Lecture6 audio

LECTURE 7

THE USES OF DIGITAL AUDIO IN
MULTIMEDIA

Prepared by
Razia Nisar Noorani

1

Objective

q What is sound?
• Waveforms and attributes of sound
q Capturing digital audio
• Sampling
q MIDI (Musical Instrument Digital
Interface)

2

Sound
 Sound comprises the spoken word, voices, music and
even noise.

 It is a complex relationship involving a vibrating object
(sound source), a transmission medium (usually air), a
receiver (ear) and a perceptor (brain). Example
banging drum.

 As the sound vibrates it bumps into molecules of the
surrounding medium causing pressure waves to travel
away from the source in all directions

3

Sound
 So, Sound are rapid vibrations that are transmitted as
variations in air pressure.

4

Waveforms
 Sound waves are manifest as waveforms
 A waveform that repeats itself at regular intervals is

called a periodic waveform
 Waveforms that do not exhibit regularity are called

noise

 The unit of regularity is called a cycle
 This is known as Hertz (or Hz) after Heinrich Hertz

 One cycle = 1 Hz

 Sometimes written as kHz or kiloHertz (1 kHz =

1000 Hz)

5

Waveforms

Time for one cycle

distance
along wave
Cycle

6

The characteristics of sound waves
 Sound is described in terms of two characteristics:
 Frequency

 Amplitude (or loudness)

 Frequency
 the rate at which sound is measured

 Number of cycles per second or Hertz (Hz)

 Determines the pitch of the sound as heard by our

ears
 The higher frequency, the clearer and sharper the

soundthe higher pitch of sound

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 Amplitude
 Sound’s intensity or loudness

 The louder the sound, the larger amplitude.

 In addition, all sounds have a duration and successive
musical sounds is called rhythm

8


Amplitude Time for one cycle

pitch distance
along wave
Cycle

9

Example waveforms

Piano

Pan flute

Snare drum

10

Capture and playback
of digital audio
Air pressure
variations Digital to
Analogue
Converter
Converts
back into
Captured via voltage DAC
microphone

Signal is
converted into
binary
Analogue (discrete form)
to Digital ADC 0101001101 Air pressure
Converter 0110101111 variations

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The Analogue to Digital
Converter (ADC)
 An ADC is a device that converts analogue signals into
digital signals

 An analogue signal is a continuous value
 It can have any single value on an infinite scale

 A digital signal is a discrete value
 It has a finite value (usually an integer)

 An ADC is synchronised to some clock

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The Analogue to Digital
Converter (ADC)
 It will monitor the continuous analogue signal at a set rate
and convert what it sees into a discrete value at that
specific moment in time

 The process to convert the analogue to digital sound is
called Sampling. Use PCM (Pulse Code Modulation)

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Digital sampling
Sampling frequency

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Digital sampling
Sampling frequency

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Sampling
 Two parameters:

Sampling Rate
 Frequency of sampling
 Measure in Hertz
 The higher sampling rate, higher quality sound but size storage is
big.
 Standard Sampling rate:
- 44.1 KHz for CD Audio
- 22.05 KHz
- 11.025 KHz for spoken
- 5.1025 KHz for audio effect

16

Sampling

Size sample
The resolution of a sample is the number of bits it uses
to store a given amplitude value, e.g.
 8 bits (256 different values)
 16 bits (65536 different values)
 A higher resolution will give higher quality but will require more
memory (or disk storage)

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Quantisation
 Samples are usually represented the audio sample
as a integers(discrete number) or digital

15

0

S a m p le p o in t s
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Calculating the size
of digital audio
 The formula is as follows:

rate × duration × resolution × number of channels
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 The answer will be in bytes
 Where:
 sampling rate is in Hz

 Duration/time is in seconds

 resolution is in bits (1 for 8 bits, 2 for 16 bits)

 number of channels = 1 for mono, 2 for stereo, etc.

