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noobie question :)

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#1
Hi,

I try the great free software 'rew', but i'm a noobie :

Measurement after MOTU 8A DAC (after ADC calibration - loopback ):

Capture d’écran 2019-11-09 à 12.23.18.png


And RTA measurement of same 1000 Hz at 90 DBA, after DAC, after ADAU 1701 CROSSOVER in RCF HD-32 active speakers, and after CLASS-D amplifier of those active speakers (several step THD audio degradation) : on air (with REW and UMIK-1 MIC USB) in my living room :

Capture d’écran 2019-11-09 à 14.15.54.png



Is the THD "on air" enough good ?

Regards,

Olivier
 

MediumRare

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#2
Others can give you a technical answer, but for just this one tone, yes, it’s below any audible level. Some say 1% is the threshold, some 0.5%. You’re at 0.19% What is your full spectrum (20-20) THD?
 
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#3
Others can give you a technical answer, but for just this one tone, yes, it’s below any audible level. Some say 1% is the threshold, some 0.5%. You’re at 0.19% What is your full spectrum (20-20) THD?
Hi,

Thanks :)

I search for full spectrum in REW.... But happy that for 1000 Hz, it's not bad at all. :)
 

pozz

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#4

MediumRare

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#6
Are you sure about that? It looked to me you are comparing component measurements with in-room measurements. I may be wrong about that.
Right : :)
Of course, i compare the degradation between original component measurements (loopback) with in-room measurements (RTA mic on air ), DSP/AMPLIFIER/HP add NOISE.....and Harmonics
 
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sergeauckland

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#8
One thing to bear in mind about in-room THD measurements is that the room response affects what you measure. If, for example, the room (or an uncalibrated microphone) has a rise in response at 2kHz, then the 1kHz THD measurement will show a worse number than if the room or microphone were flat. Similarly of course, if the room's response drops at 2k, you'll get better figures.

Without a calibrated room or loudspeaker, it's hard to know what is the real number. Regardless, your THD number is pretty good, so nothing to worry about.

I had the same problem when I tried to measure the THD of my 'speakers

S
 
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#9
One thing to bear in mind about in-room THD measurements is that the room response affects what you measure. If, for example, the room (or an uncalibrated microphone) has a rise in response at 2kHz, then the 1kHz THD measurement will show a worse number than if the room or microphone were flat. Similarly of course, if the room's response drops at 2k, you'll get better figures.

Without a calibrated room or loudspeaker, it's hard to know what is the real number. Regardless, your THD number is pretty good, so nothing to worry about.

I had the same problem when I tried to measure the THD of my 'speakers

S
Thanks :)

I understand that with a good calibrated room (the mic is already calibrated and flat), the THD could be better. So it's already not bad at all. It's promising.
I search to identify the quality of plate CLASS D amplifier of my active RCF HD-32A speakers.
I know it's an ADAU1701 for CROSSOVER AD/DA (not a great dac), I think it could be ICE POWER amplifier module, but i don't know.

Regards,
 

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#10

MediumRare

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#11
One other thing to consider is the contribution of the UMIK-1. A quick anechoic measurement of its noise floor was posted here: https://www.minidsp.com/forum/umik-questions/9266-noise-floor#9909

I haven't seen similar distortion measurements posted anywhere, or its response to specific SPL.
Can you explain what we are seeing? I'm confused by the vertical axis and the spikes at 1k, 2k, etc. Is that all noise or is it measuring a very low 1 k tone, or are those spikes from the USB or....?
 

pozz

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#12
Can you explain what we are seeing? I'm confused by the vertical axis and the spikes at 1k, 2k, etc. Is that all noise or is it measuring a very low 1 k tone, or are those spikes from the USB or....?
The vertical axis is in dB SPL. What you're seeing is the self-noise of the UMIK-1 in the RTA window of REW with nothing playing (using a 48kHz sampling rate). USB packet noise typically results in an 8kHz spike. Everything else belongs to the UMIK-1. It has a lot of circuitry built into its housing so that spectrum is to be expected.
 

sergeauckland

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#13
Can you explain what we are seeing? I'm confused by the vertical axis and the spikes at 1k, 2k, etc. Is that all noise or is it measuring a very low 1 k tone, or are those spikes from the USB or....?
I'm also confused by those spikes. They shouldn't be there, so there's something in that room (or bleeding in from outside) generating a low level 1kHz tone, plus its harmonics. I'm pretty certain microphone can't generate that sort of noise at specific frequencies.

S
 

pozz

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#14
I'm also confused by those spikes. They shouldn't be there, so there's something in that room (or bleeding in from outside) generating a low level 1kHz tone, plus its harmonics. I'm pretty certain microphone can't generate that sort of noise at specific frequencies.

S
If you look up complaints about the UMIK-1 on the miniDSP forums you'll see those strange spikes referenced. No idea what's actually causing them.
 

sergeauckland

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#15
If you look up complaints about the UMIK-1 on the miniDSP forums you'll see those strange spikes referenced. No idea what's actually causing them.
Had a brief read, but still not convinced the mic is generating those spikes. They are too precise at 1kHz to be anything 'natural' which makes me wonder if the mic was picking up from the laptop it was connected to or anything ambient like a 1k tone elsewhere in the building breaking through. SMPS breakthrough is common, but at a much higher frequency.

I don't have a UMIK as I use a calibrated ECM8000, but certainly that doesn't have any spikes, nor does any other microphone I've ever used for recording.

S
 

pozz

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#16
https://www.minidsp.com/forum/umik-questions/8678-re-low-quality-design-or-faulty-mic

This is the thread is one I had in mind. The first one I posted has a staff member admitting in response to a question by @edechamps that a batch of UMIK-1s had productions issues. From the anechoic measurements it seems that they addressed them, but not completely.

I doubt it's the microphone capsule itself. Maybe it's poor upstream separation, but then the housing has a preamp, DAC ADC and USB board, all digitally controlled.
 
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sergeauckland

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#17
https://www.minidsp.com/forum/umik-questions/8678-re-low-quality-design-or-faulty-mic

This is the thread is one I had in mind. The first one I posted has a staff member admitting in response to a question by @edechamps that a batch of UMIK-1s had productions issues. From the anechoic measurements it seems that they addressed them, but not completely.

I doubt it's the microphone capsule itself. Maybe it's poor upstream separation, but then the housing has a preamp, DAC and USB board, all digitally controlled.
Yes, but it doesn't explain why the 1kHz spikes. All that a USB microphone has is a preamp, and ADC and a USB driver of some sort. DSP is optional. What generates low level 1kHz spikes? Would anyone be daft enough to leave a 1kHz tone generator running internally in the DSP? What for? I'm not convinced by any of this .

S
 

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#20
How can SPL be negative?
0 dB SPL refers to a 20 micropascal variation in atmospheric pressure (standard at 101.325 pascals).

If you look at the equal loudness contours, you'll see that our sensitivity is at its maximum at 2kHz— 4kHz, which dip below 0 dB. Threshold of pain, 120 dB SPL, is 20 pascals. There is of course individual variation, but dynamic range of hearing is basically 1000000:1.

Do you feed the ECM8000 into a laptop and, if so, how do you do that? I have one with an XLR out. Also, do you know how much changed with the calibration? Mine isn't and I don't have a serial number.
Cross Spectrum Labs does calibrations of measurement microphones. This is their snapshot of the various curves that they've produced for the ECM8000.

download.jpg
 
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