On 05/19/2014 11:58 AM, John Larkin wrote:
On Mon, 19 May 2014 11:42:49 -0400, Phil Hobbs
wrote:
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
I didn't say LC. I was thinking 1st order, RC, because that case is easy to
think about.
But it could be done to a higher-order filter, just more work.
Point is, that the answer to Jim's question is that, yes, it can be done within
reasonable practical limits.
First order RC filters don't have real delay--you can cancel out a
lowpass with a highpass, over whatever bandwidth you like, if you can
stand the loss.
Cheers
Phil Hobbs
--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC
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