technical noise measurement question
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 10th January 2008, 11:35 PM #1 cuibono   diyAudio Member     Join Date: May 2006 Location: City of Angles technical noise measurement question Hello all - I asked this in Digital, and got no reply, so maybe I'll get lucky here if I rephrase it: I'm measuring the noise floor of an USB ADC (the zoom H2) with Visual Analyzer, which is a software oscilloscope, with a built in spectrum analyzer. The spectrum analyzer (broadband, 20-20kHz) shows noise down at -120dB all the way across. The scope gives a single number readout of the signal, saying it is about -75dB. How do they get between the two numbers? I'm assuming you have to do some sort of power integration, the math of which is beyond me, but I hope someone could give me a simple explanation of why the numbers are different. But more so, can someone tell me if my noise floor is at -120dB or -75dB? Thanks __________________ double complete rainbow all the way!!
 10th January 2008, 11:45 PM #2 SY   On Hiatus     Join Date: Oct 2002 Location: Chicagoland You have it. The total noise is an RMS summation of the noise energy contained within the specified bandwidth. It's usually calculated by separating the bandwidth into a number of bins of roughly constant noise density, then RMS summing the noise density in each bin times the bin's bandwidth. The other issue is "dB with respect to what." -120dB is probably the noise with respect to full scale of the ADC. -75dB is probably the noise with respect to some signal that the scope is locked into or perhaps with respect to some standard level. I'm just speculating on that, but you do have to be careful to use the same reference in the dB calculations. __________________ "You tell me whar a man gits his corn pone, en I'll tell you what his 'pinions is."
 11th January 2008, 12:32 AM #3 cuibono   diyAudio Member     Join Date: May 2006 Location: City of Angles Thanks! The software seems well designed, but I can't figure its units out - the spectrum analyzer is labeled 'dBpp', and the scope just 'dB'. It seems like dBFS would be the appropriate unit. Also, I clipped the signal at 0dB on the analyzer, suggesting it is showing dBFS, and the scope was showing +/-1, which is either V or %, neither of which quite make sense... __________________ double complete rainbow all the way!!
 11th January 2008, 12:39 AM #4 cuibono   diyAudio Member     Join Date: May 2006 Location: City of Angles Interestingly, the spectrum analyzer noise floor drops when I change the number of samples for the FFT. Maybe that shouldn't be surprising though. My only real concern is if the device under test is too noisy. My standard soundcard shows a noise level of -87dB, and I have had no problems with noise. Now the Zoom H2 (which is incredibly well designed overall) has a noise floor of -75dB, and initial listenings with headphone show an unacceptable level of noise. Where the noise is coming from, I haven't yet determined. By the way, if the noise turns out to not be a problem, I would recommend the H2 highly. It is extremely well thought out, and makes recording very easy. Assuming it all works out, I plan on making my own test recordings - there is no better way to compare the reproduction if you know the original.... __________________ double complete rainbow all the way!!
 17th January 2008, 03:44 AM #6 Iain McNeill   diyAudio Moderator Emeritus     Join Date: Oct 2007 Location: Santa Cruz, California As SY says, the -75dB(FS) is the sum of all the noise from the lowest frequency bin of the FFT (=sample rate / fft size) up to nyquist (=sample rate / 2) As you divide this noise energy into more and more bins you get less and less in each bin. So if you divide the -75dB into 2 bins you'll get -78dB in each (we're talking power so only a 3dB drop), 4 bins -81dB in each etc etc etc. That's why a noise figure only has meaning if you know the bandwidth.
 17th January 2008, 03:48 AM #7 jackinnj   diyAudio Member     Join Date: Apr 2002 Location: Llanddewi Brefi, NJ to get an idea of the noise which is relevant to listening you should download the ITU CCIR ARM-486 paper -- the CCIR curve corresponds most acurately to the annoying aspects of noise (at least according to Dolby) -- and as I have stated on previous occasions, there is a very high R^2 to the pitch used by a screaming wife.
 20th January 2008, 05:08 PM #9 EC8010   diyAudio Moderator Emeritus     Join Date: Jan 2003 Location: Near London. UK It's as well to remember that the only certain thing about a 24 bit ADC is that it produces 24 bits. On most of them, at least the four least significant bits are rubbish. __________________ The loudspeaker: The only commercial Hi-Fi item where a disproportionate part of the budget isn't spent on the box. And the one where it would make a difference...
jackinnj
diyAudio Member

Join Date: Apr 2002
Location: Llanddewi Brefi, NJ
Quote:
 Originally posted by EC8010 It's as well to remember that the only certain thing about a 24 bit ADC is that it produces 24 bits. On most of them, at least the four least significant bits are rubbish.
bit 24 is 59.6nV, bit 23 is 119.2nV -- that's a pretty big gulp of water.

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