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almikel   

(edit 21/4/17) - I've made some edits below based on finding some errors - noting each as an edit just makes it messy,

I've left errors due to my lack of understanding - these are addressed by Dave's comments.

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I've posted similar information a couple of times recently, but thought it worthy of its own thread.

 

Tools like REW, ARTA, Holm Impulse can show the phase response of speakers, rooms etc.

 

Often phase plots look like this (my tapped horn sub measured indoors at the listening position)

58f18d604da32_subonlyphaseonlywrappedphase.jpg.dc70459a7ba87c46275e3d2ac5d6d4fb.jpg

The dotted vertical lines do not represent instantaneous changes in speaker phase - they represent a way to keep the graph on the page known as "wrapped phase".

 

The left axis scale is dB and is irrelevant for this graph - phase is on the right axis in degrees

When the graph hits (in this case) +180 degrees on the right hand scale, the dotted vertical line drops to -180 degrees to keep it on the page, and the curve continues to rise.

The peak around 35-38Hz, and the peak just under 60Hz are  360 degrees apart (ie 38Hz is 180 degrees phase lead, and 60Hz is 540 degrees phase lead) - the graph is "phase wrapped" to keep it on the same page for easier viewing.

This allows more detail to be shown rather than having a phase plot covering over 1000 degrees - to know the actual phase you need to count the vertical lines - each one represents 360 degrees.

The same graph shown "unwrapped" below

58f1910692b69_subonlyphaseonlyunwrapped.jpg.37be26b414f962eab5dd5cf6e86c7ca3.jpg

same measurement, just unwrapped shows a steadily rising phase from 30Hz - note the scale on the right hand side- 1080 degrees - REW wouldn't let me zoom out further.

 

Research has demonstrated that smoothly changing phase is in-audible - lucky really, if we could hear the phase change above we wouldn't have hi-fi as we know it.

 

However phase changes stop being smooth when we add multi-way speakers, crossovers and rooms

 

Lots has been said about "minimum phase" and I'll attempt to add some value to the discussion on this - but some of my measurements confuse me - so hoping the SNA community can help me also :thumb:

 

A dead flat frequency (amplitude) response across the audible range (say 20Hz to 20kHz) is an unachievable target.

Anytime you have a wiggle in the frequency response, you have phase changes.

Look at measurements of speakers - where their response deviates most from flat is where the phase changes most.

 

outdoor close miked measurement of my sub

58f19be85329c_FRandUnwrappedPhase.jpg.6e8ba45b5e3878cddb2062821eebaf34.jpg

 

apologies for similar colours - top trace is FR (dark blue) and light blue trace is phase.

Note the phase plot (light blue) is more steep where there are larger  changes in FR.

 

Minimum Phase - what is it?

I still grapple with an easy to understand definition...and always fail :emot-bang:

Consider our stereo systems:

  • You can define them in the time domain (we listen to our systems in the time domain) - the normal measurement is an impulse response - this describes the system in a single measurement - any ringing in the time domain will cause wiggles in the frequency domain
  • You can define them in the frequency domain based on amplitude and phase - the classic Frequency Response curve (more correctly known as Amplitude Response), only describes part of the system. To properly define a system in the frequency domain requires both an amplitude and a phase response.

 

You can convert between the time domain and frequency domain using a Fourier Transform.

From an impulse response you can calculate both the frequency (amplitude) response and phase response using the Fourier Transform.

To calculate the impulse response you need both the amplitude response and the phase response (still using a Fourier Transform).

 

In a Minimum Phase system (and only in a minimum phase system), you can calculate the Phase Response from the Amplitude Response, and you can calculate the Amplitude Response from the Phase Response.

Remember you need both Amplitude Response and Phase Response to calculate (via the Fourier Transform) the Impulse Response.

So with minimum phase systems:

  • if you have either the amplitude response or the phase response you can determine the impulse (time) response

 

With Minimum Phase systems, a peak in the frequency response, can be EQ'd by an opposite "cut" using minimum phase EQ (Parametric EQ - PEQ) and correct both amplitude and phase.

If treating minimum phase effects, dips can also be boosted via PEQ and correct both the amplitude and phase response. 

 

BUT is a room minimum phase?

Unfortunately in normal rooms, regions where the response is "minimum phase" is unpredictable.

Tools such as REW can measure minimum phase.

