# Running Measurement Sweep Through Headphone Jack?



## jake5 (Apr 28, 2013)

Just getting started with a netbook and a UMIK-1 here.

Can I count on my netbook's headphone output to be a reliable source for generating a measurement sweep?


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## JohnM (Apr 11, 2006)

Headphone output is fine, there is some chance the netbook itself could struggle though. Sweeps should vary smoothly with no gaps, clicks or stutters, so keep an ear out for anything that sounds odd. Beyond that problems tend to be evident in the measurements, if you post the .mdat file from a measurement here we can check all is well with it.


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## jake5 (Apr 28, 2013)

The netbook has been suuped up with a nice ssd, etc. so it's actually a decent performer.

Regarding the headphone jack, however, I had imagined it could easily color the output by being non-linear. How do we know this is a non-issue?


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## JohnM (Apr 11, 2006)

Computers have good audio interfaces pretty much universally, see this thread for some discussion on that.


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## jake5 (Apr 28, 2013)

"good audio interfaces" would imply good input sensitivity as well. But like the OP in the thread you linked, my netbook's soundcard input was far from flat. 

Your post about AC 97 specs for FR makes me feel optimistic, however, as my chipset has the Intel HD Audio spec (which is hopefully an example of meeting/exceeding the earlier AC 97 spec as you had mentioned).

Could I somehow calibrate my desktop's soundcard in REW, then connect my netbook to test the netbook's linearity?


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## JohnM (Apr 11, 2006)

jake5 said:


> my netbook's soundcard input was far from flat.


How did you measure that?



> Could I somehow calibrate my desktop's soundcard in REW, then connect my netbook to test the netbook's linearity?


Only if you could run some application on the netbook that turned it into an audio path, so audio data was piped from input to output allowing it to be included in a loop for the desktop audio.


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## jake5 (Apr 28, 2013)

JohnM said:


> How did you measure that?
> 
> Only if you could run some application on the netbook that turned it into an audio path, so audio data was piped from input to output allowing it to be included in a loop for the desktop audio.


Soundcard measured doing a calibration when I was considering a non-USB mic.

I went ahead and calibrated my desktop's soundcard on REW and it looks quite good though I'm not sure about the rolloff ~20khz (see attached REW shot). I then found a free spectrum analyzer program online at:
http://www.diyaudio.com/forums/pc-b...ope-spectrum-analyzer-function-generator.html

I ran this program on my desktop with my netbook plugged into the desktop's line-in. I'm admittedly over my head with the free spectrum analyzer program, but it was registering the various test tones I was playing on my netbook. Attached are screenshots of pink noise, 250hz test tone, and a silent track. (pictured in order - apologies for the blurry shots)

ALSO:

Checking out the advanced sound playback options (default device is Realtek HD Audio output) in Windows XP, there is an option to select the speaker setup to match what is being run - options like:

no speakers, stereo headphones, desktop stereo speakers, etc.

I can only assume these various options may affect output linearity as why else would there be multiple options for stereo outputs?

Not sure what option might be the best bet for a linear output.


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## JohnM (Apr 11, 2006)

Run the REW RTA on your desktop set up like this:



then play the REW Pink PN signal with Sequence Length set to 65536 on your netbook with its output connected to your desktop's line in. Make sure both desktop and netbook are set to the same sample rate (e.g. 48k). You can see what effect, if any, the speaker settings have but their main purpose was to provide information for adjusting other audio effects (3D sound etc).


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## jake5 (Apr 28, 2013)

JohnM said:


> Run the REW RTA on your desktop set up like this:
> 
> 
> 
> then play the REW Pink PN signal with Sequence Length set to 65536 on your netbook with its output connected to your desktop's line in. Make sure both desktop and netbook are set to the same sample rate (e.g. 48k). You can see what effect, if any, the speaker settings have but their main purpose was to provide information for adjusting other audio effects (3D sound etc).


here's my result


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## JohnM (Apr 11, 2006)

Looks pretty flat to me.


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## jake5 (Apr 28, 2013)

Thanks very much, that was quite easy to do. Just out of curiosity, why was this method not recommended in response to post #5? 

Or was your response below (from post 6) simply another way of wording your suggestion in post 8?
"Only if you could run some application on the netbook that turned it into an audio path, so audio data was piped from input to output allowing it to be included in a loop for the desktop audio."


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## JohnM (Apr 11, 2006)

jake5 said:


> Just out of curiosity, why was this method not recommended in response to post #5?


