# Soundcard Calibration Samplerate



## BlueFlowers (Sep 26, 2012)

I too had the "excessive variation" error trying to calibrate my soundcard (Pci Prodigy HD2 Advance).

Not until I switched the samplerate to higher than 44.1 kHz the problems disappeared and things went fine.

When I load up the correction file however, created at 96 kHz, I do get a popup from REW saying that my soundcard is currently set to a different sample rate than the calibration file that I loaded, and that "calibration data measured at a different rate won't provide accurate correction".









My soundcard switches its sample rate automatically to what is fed to it. If the test tones from REW are at 44.1 kHz then it will operate at that rate and thus the soundcard calibration file will

a) be inaccurate ?
or
b) not be used perhaps?


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## Barleywater (Dec 11, 2011)

For most measurement work working at 44.1kHz or 48kHz sample rates works quite well. Make soundcard calibrations for these rates and use them. If buffer size is too small odd behavior is sometimes encountered. I do have an E-MU 0404 USB, that also behaves badly when buffer is set to maximum. So a little experimentation may help.

Regards,

Andrew


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## BlueFlowers (Sep 26, 2012)

Hi Andrew, thanks for your reply.
Creating a calibration file with 48khz, 88 or 96 khz is no problem. Anything but 44.1 khz works. 
My concern is if REW sends out its testsignals at 44.1 and my correction isn't created at 44.1khz, what happens?


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## Barleywater (Dec 11, 2011)

If other audio applications, such as media players, are active they may control sample rate on soundcard. You appear to be using ASIO drivers. I have two external cards, and sometimes I find it necessary to power cycle card, and close REW to get drivers to play nice. With PCI card this equates to re-boot.

Andrew


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## BlueFlowers (Sep 26, 2012)

Andrew thanks again.
I don't think we completely understand each other. 

When calibrating my sound card at 44.1 khz I kept getting the excess variation error (too much db difference between channels), irrespective of buffer size.

When I set the REW samplerate to 48, 88 or 96, without changing anything else, the error disappeared, so I just went ahead and created a correction filter for 96 khz.

I don't use any other mediaplayers at the same time (learned that the hard way: asio + asio = BSOD, lol).

My concern again is, if REW sends out a test tone at a sample rate I haven't created a correction filter for, how can the correction work? I don't see an option to choose what sample rate the signal is outputted in from REW, so I presume it's in 44.1 khz as I've read in a post.

Kind regards

edit:
I notice when starting up REW it puts the sample rate of my sound card at 44.1 khz, even if it was used at a different rate beforehand.
However, in the preferences window i can change the samplerate from the default 44.1 khz to anything I like, in this case 96 khz, and then load the correction file without the error. 

I'll just have REW working at 48, 88, or 96 khz then i suppose. Doesn't matter really.
Much ado about nothing in the end I guess.


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## JohnM (Apr 11, 2006)

REW attempts to restore whatever sample rate you last selected on the soundcard preferences, so if you set it to 48k (or whatever you want to use) it should set that rate next time you start it up.


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## BlueFlowers (Sep 26, 2012)

Indeed. Thanks John. And congrats on a great program and forum!

Do you have any idea as to why I can't just run the calibration succesfully at the default 44.1, but without any problems at the other sample rates?


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## JohnM (Apr 11, 2006)

No, what does the response at 44.1k look like?


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## BlueFlowers (Sep 26, 2012)

Was away for the weekend, sorry for the delayed reaction.

I can't get a response at 44.1, thats the problem.










EDIT:
Silly me, obviously I can get the response, just can't create a calibration file of it. Seems like at 44.1 it drops off very steeply before 20khz.








However, when i hook up the soundcard to my Mic100 preamp I do manage to calibrate the combo even at 44.1 khz (graph is for 96).









I bought the Mic100 for REW, only later to find out a different preamp would have been better.
When comparing these readings, one for the soundcard only, the other one for the soundcard ran through the preamp, would you say the Mic100's response is usable for REW?

One thing I notice, for the soundcard/preamp combo there is quite a large difference in response flatness for 44.1 (+ 0.4 / - 4.6 db, red) compared to 96khz (+ 0.8 / -1.0 db, blue).









Kind regards


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## BlueFlowers (Sep 26, 2012)

Tried out various connections as I read on the forum that connecting through XLR - mini jack sometimes doesn't work out.

