Read here : https://www.roomeqwizard.com/help/help_en-GB/html/minimumphase.html

 

Excess Phase is a measurement in REW that calculates the difference in phase between the calculated phase of a minimum phase system based on the measured Impulse Response and the actual phase response of the measured system.

Excess Phase shows which areas are "minimum phase" - flat areas indicate "minimum phase".

Below is an outdoor measurement of my sub - black is "Excess Phase" - not flat is non "minimum phase"

58f1a0f5cec43_inclexcessphase.jpg.ca7b14fb8ed71c3c56cf666818d3e687.jpg

 

My observations:

  • I would have expected a flat area of minimum phase between 35-55Hz based on the flattish FR - not the case.
  • I did not expect the area between 70 and 80Hz to be flat for minimum phase in the middle of a dip in FR
  • the only relatively flat areas of minimum phase are below 25Hz and between 70-80Hz

 

Bringing the sub into the room gives different results

Setup:

  • in room measurement at the listening position
  • sub + mid bass
  • low pass Xover on sub around 50Hz or so 
  • no high pass on the 18" mid bass (but loads of linear phase speaker correction to push the 18's lower)
  • Dark blue is FR, Light Blue is phase, black is Excess Phase
  • Measurement is mic at the listening position, sub and left main 18" driven together (in their range)

58f1c35e4d72b_inroomsub18sFRPhaseandExcPhase.jpg.836430d0613896c9840ef9e2779ed7cc.jpg

 

Observations:

  • phase changes most when FR wiggles most (see below 20Hz, 90-100Hz, big dip around 250Hz
  • based on Excess Phase, EQ should work well <100Hz, but not above 100Hz

 

All graphs are of measurements taken some time ago - and I did some cherry picking to best describe what I was trying to show.

 

I particularly don't understand how an outdoor measurement of my sub could demonstrate less minimum phase  behaviour than the indoor measurement of the sub integrated with my mid bass speakers - would be great if the SNA community could help me understand why???

 

cheers

Mike

Edited by almikel
clarification

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35 minutes ago, almikel said:

Below is an outdoor measurement of my sub (as per above graphs) - black is "Excess Phase" - not flat is non "minimum phase"

 

Not zero is non-minimum-phase.

 

Edited by davewantsmoore

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almikel   
11 minutes ago, davewantsmoore said:

 

Not zero is non-minimum-phase.

 

likely correct with measuring a single speaker, but the  the REW page on Minimum Phase quotes "Anywhere the excess group delay plot is flat is a minimum phase region of the response".and using the Excess Phase graph "flat areas" for determining time alignment issues between sub and mains.

https://www.roomeqwizard.com/help/help_en-GB/html/minimumphase.html

58f1dd3a6b308_REWminphase.thumb.JPG.247e4245e8a8ca04f314ecd70e126a49.JPG

 

Is the REW guide wrong?

 

Mike

 

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almikel   

perhaps the "flat but non zero" area of minimum phase in my last REW graph is due to not using the "loopback" time reference? 

 

I've never mucked with this.

 

Mike

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1 hour ago, almikel said:

Is the REW guide wrong?

 

By definition, the "minimum phase", is when there is zero excess phase.

 

Positive, but flat excess phase, is saying that the phase has whatever amount of wiggles it is supposed to have when comparing it to the amplitude response ..... but it is delayed.    This could be telling you EQ is not a good idea   (it's hard to generalise).

 

Moving on to your final chart.    You have altered the phase (independently of the amplitude response) with your filters.   The phase has been corrected flat, even though the amplitude is rising.....  and so becuase of this, the excess phase is not flat.     So your conclusion about EQ above 100Hz becuase of this isn't justified.

 

 

In order to make a "should EQ be ok here?" assessment ....  you should not have any non-minimum phase EQ/filters enabled, because this adjusts the relationship between amplitude and phase, which is what you are trying to see   (to see for example, if something like a reflection is causing the wiggle in your measurement).

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almikel   

cheers Dave,

your insight/knowledge is always an order of magnitude above mine.

