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From: stadlin@hou2h.UUCP (Art Stadlin)
Newsgroups: net.audio
Subject: The Sampling Theorem and the CD
Message-ID: <771@hou2h.UUCP>
Date: Wed, 16-Jan-85 14:46:23 EST
Article-I.D.: hou2h.771
Posted: Wed Jan 16 14:46:23 1985
Date-Received: Thu, 17-Jan-85 12:54:59 EST
Organization: AT&T Consumer Products, Neptune
Lines: 36


	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.

-- 
  \\\
   \\\\                                  Art Stadlin
    \\\\\\________!{akgua,ihnp4,houxm}!hou2h!stadlin