Using Cross Corelation to lower influence of ADC for DAC measurements

[KSTR]

Of course. I didn't mean the adc chips themselves. I meant the cosmos without dac connected and with its xlr inputs shorted. If the noise of the 2 channels is primarily uncorrelated, would it not tend towards 24 bits 144dB?

It would go to -infinity after infinite amount of time if the noise is completely uncorrelated.
In a "quick" test it took about 20k averages to go below -144dBFS (20Hz...20kHz bandwidth).

 

[KSTR]

It is possible to make
the measurement of the 2 channels in delayed time.

But that de-correlates (and thus reduces) the DUT's noise, which typically is not what we want.

 

[KSTR]

This would mean 2 sliding notch filters that match fairly well (phase is my concern in case the notch is deep).

Yes, the lower harmonics might see a phase difference between channels when the notches are not 100% the same. But as far as I have seen even a 10 degree delta does not cause any significant error.

 

[FrenchFan]

Yes, that's probably it.
Grade 0 should be -129dBFS A with open inputs in mono mode.
So -126 or so in stereo.
Could be lower with input shorted, but I don't know how much.

This has nothing to do with CC, though.

If a little bit.

When we have this noise value, which I grant you,
is only an approximation of the noise on a real measurement,
We can with 5*log(N) approximate the noise floor of the ADC

For jamesdyson with 6346 correl, so -19 dB of correlation gain
on his last measurement

We can say that the noise of the ADC = -123.49 -19 = -142.49 dBFS

Like this we know approximately the distance between the noise
of the ADC and that of the measurement.

For jamesdyson he has left if we look at his last measurement
DAC noise (-134dB) - ADC noise (-142.49) = 8.5 dB of noise margin
his measurement really tends towards an increasingly accurate value.

 

[FrenchFan]

But that de-correlates (and thus reduces) the DUT's noise, which typically is not what we want.

Right, there I am bad
Thank to correct

 

[FrenchFan]

Yes, the lower harmonics might see a phase difference between channels when the notches are not 100% the same. But as far as I have seen even a 10 degree delta does not cause any significant error.

Isn't the phase corrected on the notch filter?

 

[nanook]

Yes, the lower harmonics might see a phase difference between channels when the notches are not 100% the same. But as far as I have seen even a 10 degree delta does not cause any significant error.

Using two notches that do not match properly would thus mainly introduce errors in the frequency region of the fundamental (because here the phase walk is pretty steep, particularly for a deeper notch than the moderate 30dB the APU is adjusted to).

The calibration in REW does account for amplitude and phase:
-> The noise of the source gets amplitude and phase aligned for both channels (good resolution in the notch region presumed)
(If I remember correctly, the calibration in REW is common for all channels, so this isn't possible as-is, right?)
The ADC noise is still uncorrelated.

So with an individual calibration of both channels even this should work in principle.
(This basically is what I meant for a possible solution for the AP, just that it would have to cover the frequency range)

 

[jamesdyson]

jamesdyson has a grade "0" ADC

The noise curve represents, I think,
the measurement of the noise, with input at "0" in
stereo mode, this corresponds to 123.49dBFS / 125.6 dBFSA

Which corresponds to the noise for the cross correlation
measurement.

In "mono" mode we will certainly see ~ 128 dBFSA

On my iso B I have ~ 122 dBFS ~ 124dBFSA in "stereo"

@jamesdyson, I noticed that by increasing the
sampling frequency we measure a slightly better ADC noise.

Try at 192Khz.

Yes I tried 192, sanes 96kHz. I don't have an explanation for the improvement from 48kHz.

 

[jamesdyson]

OK, so what we see us the amount of correlate noise between the 2 channels.
Not sure how usefull that is, though.
Dud you try the same in 32 bits ?

Yes. It was simply a demonstration of the effectiveness of the cross correlation. The ADC noise is effectively eliminated.

 

[Rja4000]

Yes I tried 192, sanes 96kHz. I don't have an explanation for the improvement from 48kHz.

This is documented by Yvan.
The ADC chip has slightly better performance when a bit hot, and increasing sampling frequency makes it heating.
He even sold, at some point. a resistor to heat the chip and regulate its temperature.

(Edit: still sold here)

 

[jamesdyson]

It would go to -infinity after infinite amount of time if the noise is completely uncorrelated.
In a "quick" test it took about 20k averages to go below -144dBFS (20Hz...20kHz bandwidth).

Is it correct to allow it to go below the fundamental bit depth of the system?

 

[KSTR]

Is it correct to allow it to go below the fundamental bit depth of the system?

Yes.

 

[FrenchFan]

Doesn't Apx have a measurement without notch filter??

In this case it could make the measurement in 2 passes

One for the precision of the THD (with notch)
The other for the precision of THD+N (without notch)

???

 

[nanook]

By the way, I have another one under test which is even cheaper and gives me this (with Notch) in stereo mode (Left channel here).

Using Cross Corelation to lower influence of ADC for DAC measurements

Thanks, true, KSTR and FrenchFan. Both ADC channels see a linear combination of the signal and their own imperfections. This is what the actual cc process sees. So the signal distortion and the individual ADC distortion can add such that e.g. the digitized data of one ADC channel has the same...

audiosciencereview.com

@Rja4000: Can you share the information which DAC you have measured in this post?

 

[Rja4000]

Virtins published a paper on those techniques and more