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IBM Audio Interface Library
Extended MIDI (XMIDI) Specification
Release 1.02 of 17 August 1992
Contents
Overview ........................................................... 3
Extended MIDI Controller Reference ................................. 4
Extended MIDI IFF (.XMI) Structure ................................. 11
Extended MIDI Global Timbre Library File Format .................... 13
Glossary of Audio Interface Library Terms .......................... 14
Overview
The term 'Extended MIDI,' or 'XMIDI,' refers to the set of features added to the base Musical
Instrument Digital Interface (MIDI) data specification by the drivers and utility programs
included with the Audio Interface Library's Version 2.00 release. The XMIDI standard brings
many powerful features to the Audio Interface Library which would otherwise be absent in a
system conforming to the original MIDI standard.
Composers who are interested in getting started with XMIDI should pay particular attention to
the descriptions of the utility programs MIDIFORM, GLIB, MIDIECHO, and CAKEPORT
in the Tools Reference section of this manual, before moving on to the specific discussions of
the Extended MIDI controllers below. It is strongly recommended that musicians with even the
slightest programming knowledge (or interest) examine the Application Program Interface
Reference section as well, in order to gain the best possible understanding of the Audio
Interface Library's capabilities.
Programmers seeking to master effective Audio Interface Library application design are also
advised to familiarize themselves with the Tools Reference section, followed by a thorough
perusal of the Application Program Interface Reference and Technical Notes. Special attention
should be given to the Programming Examples section as well. The chapters in this section on
the two primary XMIDI file formats are provided for reference only.
Users planning to upgrade Audio Interface Library Version 1 MIDI applications to the XMIDI
standard should begin with an understanding of the MIDIFORM and GLIB programs and how
they create and manage the new system's data file formats. Virtually all of the Version 1 API
functions have been retained, but many have undergone revision or syntactic changes as part of
the Version 2.0 release. A careful review of the Application Program Interface Reference and
Programming Examples sections will ease the transition to the new API.
Extended MIDI Controller Reference
XMIDI provides a number of additional MIDI-style Control Change messages which greatly
extend the range of composition and arrangement options available to the musician and
application programmer. Below is a list of Extended MIDI Control Change messages available
in the current Audio Interface Library release, together with explanations of their uses in
XMIDI sequences.
<EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD>Ŀ
<EFBFBD> Controller <20> Description <20>
<EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD>͵
<EFBFBD> 110 (6eh) <20> Channel Lock <20>
<EFBFBD> 111 (6fh) <20> Channel Lock Protect <20>
<EFBFBD> 112 (70h) <20> Voice Protect <20>
<EFBFBD> 113 (71h) <20> Timbre Protect <20>
<EFBFBD> 114 (72h) <20> Patch Bank Select <20>
<EFBFBD> 115 (73h) <20> Indirect Controller Prefix <20>
<EFBFBD> 116 (74h) <20> For Loop Controller <20>
<EFBFBD> 117 (75h) <20> Next/Break Loop Controller <20>
<EFBFBD> 118 (76h) <20> Clear Beat/Bar Count <20>
<EFBFBD> 119 (77h) <20> Callback Trigger <20>
<EFBFBD> 120 (78h) <20> Sequence Branch Index <20>
<EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD>
Channel Lock (110)
< 64: Release previously locked channel and restore default channel
number mapping
>= 64: Seize ("lock") an unlocked, unprotected physical MIDI channel
(2-9) for assignment to this logical channel # (11-16)
The Channel Lock controller is one of the more powerful Extended MIDI controllers. Channel
locking allows the XMIDI drivers to play multiple XMIDI sequences simultaneously, which in
turn permits the creation of impressive sound effects and musical "mixes."
When used with a value greater than or equal to 64, the Channel Lock controller attempts to
seize, or "lock," the highest MIDI channel number recognized by the driver's supported
synthesizer which has the fewest actively playing notes. Channels which are already locked, or
which have been "protected" against locking by use of the XMIDI Channel Lock Protect
controller (111), are immune to seizure. If no available MIDI channels are eligible for locking
under these criteria, the controller will have no effect. (This will occur only if all unprotected
channels are already locked.)
