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Author Topic: Frequency dependent delay or not?  (Read 1563 times)

Marcel de Graaf

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Re: Frequency dependent delay or not?
« Reply #10 on: July 21, 2017, 03:16:22 pm »


Consider the "hydrogen atom" of linear systems: the first-order low-pass. It has phase = 0 at zero frequency and phase = -pi/2 at infinite frequency. In the transition region the phase has a negative slope, and the delay is positive.


Regarding absolute phase its true that you should read phase=0 at zero frequency. Systems are formed by casuality. It makes sence, because a signal can never arrive before it is emitted at the system.

Systems (we know off) always have a low pass, even at infinity high frequencys. What normally is done is that the time=zero marker is at the peak of the impulse response.
Knowing that a system has a low pass it would not be logical because we know there was energy before the peak (in very low magnitude).

Have the wright time=zero marker should keep the phase plot going negative with increasing frequency.

Delay is indeed very confusing but it can be far more. Most analyzers will show a phase plot from extracting it from the magnitude response. The plot is than at 0 degrees where the magnitude is flat. In a electronic system this is wright but acoustic can be very different. Acoustic air has impedance with a changing resistance. This changing resistance is compensated with the wright loudspeaker parameters (compensating it with the imaginairy parts) to make the magnitude response flat. It`s stil a minimum phase system BUT not having the phase plot at the 0 degrees line.

I think this is what barry mentioned in his post.

grtz. Marcel
« Last Edit: July 21, 2017, 03:22:42 pm by Marcel de Graaf »
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Merlijn van Veen

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Frequency dependent delay or not?
« Reply #11 on: July 21, 2017, 03:26:05 pm »

Dear all,

As much as I appreciate your knowledgeable input, we're drifting into abstracts which is the very thing I wanted to avoid.

That's why I avoid poles, zeros and polynomials like the plague in class.

My question is about the practical physical implications. Maybe the attached illustration makes sense.

What would you say to a layman, a novice, an uninitiated, a mere mortal?
« Last Edit: July 21, 2017, 03:36:12 pm by Merlijn van Veen »
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Barry Singleton

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Re: Frequency dependent delay or not?
« Reply #12 on: July 21, 2017, 11:58:29 pm »

  I don't know if that graphic really captures it Merlijn, a stretch maybe in the very most basic sense. This is a tough concept to really grasp.

  I think more about single devices that smear time like the three poles one runs across in  large format compression drivers. Yeah I had to use the P word.

  I will have to re-read Heyser again tonight.

  This is like explaining fission by saying the equal sign in E=Mc˛ is the bridge that if you send mass fully across and convert to energy, a piece of plutonium the size of a ChapSick can destroy a city.

 While that's functionally true, does it help anyone really understand the worlds most famous equation?

  I for one would love to see group delay made truly explainable without all the math but I am not sure how to do it.

Barry.
   

 

 

 

 
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Merlijn van Veen

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Re: Frequency dependent delay or not?
« Reply #13 on: July 22, 2017, 01:57:56 am »

Thanks for proving my point. Because this is what keeps happening over and over again AFAICT. Everybody agrees that Group Delay is the first derivative of the phase response (either with or without minus sign) and that the unit must be time. But when it comes to explaining what that time entails in plain understandable language, "shit hits the fans" (pun intended).


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Merlijn van Veen

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Re: Frequency dependent delay or not?
« Reply #14 on: July 22, 2017, 03:21:26 am »

Group delay is -d phi/dw, where phi is phase and w is frequency. The article omits the minus sign which, while perhaps irrelevant to this philosophical discussion, is necessary to get the math right. -F

I stand corrected :-[

Thanks for making me think Frank!

In order to maintain causality and keep past, present and future sorted the minus sign is mandatory.

The topic swerve turns out to be serendipity :)
« Last Edit: July 22, 2017, 04:05:06 am by Merlijn van Veen »
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Ivan Beaver

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Re: Frequency dependent delay or not?
« Reply #15 on: July 22, 2017, 08:50:38 am »

Dear all,

As much as I appreciate your knowledgeable input, we're drifting into abstracts which is the very thing I wanted to avoid.

That's why I avoid poles, zeros and polynomials like the plague in class.

My question is about the practical physical implications. Maybe the attached illustration makes sense.

What would you say to a layman, a novice, an uninitiated, a mere mortal?
Your illustration is the whole idea behind the BBE sonic maximizer.

They "assumed" that all loudspeakers (or at least the ones for their target audience), had the same time offsets between woofers and mids and horns.

So the basic concept was to split the signal up into 3 bands, add some specific delay to 2 of the freq bands, and then mix them back together.

This concept DOES WORK, and I have done it.  HOWEVER, it MUST be for a SPECIFIC cabinet-and is not universal.  And it only works in a true point source/single source type cabinet-NOT one in which the lows/mids/highs are located in different places on the cabinet.

