Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Posting-Version: version B 2.10.1 6/24/83; site mit-eddie.UUCP Path: utzoo!watmath!clyde!burl!ulysses!mhuxr!ihnp4!mit-eddie!gs From: gs@mit-eddie.UUCP (Gordon Strong) Newsgroups: net.audio Subject: Re: CD Reflections Message-ID: <3411@mit-eddie.UUCP> Date: Sat, 12-Jan-85 03:25:34 EST Article-I.D.: mit-eddi.3411 Posted: Sat Jan 12 03:25:34 1985 Date-Received: Sun, 13-Jan-85 07:08:56 EST References: <15100001@hpfcmp.UUCP> Organization: MIT, Cambridge, MA Lines: 26 Re: a 2x sampling rate is not good enough to reconstruct a sine wave. I'm afraid it is. The 2x rate is called the Nyquist Rate and is the minimum sampling rate necessary to prevent aliasing. It has been proven rigorously that any waveform sampled at or above twice the highest frequency can be reconstructed. This is part of the sampling theorem and is the basis of much of modern communication theory. Since this is the accepted minimum, it is not the same 2x rate that is usually referred to in the CD literature. The highest audio frequencies are 22.1KHz, so the sampling rate must be 44.2KHz. When CD manufacturers refer to a sampling rate of 2x, they usually mean 88.4KHz (that's what Yamaha says for my CD-2). The use of a higher sampling rate (oversampling) is used so that the low pass filter used to reconstruct the original waveform need not have a severe slope in the transition band. The claim is that a gentler slope reduces phase and group delay effects. For those that care, the CD-2 using 2x oversampling requires a 7th order filter to reconstruct the signal. That's still a bit steep for some. Several manufacturers offer CD players that use 4x oversampling. I haven't heard of any that use anything higher. I don't know if anyone has anything in the works or even what the theoretical limit is. If anyone has some information on this, I'd like to hear it. Gordon Strong {decvax!genrad, ihnp4}!mit-eddie!gs GS@MIT-XX