software with a common interface through which
hardware may be accessed, and this simplifies the hardware compatibility
issue. Before a synthesizer is used, a suitable driver must be installed
using the Windows Driver applet within the Control Panel. The device
drivers supplied with Windows 3.1 include a driver for the MPU-401/LAPC-1
MIDI interface, and a driver for the original AdLib FM synthesizer
card. Most other MIDI interfaces and/or synthesizers are shipped with
their own Windows drivers.
When a MIDI interface or synthesizer is installed in the PC and a
suitable device driver has been loaded, the Windows MIDI Mapper applet
will appear within the Control Panel. MIDI messages are sent from
an application to the MIDI Mapper, which then routes the messages
to the appropriate device driver. The MIDI Mapper may be set to perform
some filtering or translations of the MIDI messages in route from
the application to the driver. The processing to be performed by the
MIDI Mapper is defined in the MIDI Mapper Setups, Patch Maps, and
Key Maps.
MIDI Mapper Setups are used to assign MIDI channels to device drivers. For
instance, If you have an MPU-401 interface with a General MIDI synthesizer
and you also have a Creative Labs Soundblaster card in your system,
you might wish to assign channels 13 to 16 to the Ad Lib driver (which
will drive the Base-level FM synthesizer on the Soundblaster), and
assign channels 1 - 10 to the MPU-401 driver. In this case, MPC compatible
MIDI files will play on both the General MIDI synthesizer and the
FM synthesizer at the same time. The General MIDI synthesizer will
play the Extended arrangement on MIDI channels 1 - 10, and the FM
synthesizer will play the Base arrangement on channels 13-16. The
MIDI Mapper Setups can also be used to change the channel number of
MIDI messages. If you have MIDI files which were composed for a
General MIDI instrument, and you are playing them on a Base Multitimbral
Synthesizer, you would probably want to take the MIDI percussion data
coming from your application on channel 10 and send this information
to the device driver on channel 16.
The MIDI Mapper patch maps are used to translate patch numbers when
playing MPC or General MIDI files on synthesizers which do not use
the General MIDI patch numbers. Patch maps can also be used to play
MIDI files which were arranged for non-GM synthesizers on GM synthesizers. For
example, the Windows-supplied MT-32 patch map can be used when playing
GM-compatible .MID files on the Roland MT-32 sound module or LAPC-1
sound card.
The MIDI Mapper key maps perform a similar function, translating the
key numbers contained in MIDI Note On and Note Off messages. This
capability is useful for translating GM-compatible percussion parts
for playback on non-GM synthesizers or vice-versa. The Windows-supplied
MT-32 key map changes the key-to-drum sound assignments used for General
MIDI to those used by the MT-32 and LAPC-1.
Some MIDI applications, such as MIDI sequencer software packages,
can be set to make use of the MIDI Mapper, or to address the device
driver directly (bypassing the MIDI Mapper). Other Windows applications
always utilize the MIDI Mapper.
Summary
The MIDI protocol provides an efficient format for conveying musical
performance data, and the Standard MIDI Files specification ensures
that different applications can share time-stamped MIDI data. The
storage efficiency of the MIDI file format makes MIDI an attractive
vehicle for generation of sounds in multimedia applications, computer
games, or high-end karaoke equipment. The General MIDI system provides
a common set of capabilities and a common patch map for high polyphony,
multi-timbral synthesizers. General MIDI-compatible Synthesizers
employing high quality wavetable synthesis techniques provide an ideal
MIDI sound generation facility for multimedia applications.