Path: utzoo!news-server.csri.toronto.edu!cs.utexas.edu!uunet!mcsun!hp4nl!orcenl!mberg From: mberg@dk.oracle.com (Martin Berg) Newsgroups: comp.dsp Subject: Re: Discreet Sampling Message-ID: <1285@dkunix9.dk.oracle.com> Date: 7 Mar 91 18:08:32 GMT References: <1991Feb28.194203.27097@alzabo.ocunix.on.ca> <11588@pasteur.Berkeley.EDU> Reply-To: mberg@dk.oracle.com (PUT YOUR NAME HERE) Organization: Oracle Denmark Lines: 26 In article <11588@pasteur.Berkeley.EDU> jbuck@galileo.berkeley.edu (Joe Buck) writes: >Of course, in the real world there's no such thing as an ideal >lowpass filter, so you're simply not going to be able to reproduce accurately >a signal that close to the Nyquist frequency. That's why CDs run at 44.1 KHz >and only attempt to reproduce signals up to 20 KHz. The extra 2.1 KHz >bandwidth is to allow for filter rolloff (and even then you need to get >fancy to get that steep a rolloff). One of the consequenses of an ideal lowpass filter is that it would have an infinite time-responce - thus never give an answer. A real-world lowpass filter also gives an delayed answer - the steeper the longer. This means that the extremely steep filter required for the original question in other words would be able to 'store' (computer language ! ) all the information about the signal to eliminate the 'beating'. A FIR-filter with 20000 (40000 ?) taps may be able to implement the required filter - but talk about hardware requirements ! My usage of terms may be somewhat primitive, but it's been a long time since I attented classes i DSP. Martin Berg Oracle Denmark