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Using Cross Corelation to lower influence of ADC for DAC measurements

Would it be fair to say that Amirs AP555 is now being outperformed by these top end dacs? It's limit being below 124dB.
 
Would it be fair to say that Amirs AP555 is now being outperformed by these top end dacs?

The DAC based generator in the APX555 is easily outperformed by many DACs, but the high perfrormance analogue sine generator in Amir's particular unit is, from what I can see, a particularly good one. THD is off the chart low. Noise is the limitation now.

The analyzer generators can of course drive to much higher levels (~26V) and that means there is gain. With gain comes some noise at those high levels.
 
Would it be fair to say that Amirs AP555 is now being outperformed by these top end dacs? It's limit being below 124dB.
Hi jamesdyson

Thanks for the feedback.

When the correlation converges CC and CCmagnitude
get closer I see that the diff ~0.3db.

Here we can say that the measurement is correct,
and that your DAC really has THD+N = -130dBFS
and a little more in dBFSA.

Concerning the APx 555 for the THD+N it's over for him,
but concerning the THD he is still in the race
with his system with 2 ADC + sliding filter,
in addition he does a lot of other things.

I think like @restorer-john

We need to tackle the distortion measurement like APx does
we have 2 ADCs on the E1DA cosmos, now we need ideas
+ an E1DA cosmos APU that filters at 1000 and 10000Hz to at least
make these measurements on 2 frequencies .

Anyway I think APx will have a bell ringing about
cross correlation and that the implementation is only a matter of
time .

It can do the same principle today as with
the E1DA cosmos APU to gain dB for the THD+N measurement,
it has notch filters .
 
The DAC based generator in the APX555 is easily outperformed by many DACs, but the high perfrormance analogue sine generator in Amir's particular unit is, from what I can see, a particularly good one. THD is off the chart low. Noise is the limitation now.

The analyzer generators can of course drive to much higher levels (~26V) and that means there is gain. With gain comes some noise at those high levels.
Agreed. The AP ADC does also seem to be a limitation. It looks like the E1DA can also outperform it even without cross correlation.
 
.

Concerning the APx 555 for the THD+N it's over for him,
but concerning the THD he is still in the race
with his system with 2 ADC + sliding filter,
in addition he does a lot of other things.
Yes of course, the E1DA is not a complete measurement system and the AP does many additional and necessary things.
However it cannot accurately measure the best DACs.
 
Agreed. The AP ADC does also seem to be a limitation. It looks like the E1DA can also outperform it even without cross correlation.
The AP does all this with proper input protection and gain ranging.

If it's about sine or sweeps, the tracking notch of the AP is a great feature.

If it comes to multitone, the E1DA ADC will likely beat it - for sure when using cross-correlation.
 
Anyway I think APx will have a bell ringing about
cross correlation and that the implementation is only a matter of
time .
Ideally, one would include the whole signal chain from the DUT downstream into the CC process, that is, effectively two identical channels strips with input stage, notch and makeup gain going into a stereo ADC to fully exploit CC noise reduction. As mentioned earlier, I don't know if APX series has dual notches and gain stages, though. The older range of analyzers don't.
 
The AP does all this with proper input protection and gain ranging.

If it's about sine or sweeps, the tracking notch of the AP is a great feature.

If it comes to multitone, the E1DA ADC will likely beat it - for sure when using cross-correlation.
Yes it does have many necessary features, but its clear that Amirs unit is at the top end of the AP ADC tolerance range but can't get past 124dB. Typical is 120dB. its pretty clear that its unable to measure the best DACs accurately. Steady sine or sweep. The signal generator is superb. Again Amirs does appear to be at the top end of the tolerance range but still doesn't get past 122dB.
 
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That's simply measuring the ADC noise floor with its input shorted.
How could that work ?

And what figure are you looking at ?

-125.6 dBFS (A) ?
For a class 0, that should rather be -129dBFS (A)

Of course, it can't be -143.3dBFS A either

I really don't get it.

1724604659011.png
 
A test with Virtins Multi Instrument 3.9.11
Source DAC D50 III Mono

997Hz 0dBFS 44.1kHz
ADC RME ADI-2/4 19dBu range
FFT Size 32k, Dolph Chebyshev 200
BW 20-20k Hz
AES-17 1 octave notch

View attachment 387115

Same with REW 5._40 Beta 50


View attachment 387113

As may be seen, both measure the same SNR (0.1dB difference is not significant)

REW is measuring lower THD and therefore gets higher SINAD (more in line with Notch measurementt)
REW also converges much faster.

But it's good to see this implemented in MI still.

I re-tested with version 3.9.11.1 which adds "In phase" Cross correlation averages

1724605839602.png


Now, with that option, convergence speed and result is similar to REW's "Cross correlation averaging"
(When unchecked, it's similar to REW's CC Mangitude.)

1724606031547.png
 
How could that work ?

And what figure are you looking at ?

-125.6 dBFS (A) ?
For a class 0, that should rather be -129dBFS (A)

Of course, it can't be -143.3dBFS A either

I really don't get it.
Measuring the ADCs noise floor with cross-correlation straight away is not possible to my opinion. You need to account for the noise reduction = f(#averages).

JamesDyson probably meant the noise floor of the acquisition system (in contrast to the noise floor with the DAC connected) ?
 
Ideally, one would include the whole signal chain from the DUT downstream into the CC process, that is, effectively two identical channels strips with input stage, notch and makeup gain going into a stereo ADC to fully exploit CC noise reduction. As mentioned earlier, I don't know if APX series has dual notches and gain stages, though. The older range of analyzers don't.
This would mean 2 sliding notch filters that match fairly well (phase is my concern in case the notch is deep).
With software and thorough individual calibration this should be feasible.
 
Measuring the ADCs noise floor with cross-correlation straight away is not possible to my opinion. You need to account for the noise reduction = f(#averages).

JamesDyson probably meant the noise floor of the acquisition system (in contrast to the noise floor with the DAC connected) ?
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?
 
How could that work ?

And what figure are you looking at ?

-125.6 dBFS (A) ?
For a class 0, that should rather be -129dBFS (A)

Of course, it can't be -143.3dBFS A either

I really don't get it.

View attachment 388517
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.
 
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.

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.
 
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?
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 ?
 
This would mean 2 sliding notch filters that match fairly well (phase is my concern in case the notch is deep).
With software and thorough individual calibration this should be feasible.

"WRONG PROPOSITION"

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

It can be done with an E1DA adc, it is necessary
to record on the same channel 2 *wav at
different times (in mono)

The ADC noise will be decorrelated in time therefore on the FFT.

Well, it is necessary to double the measurement time, OK

But if you use the E1DA ADC measurement in "mono"
we gain in addition "3db" it will be a plus the day
when the DACs will go below the 135dBFS of THD + N

In addition it will be necessary to adjust only one ADC and only one
notch filter.

We can take by software take alternately on a mono signal
a block of sample and the next one and correlate them, the noise will be
different in time from one sample to another. It will certainly be necessary
to look closely at the possibility of overlap.

Well, I think there are still some ideas that will emerge.
 
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