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.


--

John Larkin Highland Technology Inc
www.highlandtechnology.com jlarkin at highlandtechnology dot com

Precision electronic instrumentation