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					                What is sound?
• Sound is a wave phenomenon like light, but is macroscopic
  and involves molecules of air being compressed and
  expanded under the action of some physical device.
• Sound is a pressure wave, thus it takes on continuous
  values. We can detect sound by measuring the pressure
  level at a location, using a transducer變頻器 to convert
  pressure to voltage levels.
                     Sampling
• To represent waveforms on digital computers, we
  need to digitize or sample the waveform.
• Sampling means measuring the quantity we are
  interested in, usually at evenly-spaced intervals. The
  audio signal is sampled in each dimension尺度: in
  time, and in amplitude.
    1.   Sampling in the time dimension, using measurements only
         at evenly spaced time intervals, is simply called, sampling.
         The rate at which it is performed is called the sampling
         frequency.
    2.   Sampling in the amplitude or voltage dimension is called
         quantization量化.




Sampling in time        Sampling        Sampling in amplitude           Quantization

side effects of digitization:
   – introduces some noise
   – limits the maximum upper frequency range
                Sampling Rate
• The sampling rate (SR,measured in Hertz) is the rate
  at which amplitude values are digitized from the
  original waveform.
• The sampling frequency or sampling rate defines
  the number of samples per second taken from a
  continuous signal to make a discrete signal.
   – CD sampling rate (high-quality):
     SR = 44,100 samples/second(44.1kHz)
   – medium-quality sampling rate:
     SR = 22,050 samples/second
   – phone sampling rate (low-quality):
     SR = 8,192 samples/second
 Sampling Rate

• Higher sampling
  rates allow the
  waveform to be more
  accurately
  represented.
Audio Quality vs. Data Rate

				
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posted:11/28/2011
language:English
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