 

20 minutes ago, davewantsmoore said:

Positive, but flat excess phase, is saying that the phase has whatever amount of wiggles it is supposed to have when comparing it to the amplitude response ..... but it is delayed.    This could be telling you EQ is not a good idea   (it's hard to generalise).

so potentially useful to show time alignment issues but not much else (and I use different tools to achieve time alignment)

 

25 minutes ago, davewantsmoore said:

Moving on to your final chart.    You have altered the phase (independently of the amplitude response) with your filters.   The phase has been corrected flat, even though the amplitude is rising.....  and so becuase of this, the excess phase is not flat.     So your conclusion about EQ above 100Hz becuase of this isn't justified.

light bulb moment - thankyou - but I'll ask questions after the next quote

 

28 minutes ago, davewantsmoore said:

In order to make a "should EQ be ok here?" assessment ....  you should not have any non-minimum phase EQ/filters enabled, because this adjusts the relationship between amplitude and phase, which is what you are trying to see   (to see for example, if something like a reflection is causing the wiggle in your measurement).

So my linear phase EQ is all about speaker correction which I've always done independently from room correction.

My 18" mid bass have been corrected (and pushed lower) based on "in room" but close miked measurements.

Obviously amplitude and phase have been mucked with independently in this process - the target being a flat frequency response for the speaker (let's say I pushed the 18's down to 40 Hz using linear phase FIRs - close miked).

My sub has no FIRs applied and runs between 20 - 50hz (I think a 48dB LR 50Hz low pass)

 

Now I put the mike back in the listening position to do room measurement and correction using PEQ.

I run a sweep from the listening position using REW to look at excess phase to assist me in determining where EQ may be applied

- are the listening position measurements of excess phase once the TD18s take over invalid (say 60 - 250Hz) to assist in determining where PEQ can be applied?

- if using linear phase FIRs for Xover and speaker correction for all speakers (incl sub), is measuring excess phase useful at all in determining if PEQ can be applied?

 

Modern DSP is heading down a linear phase filter path.

Excess Phase has been a useful tool in knowing when to apply PEQ and when not to.

If linear FIRs muck all that up, what is left to determine how to apply sensible EQ - minimum phase or linear phase?

 

As per other recent threads - well applied EQ can do great good, but poorly applied even greater evil.

 

cheers

Mike

 

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11 hours ago, almikel said:

- are the listening position measurements of excess phase once the TD18s take over invalid (say 60 - 250Hz) to assist in determining where PEQ can be applied?

 

Yes.      However "determining where EQ can be applied" .... is very 'binary'.    It isn't as cut and dry as that.     Positive excess phase doesn't say you can't (or can) use EQ.... it is just an indication that something might be in your measurement that makes it more complex than it might seem.       It usually IS more complex than it seems.... which doesn't mean don't use EQ.  Just means take care.

 

 

 

12 hours ago, almikel said:

If linear FIRs muck all that up, what is left to determine how to apply sensible EQ - minimum phase or linear phase?

 

Turn off all your EQ.    Now your source is (supposed to be) minimum phase.      Measure....   and see where it's not.

 

Where you have excess phase, this can imply that some type of reflection is in the measurement, and maybe complicating the amplitude response .... and hence a peak or dip, is not a "simple" peak or dip that can be knocked up and down with EQ .... at least not without careful consideration (eg. measurements from lots of locations, and time windows).

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PS.    If you are not using a timing reference... then in REW don't forget to click the "estimate IR delay button" to remove delay.

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Keith_W   

This is a beautiful thread, with a beautifully explained post on phase. More people should read it ... so ... bump. 

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Nigel   

Thank Yous chasp,

it will take many reads and my own measurements to begin to understand what you might be talking about. Concepts on concepts....

This thread may become a reference.

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almikel   
On 4/18/2017 at 7:34 PM, Keith_W said:

This is a beautiful thread, with a beautifully explained post on phase. More people should read it ... so ... bump. 

 

7 hours ago, Nigel said:

Thank Yous chasp,

it will take many reads and my own measurements to begin to understand what you might be talking about. Concepts on concepts....

This thread may become a reference.

When I joined SNA way back when, I didn't understand phase much at all.

A lot of what I've learned has been from SNA directly or inspired by SNA to make me look further.

On 4/15/2017 at 5:52 PM, almikel said:

Anytime you have a wiggle in the frequency response, you have phase changes.

To coin a new SNA term - this is a daveism (@davewantsmoore) - reading similar in a dave post a long time ago had a profound impact on my understanding of the relationship between FR and phase - if my recollection is correct Dave even used the technical term "wiggle".

 

I greatly enjoy my membership in the SNA community - adding something is what SNA is all about.

 

cheers

Mike

 

 

 

 

 

 

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