Forums

negative group delay

Started by bharat pathak March 12, 2008
Hi,

   I am seeing some negative group delay at 
   1000, 2000, 3000, 4000, 5000 Hz. The example
   filter listed below with matlab/octave code.

   any clue why this is happening? interesting
   part is when I feed 1000 hz sine wave sampled
   at 10000 hz I don't see the behaviour of 
   negative group delay.

Regards
Bharat


%----------------------------------------------------------------


clear;
close all;

h1      = [10 9 8 7 6 5 4 3 2 1]/55;
Fs      = 10000;

[gd, f] = grpdelay(h1, 1, 8192, Fs);
figure; clf;
plot(f, gd);

On Mar 12, 5:58 pm, "bharat pathak" <bha...@arithos.com> wrote:
> Hi, > > I am seeing some negative group delay at > 1000, 2000, 3000, 4000, 5000 Hz. The example > filter listed below with matlab/octave code. > > any clue why this is happening? interesting > part is when I feed 1000 hz sine wave sampled > at 10000 hz I don't see the behaviour of > negative group delay.
Individual sine waves don't experience group delay, they experience phase delay. -- Oli
On Mar 12, 11:04 am, Oli Charlesworth <ca...@olifilth.co.uk> wrote:
> On Mar 12, 5:58 pm, "bharat pathak" <bha...@arithos.com> wrote: > > > Hi, > > > I am seeing some negative group delay at > > 1000, 2000, 3000, 4000, 5000 Hz. The example > > filter listed below with matlab/octave code. > > > any clue why this is happening? interesting > > part is when I feed 1000 hz sine wave sampled > > at 10000 hz I don't see the behaviour of > > negative group delay. > > Individual sine waves don't experience group delay, they experience > phase delay.
Yes. To see the group delay you will have to modulate your sine wave while staying completely within a bandwidth where the group delay is fairly constant, and then compare the before and after filtering locations of your modulation envelope.
Ron N. wrote:
> On Mar 12, 11:04 am, Oli Charlesworth <ca...@olifilth.co.uk> wrote: >> On Mar 12, 5:58 pm, "bharat pathak" <bha...@arithos.com> wrote: >> >>> Hi, >>> I am seeing some negative group delay at >>> 1000, 2000, 3000, 4000, 5000 Hz. The example >>> filter listed below with matlab/octave code. >>> any clue why this is happening? interesting >>> part is when I feed 1000 hz sine wave sampled >>> at 10000 hz I don't see the behaviour of >>> negative group delay. >> Individual sine waves don't experience group delay, they experience >> phase delay. > > Yes. To see the group delay you will have to modulate your > sine wave while staying completely within a bandwidth where > the group delay is fairly constant, and then compare the > before and after filtering locations of your modulation > envelope.
No modulation needed. three signals close in frequency and amplitude will produce beats that _look_ like modulation and illustrate the effect. Jerry -- Engineering is the art of making what you want from things you can get. &#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;
On Mar 12, 4:42 pm, Jerry Avins <j...@ieee.org> wrote:
> Ron N. wrote: > > On Mar 12, 11:04 am, Oli Charlesworth <ca...@olifilth.co.uk> wrote: > >> On Mar 12, 5:58 pm, "bharat pathak" <bha...@arithos.com> wrote: > > >>> Hi, > >>> I am seeing some negative group delay at > >>> 1000, 2000, 3000, 4000, 5000 Hz. The example > >>> filter listed below with matlab/octave code. > >>> any clue why this is happening? interesting > >>> part is when I feed 1000 hz sine wave sampled > >>> at 10000 hz I don't see the behaviour of > >>> negative group delay. > >> Individual sine waves don't experience group delay, they experience > >> phase delay. > > > Yes. To see the group delay you will have to modulate your > > sine wave while staying completely within a bandwidth where > > the group delay is fairly constant, and then compare the > > before and after filtering locations of your modulation > > envelope. > > No modulation needed. three signals close in frequency and amplitude > will produce beats that _look_ like modulation and illustrate the effect.
Well, I sort of agree. But call any two of those sinusoids the sidebands produced by complex modulation of the third, and I think the sum is identical to a form of modulation with carrier.
Ron N. wrote:
> On Mar 12, 4:42 pm, Jerry Avins <j...@ieee.org> wrote: >> Ron N. wrote: >>> On Mar 12, 11:04 am, Oli Charlesworth <ca...@olifilth.co.uk> wrote: >>>> On Mar 12, 5:58 pm, "bharat pathak" <bha...@arithos.com> wrote: >>>>> Hi, >>>>> I am seeing some negative group delay at >>>>> 1000, 2000, 3000, 4000, 5000 Hz. The example >>>>> filter listed below with matlab/octave code. >>>>> any clue why this is happening? interesting >>>>> part is when I feed 1000 hz sine wave sampled >>>>> at 10000 hz I don't see the behaviour of >>>>> negative group delay. >>>> Individual sine waves don't experience group delay, they experience >>>> phase delay. >>> Yes. To see the group delay you will have to modulate your >>> sine wave while staying completely within a bandwidth where >>> the group delay is fairly constant, and then compare the >>> before and after filtering locations of your modulation >>> envelope. >> No modulation needed. three signals close in frequency and amplitude >> will produce beats that _look_ like modulation and illustrate the effect. > > Well, I sort of agree. But call any two of those sinusoids > the sidebands produced by complex modulation of the third, > and I think the sum is identical to a form of modulation > with carrier.
Modulation (according to me) is a non-linear process (multiplication, rectification, etc). Summation is not. Both produce envelopes, but the details of those envelopes differ. Jerry -- Engineering is the art of making what you want from things you can get. &#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;