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Ukko Kari

Sound card and measurement mic upgrade

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Eventually ( after the new year ) I will be looking at a new sound card and a measurement mic. Currently I have a Behringer ECM8000, and UCA-202 sound card.

From this post on the HT Shack, it appears the self noise of the ECM8000 is higher than I would like to see:

http://www.hometheatershack.com/forums/spl-meters-mic-s-calibration-sound-cards/10224-ecm8000-microphone-measuring-techniques-usage-discussion-10.html#post175019

I can pick up the Focusrite Scarlett 2i2 without breaking the budget, and from measurements that notnyt posted, looks like a great interface.

http://www.avsforum.com/forum/155-diy-speakers-subs/1780914-measuring-amplifiers.html

Any solid recommendations for mics that aren't stupid expensive, yet have decent response to at least 20khz? Low self noise and the ability to use common 48v phantom power would be great. 

 

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53 minutes ago, Ukko Kari said:

I can certainly wait a bit longer to grab something in that price range. Have a link?

http://www.rationalacoustics.com/store/microphones/isemcon-emx-7150.html

I have the EMM-7101 but it's not quite as "plug and play" as the 7150 because it requires a cable to convert from traditional XLR.

What's great about these mics is the FR.  For bass frequencies you don't even need a calibration file, and they can handle far greater SPL levels than the typical ECM/EMM/UMM/UMIK stuff.  

You'll have to research the noise floor, though.  Not something I've looked into much.  

 

Anything from EW is also fantastic:

http://www.rationalacoustics.com/store/microphones/earthworks-s30.html

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I have done some more research on sound cards, and came up with some interesting information. The Behringer UCA-202 has reversed polarity on both the input and outputs, the only one in the test to exhibit this phenomenon, according to this website:

http://www.daqarta.com/dw_gguu.htm

Polarity:

The Polarity tests were done using a separate scope. The Generator was set to produce a biphasic Pulse waveform, such that the positive phase preceded the negative phase, and there was a dwell time at zero before the next pulse.

The output had normal polarity if the positive phase appeared first on the scope. A loopback cable was then used to feed the output back to the input, and if the Input waveform seen by Daqarta also showed the positive phase first, the input likewise had normal polarity. This was the case with all devices except the Behringer UCA202, which inverted both the output and input (which means that it could not have been detected with a loopback alone).

 

Read this thread on DIY audio, might have second thoughts on the Focusrite Scarlett 2i2 sound card.

http://www.diyaudio.com/forums/equipment-and-tools/301166-focusrite-scarlett-2i2-2nd-gen-measurements-whats.html

 

 

 

This thread has me looking at another Behringer:

http://www.diyaudio.com/forums/equipment-and-tools/312454-usb-audio-interface-measurement.html

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Reversed polarity is pretty common with a variety of electronic devices and is easy to fix in measurements.  My Motu 16A reverses polarity.  I don't think it's a big deal, and I'm not aware of any empirical evidence that polarity inversion is audible at all, as long the polarity is consistent for all output transducers.  The exception would be for crossovers that are designed to be inverted. 

Though this does highlight a more general problem when using systems with speakers of different types, even if they are from the same product line.  The issue is not really timbre matching but rather phase matching.  When the crossovers are not identical, there will be phase mismatch over at least some of the range, and this definitely is audible.  I have discovered that multiple speaker integration in a system with biquad EQ capability can be improved by using all-pass filters.

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Regarding reversed polarity, consider the example of simply using the device for playback:

Instead of a forward motion on the speaker diaphragm, you will have a rearward movement. This will change our perception of the sound of a completed system if this is not corrected, especially in the bass, not so much in the region where wavelengths are short, and small head movements will cause many rotations of phase.

 

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31 minutes ago, Ukko Kari said:

Regarding reversed polarity, consider the example of simply using the device for playback:

Instead of a forward motion on the speaker diaphragm, you will have a rearward movement. This will change our perception of the sound of a completed system if this is not corrected, especially in the bass, not so much in the region where wavelengths are short, and small head movements will cause many rotations of phase.

 

Floyd Toole has a short section on the issue of absolute polarity in Sound Reproduction 3rd ed. in which he cites two studies.  One of them appears to be substantially flawed, relying exclusively on LP records.  The second, relying on CDs concluded "polarity inversion is not easily heard with normal complex musical program material [... but] is audible in many select and simplified musical settings."

However, Dr. Toole also notes that "there appears to be no audio or film industry standards that ensure the deliver of suck absolute polarity sounds from the microphones, through the extensive electronic manipulations in control rooms or dubbing stages, which may be different for different components of a mix, and finally through the playback electronic and loud-speakers at home."

