Difference; Internal Vs External Cal Sweep - Home Theater Forum and Systems - HomeTheaterShack.com

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post #1 of 6 Old 02-21-15, 09:36 PM Thread Starter
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Difference; Internal Vs External Cal Sweep

Difference; Internal Vs External Cal Sweep:
Maybe someone can help explain the source of an issue I have been struggling with. I need a wav file for both the REW 44kHz calibration sweep and the REW measurement sweep I use (12-20k).

I have noted for that SPL and phase responses are different for a test signal sent from my Saffire 2i2 Analog out Vs one of the “same” signal sent from a media player (Foobar). There would be expected to a small difference due to the extra dac/adc process when using the 2i2 analog output, but the difference I found is much more significant than I expected. The 2i2 analog method provides a flatter SPL and phase response than the digital file does. This is counterintuitive for me. I would expect the digital version to be slightly better.

Below are the SPL and Phase charts for the 2 measurements and the mdat file. Trace 1 (red) is the analog Soundcard measurement and trace 2 (green) is the digital Soundcard measurement. These measurements were used to create my internal and external soundcard calibration files from. They actual enclude my entire measurement chain Vs entire playback chain.

Difference; Internal Vs External Cal Sweep-spl.png

Difference; Internal Vs External Cal Sweep-phase.png

External Sweep.mdat

The reason for the concern is that I have been doing all my setups with the normal REW analog calibration, but all my program material is all digital sourced so the 2i2 isn’t in the playback chain. It impacts both my house curve and my rePhase phase correction filter creation and testing. The digital version has is a +2.9dB bulge around 14kHz and there is and extra 360° phase rotation through the HF. Since the analog looks normal and digital looks wrong, I have been trying to create a good external calibration file for some time now. The problem still remains however. I have been trying to understand what may be wrong with my creation of the digital version of the calibration sweep. I am still guessing that is the problem.

Below attached is the process I am using to create the wav file version of the REW calibration sweep. Any thoughts you have on what may be wrong my process or how this file could be created accurately would be appreciated. Below is a screen shot of the Audacity captured sweep and I can post the actual sweep file if that will help.

External Signal Capture Process.txt

Difference; Internal Vs External Cal Sweep-audacity.png

Has anyone created a wav file that matches the REW 44k calibration signal?
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post #2 of 6 Old 02-22-15, 01:01 PM
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Re: Difference; Internal Vs External Cal Sweep

Are you sure there isn't some kind of EQ or processing active in Foobar? A 3 dB boost is much too high to be any kind of path flatness issue and there is a sharp rise in 2nd harmonic distortion that starts at 1.35 kHz and reached nearly 5% by 10 kHz. You could check the flatness using the RTA and the Pink PN test signal.
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post #3 of 6 Old 02-22-15, 01:44 PM Thread Starter
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Re: Difference; Internal Vs External Cal Sweep

Thanks! I'll investigate.
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post #4 of 6 Old 02-22-15, 06:23 PM Thread Starter
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Re: Difference; Internal Vs External Cal Sweep

Problem found. That helped. The PinkPN over DLNA confirmed that the SPL was identical to the sweep. Both had that same peak. So your thought was correct. The sweep file was not the issue.

I finally identified the cause. My new AV pre-pro has a setting called M-DAX. I never heard of that before. It is on by default on the 2 inputs I use for the digital sweeps. It Is off by default for the other inputs. Once I set to off, the analog sweep and digital sweep are now both flat.

Now maybe my streamed music will finally go back to sounding normal.
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post #5 of 6 Old 02-22-15, 07:18 PM
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Re: Difference; Internal Vs External Cal Sweep

As I understand it, the reason to play the sweep waveform from foobar is that it is closest to "real life" use of the system for music playback. And the question is how to use the Saffire 2i2 when needed, with appropriate calibration file, without negating the accuracy of the "real life" setup. Is that the idea?


First the recorded sweep waveform. I created one using REW's output looped through Reaper, via Reaper's built-in ReaRoute ASIO to make all the digital connections in a single computer, I just ran a 10 Hz to 22 kHz measurement sweep at -20 dbFS and recorded the sweep signal in Reaper at the same time. Zooming way in on the REW measurement sweeps, there is zero amplitude or phase error, they are perfectly flat.

Then I played back the sweep captured in Reaper, clicking on REW's Start Measuring button just before it started, and let REW measure that recorded sweep. Zooming way in on the measurement sweeps, there is zero phase error and a tiny ripple in the frequency response between 10 Hz and 20 Hz, amounting to about 0.01 dB of error.

Then I played back the sweep captured in Reaper on computer A, via SPDIF to computer B and let REW measure that recorded sweep in computer B. Again, near zero error in the measurements, just a tiny bit of ripple in the 10 Hz region of the FR curve, about 0.01 dB.

So the recorded sweep looks good and can be used with REW accurately, but all of this has been without a calibration file, which appears perfectly legit if it is not needed.


That calibration file will be needed with the Saffire 2i2, but it seems like it needs to compensate only for the converters and output stage of your amp and for the input stage of the Saffire 2i2. Starting out with a perfect recorded sweep like the one described above, what would happen if you used that sweep and ran a calibration curve through your amplifier output, through an attenuator, back to your Saffire 2i2 input? Wouldn't the generated calibration curve now be compensating for all electronics amplitude and phase errors?

There will be a difference between the above setup with cal curve and the above setup using a regular cal file for the Saffire 2i2, and that difference is in the inclusion of the Saffire 2i2 output stage rather than your amp output stage in the cal file. As John points out, it should be a small difference, a 3 dB peak indicates a larger error than I would expect to see.

Hope this all makes a little sense, and also hope I haven't totally missed an important point along the way.

Edit: Late to the party and totally out of sync, that's me. Glad you found the problem. It was an interesting exercise thinking through your situation anyway.
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post #6 of 6 Old 02-22-15, 09:17 PM Thread Starter
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Re: Difference; Internal Vs External Cal Sweep

Yes, I was trying to make the DLNA to P-Amp path measure Flat for both SPL and Phase. It seems I run into all sorts of minor issues in achieving that. I think they are all resolved now. The REW sweep file from DLNA is now flat for both SPL and Phase.

I use my analog 2i2 for all timing/delay work because loopback is needed. I also use it for EQ as it is much more convenient. The external sweep is really only needed for confirming my rePhase phase correction FIR filter that I implement in Foobar. I have always had some issues with Phase from DLNA not matching the analog sweep. Now it all seems to working fine.

I just figured out how to do that in Holm also, so now I am all set I hope. This all gets very confusing for me.
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