After a MIDI channel has been successfully locked, the driver maps the locked channel onto
the MIDI channel in which the Channel Lock controller itself appeared. This means that all
subsequent MIDI events sent on the same channel as the Channel Lock controller (the "logical"
channel) will instead be transmitted on the newly locked (or "physical") channel.
This concept is perhaps best illustrated with a typical application of channel locking. Below is
a list of events comprising a short XMIDI sequence to create a "machine gun" sound effect.
(Timing and note velocity information has been omitted for simplicity.)
Ch 02 Controller 114 = 1 ;XMIDI Patch Bank Select = 1
Ch 02 Patch = 5 ;MIDI Program Change to patch 5 ("gunshot")
Ch 02 Pitch Wheel 0 ;Pitch bend centered (no bend)
Ch 02 Controller 1 = 0 ;Modulation = 0
Ch 02 Controller 7 = 127 ;Volume = 127
Ch 02 Controller 10 = 64 ;Panpot = 64 (center of stereo field)
Ch 02 Controller 116 = 5 ;FOR loop = 1 to 5...
Ch 02 Note C 5 on ;...sound a brief "gunshot" note...
Ch 02 Note C 5 off
Ch 02 Controller 116 = 127 ;...NEXT loop
This "machine gun" sequence would be ideal for use as part of an arcade game's sound-effects
arsenal, except for one serious drawback: what happens if MIDI channel 2 is already being
used by a background music sequence, or perhaps another sound effect? The controller values
and patch change in the sequence above would certainly have an unfavorable effect on a
romantic violin solo playing on channel 2. The problem could be solved by setting aside a
block of active MIDI channels for use with overlaid sound-effects sequences, but this might be
an unacceptable compromise for applications requiring playback of any but the simplest XMIDI
sequences.
Channel locking and remapping is a much more satisfactory solution. When composing
sequences for the XMIDI standard, the composer could assign all tracks in sound effects and
other overlaid sequences to MIDI channels in the range 11 through 16. MIDI channels in this
range are not normally recognized by synthesizers supported by the AIL. But if an XMIDI
Channel Lock controller is then inserted before the first Channel Voice event in the channel,
the AIL driver can temporarily reassign a physical MIDI channel to the out-of-range logical
channel number, preventing the channel's use by any other sequence being played for as long
as it is needed by the sequence which "locked" the channel. A not-so-obvious benefit of this
scheme is that all sound-effects sequences can safely share the same logical channel numbers
between 11 and 16, since only the physical channel numbers recognized by the synthesizer are
vulnerable to usage conflicts.
With channel locking installed, the machine gun sound-effect sequence might look like this:
Ch 11 Controller 110 = 127 ;XMIDI Channel Lock (seize channel)
Ch 11 Controller 114 = 1 ;XMIDI Patch Bank Select = 1
Ch 11 Patch = 5 ;MIDI Program Change to patch 5 ("gunshot")
Ch 11 Pitch Wheel 0 ;Pitch bend centered (no bend)
Ch 11 Controller 1 = 0 ;Modulation = 0
Ch 11 Controller 7 = 127 ;Volume = 127
Ch 11 Controller 10 = 64 ;Panpot = 64 (center of stereo field)
Ch 11 Controller 116 = 5 ;FOR loop = 1 to 5...
Ch 11 Note C 5 on ;...sound a brief "gunshot" note...
Ch 11 Note C 5 off
Ch 11 Controller 116 = 127 ;...NEXT loop
Ch 11 Controller 110 = 0 ;XMIDI Channel Lock (release channel) The sequence's new final event, a Channel Lock controller with the value 0, is actually
unnecessary since all channels locked by a sequence are released automatically as soon as the
sequence ends or playback is stopped. However, in sequences with multiple tracks, it is a
good idea to explicitly release each locked channel as soon as it is no longer needed by the
sequence, instead of waiting for all tracks to expire. This allows the XMIDI driver to restore
the channel to the sequence from which it was "stolen," minimizing the disruption to the
background music.
When a locked channel is released, either through a Channel Lock controller with a value
below 64 or through a C call to the analogous AIL_release_channel() function, the timbre
bank, patch number, pitch wheel, and MIDI controllers in the channel are updated to their
correct values before being returned to the service of the original sequence. This relieves the
application of the responsibility of keeping track of these values while a channel is locked.