Going from my experience, my own playback system has been configured with both absolute polarities, and I've never noticed a different myself, including with bass.

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From Ethan Winer:

http://ethanwiner.com/myths.html

Myth: Absolute microphone or speaker polarity makes an audible difference.

Fact: While nobody would seriously argue that it is okay to reverse the polarity of one signal in a stereo pair, I've never been able to determine that reversing the polarity of one signal - or both if stereo - ever makes an audible difference. Admittedly, it would seem that absolute polarity might make a difference in some cases, for example, when listening to a bass drum. But in practice, changing the absolute polarity has never been audible to me.

You can test this for yourself easily enough: If your console offers a polarity-reverse switch, listen to a steadily repeating bass drum hit and then flip the switch. It is not sufficient to have a drummer go into the studio and hit the drum while you listen in the control room, because every drum hit is slightly different. The only truly scientific way to compare absolute polarity is to audition a looped recording or drum sample, to guarantee that every hit is identical.

Important Update: Mike Rivers from Recording magazine sent me a test Wave file that shows absolute polarity can be audible in some circumstances. The polarity.wav file (87k) is a 20 Hz sawtooth waveform that reverses polarity in the middle. Although you can indeed hear a slight increase in the low end fullness after the transition point, I'm still not 100 percent certain what this proves. I suspect what's really being shown is a nonlinearity in the playback speaker, because with a 50 Hz sawtooth waveform there is no change in timbre. However, as Mike explained to me, it really doesn't matter why the tone changes, just that it does. And I cannot disagree with that.

More Update Info: After discussing this further with Mike in the rec.audio.pro newsgroup I created two test files you can download and audition yourself. The Kick Drum Wave file (324 KB) contains a kick drum pattern twice, with the second reversed. Play it in SoundForge or any audio editor that has a Loop mode, so you can play it continually to see if you hear a difference. The Voice Wave file (301 KB) is the same but with me speaking, because Mike says reversing polarity on a voice is surely audible. I don't hear any difference at all. However, I have very good loudspeakers in a room with proper acoustic treatment. As explained above, if your loudspeakers can't handle low frequencies properly that could account for any difference you might hear.

 

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Interesting.  Thanks for posting this.  I cannot hear any difference on the kick drum sample.  On the 50 Hz sawtooth, I am hearing differences, but I suspect this test is flawed.

First of all, the most distinct difference is at the inversion points.  There is only one inversion point in the sample itself.  The other inversion point is introduced by repeating the waveform.  I used Audacity to repeat the waveform into a longer sample to avoid any artifacts from the playback program itself, but it had no effect on the sound.  Depending on how the waveform was generated, there may be an extra sample at the end that should be omitted when repeating it.  I think I can hear a very slight difference in the continuous part, away from the inversion points, but it's really too short for me to tell for certain.  The perceptual influence of the inversion points seem to be very strong and have lingering effects.

There is another potential problem.  The wave form data reaches full-scale in a number of places at both the (+) and (-) ends.  A 16-bit digital waveform is inherently asymmetric in the sense that full-scale on the negative side is about 0.0027 dB higher than on the positive side.  More significantly, the true waveform peak may actually exceed full-scale leading to an amplification of the asymmetry.  Also, it's possible that the waveform was not merely peak-normalized after its generation but was clipped during the generation itself.  One reason I have to believe this is that when viewing the waveform in Audacity, the pattern of peaks is irregular about each full-scale peak.  This suggests that the inverted waveform is not a perfect mirror image of the original.

I could try generating a cleaner waveform, but I don't know how important this is.  I would not be surprised if absolute polarity was inaudible in reality, and those occasions where people can hear it are due to unexpected asymmetries in either the source material or in asymmetric distortion in the recording and/or playback system.

Edit: The sample rate of polarity.wav 11025 Hz.  If the wave is precisely 50 Hz, then the waveform pattern should precisely repeat every 2 cycles because (50 / 2 = 25) divides evenly into 11025.0.  This waveform is obviously flawed in a number of ways for the purpose of this test.

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Just an update, I was gifted a Behringer UMC204HD at xmas. I could not get it to play nice with Windows 10, even with the latest drivers, not sure if it was defective or not, but searching the internet other people had similar experiences, swirling garbled audio, regardless of connection. It went back to the vendor, and there is a credit to use up.

Still thinking of the Scarlett 2i2 2nd gen, any other suggestions?

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