Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Posting-Version: version B 2.10 5/3/83 based; site hou2h.UUCP Path: utzoo!watmath!clyde!burl!ulysses!mhuxr!mhuxj!houxm!hou2h!stadlin 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