Conversely, when a channel is locked by a Channel Lock controller or its complementary
AIL_lock_channel() function, the MIDI Sustain (Hold1) controller (64) in the channel is
forced to its "off" condition and all notes playing in the channel are silenced. No other
controller values or MIDI messages are initialized! It is essential for a sequence (or an
application) which has just received a locked channel to initialize all MIDI parameters,
including MIDI controllers likely to contain undesirable values, the XMIDI Patch Bank Select
controller and MIDI Program Change number, and the MIDI pitch wheel, to appropriate
values. (Avoid the temptation to accomplish this with MIDI Channel Mode messages such as
All Notes Off or Reset All Controllers. Channel Mode messages may be interpreted differently
by various synthesizers, and the XMIDI driver code which maintains sequence state tables and
other data structures makes no attempt to interpret them at all.)
Channel Lock Protect (111)
< 64: Channel is eligible for temporary reassignment by
AIL_lock_channel() or Channel Lock controllers
>= 64: Channel may not be locked (seized for reassignment)
Occasionally it may be desirable to prevent any sequence or application process from
interrupting traffic on a given MIDI channel. Depending on its value, the XMIDI Channel
Lock Protect controller will either render the physical MIDI channel in which it appears
immune to seizure, or allow it to be considered for seizure when a Channel Lock controller
message or AIL_lock_channel() call is issued.
Voice Protect (112)
< 64: Voices playing in channel assume normal priority (release voice
protection)
>= 64: Voices playing in channel assume highest possible priority (assert
voice protection)
Available on some synthesizers, voice protection is an XMIDI feature which allows a sequence
to discourage the polyphonic voice assignment algorithm from "stealing" synthesizer voices
from notes played in a given MIDI channel when no free voices are available. If the Voice
Protect controller's value is less than 64, voices assigned to notes playing in the channel will be
considered for reassignment to play incoming note events. This is the default condition for
each channel. Otherwise, the "voice priority" of notes in the channel will constantly assume its
maximum value, preventing the channel's notes from being robbed of their voices. Voice
protection should be used conservatively and released as soon as possible, since the synthesizer
emulation code works best when allowed to make its own decisions regarding voice allocation.
Timbre Protect (113)
< 64: Timbre currently assigned to this channel's patch may be
"discarded" (if least-recently-used) to make room in local timbre
cache or synthesizer memory
>= 64: Timbre may not be discarded from cache or synthesizer memory
For information on timbre protection, refer to the descriptions of the AIL_protect_timbre() /
AIL_unprotect_timbre() functions in the Application Program Interface Reference section of
this manual.
Patch Bank Select (114)
This controller may be used to "extend" the scope of MIDI Program Change (or Cakewalk
"Patch") messages. Normally, any of 128 synthesizer patches may be selected with a Program
Change message; however, the Patch Bank Select controller instructs the AIL XMIDI driver to
associate custom timbres in any of 127 banks with synthesizer patches which are subsequently
selected in a channel.
By convention, patch bank 0 is reserved for the Roland MT-32's built-in patch/timbre
combinations, and for the Ad Lib-style instruments intended to emulate those patches. Custom
Roland timbres (and their Ad Lib counterparts) should be placed in patch banks 1 through 126.
Timbre bank 127 is reserved for custom "drum kit" timbres to be used when playing note
events on MIDI channel 10.
When a MIDI Program Change (or "patch") event is encountered, the XMIDI synthesizer
driver looks in its collection of timbres from the Global Timbre Library for a custom timbre
whose "patch number" is the specified Program Change number and whose "bank" is equal to
the current Patch Bank Select controller value. If the timbre is found, the synthesizer is
automatically instructed to associate the timbre with the patch which was just selected by the
Program Change event. Under normal circumstances, the search for the requested timbre
should not fail, since the application should have been informed in advance of the need for the
timbre via the AIL_timbre_request() function. (See the Application Program Interface
Reference section of this manual for more information on XMIDI timbre management.)
