Tony_J

rePhase

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10 minutes ago, MartinC said:

That shows that the two microphones have similar phase responses but not that this similar response is linear.

How similar were the amplitude responses of the two microphones?

Looking at the calibration files, within about 0.3db.

Edit:  Sorry, I'm technically wrong... 0.3db is the variation in the range of frequencies that I checked (from about 300hz upward).

Looking at the lowest and highest frequencies, the variation can be as much as about 1.8db but is down to 0.000 for a good chunk of the range.

Edited by rv295

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20 hours ago, rv295 said:

Looking at the calibration files, within about 0.3db.

Edit:  Sorry, I'm technically wrong... 0.3db is the variation in the range of frequencies that I checked (from about 300hz upward).

Looking at the lowest and highest frequencies, the variation can be as much as about 1.8db but is down to 0.000 for a good chunk of the range.

The more similar the amplitude the responses the more similar the phase responses are likely to be I think. It would be interesting if the biggest differences in your phase measurements were where the amplitude responses differed most?

Below is what the amplitude response of my UMIK-1 looks like, with I assume the values in dB. It's reasonably flat but I'd be a little surprised if the phase response is totally insignificant. I've found a couple of old threads on the miniDSP forum about this but no actual data - I'll start a thread there in a bit to see if I can find anything out.

image.png.7e098bbbb10b87c750c39b8b482b8ded.png

Edited to replace image.

Edited by MartinC

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6 minutes ago, MartinC said:

The more similar the amplitude the responses the more similar the phase responses are likely to be I think. It would be interesting if the biggest differences in your phase measurements were where the amplitude responses differed most?

Below is what the amplitude response of my UMIK-1 looks like, with I assume the values in dB. It's reasonably flat but I'd be a little surprised if the phase response is totally insignificant. I've found a couple of old threads on the miniDSP forum about this but no actual data - I'll start a thread there in a bit to see if I can find anything out.

UMIK%20respose_zpsgl4wcyhf.jpg

The phase and amplitude would directly relate, so it would be easy to work out what the effect of each db variation would be on each frequency.  From an educated guess (simply based on seeing the result that parametric adjustments have on the phase), we'd be talking less than a degree from these kind of variations. I don't have the time to do the maths right now, but would be happy to confirm my guess when I have the time.

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To get an idea of how much these db variations will affect the phase of the mic calibration, simply open an impulse response measurement in rephase, make a parametric adjustment at the chosen frequency that matches with the calibration file and look at the resulting adjustment in the phase prediction. 

It's actually of little concern to me as I only use microphones to check things now.   The Linn system calculates the precise phase of each frequency through a combination of TS parameters and driver impedance measurements.

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32 minutes ago, rv295 said:

To get an idea of how much these db variations will affect the phase of the mic calibration, simply open an impulse response measurement in rephase, make a parametric adjustment at the chosen frequency that matches with the calibration file and look at the resulting adjustment in the phase prediction. 

I don't think you can universally assume a particular amplitude response change will be associated with a particular phase response change (if it was there would never be any need for phase calibration). FIR filters won't introduce non-linear phase shifts whilst IIR filters would for example. It's the difference from linearity in the phase response that matters. My point being I'm not sure that looking at examples in rephase will be able to tell us about the microphone's response.

Edited by MartinC

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57 minutes ago, MartinC said:

I don't think you can universally assume a particular amplitude response change will be associated with a particular phase response change (if it was there would never be any need for phase calibration). FIR filters won't introduce non-linear phase shifts whilst IIR filters would for example. It's the difference from linearity in the phase response that matters. My point being I'm not sure that looking at examples in rephase will be able to tell us about the microphone's response.

I'm pretty sure the calibration filters are applied via a parametric equation in REW so yes, you could easily work out the result of the calibration filter on the phase.  Phase directly relates to the timing and the resolution of the mic will depict how accurate it can sample time, so as I said earlier... unless microphones are fundamentally flawed then I'm not going to worry about it.

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15 hours ago, rv295 said:

I'm pretty sure the calibration filters are applied via a parametric equation in REW so yes, you could easily work out the result of the calibration filter on the phase.  Phase directly relates to the timing and the resolution of the mic will depict how accurate it can sample time, so as I said earlier... unless microphones are fundamentally flawed then I'm not going to worry about it.

My point was that you can get different filters with similar amplitude response but different phase response so theree is not a universal relationship between the two. The same goes for systems like loudspeakers and microphones, although for similar designs I think an appropriate electrical model could be produced to approximate the relationship.

Edited by MartinC

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1 hour ago, MartinC said:

My point was that you can get different filters with similar amplitude response but different phase response so theree is not a universal relationship between the two. The same goes for systems like loudspeakers and microphones, although for similar designs I think an appropriate electrical model could be produced to approximate the relationship.

I've not seen any evidence of this but my experience is rather limited with only 2 different microphones.

I'm also struggling to see how it's possible unless it comes via the calibration adjustments.  Microphones aren't pre-phase aligned, they detect the phase via pressure and time - unless the mic can't sample often enough to do it accurately,  I'm not keeping up with your thinking but that could be me being thick or having gaps in my understanding of the subject at hand.

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5 hours ago, rv295 said:

I'm also struggling to see how it's possible unless it comes via the calibration adjustments.  Microphones aren't pre-phase aligned, they detect the phase via pressure and time - unless the mic can't sample often enough to do it accurately,  I'm not keeping up with your thinking but that could be me being thick or having gaps in my understanding of the subject at hand.

