On 05/19/2014 11:29 AM, John Larkin wrote:
On Mon, 19 May 2014 07:44:45 -0400, Phil Hobbs
wrote:
On 05/18/2014 05:57 PM, John Larkin wrote:
On Sun, 18 May 2014 15:15:14 -0400, Phil Hobbs wrote:
On 5/18/2014 1:32 PM, Jim Thompson wrote:
Does there exist a dual to an AllPass, that is, a structure in which
amplitude varies with frequency, but phase does not?
...Jim Thompson
Wouldn't it be nice. No, in general that violates causality. (I'll be
a bit less dogmatic about that than previously, since we talked about
this several months back.)
A symmetric FIR filter centered around t=0 might qualify, if you don't
need it to be real-time.
Cheers
Phil Hobbs
Couldn't you make a lowpass filter, then fix its phase shift with an all-pass?
You can flatten it out, or fix it in some restricted region (as in your
example from last year) but you can't get rid of it everyplace. Group
delay != real delay, but it's the leading order approximation.
Cheers
Phil Hobbs
Jim didn't specify exactly zero phase, over an infinite frequency span; neither
request would be reasonable. But it should be practical to make a lowpass
filter, followed by an allpass network, that has minimal phase shift up to, say,
10 or 20x Fc.
I doubt that very much. If you can do that to an LC-type lowpass,
you'll put Tim Wescott and Co. out of business.
Cheers
Phil Hobbs
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