Overview of Multimedia• A multimedia presentation might contain:

– Text– Animation– Digital Sound Effects– Voices– Video Clips– Photographic Stills– Music

Overview of Sound• Digital 數碼 versus Analogue 模擬• Conversion from Analogue to Digital

– Sound Levels– Sampling Frequency

• Sound Quality• Format

Digital 數碼 versus Analogue 模擬

• Simulate sound with mechanical tracks or magnetical device

• Distorts when transmitting

• Simulate sound with a series of digits

• Some means to compensate the distortion, therefore No distortion when transmitting

Digital Audio Capture

Analog (continuous) signal

digital (discrete) signal

Audio casette

Audio CD

When the amplitude of the analog signal is sampled at the instant, it is converted into a digital signal consisting of a number of digits. It is an approximation.

Factors affecting the conversion process

1) Number of bits used to represent different amplitudes of a sound signal

The greater the number of bits, the better the approximation.

4 bits 24 = 16 levels

8 bits 28 = 256 levels

16 bits 216 = 65536 levels (for CD player)

Factors affecting the conversion process

2) The sampling frequency (number of samples taken in one second)

The higher the sampling frequency, the better the approximation.

Nyquist's sampling theorem

If sampling frequency > 2 highest frequency,

the signal can be reconstructed with no distortion.

Sound Quality• Telephone quality speech

– Range: 300 to 3400 Hz– sampling frequency = 8000 Hz > 2 3400 Hz

• CD quality sound– Up to 20 kHz– sampling frequency = 44.1 kHz > 2 20 kHz

• Audio frequency (AF): The band of frequencies (approximately 20 Hz to 20 kHz) that, when transmitted as acoustic waves, can be heard by the normal human ear.

Factors affecting the conversion process

3) Number of channels

One channel: mono

Two channels: stereo

file size of stereo sound file = 2 file size of the mono sound file.

Bit rate

= (number of bits / second) at the output

= number of bits used in sampling

sampling frequency number of channel

A Common Measure of Sound Quality

Example of bit rate calculation

For CD,

number of bits: 16

channel: 2 (stereo)

sampling frequency: 44.1 kHz

Bit rate

= 16 2 44.1k = 1411 Kbits / second

= 176 Kbytes / second

A song of 4 minutes long = 176 Kbytes 4 60

= 41 Mbytes

Example of bit rate calculation

For telephone quality speech,

number of bits: 8

channel: 1 (mono)

sampling frequency: 8 kHz

Bit rate

= 8 1 8k = 64 Kbits / second

= 8 Kbytes / second

Compression• Sound files are too large to

transmit and save• Different means to compress the

files are therefore needed• Still reasonable sound quality is to

be maintained.

Common audio file formats

• MIDI (Musical Instrument Digital Interface)

• MP3 (MPEG-1 Audio Layer-3) (*.mp3)• Wave file (*.wav)• Windows Media Audio (*.wma)• RealAudio and Shockwave (*.rm)

MIDI (Musical Instrument Digital Interface)

• designed for recording and playing back music on digital synthesizers.

• records information about how music is produced: note-ons, note-offs, key velocity, pitch bend and other methods of controlling a synthesizer.

• Advantage: very small file size.• Disadvantage: lack of specific sound contro

l.

MP3 (MPEG-1 Audio Layer-3) (*.mp3)

• a standard technology and format for compressing a sound sequence into a very small file (about 1:12) while preserving the original level of sound quality.

• reduces data about sound that most listeners cannot perceive.

Wave file (*.wav)• audio file format created by

Microsoft. • become a standard PC audio file

format for everything from system and game sounds to CD-quality audio.

Windows Media Audio (*.wma)

• Used in Windows Media Player• Use different compression rates

品質 位元速率 (Kbps) 每 650 MB 或 74 分鐘長的 CD 之磁碟空間 (MB)

最小大小 48 22小 64 28中 96 42中高 128 56高 160 69最佳品質 192 86

RealAudio and Shockwave (*.rm)

• designed to allow real-time playback of audio directly from a Web site.

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