REW J-test measurement on RME ADI-2

[onlyoneme]

I've just used REW generator to measure j-test 96kHz on USB ADI-2 input with pre-FX loopback to USB output. And I've got something like that:

Do you think it's fine here with all those spikes?

 

[staticV3]

The spikes are part of the J-test signal.

 

[onlyoneme]

The spikes are part of the J-test signal.

What do you mean? I can read that
"The jitter test signal consists of a square wave at one quarter of the sample rate", which is 24 kHZ for 96 kHz test.

 

[staticV3]

Please post the entire quote:

The jitter test signal consists of a square wave at one quarter of the sample rate at half full scale amplitude modulated by a square wave at 1/192 of the sample rate that alternates between 0 and -1 lsb at the 24-bit level.

Main component: 96kHz / 4 = 24kHz
Secondary component: 96kHz / 192 = 500Hz

The spikes that you're seeing is the secondary square wave at 500Hz, which is part of the J-Test signal.

 

[onlyoneme]

Please post the entire quote:

Main component: 96kHz / 4 = 24kHz
Secondary component: 96kHz / 192 = 500Hz

The spikes that you're seeing is the secondary square wave at 500Hz, which is part of the J-Test signal.

So 24 kHz and 500 Hz can be clear, but what about 1.5 kHz, 2.5, 3.5 etc? Odd harmonics of 500 Hz?

 

[staticV3]

That's what a square wave looks like when you run it though the FFT. One fundamental sine wave plus an infinite amount of odd harmonics.
You can learn about that here:

 

[onlyoneme]

That's what a square wave looks like when you run it though the FFT. One fundamental sine wave plus an infinite amount of odd harmonics.
You can learn about that here:

Thx for those movies, I'll watch them carefully.

I've generated simple square wave in REW and I've seen indeed those odd harmonics as well. So if I understand correctly, odd harmonic are completely expected to be here in case of square wave because of Fourier series approximation, right? Which means j-test case too, I guess.

 

[pma]

The spikes are part of the J-test signal.

Correct, speaking about 16-bit jitter test signal. It is 11025Hz sine + 229,6875Hz toggle square least significant bit for 44.1/16 format.

 

[restorer-john]

Don't link the square wave wave testing of audio products- it's got some serious errors.

The reason the 100Hz square wave slopes down at this point:

Has nothing to do with not having enough harmonics as Amir states. The reason is he has set the the AC coupling on the Audio Precision. (~10Hz)

A downward slope on the top of a square wave is caused by inadequate low frequency extension, not a lack of harmonics. The commentary around this is completely 100% wrong. Maybe if he'd spent more time actually using 100Hz square waves he'd understand the CD format makes a virtually perfect 100Hz square wave.

Here's an actual 100Hz square wave out of a CD player. Note: DC coupled...

Here's a nice visual:

 

[Blumlein 88]

I would also mention that loopbacks in the same device won't show jitter because the same clock is being used for ADC and DAC. Only with separate clocks will you see jitter.

So a loopback with locked clocking shows you what a no jitter J-test would look like.

 

[morillon]

these precautions are important many cases concerning the dc -ac
but for all that it seems to me that will have little impact in jtest measurement with a square at more than 200hz... right?
(
on the other hand, I have more and more doubts about the source jitter observation measurements of sources....
very simple because linked to nature, efforts,
done at the toslink spdif digital inputs....
if this is important, the acquisition is done after the pll etc, the observation becomes not significatives.. in any case there will be a leveling of the capacities of measurements...
What do you think ?)

 

[Rednaxela]

Perhaps change the thread title from

REW J-test measurement on RME ADI-2​

to

REW J-test measurement questions/guidance​

?

 

[onlyoneme]

Perhaps change the thread title from

REW J-test measurement on RME ADI-2​

to

REW J-test measurement questions/guidance​

?

Not sure if it changes anything but no problem if you tell me how to do that.

 

[onlyoneme]

I would also mention that loopbacks in the same device won't show jitter because the same clock is being used for ADC and DAC. Only with separate clocks will you see jitter.

So a loopback with locked clocking shows you what a no jitter J-test would look like.

ADC wasn't involved here at all as external signal on DAC USB input has been passed directly to USB output to allow capture on my PC. Not sure if it's a kind of loopback you were referring about.

 

[Rednaxela]

Not sure if it changes anything

Good question. I guess to take away the suggestion that RME ADI DACs need to be J-tested or that doing so would reveal anything yet unknown about them.

but no problem if you tell me how to do that.

Don't know. Might take a mod, but please don't worry too much about it.

Sorry for bringing it up. Can't help myself this morning.

 

[amirm]

I've just used REW generator to measure j-test 96kHz on USB ADI-2 input with pre-FX loopback to USB output. And I've got something like that:

So you are just looping back digital samples? If so, then you are (correctly) decomposing the j-test signal. It has nothing to do with Jitter in RME.

 

[onlyoneme]

So you are just looping back digital samples? If so, then you are (correctly) decomposing the j-test signal. It has nothing to do with Jitter in RME.

I was rather interested in investigation of interface jitter when feeding my DAC. Is it an incorrect assumption?

 

[onlyoneme]

If I understood correctly those spikes below the yellow line are part of the test signal:

But what about those ones with question marks?

 

[Blumlein 88]

Can you quite specifically describe the signal flow? Are you feeding a signal to the RME DAC, and capturing the result with the built-in sound input to your PC? Or is it something else.

 

[onlyoneme]

Can you quite specifically describe the signal flow? Are you feeding a signal to the RME DAC, and capturing the result with the built-in sound input to your PC? Or is it something else.

I think it's like you said above. I feed my DAC on its spdif or usb input with the external signal. And I use one of possible ADI-2 options to pass it directly to the usb output to be able to capture it on my PC.