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Really low level linearity

Blumlein 88

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#1
Had some time to look at the linearity of a couple of my devices. Was surprised at how well the devices measured out. No reason to post full results. Both were within +/- .02 db FS down to -120 db. I made some signals that go the full distance beyond just the top 20 bits. One DAC was a bit more up and down with at least 1 db swings for the bits below -120 db. The other was even better.

Here is a screen shot of the LSB. Theoretically it would be -138.47 db. Here there is a .07 db offset due to gain settings on the ADC. So it shows -138.74 which is actually -138. 67. Of course at those levels the noise of the gear cause some fluctuations of a .5 db or sometimes more. This is more or less the average over 10 seconds.

I used a 12 khz sine wave for the test signal. So the black cursor at 12 khz is what the readings on the side reference. 128 K FFT. The DAC is my Focusrite Forte being measured by an Antelope Audio Zen Tour. The Zen Tour DAC was also very good though having larger fluctuations below the 20th bit.

Forte 24 bit LSB.png


Here is the same Forte showing the 20th bit.

Forte 20th bit.png


So here are the correct levels and the average levels for my Forte.

.......................Forte

120.41......... 120.48

126.43..........126.57

132.45..........132.67

138.47..........138.67

Now these lower levels fluctuate around and this is my eyeball averaging. I should probably run them thru REW and let it do a real longer term average. But I was surprised these are so close you basically couldn't tell it from perfect for the noise. It appears to indicate excellent linearity all the way to the Least Bit though not low enough noise one should call it of 24 bit resolution. The gear might qualify for 19 bit resolution looking at noise levels. Of course this is what sigma delta DACs are able to provide. Good luck getting multi-bit R2R ladder DACs to approach something like this.

I'm not suggesting any need to regularly measure below -120db. For one thing the noise floors corrupt exact values below that point. The measurements of both DACs were very steady at -120db and above. Below that noise increasingly had some influence. This was just a lark to satisfy my own curiosity.
 
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RayDunzl

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#2
Let's see, -138dB...

If somebody shouts at you floating a meter away in free space and registers 0dB (full scale) on your shoutometer......

Then they back off another meter... -6dB... and so on...
4 m -12dB
8 m -18dB
16 m -24dB
.
4194 km -132 dBfs
8388 km -138 dBfs 5212 miles

Back to you... Over...
 

Blumlein 88

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#3
Let's see, -138dB...

If somebody shouts at you floating a meter away in free space and registers 0dB (full scale) on your shoutometer......

Then they back off another meter... -6dB... and so on...
4 m -12dB
8 m -18dB
16 m -24dB
.
4194 km -132 dBfs
8388 km -138 dBfs 5212 miles

Back to you... Over...
That implies to get really good audio performance in room you need to control all sources of sound for a diameter of 10,246 miles. Or purchase my black hole sound absorbing panels.
 

amirm

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#4
Yes, the trickiness is that the variance gets large from each capture at depths approaching and going beyond -120 dB.

That aside, you know see why I am so strict with my 0.1 dB criteria. :) It is not hard or impossible to do very well here.
 

Blumlein 88

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#6
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