Overview of Multimedia

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Overview of Multimedia

• A multimedia presentation might contain
• Text
• Animation
• Digital Sound Effects
• Voices
• Video Clips
• Photographic Stills
• Music

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Overview of Sound

• Digital ?? versus Analogue ??
• Conversion from Analogue to Digital
• Sound Levels
• Sampling Frequency
• Sound Quality
• Format

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Digital ?? versus Analogue ??

• 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

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Digital Audio Capture Analog (continuous)
signal digital (discrete) signal
Audio casette
Audio CD
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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.
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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)
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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.
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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.

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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.
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A Common Measure of Sound Quality
Bit rate (number of bits / second) at the
output number of bits used in sampling ?
sampling frequency ? number of channel
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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
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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
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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.

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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)

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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 control.

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

• a standard technology and format for compressing
  a sound sequence into a very small file (about
  112) while preserving the original level of
  sound quality.
• reduces data about sound that most listeners
  cannot perceive.

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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.

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Windows Media Audio (.wma)

• Used in Windows Media Player
• Use different compression rates

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RealAudio and Shockwave (.rm)

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

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