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Author Topic: SQ/AR2412 Traces  (Read 1229 times)

Mark Wilkinson

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Re: SQ/AR2412 Traces
« Reply #10 on: July 24, 2018, 11:00:07 am »

I think I found what you are talking about.

http://www.rationalacoustics.com/forums/showthread.php?1094-ref-to-gen-delay-time-problems

"Transfer Function measurements can also be referenced internally to Smaart's signal generator by enabling "Allow Multi-Device Transfer Function"

Mark, good discussion. I agree with your math. 1/8" seems about right.

Does that mean that we can't measure up to 20kHz phase response accurately? Or am I missing something with the REW discussion?

Can someone explain in more layman's terms how subsampling works? Is it simply using interpolation between samples to obtain the expected value?

Nathan, excellent find.   I had no clue that's what Multi Device Transfer could do..
Seems I need to be hitting the manuals harder !

I think with Smaart, we can measure 20kHz accurately, to no worse than 1/2 sample off, at 48kHz (assuming we are using MTW transfer windowing with its 48k requirement.)
So this is the same 75 degrees from 96k sampling numbers above....which means to me, and I see every now and then based on random luck with mic position, that if the mic just happens that it's positioned halfway between inferred sample timing, the phase trace at 20kHz will tail upwards from flat by about 30 degrees, or downwards by 30 odd degrees, as we bump delay finder one sample.

Obviously a sample off is no big deal in the real world at all...heck, I was trying to locate a line array last week and couldn't even pin the damn thing to within 30 samples.   
I just love precision and knowing how things really work....

My take on sub-sampling is that it just re-samples the impulse response, keeping the mag and phase relationship through the spectrum the same, while forcing the very upper end of the freq spectrum (nyquist freq maybe) to zero degree phase.
But that take is just one hell of a layman's guess ....
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Nathan Riddle

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Re: SQ/AR2412 Traces
« Reply #11 on: December 04, 2018, 08:52:02 pm »

Brought upon by these developments:

https://forums.prosoundweb.com/index.php/topic,169366.msg1562589.html#msg1562589

Qu to SQ to QU measurements.
And
Qu to SQ measurements.

QU-IP1 > SQ-IP1 > SQ-MainL > QU-IP17 > QU-MainL
And
QU-IP1 > SQ-IP1 > SQ-MainL

Note the differences in impulse responses. There's a reason SQ sounds better...
« Last Edit: December 04, 2018, 08:55:33 pm by Nathan Riddle »
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Nathan Riddle

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Re: SQ/AR2412 Traces
« Reply #12 on: December 04, 2018, 08:52:49 pm »

SQ at various levels of pre-amp gain. It handles being buried in the red (red constantly on) very well IMO.

Eh, my Focusrite Gen2 was clipping. Might disregard those other two images.

Still... looks great IMO.

...I really need Langston's suggestion for real testing: RME Babyface Pro (stupid name)
« Last Edit: December 04, 2018, 08:58:33 pm by Nathan Riddle »
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Frank Koenig

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Re: SQ/AR2412 Traces
« Reply #13 on: December 04, 2018, 10:03:39 pm »

REW will do sub-sample alignment of impulse response

Indeed, sub-sample time adjustment can be performed by interpolation in the time domain or, alternatively, in the frequency domain by multiplying the spectrum by a linear-phase all-pass function representing an arbitrarily small time shift. The resolution of this time shift is limited only by finite register length (finite precision) effects. Why anyone would need a time shift of less than one sample, for anything other than prettying up a phase plot, is another question.

For purely electronic measurements where you're dealing with what is essentially a well behaved linear filter you call always fall back onto the old sine-wave generator and scope to sort things that are giving weird results. Fourier-based analyzers can get wonky in the top octave at fs = 48 kHz. Problems such a clipping, or other gross nonlinearities, will be obvious on a scope. A loop back test of the analyzer is always a good idea.

--Frank

PS: The cool thing is that shifting an impulse response by a fractional sample amount will make it look completely different, while shifting it by an integer number of samples will leave it unaltered except for the shift. The sampling theorem at work...
« Last Edit: December 04, 2018, 10:10:45 pm by Frank Koenig »
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Mark Wilkinson

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Re: SQ/AR2412 Traces
« Reply #14 on: December 05, 2018, 01:38:21 pm »

Why anyone would need a time shift of less than one sample, for anything other than prettying up a phase plot, is another question.



Lol...and agreed! 

The anal, gotta have precise repeatable measurements part of me, wasn't happy till I figured out why phase at 20kHz using Smaart never seemed to wanna be at zero.   At least now I know why, and not to care...

But I do think fine timing is important...I believe most of the 96kHz sampling hullabaloo being discussed in conjunction with mixers, is probably more about being able to maintain tighter phase alignment throughout all the routing, than any other audio quality issue.

Oh, in addition to REW handling sub-sample timing on the measurement side, it appears that q-sys handles it too, on the signal side.
 

@Nathan,  sorry...I'm not getting what you're seeing / trying to show, with your impulse plots.... 

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Nathan Riddle

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Re: SQ/AR2412 Traces
« Reply #15 on: December 05, 2018, 02:50:57 pm »

Lol...and agreed! 

The anal, gotta have precise repeatable measurements part of me, wasn't happy till I figured out why phase at 20kHz using Smaart never seemed to wanna be at zero.   At least now I know why, and not to care...

But I do think fine timing is important...I believe most of the 96kHz sampling hullabaloo being discussed in conjunction with mixers, is probably more about being able to maintain tighter phase alignment throughout all the routing, than any other audio quality issue.

Oh, in addition to REW handling sub-sample timing on the measurement side, it appears that q-sys handles it too, on the signal side.
 

@Nathan,  sorry...I'm not getting what you're seeing / trying to show, with your impulse plots....

The length of the impulse response 'ringing' is significantly less for the SQ compared to QU.
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