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Sound/Audio

Contents

The Nature of Sound

Computer Representation of Sound

Computer Music MIDI

Summary MIDI versus digital audio

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The Nature of Sound

Sound is a physical phenomenon produced by the vibration ofmatter and transmitted as waves.

Perception of sound by human beings is a complex process;involves three systems

The sourcewhich emits sound; The medium through which the sound propagates;

The detectorwhich receives and interprets the sound

Every sound is comprised of waves of many differentfrequencies and shapes

Simplest sound we can hear is a sine wave

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Sound waves can be characterised by the following attributes

Period Frequency Amplitude Bandwidth

Pitch Loudness Dynamic

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Pitch and frequency

Period: interval at which a periodic signal repeats regularly

Pitch: perception of sound by human beings; measures how

high is the sound as it is perceived by a listener

Frequency:

The unit is Hertz (Hz) or kiloHertz (kHz)

Musical instruments are tuned to produce a set of fixed pitches.

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Amplitude and Loudness Amplitude: measure of sound levels; for a digital sound, amplitude is

the sample value. Sounds have different loudness because they carry different amount

of power; unit of power is watt; intensity of sound is the amount ofpower transmitted through an area of 1m2 oriented perpendicular tothe propagation direction of sound

There are two limits of sensitivity to a tone at a given frequency:

A lower limit - the threshold of hearing 10-12 Watt/m2 , and

An upper limit - the threshold of feeling 1 Watt/m2 .

The relative intensity of two different sounds is measured using the

unit Bel or more commonly desiBel (dB)

Very often, we compare a sound with the threshold of hearing

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Comparing intensity, sound level and perceived loudness

Intensity (watt/m2

) Perceived Loudness SL (dB)

1 Threshold of Feeling 120

10-3 Extremely loud (fff) 90

10-4 Very loud (ff) 80

10-5 loud (f) 70

10-6 Moderately loud (mf) 60

10-7 Soft (p) 50

10-8 Very soft (pp) 40

10-9 Extremely soft (ppp) 30

10-12 Threshold of Hearing 0

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Dynamic and Bandwidth

Dynamic range means the change in sound levels. E.g. a large

orchestra can reach 130 dB at its climax and drop to 30dB at its

softest, giving a range of 100dB

Bandwidth is the range of frequencies a device can produce or a

human can hear.

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Computer Representation of Sound

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Quality vs. file size

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Audio File Formats

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Computer Music - MIDI

Musical

Instrument

Digital

Interface

A communication standard developed in the early 1980 for

electronic instruments and computers.

Specifies the hardware connection between equipments as well

as the format in which the data are transferred between theequipments

Common MIDI devices include music synthesisers, modules,

and MIDI devices in common sound cards

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MIDI Hardware

An electronic musical instrument or a computer which has MIDIinterface should have one or more MIDI ports. The MIDI ports on

musical instruments are usually labelled with:

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MIDI Data

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MIDI Channels and Modes

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Instrument Patch

Each MIDI device is usually capable of producing sound

resembling several real instruments and/or noise effects (e.g.telephone, aircraft). Each instrument or noise effect is known as a

patch, orpreset.

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MIDI Files

Each MIDI file contains a number of chunks.

There are two types of chunks

Header chunk: information about entire file; the type of MIDI file,

number of tracks and the timing

Track chunk: the actual data of MIDI track

Three types of MIDI files

0 single multichannel track

1 one or more simultaneous track of a sequence

2 one or more sequentially independent single-track patterns

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Tracks, Channels and Patches

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How MIDI sounds are synthesised

A simplistic view

The MIDI device stores the characteristics of sounds produced bydifferent sound sources

The MIDI messages tell the device which kind of sound, at which

pitch is to be generated, how long the sound is played and other

attributes the node should have

Two ways of synthesizing sound

FM Synthesis: frequency modulation; using one sine wave to

modulate another sine wave, thus generating a new wave whichis rich in timbre

Wave-table synthesis: stores representative digital sound

samples. It manipulates these sample, e.g. by changing the pitch,

to create the complete range of notes.

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MIDI sound attributes

The shape of amplitude envelop has great influence on the

resulting character of sound

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The Amplitude Envelop

Delay: the time between when a key is played and when the

attack phase begins

Attack: the time from no sound to maximum amplitude

Hold: the time envelop will stay at the peak level before startingthe decay phase

Decay: the time it takes the envelop to go from the peak level to

the sustain level

Sustain: the level at which the envelop remains as long as a key

is held down

Release: the time it takes for the sound to fade to nothing

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The Amplitude Envelop

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MIDI Software

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Summary: MIDI vs. Digital Audio