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