If you go to far with the delay it is just as bad as when you started.  Or if you move around in the coverage pattern, the delay times will need to change, which they can't.

I do like your illustration.

One size DOES NOT fit all in this case.
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Mark Wilkinson

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Re: Frequency dependent delay or not?
« Reply #16 on: July 22, 2017, 08:54:48 pm »

But when it comes to explaining what that time entails in plain understandable language, "shit hits the fans" (pun intended).


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Yes, "shit hits the fan" and then keeps on spreading......

Don't know the simple answer with complete certainty yet, but my best guess is that time alignment means alignment at 0 degrees ...when wave forms begins to rise, and not at peak magnitude.  I'd point to stuff from Charlie Hughes.
 
I applaud your quest for simple understanding, cause i really think when something is properly defined and completely understood, it depends
gets washed away into clear simplicity
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Doug Fowler

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Re: Frequency dependent delay or not?
« Reply #17 on: July 23, 2017, 03:10:47 pm »

Thanks for proving my point. Because this is what keeps happening over and over again AFAICT. Everybody agrees that Group Delay is the first derivative of the phase response (either with or without minus sign) and that the unit must be time. But when it comes to explaining what that time entails in plain understandable language, "shit hits the fans" (pun intended).


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I can tell you with great certainty that beginning measurement students absolutely do not understand that different frequencies are arriving at different times.  But you already knew that.... :-)

Are we talking science or simple field alignments?  For the overwhelming majority of FFT software users, "frequency dependent delay" gets the point across.  This is where the group delay view becomes so useful. Rather than "looking down the phase tube" just display group delay to make the point.

Now, it's quite obvious to novices that the subwoofer is 20-30ish msec behind the rest.  If you zoom in sufficiently one can see more delay present as we go lower in frequency, at least in a typical LF section of (for example) a 3 way loudspeaker. 

So for this majority of users, the biggest hurdle is the legendary sub alignment problem. I posted elsewhere in the forum about a method for using group delay to quickly get "in the neighborhood" re: delay value at crossover at the chosen alignment place.   Yes it's group delay, s/n issues can cause it to display negative time in some places, but it's a fast path to a proposed delay value which can be fine tuned with the phase display.

The user can then decide if this delay value is reasonable (no 88 msec is not reasonable:-), inspect phase which should be close at this point, and determine if it sums.  I'll leave the "how do I choose my alignment spot" to Merlijn and his excellent recent piece on this. 

This leaves (at least in my old roadie mind) the next logical step, unrelated to the alignment task but still something most want to know:

What mechanical/physical/electrical circumstances cause every loudspeaker ever constructed (thanks 6o6) to display this behavior?

If we can explain this in a way that makes sense without overloading beginners' already roasted brains, the logical conclusion is the demonstration, and in my experience group delay view is very helpful.
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Uwe Riemer2

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Re: Frequency dependent delay or not?
« Reply #18 on: July 28, 2017, 05:22:58 pm »

sorry for being late, but in July there are more important questions like getting sleep or food, also I had to create a new account, because I couldnt get the forum server to answer my request for a new password, seems my old mail adress does not work in this case.

But now back to topic:
Yes I think group delay is frequency dependant delay and Merlins nice pictures nails this.
Further I think, that Anselm Goertz in the linked article never contradicted to this view and that his explanation is equivivalent and comes from the attempt to measure Group delay directly instead of trying to derive it from the Phase response. There are some practical reasons to do this:
Measuring Phase without a reflection free envirement is already difficult and done with the help of Windowing, but still especially in Systune will often create a function ( Phase over frequency ) which is overall not differentiable ( my Math english sucks ), means the derivation of the function does not exist at certain points, therefore sometimes the stupid computer will generate negative values, which hold no meaning.

Measuring the group delay directly like described by Anselm Goertz is done with a pulse, which to me looks like a single frequency( lets call it basis frequency ) modified by a window function, which seems like an attempt to switch a single frequency smoothly on and off without creating too much new frequencies.

Personnally I hate Math so someone else should verifiy or falsify this proposition by performing a Fourier Transform on this pulse.
But if this should be true, the delay of the bulk energy of this pulse through the DUT will represent roughly the Group delay at above mentioned basis frequency

Back to sleep now
Uwe


« Last Edit: July 28, 2017, 06:09:52 pm by Uwe Riemer2 »
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Uwe Riemer2

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Re: Frequency dependent delay or not?
« Reply #19 on: July 30, 2017, 06:10:28 pm »

I almost forgot
the sound of silence without any group delay,

which brings me to a question for Ivan and the BBE thingy:
how would they ( BBE ) achieve splitting the audio without creating group delay, a problem they want to cure according to you?

P.S. apologies for many spelling errors in my post above, especially the missing J

Uwe

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