AudioImage/Graphics Movie AnimationText Audio

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Agenda

• Properties of sound

• Analog audio

• Digital audio

• Audio file formats

• Sound recording

• Sound editing

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Introduction

• Audio is the reproduction of sound - important means of communication.

• There are a number of advantages of using audio.

• most obvious: interface for visual disabilities

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Properties of Sound

• Sound is a pressure wave created by successively vibrating matter back and forth.

• The sound wave is in the form of a sine wave.

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Frequency

• The number of air pressure oscillations per second at a fixed point by a sound wave

• Measured in Hertz (Hz) - number of waves per second

• Audible range (human) = 20 Hz - 20 kHz

• this is where the sensation of pitch originates

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Frequency & PitchFr

eque

ncy

Pitc

h

Frequency

Pitch

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Wavelength

• The distance between waves

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Amplitude

• The height of the wave

• Measured in decibels (dB or dBA)

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Intensity vs. Loudness• Amplitude can be thought of as the intensity of

sound and is measurable.

• Loudness of sound is a perception.

• Sound can be perceived differently at the various frequencies (1 kHz-4 kHz is more sensitive than the lower and the higher)

• Individuals also have different hearing capacities. What is loud to one person may not be loud to another, which why loudness is a perception and cannot be measured.

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dBA Sound

10 Normal breathing

20 Whispering at 5 ft

30 Soft whispering

50 Rainfall

60 Normal conversation

110 Shouting in the ear

120 Thunder

110-120 Rock concert

130 Air raid

180 Rocket launching from pad

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Analog Audio

• analog = the measurement of continuous change or movement over a period of time

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Digital Audio

• The reproduction of sound using digital information

• The value of the signal is sampled at regular intervals by an analog-to-digital converter (ADC)

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Basic Terminology & Properties of DA

• Channels

• Sampling rate

• Bit depth

• Quality and file size

• Compression

• Audio codecs

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ChannelsStereo

Mono

An uncompressed

stereo digital file

will be twice the

size of a mono file.

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Sampling Rate

• ADC - the signal is sampled at regular intervals

• The value of each sample is rounded to the nearest integer in a scale that varies according to the resolution of the signal

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Sampling Rate

• The accuracy the reproduction to the source depends on how often the sample is taken.

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Common DA Sampling Rate

8,000 Hz telephone, adequate for human speech

22,050 Hz radio

32,000 Hz miniDV digital camcorder

44,100 HzCD, commonly used with MPEG-1 audio (VCD, SVCD, MP3)

48,000 Hzdigital sound used for digital TV, DVD, DAT, films and professional audio

96,000 or 192,400 Hz DVD-audio and HD-DVD audio tracks

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Bit Depth• The number of bits used to describe each of

the samples taken from the sampling rate

• the potential accuracy of a digital audio

• Higher bit-depth audio will sound better than lower one

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Quality & File Size

• Make the highest level (if possible) ~ 16-bit 44kHz Stereo

• this produces a large file size, however, high audio clarity

• Chances to manipulate such file (to make it smaller) :

• reduce the sampling rate

• make it mono

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Compression

• Digital audio files can also be compressed to reduce file size

• MPEG-1 Audio layer-3 (MP3) is a common

LossyMP3

Real Audio

LosslessWMA

Audio Quality

File Size

“later editing”

“transmission”

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Audio Codecs• An audio codec (coder/decoder) is a piece of

software or a driver that provides support for certain audio formats for certain system.

codec y

codec x

codec x ✘✔Audio codecs

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Audio File Formats• An audio file format is a

container format for storing data on a computer system.

• AIF format

• developed by Apple Computer

• supported by most web browsers

• AU format

• one of the most common format on the web

• created by Sun Microsystems

• WAV format

• Microsoft’s audio format

• also one of the most common format on the web

• MP3

• most widespread form of audio on the Internet

• playable on various players

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MIDI Format

• Musical Instrument Digital Interface (MIDI) is typically used to play music.

• MIDI files contain a set of instruction that the computer sends to a sound card, synthesizer, or other device.

• Such instructions (note and instrument description) describe how to play the sound, rather than recording an actual rendition of the sound.

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Sound Recording

sound recorder(mic, tape recorder, MD, etc)

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Sound Editing• Audacity - it’s GNU GPL license, it’s free, it’s

cross platform.

• Tracks

• Waveform display

• Selection

• Trim

• Amplify

• Pitch

• and many more....27