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REW J-test measurement on RME ADI-2
- Thread starter onlyoneme
- Start date
- Thread Starter
- #3
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.
- Thread Starter
- #5
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?
- Thread Starter
- #7
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.
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pma
Major Contributor
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
Grand Contributor
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:
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:
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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.
So a loopback with locked clocking shows you what a no jitter J-test would look like.
morillon
Major Contributor
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 ?)
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 ?)
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- Thread Starter
- #13
Not sure if it changes anything but no problem if you tell me how to do that.
- Thread Starter
- #14
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
Major Contributor
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.
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.
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.
- Thread Starter
- #17
I was rather interested in investigation of interface jitter when feeding my DAC. Is it an incorrect assumption?
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.
- Thread Starter
- #20
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.
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