Beginner questions - getting consistent RTA traces

dch53

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Hi. I've been playing around with REW for a couple of days. At the moment I'm only measuring my soundcard interface.

It's pleasingly easy to get frequency response and distortion curves. Here's a sample:

Soundcard interface loopback 2V scale -25dB.png


THD at 1kHz of 0.0087%.

When I analyse a 1kHz tone in RTA and all is running well I get:

RTA 1kHz tone good.png


THD is shown as 0.0018%.

Why the difference when using RTA?

Now, things often don't go well. Here are a couple of images:

RTA 1kHz tone poor 1.png


RTA 1kHz tone poor 2.png


The trace returns to normal but what causes these errant curves? Noise?

I suspect that whatever causes these cause the same problem with the stepped sine curves. Here's what it looks like when all is going well:

RTA stepped sine good.png


but other averages produce curves like this that mess things up:

RTA stepped sine poor 1.png


RTA stepped sine poor 2.png


Again, noise is high some I'm guessing that's the cause. Sometimes the noise must persist for a while because the curve only gradually returns to what it should be.

Should I be looking at an external USB soundcard instead of my interal Juli@ card?

Thanks very much!

Dave.
 

John Mulcahy

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That sweep distortion measurement is in the noise floor until about 2 kHz, there is an option in the graph controls to mask data below the noise floor. Longer sweeps are a little better, but the RTA can see much lower distortion levels. The glitches in the RTA plots would typically be due to missing or skipped data. If you are using ASIO drivers try increasing the ASIO buffer size. Problems can also occur if the sample rate REW is set to is not the same as the sample rate the card is running at per the OS settings, in which case the OS will be resampling and there is again some risk of repeated or skipped data.
 

dch53

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Thanks very much for the reply John. Here's my soundcard setup:

upload_2019-2-16_19-16-31.png


Something to note is that clicking on the ASIO Control Panel button elicits no response.

Anyway, I have 96kHz selected there and the same set in the Juli@ control panel:

upload_2019-2-16_19-19-18.png


Regards, Dave.
 

John Mulcahy

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Something to note is that clicking on the ASIO Control Panel button elicits no response.
That is down to the ASIO driver I'm afraid, it is part of the ASIO specification but the driver might not implement it. Buffer size maybe tucked away in the Juli@ Config menu somewhere.
 

dch53

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Indeed it is:
upload_2019-2-16_22-18-29.png


I'll play around with the latency settings tomorrow. There's nothing else to configure.
 

John Mulcahy

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That's the one, different drivers label it differently but basically it is the size of the data blocks the driver provides. Fine to use the max for audio measurements.
 

dch53

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That's the one, different drivers label it differently but basically it is the size of the data blocks the driver provides. Fine to use the max for audio measurements.
Thanks John. Setting the latency to 2048 samples seems to have fixed the problem.
 
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