stadlin@hou2h.UUCP (Art Stadlin) (01/16/85)
Bring out the textbook...
+++++++++++++++++++++
The SAMPLING THEOREM states that, if a function of time f(t)
contains no frequencies higher than W hertz, f(t) is
*completely* determined by giving the value of the function
at a series of points spaced 1/2W seconds apart.
"It is important to note that the theorem makes no mention
of the time origin of the samples. The time origin is
unimportant; it is only the *spacing* of the samples which
matters."
+++++++++++++++++++++
MY CONCLUSION:
The analog source signal (orchestra, etc.)
contains frequencies above human hearing.
Therefore, if it is desired to sample this signal
at a 40~kHz rate, the analog signal must first be
low-pass filtered to assure no frequency
components greater than 20~kHz.
Since a "perfect" 20~kHz low-pass filter is very
expensive, the recording industry probably uses a
"less-than-perfect" low-pass filter at, say,
35~kHz. Then the signal is sampled at an 80~kHz
rate, just to be sure.
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\\\\ Art Stadlin
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