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.
• 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
2. Sampling in the amplitude or voltage dimension is called
Sampling in time Sampling Sampling in amplitude Quantization
side effects of digitization:
– introduces some noise
– limits the maximum upper frequency range
• The sampling rate (SR,measured in Hertz) is the rate
at which amplitude values are digitized from the
• 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
• Higher sampling
rates allow the
waveform to be more
Audio Quality vs. Data Rate