"robert bristow-johnson" <rbj@audioimagination.com> wrote in message news:BE036569.3A84%rbj@audioimagination.com...> in article o7Cdnej2p4lKaUDcRVn-tw@centurytel.net, Fred Marshall at > fmarshallx@remove_the_x.acm.org wrote on 01/06/2005 21:32: > >> "Jon Harris" <goldentully@hotmail.com> wrote in message >> news:343c9pF455tmaU1@individual.net... >>> "Fred Marshall" <fmarshallx@remove_the_x.acm.org> wrote in message >>> news:t4-dnYJRBdvqg0HcRVn-hA@centurytel.net... > ... >>>> Maybe this would be something like a FIR notch filter at fs/2. >>> >>> What is the difference bewteen a notch filter at fs/2 and a low-pass >>> filter with >>> cut-off very close to fs/2 (assuming real filter coefs)? >> >> ***Well, I did say "maybe"..... I don't think there's a difference. >> And, I >> agree with your comments regarding implementation. > > there is no difference if the LPF was designed in the s-plane and > converted > to z-plane by use of the bilinear transform. the BLT maps s=j*inf to > z=exp(j*Nyquist). but if the LPF was converted to H(z) using, say, > impulse > invariant, then frequency response aliasing occurs and the notch does not > go > down to -inf dB.Yeah, I was assuming FIR design on the unit circle in the z plane - as is often the case. Fred
Why is it bad to have spectral samples nonzero at fs/2?
Started by ●January 3, 2005
Reply by ●January 7, 20052005-01-07
Reply by ●January 7, 20052005-01-07
in article 34807vF482h3eU1@individual.net, Jon Harris at goldentully@hotmail.com wrote on 01/07/2005 12:46:> "robert bristow-johnson" <rbj@audioimagination.com> wrote in message > news:BE036569.3A84%rbj@audioimagination.com......>> there is no difference if the LPF was designed in the s-plane and converted >> to z-plane by use of the bilinear transform. the BLT maps s=j*inf to >> z=exp(j*Nyquist). but if the LPF was converted to H(z) using, say, impulse >> invariant, then frequency response aliasing occurs and the notch does not go >> down to -inf dB. > > Agreed, Robert, though I was actually thinking of an FIR notch filter in this > case.... in article VLadnYfFLqsge0PcRVn-ig@centurytel.net, Fred Marshall at fmarshallx@remove_the_x.acm.org wrote on 01/07/2005 14:45: ...> Yeah, I was assuming FIR design on the unit circle in the z plane - as is > often the case.well, we can pretend that i am actually literate and that i really do read through your posts. :-\ -- r b-j rbj@audioimagination.com "Imagination is more important than knowledge."