The default Patch Bank Select controller value is 0. Therefore, AIL applications which do not
require custom Roland MT-32 timbres may disregard this controller altogether. Otherwise,
care must be taken to explicitly specify the Timbre Bank Select controller value to be used in
each channel of every sequence and sound effect used by the application, since the default
value of 0 is asserted only upon driver initialization -- not when a channel is locked or when a
sequence stops.
Indirect Controller Prefix (115)
This controller specifies that the next Control Change message encountered in this channel
should obtain its value from the Indirect Controller Array's nth byte entry, where n is the
indirect controller prefix's own value. For more information on the Indirect Controller Array,
see the AIL_register_sequence() function description in the Application Program Interface
Reference section of this manual.
For Loop Controller (116)
1-127: Define beginning of MIDI data block to be played a total of v
times, where v is the For loop controller's value
0: Define beginning of MIDI data block to be played indefinitely
This controller allows the creation of a loop within an XMIDI sequence. Its use is almost self-
explanatory, especially for those familiar with BASIC or C's "for...next" construct. A loop
value of 1 causes the block of MIDI/XMIDI events between the For controller and its
corresponding Next/Break controller to be executed precisely once -- as if the controllers did
not exist. Values from 2 to 127 cause the event block to be executed from 2 to 127 times. A
value of 0, on the other hand, causes the event block to loop indefinitely, interruptable only at
the application's request.
There are four rules to bear in mind when using For...Next/Break loops:
* At least one XMIDI 'quantization interval' must separate the For and
Next/Break loop controllers. In practice, this means that more than
1/120 second (8.4 milliseconds) of real time must elapse between these
two events during playback. Otherwise, the application will "hang"
forever with interrupts disabled, since neither the For nor the
Next/Break controller automatically terminates the current interval.
* The branching effect caused by a Next/Break controller does not result
in the note queue being flushed, unlike the operation of the
AIL_branch_index() function. Only the sequence pointer is adjusted by
the Next/Break controller. Refer to the AIL_branch_index() function
description in the Application Program Interface Reference for
additional notes about sequence branching in general.
* Multi-track MIDI Format 1 files are converted to MIDI Format 0 during
the MIDIFORM program's XMIDI compilation process. This implies that
loops placed on a single track will actually cause events in all other
tracks to be repeated as well. In situations where it is desired to
loop tracks independently of each other, a possible solution is to
split the tracks into multiple XMIDI sequences and have the application
play each sequence simultaneously.
* For...Next/Break controller loops may be nested up to 4 levels deep.
Next/Break Loop Controller (117)
< 64: (Break) Loop iteration stops, regardless of the current For loop
count
>= 64: (Next) Decrement innermost For loop count and, if nonzero, branch
immediately to the innermost For controller event
One application of the Indirect Controller Prefix controller (q.v.) would be to turn a Next
controller into a Break controller by providing a dynamic controller value as specified above.
This feature may be used to introduce elements of application program control into sound-effect
sequences. Otherwise, the Next controller should have a value greater than or equal to 64, in
order to function in the manner detailed in the preceding For Loop Controller description.
Clear Beat/Bar Count (118)
(Value unused, 0-127)
This controller resets the current beat and measure counters to 0:0. See the AIL_beat_count()
/ AIL_measure_count() function description in the Application Program Interface Reference
section for information on the beat and measure counters.
Callback Trigger (119)
(Value = trigger_value parameter passed to callback function, 0-127)
See the AIL_install_callback() function description in the Application Program Interface
Reference section for information on callback functions.
Sequence Branch Index (120)
(Value = marker_number parameter passed to AIL_branch_index() function,
0-127)
See the AIL_branch_index() function description in the Application Program Interface
Reference section for information on sequence branching.
* * *
In addition to the controllers listed here, some synthesizer drivers (such as the Roland MT-32
family) provide several other XMIDI Control Change message types. These controllers allow
the musician to access the synthesizer's set of memory-addressable features, without the need to
deal directly with MIDI System Exclusive messages. For more information about synthesizer-
specific XMIDI controllers, as well as a list of general XMIDI controllers supported by each
driver, refer to the Technical Notes section of this manual. Extended MIDI IFF (.XMI) Structure
XMIDI sequence files are "preprocessed" MIDI sequence files. Created by the MIDIFORM
program described in the Tools Reference section of this manual, XMIDI files are stored in
accordance with Electronic Arts' Interchange File Format (EA IFF 85) standard. Below is a
summary of the XMIDI format's technical details, which is presented as an aid to application
developers with specialized needs not supported directly by the Audio Interface Library.
