Lecture Eight Quality

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Lecture EightQuality Sampling Frequencies
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Digital Sound Quality

• Nyquist Sampling Theorem
• faithful sampling requires a sampling frequency
  twice that of the highest frequency in the signal
• Human ear 20Hz to 20kHz (approximately)
• Audio CD sampled at 44.1kHz
• Two issues sampling frequency number of bits
  per sample

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Number of bits per sample

• Affects the quality of the digital sound
• 8 bits gt range split into 256 steps
• Issue also of mono and stereo (2 tracks)

Dynamic Range
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Example 1

• CD-quality sound recording
• 44.1kHz sampling frequency, 16-bits per sample,
  stereo sound
• 1 second has 44,100 samples of 2 bytes each
• 88,200 bytes x 2 channels
• 176,400 bytes / second (or 172.3kB/s)

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Example 2

• Telephone speech has a narrow bandwidth of around
  3.6kHz
• 1 second has 7200 samples at 8 bits mono
• 7200 bytes / second
• 7kB/s
• (and could be much less if compression is used)

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Example 3

• A CD-ROM can store 650MB of data or 74 minutes of
  stereo music at 44.1kHz sample rate.What is the
  sample size?
• 650 / 74 8.78 MB / minute 150kB/s
• 88,200 samples per second, so
• 150 1024 8 / 88200 14 bits per sample

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Final Example

• One minute of sound must be recorded using a
  maximum of 2.5MB of memory.What are the sampling
  options?
• Available for 1 second is 2.51024/60 42.67kB
• OPTION 1 8-bit 42.67kHz rate, mono
• OPTION 2 16-bit 21.33kHz rate, mono
• OPTION 3 8-bit 21.33kHz rate, stereo
• OPTION 4 16-bit 10.67kHz rate, stereo