Introduction to Acoustics

0
–30
(dB)
–60
0 1.5 3
4.5
(kHz)
Fig. 17.44 Banded decomposition of a struck-bar spectrum
sampling in time, space, and frequency to model stiff
one-dimensional systems [17.38]. This can be viewed
as a hybrid of modal and waveguide synthesis, in that
each waveguide models the speed of sound in the region
around each significant mode of the system. As an example,
Fig. 17.44 shows the spectrum of a struck marimba
17.9 Music Description and Control
The musical instrument digital interface (MIDI) standard,
adopted in 1984, revolutionized electronic music,
and also profoundly affected the computer industry
[17.39]. A simple two-wire serial electrical
connection standard allows interconnection of musical
devices and computers over cable distances of up to
15 m (longer over networks and extensions to the basic
MIDI standard). The MIDI software protocol is best described
as musical keyboard gestural, meaning that the
messages carried over MIDI are the gestures that a pianist
or organist uses to control a traditional keyboard
instrument. There is no time information contained in
the basic MIDI messages; they are intended to take ef-
MIDI serial data transmission
31.25kBaud, 1 Start, 8 Data, 1Stop bit
Asynchronous bytes
“Mark”
1sssssss 0ddddddd
If first bit = 1 then status byte
If first bit = 0 then data byte
Typical message: 10010000 00111100 01000000
Meaning: NoteOn Note = 60 Velocity = 64
Chan= 0 (middle C) (1/2 sort of)
Fig. 17.46 MIDI software transmission protocol
•••
Controls
Force
Velocity
Position
Computer Music 17.9 Music Description and Control 735
Mallet
/bow
+
BP
BP
BP
Delay
Delay
Delay
Fig. 17.45 Banded waveguide model
Sound output
bar, with additional bandpass filters superimposed on
the spectrum, centered at the three main modes. In the
banded waveguide technique, each mode is modeled by
a bandpass filter, plus a delay line to impose the correct
round-trip delay, as shown in Fig. 17.45.
fect as soon as they come over the wire. As such it is
a real-time gestural protocol, and can be adapted for realtime
non-musical sound synthesis applications. There
are complaints about MIDI, however, mostly related to
limited control bandwidth (approximately 1000–1500
continuous messages per second maximum), and the
keyboard-centric bias of the messages.
Basic MIDI message types include NoteOn and
NoteOff, sustain pedal up and down, amount of modulation
(vibrato), pitch bend, key pressure (also called
AfterTouch), breath pressure, volume, pan, balance, reverberation
amount, and others. NoteOn and NoteOff
messages carry a note number corresponding to a particular
piano key, and a velocity corresponding to how
hard that key is hit. Figure 17.46 shows the software serial
format of MIDI, and gives an example of a NoteOn
message.
Another MIDI message is program change, which is
used to select the particular sound being controlled in
the synthesizer. MIDI provides for 16 channels, and the
channel number is encoded into each message. Each
channel is capable of one type of sound, but possibly
many simultaneous voices (a voice is an individual
sound) of that same sound. Channels allow instruments
and devices to all listen on the same network, and choose
to respond to the messages sent on particular channels.
General MIDI (1991), and the standard MIDI file
specifications serve to extend MIDI and made it even
more popular [17.40]. General MIDI helps to assure
Part E 17.9