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Default Dual to AllPass.?

Does there exist a dual to an AllPass, that is, a structure in which
amplitude varies with frequency, but phase does not?

...Jim Thompson
--
| James E.Thompson | mens |
| Analog Innovations | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| San Tan Valley, AZ 85142 Skype: Contacts Only | |
| Voice480)460-2350 Fax: Available upon request | Brass Rat |
| E-mail Icon at http://www.analog-innovations.com | 1962 |

I love to cook with wine. Sometimes I even put it in the food.
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Default Dual to AllPass.?

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

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC
Optics, Electro-optics, Photonics, Analog Electronics

160 North State Road #203
Briarcliff Manor NY 10510

hobbs at electrooptical dot net
http://electrooptical.net
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Default Dual to AllPass.?

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


I was hoping you had a change of heart ;-)

(I've decided, therefore, that I've been fed some bad data from a real
device :-(

...Jim Thompson
--
| James E.Thompson | mens |
| Analog Innovations | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| San Tan Valley, AZ 85142 Skype: Contacts Only | |
| Voice480)460-2350 Fax: Available upon request | Brass Rat |
| E-mail Icon at http://www.analog-innovations.com | 1962 |

I love to cook with wine. Sometimes I even put it in the food.
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Default Dual to AllPass.?

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?


--

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

Precision electronic instrumentation
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Default Dual to AllPass.?

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



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Default Dual to AllPass.?

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

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC
Optics, Electro-optics, Photonics, Analog Electronics

160 North State Road #203
Briarcliff Manor NY 10510

hobbs at electrooptical dot net
http://electrooptical.net
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Default Dual to AllPass.?

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.


--

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

Precision electronic instrumentation
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Default Dual to AllPass.?

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



--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC
Optics, Electro-optics, Photonics, Analog Electronics

160 North State Road #203
Briarcliff Manor NY 10510

hobbs at electrooptical dot net
http://electrooptical.net
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Default Dual to AllPass.?

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
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Default Dual to AllPass.?

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
Optics, Electro-optics, Photonics, Analog Electronics

160 North State Road #203
Briarcliff Manor NY 10510

hobbs at electrooptical dot net
http://electrooptical.net
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