Didn't occur to me at the time


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## jake5 (Apr 28, 2013)

JohnM said:


> Didn't occur to me at the time


Gotcha. Just wanted to make sure I wasn't missing anything. I tried some of the various speaker modes in XP (headphones, laptop, desktop, etc) and as you predicted, I observed zero change. 

I've started taking measurements and have found the pink noise rta to vary significantly from the sweep. This is being done on right side mids/highs only. I realize the sweeps are recommended, but it's much easier to watch the rta while eq'ing w/pink. I'm not sure how this variation is accounted for, but I suppose I should just revert to the recommended method (sweeps)?

Also, I'm just seeing that I had the java drivers selected rather than ASIO, and "default" output device selected rather than "realtek hd audio or primary sound driver". I'm not completely sure what the best choices here are.


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## JohnM (Apr 11, 2006)

Wouldn't normally expect much difference between the response with a sweep or the RTA using PinkPN (bearing in mind the RTA view is somewhat smoothed due to the action of the RTA, should be close to using 1/48 octave smoothing on a sweep measurement). Can you post some example files showing the differences?

Best selecting the actual device and input/output you are using for measurement, the defaults can change if audio interfaces are connected/disconnected and selecting the actual input/output allows REW to show and adjust volume settings. The ASIO drivers allow measuring at rates other than 44.1k or 48k if your audio interface supports other rates.


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## jake5 (Apr 28, 2013)

*Sweep vs Pink Noise Curves*

The programmed measurement sweep is the top curve. I don't believe the db is accurate for the pink curve as I had it turned up to a good volume and my system definitely gets loud. It may have something to do with SPL calibration.

To me this looks like the difference between on vs off-axis curves. I am using a UMIK-1 from Cross Spectrum that has the SPL line pasted into the cal file. Per CS's recs, I have the mic pointed upward and am using the 90deg cal file.

I've noticed when starting REW it will ask if I want to use the UMIK(yes), if I want to continue using the 90deg cal file(yes), and then if I have a cal file I would like to use(if I answer no to the last one it will not load any cal file for the mic, despite my answering yes to the prior question; "would I like to conintue using..."). Not sure why anything here would change between running a sweep and testing using pink noise right afterward, but I felt this was worth mentioning anyway. Also it seems I have to calibrate the SPL every time I start the program.


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## JohnM (Apr 11, 2006)

Need to see the mdat file rather than the image, can you attach that?


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## jake5 (Apr 28, 2013)

12


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## jake5 (Apr 28, 2013)

mdat files posted


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## JohnM (Apr 11, 2006)

Looks like the RTA was set up in Spectrum mode instead of RTA mode, pink noise falls at 3 dB/octave on a spectrum view.


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## jake5 (Apr 28, 2013)

JohnM said:


> Looks like the RTA was set up in Spectrum mode instead of RTA mode, pink noise falls at 3 dB/octave on a spectrum view.


Okay, thanks. I was sure I eq'd with Pink on 1/3 smoothing and got a nice downward slope (from L to R), then came back and did a spectrum RTA a bit later (so sweep measure done after eq'ing should have also been down-sloping) - but I'll look into it.

Also, I thought perhaps because my mic has a dB value in the cal file I would not need to calibrate the SPL in REW, but it seems this needs to be done every session, correct?


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## JohnM (Apr 11, 2006)

jake5 said:


> Also, I thought perhaps because my mic has a dB value in the cal file I would not need to calibrate the SPL in REW, but it seems this needs to be done every session, correct?


If there is a cal entry in the correct format in the mic/meter cal file that is loaded there is no need to calibrate. The first line should look like:


```
"Sens Factor =-21.223dB, SERNO: 7000139"
```
including the quotes.


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## jake5 (Apr 28, 2013)

That's how it looks for me (after changing from .frd to .txt). File attached.

So with the .frd file loaded as the Calibration File in the Mic/Meter tab (and no cal file loaded in the soundcard tab), if I open the SPL meter and hit calibrate should the two dB values already be matching?



JohnM said:


> If there is a cal entry in the correct format in the mic/meter cal file that is loaded there is no need to calibrate. The first line should look like:
> 
> 
> ```
> ...


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## JohnM (Apr 11, 2006)

If you hit calibrate you should get a message saying calibration is not required. You also need to have the UMIK selected as the input device and Microphone as the input, so that REW can read the volume setting and allow for that.


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