# harmonics on impulse response



## Drudge (Sep 1, 2008)

Hey guys,I finally had access to a laptop again to take some new measurements:yay2: and I noticed that this time around that I have 1st and 2nd order harmonic distortion on my impulse graphs:sad2: that I did not have on previous measurements.

Is the distortion higher in level than what would be considered normal?I used the same computer I used before and none of my equipment has changed other than eq settings.I'm actually using less eq than previously now that I've added additional treatment.

Any Ideas on what might cause this?


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## brucek (Apr 11, 2006)

> Any Ideas on what might cause this?


Be sure that your soundcard isn't producing the harmonics by taking a measure of a simple loopback cable, and then examining the impulse response. You will see the base harmonics that the card itself is introducing. Certainly a level that is too high (clipping) will cause harmonics.

brucek


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## Drudge (Sep 1, 2008)

Actually when I was taking measurements REW's spl meter would briefly flash clip with some measurements but it would say[ levels ok 3dB headroom] so I don't know if that might have been the problem. 

The soundcard checks out fine.


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## brucek (Apr 11, 2006)

> REW's spl meter would briefly flash clip with some measurements


Yeah, that would do it.

Usually, until your sub is equalized, and you can take a measure that doesn't have large peaks that clip the SPL meter, I would ignore distortion. 

This is one of the problems of using an SPL meter for taking response measurements. A microphone (i.e. ECM8000) doesn't suffer from the constraint of having a limited range to work with before it clips the output.

brucek


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## Drudge (Sep 1, 2008)

By the way I was using the ECM8000.

Could I have had the preamps levels not set properly and that cause the problem as well?



Thanks brucek!


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## brucek (Apr 11, 2006)

> By the way I was using the ECM8000


Then if you were experiencing _REW's spl meter would briefly flash clip with some measurements _, then your input level was set too high.

brucek


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## DrWho (Sep 27, 2006)

Drudge said:


> Actually when I was taking measurements REW's spl meter would briefly flash clip with some measurements but it would say[ levels ok 3dB headroom] so I don't know if that might have been the problem.
> 
> The soundcard checks out fine.


That is bad. Try turning down the mic pre and see if the distortion subsides.
_Posted via Mobile Device_


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## Drudge (Sep 1, 2008)

Yeah, I had the mic preamp level to high.


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## Drudge (Sep 1, 2008)

I thought the levels where the problem and I had checked the soundcard properly but I didn't:duh::duh:

When I perform the loopback test the left and right levels aren't equal only the right or the left (depending on which I use) level hits the proper level with the other being lower in level.I don't know if that's normal or not but I didn't have that problem when I originally calibrated the soundcard and then everything was fine no distortion.

Here is what I get for the soundcard freq:










Here is the impulse










I didn't have this problem when I made my calibration file for my preamp and soundcard originally.


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## brucek (Apr 11, 2006)

> When I perform the loopback test the left and right levels aren't equal


Well they shouldn't be. When you do a soundcard calibration, you only connect a loopback on one channel and the other channel is left open. If you connect them both it causes interference in the other channel.

brucek


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## Drudge (Sep 1, 2008)

What I meant to say is when I hook up the loopback I used just the right channels and the RMS/Peak meters for left and right are active but not at the same levels one is lower than the other.

Just to check things I used the loopback for just the left channels and the same result only reversed with the left meter higher in level than the right.

However both left and right meters are active no matter which single side left or right I use.


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## brucek (Apr 11, 2006)

They shouldn't be. Are you sure you have all the effects turned off in the soundcard, and that you are in two speaker stereo?

brucek


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## Drudge (Sep 1, 2008)

Here is what the meters show


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## brucek (Apr 11, 2006)

Either you have an extreme amount of crosstalk or you have some effect turned on, or you have a fault. The unused channel should simply not be acting that way.

brucek


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## JohnM (Apr 11, 2006)

Make sure you are using the right kind of connectors to plug into your soundcard, if it has 1/8" sockets you need leads or adaptors with stereo 1/8" plugs, could get something like that plot if mono plugs were used.


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## Drudge (Sep 1, 2008)

I don't know whats happened :coocoo:the mini plug to rca adapters are stereo.The soundcard is a internal laptop type and the settings are only simple wave and main volume settings along with left/right balance.Line level is selected and everything was as before.

I never reperformed a soundcard calibration when I installed the latest version of REW but I don't think that is necessary correct?Well obviously I am already trying that know so that is mute.

Prior to installing this latest version all my soundcard calibrations went off without a hitch and the file that automatically loads is correct.

I am definitely puzzled by this.:scratchhead:Should I reinstall the soundcard and REW programs?


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## brucek (Apr 11, 2006)

Pull down the Settings selection at the index bar at the top of REW and select Delete Settings and Shutdown..

brucek


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## Drudge (Sep 1, 2008)

Well the good news is that I have fixed the problem with the meter:yay2:

I reinstalled the soundcard driver and BINGO!:jiggy:Now only the right meter is active and the calibration test went off without a hitch! 

Now the bad news :no:It looks like I have more distortion than before:hissyfit:


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## brucek (Apr 11, 2006)

I would die to have a noise floor that was -105dBFS, and a couple harmonics that make it to -75dBFS......... You should revel in the lovely situation....... stop fussing..


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## Drudge (Sep 1, 2008)

That is just the soundcard impulse that is not an actual measurement.

I thought if the soundcard measurement showed high distortion(what is normal) there was something wrong.

If that is a good soundcard measurement than by all means I surely don't need to fuss:bigsmile:As long as it's not my system and doesn't affect the actual measurements I'm good to go!


I always like to double check and hear from knowledgeable people about things I'm not to sure about:T


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## brucek (Apr 11, 2006)

> That is just the soundcard impulse that is not an actual measurement.


The only measurement that REW actually does is an impulse response. 

All the other graphs are derived from the impulse response.

brucek


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## Drudge (Sep 1, 2008)

Thanks for the info:T


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