The XMIDI file format preserves all of the information needed to faithfully reproduce the
MIDI sequence's original sound, while discarding elements of the MIDI storage format that are
redundant, inefficient, or otherwise unnecessary for real-time playback. An XMIDI file
containing a single translated MIDI sequence often requires from 10% to 30% less storage
space, and can be performed with substantially less background processing time.
Each XMIDI file created by the MIDIFORM program contains a single IFF "CAT " chunk of
type XMID. The CAT chunk contains at least one XMIDI sequence, whose local chunks are
stored within a "FORM" chunk of type XMID. As created by MIDIFORM, the XMIDI file
may contain a chunk of type FORM XDIR which contains information about the file's
collection of XMIDI sequences. The XDIR chunk is for the application's benefit, and is not
currently required by the XMIDI drivers.
In the expression below, <len> signifies a 32-bit "big endian" (or Motorola 680XX-style,
MSB first) chunk length, which includes neither itself nor its preceding 4-character CAT ,
FORM, or local chunk name. Square brackets enclose optional chunks; ellipses are placed
after the closing braces of chunks or data items which may be repeated.
[ FORM<len>XDIR
{
INFO<len>
UWORD # of FORMs XMID in file, 1-65535
} ]
CAT <len>XMID
{
FORM<len>XMID
{
[ TIMB<len>
UWORD # of timbre list entries, 0-16384
{ UBYTE patch number 0-127
UBYTE timbre bank 0-127 } ... ]
[ RBRN<len>
UWORD # of branch point offsets, 0-127
{ UWORD Sequence Branch Index controller value 0-127
ULONG controller offset from start of EVNT chunk } ... ]
EVNT<len>
{ UBYTE interval count (if < 128)
UBYTE <MIDI event> (if > 127) } ...
} ...
}
TIMB and RBRN are optional chunks. RBRN is included only if XMIDI Sequence Branch
Index (120) controllers appear in the original MIDI sequence. The mandatory EVNT chunk
contains the quantized sequence events. EVNT must appear as the last local chunk in FORM
XMID, while the other local chunks may appear in any order. <MIDI event> consists of any
MIDI Channel Voice, System Exclusive, or Meta-event except Note Off. The MIDI 'running
status' convention is not used. Note On events are followed by their duration in intervals,
expressed as a MIDI variable-length quantity (i.e., a string of 7-bit bytes stored most
significant byte first, with the high bit set in all bytes except the last).
Extended MIDI Global Timbre Library File Format
For information about Global Timbre Library data in general, review the discussion of the
GLIB program in the Tools Reference section of this manual, as well as the descriptions of
timbre-related functions in the Application Program Interface Reference. A 'boilerplate'
example of C code to fetch a desired timbre from any Global Timbre Library file can be found
in the sample program listing XPLAY.C.
In the expression below, ellipses are placed after the closing braces of data items which may be
repeated.
File header, one entry for each timbre in file
{
BYTE timbre patch number
BYTE timbre bank number
ULONG offset of timbre length word from start of file
} ...
Structure of each timbre as stored in file
{
UWORD timbre length word (size of timbre in bytes + 2)
{
UBYTE timbre data byte
} ...
} ...
Glossary of Audio Interface Library Terms
ADV: Abbreviation for Audio Driver, the standard MS-DOS filename suffix for AIL device
drivers. The .ADV files are actually small assembly-language programs which contain the
"intelligence" needed to support a given PC audio device.
API: Application Program Interface. The AIL API module is linked directly with the
application. It contains the Process Services (described in the API Reference section), as well
as the actual C or Pascal-callable routines which link the application to its installed audio
drivers.
Bank: A collection of timbres, or software instrument definitions. Think of a bank as a
"virtual orchestra" which contains a number of different instruments available for use by a
given driver. See also Bankfile.