There are obviously different types but microphones are essentially speakers working in reverse (sound producing movement that induces an electrical signal rather than the other way round.) You know that speaker drivers have non-linear phase responses because this is what you have taken the time to correct within your own system. There is no reason to assume that microphones somehow manage to have perfectly linear phase responses. Also, as you correctly observed above there is a relationship between amplitude and phase response such that the fact that the amplitude response shows variation indicates there will be associated phase variations. How significant any phase variation may be is the question.

I've found some comments from staff on the miniDSP forum saying that when they did provide phase calibration data for the UMIK-1s it showed there was no significant non-linearity. It would obviously be more compelling if they provided at least some examples showing this but so far I've not had a response to my request for this.

For info. the ultrasonic 'microphones' I'm more familiar with have much less flat frequency responses and do have significant phase non-linearity associated with this, which in turn leads to distortion of measured waveforms if no corrected for.

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2 hours ago, MartinC said:

There are obviously different types but microphones are essentially speakers working in reverse (sound producing movement that induces an electrical signal rather than the other way round.) You know that speaker drivers have non-linear phase responses because this is what you have taken the time to correct within your own system. There is no reason to assume that microphones somehow manage to have perfectly linear phase responses. Also, as you correctly observed above there is a relationship between amplitude and phase response such that the fact that the amplitude response shows variation indicates there will be associated phase variations. How significant any phase variation may be is the question.

I've found some comments from staff on the miniDSP forum saying that when they did provide phase calibration data for the UMIK-1s it showed there was no significant non-linearity. It would obviously be more compelling if they provided at least some examples showing this but so far I've not had a response to my request for this.

For info. the ultrasonic 'microphones' I'm more familiar with have much less flat frequency responses and do have significant phase non-linearity associated with this, which in turn leads to distortion of measured waveforms if no corrected for.

I don't see it as the same technical issues though.  

The phase variation of a driver is due to it's mechanical properties, these simply wouldn’t be an issue on a small scale diaphragm the size you get in a calibrated microphone.  Producing sound and recording sound are two very different things in terms of the engineering or physics required.

The frequencies you are experienced in, are so small, even the most minute error would result in an extreme phase shift.  That again, simply isn't the case with wavelengths that are measured in centimetres or meters.

I just can't see a calibrated measurement microphone suffering the same, or any real kind of phase shifts that are significant.

Hopefully you get an answer from minidsp, perfection is a noble pursuit but I've a feeling the gains are not there for this line of thinking to be worthwhile. Please prove me wrong as anything that improves the technical understanding of sound recording or reproduction is going to benefit everyone.

Edited by rv295
Too many spelling mistakes to ignore...

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10 hours ago, rv295 said:

I don't see it as the same technical issues though.  

The phase variation of a driver is due to it's mechanical properties, these simply wouldn’t be an issue on a small scale diaphragm the size you get in a calibrated microphone.  Producing sound and recording sound are two very different things in terms of the engineering or physics required.

The frequencies you are experienced in, are so small, even the most minute error would result in an extreme phase shift.  That again, simply isn't the case with wavelengths that are measured in centimetres or meters.

I just can't see a calibrated measurement microphone suffering the same, or any real kind of phase shifts that are significant.

Hopefully you get an answer from minidsp, perfection is a noble pursuit but I've a feeling the gains are not there for this line of thinking to be worthwhile. Please prove me wrong as anything that improves the technical understanding of sound recording or reproduction is going to benefit everyone.

 

Bear in mind that where I came into this was to try to see if there was data available to assess the scale of the effect. That is I wasn't saying I thought it was necessarily a major issue but that I'd like to actually find out rather than blindly assuming otherwise. I prefer not to make assumptions...

(Regarding ultrasonic measurements yes there are obvious differences although much smaller effective measurement volumes are used to compensate. I'm talking sub-millimetre here. I only mentioned my experience as in your last post it sounded like you were doubting there was any feasible way for any effect at all. There WILL be one but the question is how big.)

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6 hours ago, MartinC said:

Bear in mind that where I came into this was to try to see if there was data available to assess the scale of the effect. That is I wasn't saying I thought it was necessarily a major issue but that I'd like to actually find out rather than blindly assuming otherwise. I prefer not to make assumptions...

(Regarding ultrasonic measurements yes there are obvious differences although much smaller effective measurement volumes are used to compensate. I'm talking sub-millimetre here. I only mentioned my experience as in your last post it sounded like you were doubting there was any feasible way for any effect at all. There WILL be one but the question is how big.)

You are right Martin,  I don't have the knowledge to understand why a microphone would have phase irregularities (I'd only be guessing) and I get you're looking in to it and not making any claims yet.

I do understand why drivers produce a variation in phase but I can't see how these mechanical issues apply to a tiny diaphragm in a mic. They don't as far as I can see.
 
I understand the issues that come from having to create sound, but I don't know what the issues are when it comes to recording the sound.  I get the fact that any sensor has the potential to contain a margin of error but beyond that, I'm in uncharted waters and presume (as I put it badly earlier) that they had it sorted with the current tech and even the older tech.

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15 hours ago, rv295 said:

I'm in uncharted waters and presume (as I put it badly earlier) that they had it sorted with the current tech and even the older tech.

For info. for the ultrasonic work I was involved with the issue of detector phase response was one that had simply been ignored globally for decades by international experts and one of my modest scientific contributions was to raise this and demonstrate one way to correct for it. This could only be done by measuring what the phase response was. My point being the issue isn't one that can necessarily be 'sorted' by design in the same way that amplitude response variations aren't (hence the use of amplitude calibration data). Better design can of course reduce the significance of effects.

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