On Sun, 18 May 2014 14:57:31 -0700, 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 could make an approximation of a constant-delay filter (like Phil's
symmetric FIR). But you can't keep the phase constant here in the real
world.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com