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of digital audio
 Example:
Calculate the file size for 1 minute, 44.1 KHz, 16 bits,
stereo sound

8
 Where:
 sampling rate is 44,100 Hz

 Duration/time is 60 seconds

 resolution is 16 bits

 number of channels for stereo is 2

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of digital audio

8

44100 * 60 * 16 *2
8

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Digital audio editing software
 One of the most powerful and professional PC-based
packages is a tool called Sound Forge

http://www.sonicfoundry.com/

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Editing Digital Audio
 Trimming
 Splicing and assembly
 Volume adjustments
 Format conversion
 Resampling or downsampling
 Fade ins and fade outs
 Equalization
 Time Stretching
 Digital Signal processing
 Reversing Sounds

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Editing Digital Audio
 Simple audio editing software allows:
 Recording of digital audio segments
 Trimming
 Splicing and assembly
 Volume adjustments of the entire segment
 Reversing Sounds
 Copy, cut, paste and delete segments of digital audio

 Others audio editing software:
 COOL Edit Pro
 Gold Wave
 PROSONIQ SonicWORX
 Samplitude Studio 24

Audio formats
 Depend on O/S. For examples:

 AIFF (Audio Interchange File Format)
 SOU
 For Macintosh

 .WAV
 Waveform file format. For Windows/Microsoft

 .VOC
 Sound Blaster Card

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What is WAV?
q WAV is an audio file format that was developed
by Microsoft.
q It is so wide spread today that it is called a
standard PC audio file format.
q A Wave file is identified by a file name extension
of WAV (.wav).
q Used primarily in PCs, the Wave file format has
been accepted as a viable interchange medium
for other computer platforms, such as
Macintosh.
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What is WAV?
q This allows content developers to freely move
audio files between platforms for processing,
q For example. The Wave file format stores
information about
• the file's number of tracks (mono or stereo),
• sample rate
• bit depth

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What is WAV?
q Pros
q Can be played by nearly all Windows
applications that support sound
q Fast decoding
q Cons
q Very large file size

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MIDI (Musical Instrument Digital
Interface)
 MIDI is a standard for specifying a musical performance

 Rather than send raw digital audio, it sends instructions
to musical instruments telling them what note to play, at
what volume, using what sound, etc.

 The synthesiser that receives the MIDI events is
responsible for generating the actual sounds. Example:
Keyboard Piano

29

MIDI sequencers
q A MIDI sequencer
allows musicians to
edit and create
musical
compositions like a
word processor
• Cut and paste
• Insert / delete

30

MIDI Versus Wav
 Quality recording, MIDI depend to the tools
 Audio .wav easier to create compare than MIDI

 MIDI Advantages
 File Size small
 Size Storage also small

 MIDI Advantages
 Playback
 Cost and Skill

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How audio can be used effectively
Examples of uses of audio:

 Cautions and warnings
It is a good medium for alerting users to critical
information. Some uses include:
 Sounding an alarm when a limit is reached

 Alerting users when data is entered incorrectly

 Music and Sound Effects
These make multimedia interaction more real. Some
uses include:

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 Musical background for a video segment
 Birds Songs accompanying photographs in biological
field training.

 Sound-related data.
Some uses include:
 Helping mechanics diagnose engine trouble

 Training medical students to recognize different

breathing sounds

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 Direct voice communication.
Some uses include:
 Leaving a voice message for other users of an

application
 Consulting with an expert during a troubleshooting

procedure.

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Advantages and Disadvantages
of using audio
Sound adds life to any multimedia application and plays
important role in effective marketing presentations.

 Advantages

 Ensure important information is noticed

 Add interest

 Can communicate more directly than other media

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Advantages and Disadvantages
of using audio
 Disadvantages

 Easily overused

 Requires special equipment for quality production

 Not as memorable as visual media

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Summary

 There are two main types of digital audio
 Sampled audio

 Captured by sampling an analogue waveform at a

set rate
 MIDI data

 Instructions on how to perform some musical

composition
 Sampled audio requires more storage space than MIDI
information

37

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