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Author Topic: With smaart why do I have to loop back the output to my input?  (Read 626 times)

Carl Townshend

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With smaart why do I have to loop back the output to my input?
« on: January 18, 2021, 04:16:28 pm »

Hi all
With smaart why do I have to loop back the output to my input?
When I want to do a phase measurement do I need this loopback?
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Doug Fowler

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Re: With smaart why do I have to loop back the output to my input?
« Reply #1 on: January 18, 2021, 04:57:13 pm »

Hi all
With smaart why do I have to loop back the output to my input?
When I want to do a phase measurement do I need this loopback?

Read the rules, fix your display name.

Thank you for your cooperation. 
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Luke Geis

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Re: With smaart why do I have to loop back the output to my input?
« Reply #2 on: January 18, 2021, 09:51:56 pm »

Yes, it's how Smaart compares the data sent vs. the data received. If Smaart doesn't receive a signal back via the measurement mic, it will have no data to show you the phase relationship, spectral information of FFT data.
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Corey Scogin

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Re: With smaart why do I have to loop back the output to my input?
« Reply #3 on: January 18, 2021, 10:58:35 pm »

Comparing the measurement signal to a theoretical digital signal reference means the interface output and input are part of the measured system.
Comparing the direct electrical signal reference with the measurement signal reference takes the interface I/O out of the equation allowing Smaart to measure only the external system you want to measure.

When using 1-input mode (no electrical reference), Smaart is assuming the interface frequency and phase response are flat.

The reference signal also helps measure delay which is required for phase measurements. Smaart doesn't necessarily know the delay inherent in your interface's digital-to-analog and analog-to-digital converters. Comparing against the reference signal loop allows it to compensate for that delay accurately.

That's my understanding anyway. Others can add or correct me.
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Mark Wilkinson

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Re: With smaart why do I have to loop back the output to my input?
« Reply #4 on: January 19, 2021, 10:54:03 am »

Hi all
With smaart why do I have to loop back the output to my input?
When I want to do a phase measurement do I need this loopback?

hi Carl,

You don't necessarily have to loopback output to input to make a transfer function measurement.

The first thing to understand about a transfer function is that it is a comparison of Signal A, to Signal B.
These two signals can come from many places:  Smaart's internal signal generator, a line out of a mixer or processor, a microphone,..... just to name a few.

Doesn't matter, it's always just a comparison of A (reference channel) to B (measurement channel)


If you decide to use Smaart's signal generator output as the Reference signal,  you need to send the Signal Generator's output  to the Reference Channel input.
This is where loopback comes in: it's just an easy way to do that right at the soundcard.

Alternatively, you could send the Signal Generator's output to a mixer without using a loopback (at the soundcard).
And send an output from the mixer back to the soundcard's Reference Ch. input. 
Accomplishes the same thing as loopback.


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Michael Lawrence

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Re: With smaart why do I have to loop back the output to my input?
« Reply #5 on: January 19, 2021, 11:28:02 am »

Comparing the measurement signal to a theoretical digital signal reference means the interface output and input are part of the measured system.
Comparing the direct electrical signal reference with the measurement signal reference takes the interface I/O out of the equation allowing Smaart to measure only the external system you want to measure.

Exactly. Smaart does have the ability to reference the internal generator directly (software loopback) rather than a hardware loopback with a cable. The reason we don't recommend that is twofold. First, as you pointed out, Corey, the latency of the interface / driver is inside the measurement loop (and on Windows machines, certain drivers have a different latency each time they're initialized, so that's fun). But more importantly, using a software loopback, the measurement and reference signals are now on two different clocks, and you will see that clock drift manifest in your measurement, which opens the door to a whole range of unpleasantries. Results vary based on the specifics of the rig but in general it's best avoided if you have a better option. I use a short 1/4" right angle guitar pedal patch cable to go from output to input with no fuss.

Quote
When using 1-input mode (no electrical reference), Smaart is assuming the interface frequency and phase response are flat.

Carl: Single-channel measurements in Smaart include RTA, Spectrograph, and SPL. These are all obviously useful in their own way, but they can't generate a transfer function measurement that includes phase. For that, you need a pair of inputs so the analyzer can compare them to generate the magnitude and phase data.

EDIT: And Mark points out something very important: paying attention to what is inside your measurement loop, and what is not, depending on what you are trying to measure, and that will change depending on where you pull your reference from. Jamie covers that at about the 1 hr mark in this webinar: https://youtu.be/_4s_6Epc2Ng?t=3850



« Last Edit: January 19, 2021, 11:34:02 am by Michael Lawrence »
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Russell Ault

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Re: With smaart why do I have to loop back the output to my input?
« Reply #6 on: January 19, 2021, 03:37:29 pm »

[...] But more importantly, using a software loopback, the measurement and reference signals are now on two different clocks, and you will see that clock drift manifest in your measurement, which opens the door to a whole range of unpleasantries. Results vary based on the specifics of the rig but in general it's best avoided if you have a better option. [...]

If you want to see something mildly entertaining, try using the software loopback when your generator output and TF input are configured to use different hardware devices. I've never seen the delay finder get more upset!

-Russ
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Re: With smaart why do I have to loop back the output to my input?
« Reply #6 on: January 19, 2021, 03:37:29 pm »


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