I guess, although not ideal, the Behringer MIC100 preamp is usable, and preferably connected by jack - mini jack.
Another cable to waste (bought the xlr to avoid the extra tapering plug. what do I know?) :doh:

As for my previous post: still don't know why i can't make a correction filter at 44.1 with this soundcard :scratch:


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## JohnM (Apr 11, 2006)

You could make a cal file from that 44.1k response, it is unusual to be 6dB down at 20k but that doesn't stop you from using it, the response itself is clean.


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## BlueFlowers (Sep 26, 2012)

Ok ic, thanks John.

The response of the soundcard + preamp combo is a lot less clean as you see. I'm not so picky that I'd say the extra deviation of say 1 db would bother me too much. Nice 'warm' tube sound haha. But are there perhaps other implications of that fuzzy line?


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## JohnM (Apr 11, 2006)

Probably just noise in the preamp, might be some DC offset also - if you post an mdat file from a loopback with the preamp can have a look at that.


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## BlueFlowers (Sep 26, 2012)

Wow, that would be great!

View attachment Soundcard plus preamp 96 khz jacks.mdat
connected via jack - mini jack cable; looks less distorted and will probably be what I'll do

View attachment Soundcard plus preamp 96khz xlr.mdat
connected via XLR - mini jack cable, my initial option

Thanks!


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## JohnM (Apr 11, 2006)

Both traces have quite high distortion, but the XLR trace is particularly bad - from the high level odd harmonics I'd say you are heavily over-driving the input so you need to either reduce the input gain or the output level or possibly both. The jacks trace is mostly 2nd harmonic (about 7%) - I presume that is a tube pre? Would be better using solid state for this.


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## BlueFlowers (Sep 26, 2012)

I fiddled around with the Behringer Mic100 preamp, tube yes, and was able to turn the Gain off completely and only turn the Output up to about 8 o'clock. This indeed produces a much smoother calibration response :clap:









Data:
View attachment soundcard + preamp def.mdat


Onwards to the next stage then :bigsmile:

When checking levels for taking measurents, with my cross spectrum ECM8000, I have to turn the gain up to 5 o'clock, and leave the output to 8 o'clock. Otherwise the input levels in REW are just too low (I reach - 20 this way, with the db level in REW also set to -20). 
The leds on the Mic100 stay in green, so there's no clipping there, but from the previous calibration measurements I know now the signal becomes more distorted than it should. 

The pci prodigy hd2 soundcard i'm using perhaps has very low gain on its line input, reason why I have to crank up the mic preamp so hard?

Is the soundcard plus preamp calibration file I just created any good when turning up the knobs on the Mic100 so much distortion again will occur?


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## JohnM (Apr 11, 2006)

The cal file should be fine. You should not get distortion using a mic as the input, as the levels it feeds the pre are much lower than with the loopback connection.


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## BlueFlowers (Sep 26, 2012)

Seems like some progress has actually been made then, thank you John


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## Barleywater (Dec 11, 2011)

The low frequency ripple seen in Mic100 SPL traces is high level 2nd harmonic impulse response aliasing with fundamental impulse response. Combined with IR window that includes second harmonic is demonstrated with your 44.1kHz v 96kHz results. 

When performing 44.1kHz and 96kHz measurements you likely kept same sweep length. The sweep length has direct bearing on how many samples separate fundamental response from harmonic responses. So each measurement separates by same number of samples, but translates to roughly doubling time between fundamental and harmonics. IMO there is no need for 96kHz sample rate in this situation.

Close examination of your results shows 50Hz contamination and associated harmonics, likely your power lines.









Above view is your results viewed using shorter left side IR window that excludes most of 2nd harmonic's impact on fundamental. 

“…..but from the previous calibration measurements I know now the signal becomes more distorted than it should.” Mic100 is as much a tube based effect as it is a preamplifier, and your 4.5% THD is respectively quite normal. Likewise the Mic100 balanced output is likely to have high degree of impedance mismatch between + and – phases, leading to your XLR v mini-jack observations.

Regards,

Andrew


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## BlueFlowers (Sep 26, 2012)

Wow, thank you for your response Andrew.

At first I kept getting an error from REW when measuring my soundcard at 44.1 khz. When that error didn't occur at 96 khz I just went ahead and used that. Only reason really.

It's reassuring to read the THD is within the normal range. I thought turning up the gain would mess up the signal again. From a response from John earlier I've learned this not to be the case. Googling THD due to your post turned me on to wikipedia, where I've read that "For all signal processing equipment, except microphone preamplifiers, the preferred gain setting is unity. For microphone preamplifiers, standard practice is to use maximum gain.". That completely cleared it for me then.

Guess the tube choice for a mic preamp still isn't ideal, yet, for me, will suffice just fine.

Regards


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