Bankfile: An MS-DOS file containing a bank. Bankfiles are typically generated by a
synthesizer's "instrument editor" program. The Audio Interface Library includes a program
called GLIB which converts these device-specific bankfiles into an efficient, standardized
runtime file format known as the Global Timbre Library, or GTL. See the description of the
GLIB program in the Tools Reference section for details.
Channel: Most MIDI events can be associated with one of sixteen possible channels. Although
MIDI is a serial data standard at heart, it is often required to convey information which exists
as a series of events in a number of parallel, synchronous dimensions. For instance, a single
MIDI data stream may be expected to "play" an ensemble consisting of a piano, two guitars, a
drum set, and a brass section. By associating events intended for a given instrument with the
instrument's unique channel number, the composer of the MIDI sequence can prevent
acoustically unfortunate conflicts between the pianos, drums, guitars, and horns.
Controller: MIDI defines a number of control change events which affect the performance
characteristics of instruments playing in a particular channel. Each MIDI synthesizer responds
to a portion of the defined MIDI controller set. For example, the Roland MT-32 recognizes
the standard MIDI controllers for Volume (controller 7), Panpot (controller 10), Sustain
(controller 64), and Modulation (controller 1), among others.
Global Timbre Library: See Bankfile.
GTL: See Bankfile.
MIDI Channel: See Channel.
MIDI Channel Voice Message: A short representation (2-3 bytes) of a single MIDI event.
Channel Voice messages include the Note Off, Note On, Polyphonic Key Pressure, Control
Change, Program Change, Channel Pressure, and Pitch Wheel Change message types.
MIDI Controller: See Controller.
MIDI System Exclusive Message: A device-specific data packet sent to a MIDI synthesizer.
Often, system exclusive messages are used to control aspects of synthesis not otherwise covered
by the MIDI standard, such as custom instrument data, reverb control, and channel response
configuration. Most MIDI synthesizers are accompanied by literature describing what system
exclusive messages they recognize, if any.
Multi-Timbral: Refers to a synthesizer's ability to play notes with more than one type of
musical instrument at a time.
Operator: See Partial.
Partial: Also called an operator, a partial is a component of an FM or additive voice (q.v.).
On the Roland MT-32/LAPC-1 synthesizer, each voice may use from 1 to 4 of 32 available
partials. On the Ad Lib, Sound Blaster, and other YM3812-based cards, each voice requires 2
of 18 available partials. The Yamaha YMF262 (OPL3) device, used in some newer FM sound
cards, provides 36 partials which may be assigned in groups of 2 or 4 per voice.
Patch: A patch associates a timbre with a given MIDI channel. In general, the details of patch
manipulation are handled automatically by the AIL drivers. For information on a given
synthesizer's patch format, consult the programming manual for the device in question.
Patch Bank: See Bank
PCM: Abbreviation for Pulse Code Modulation, the technique used for most sampled sound
playback on the PC. A common variation which implements rudimentary data compression is
known as ADPCM, or ADaptive Pulse Code Modulation.
Polyphonic: Refers to a synthesizer's ability to play more than one voice at a time.
Sequence: Simply put, a musical work expressed as a series (sequence) of MIDI events.
Sysex: See MIDI System Exclusive Message.
System Exclusive Message: See MIDI System Exclusive Message.
Timbre: In the Audio Interface Library's parlance, a timbre (pronounced tim'-ber, or
tam'-ber) is a software definition of a musical instrument. See also Bank and Bankfile.
Timbre Cache: Some AIL XMIDI drivers require the application to set aside a small area of
memory to store instrument definitions. This area is known as the timbre cache. See the API
Reference's discussion of the AIL_default_timbre_cache_size() function for more information.
Track: A logical division of a MIDI sequence. Often, the composer associates a single MIDI
channel with each track, placing all performance data for each instrument on its own track.
Voice: As used in the Audio Interface Library, the term voice refers to the minimum
synthesizer resources needed to play a single note with a given instrument. The Ad Lib and
similar devices provide a total of 9 simultaneous voices, while the Roland MT-32/LAPC-1
synthesizer can generate from 8 to 32 voices at once, depending on the number of partials
(q.v.) used by each voice.
XMIDI: Abbreviation for eXtended MIDI. See the preceding parts of this section for details.

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