Distortion N/A

[bobkatz]

What I'm trying to do now is measure distortion of a power line! I have a wav recording I made of the power line using an attenuator and an input transformer into an interface. I can open this wav in the RTA. Currently it's in spectrum rather than bars... i can't seem to get it to display an opened wav as 1 octave bars, only as spectrum. Regardless, the spectrum looks appropriate in the RTA with a hill around 60 Hz and a rolloff in frequency response above it.

None of the smoothing options in the asterisk option seem to be working. Aha. If I make my smoothing choice in the main window it is reflected in the RTA window.

Distortion is N/A no matter what I pick. Is it possible that REW's RTA can only measure distortion of a live signal on its input?

 

[John Mulcahy]

When you open a WAV file in the RTA, or drag it onto the RTA, it will be processed according to the current RTA settings and saved as a measurement with those settings. Once the file has been processed and whilst on the RTA you can then change the mode to see how the data looks in any other mode you choose, then save that as a measurement. The smoothing choices on the RTA window are applied to data as it is received, they don't affect data that has already been captured, but as you have noted you can smooth the saved measurements.

WAV files are processed in the same way as live inputs, so if the signal has a dominant tone that will be used as the fundamental and the distortion will be shown, provided the Show distortion button is selected.

If you attach the WAV file I can check if there's anything about it that would prevent distortion capture.

 

[bobkatz]

When you open a WAV file in the RTA, or drag it onto the RTA, it will be processed according to the current RTA settings and saved as a measurement with those settings. Once the file has been processed and whilst on the RTA you can then change the mode to see how the data looks in any other mode you choose, then save that as a measurement. The smoothing choices on the RTA window are applied to data as it is received, they don't affect data that has already been captured, but as you have noted you can smooth the saved measurements.

WAV files are processed in the same way as live inputs, so if the signal has a dominant tone that will be used as the fundamental and the distortion will be shown, provided the Show distortion button is selected.

If you attach the WAV file I can check if there's anything about it that would prevent distortion capture.

Thank you, John. I'm beginning to understand how the RTA works, thanks to you. I've tried all I can to conquer the distortion measurement. Thiis is a 2448 wav recording of the power line with 60 Hz fundamental. I've tried all that I can to measure its distortion, but I get "Distortion N/A". So, attaching.

 

[John Mulcahy]

Seems OK to me:

The 2nd harmonic figure is a bit so-so since the spectrum is a little odd there, bouncing up and down, here is a view without averaging:

 

[John Mulcahy]

Here are my distortion settings:

 

[bobkatz]

Thank you, John. I'm still getting "Distortion N/A". I'm on Mac and you're on Windows. The only difference in the distortion settings I see is youhave checked "Apply ENOB amplitude correction", which I do not have available on the Mac. I don't even know what ENOB is. I also run REW on Windows 11. I'll try it there and report back.

 

[sam_adams]

I'm still getting "Distortion N/A". I'm on Mac

First off, make sure that you're on the latest beta build.

When you have the RTA window open, click the gear icon on the upper right (green arrow in pic below) to open the RTA settings. Set them like you see in the pic below. Next, click the 'Distortion settings' button (blue arrow in the pic below) and select the settings in the dialog box as indicated. Enabling the HP and LP filter settings won't change much of what you see if you set the window limits to 20 Hz to 20 KHz so it is essentially personal preference for this exercise.

Next, click the 'Show distortion' button (red arrow in the pic below) on the button bar at the top.

Next, click the 'WAV' button (yellow arrow in the pic below) and select the .wav file to analyze. You can select either the Left, Right, or both channels to analyze. Select the results to save, either Current, Peak, or Both. Both is a good choice. Click OK and the file will be processed and displayed with the distortion result. The processing of the file will generate frequency plots in the SPL & Phase window, also.

 

[bobkatz]

I thought that I had replied here, but my reply seems to be lost. Thank you John. With the same distortion settings on Mac and the same file I still get "Distortion N/A". I'll try this in REW on Windows, which I also use, and report back. A few of the distortion setting check boxes that are available on Mac do not seem to be available on Win and vice versa. I'll check my REW version :-)

 

[bobkatz]

Hooray! Thank you Sam Adams. Installing the latest Beta on Mac solved the "Distortion N/A" issue! I now get the distortion measure on mac!

 

[bobkatz]

What is the meaning of ENOB amplitude correction in this context? What is the significance of the choice of Blackman-Harris 7 window over Hann in the context of measuring THD? Of course each window will give slightly different results, so it's important to standardize on a particuilar window for a particular purpose for repeatability.

 

[John Mulcahy]

When the Apply ENOB amplitude correction option is selected the ENOB figure is normalised to the full scale level of the input so that it displays valid results for input signals below full scale. This option should be selected to obtain a figure that is valid for the interface input.

The Hann window is well suited to most measurements, offering a good tradeoff between resolution and shoulder height. If very high dynamic range needs to be resolved (very small signals close to very large signals) use the 4-term or 7-term Blackman-Harris windows. In extreme cases (e.g. measurements of digital data paths) the Dolph-Chebyshev and Cosine Sum windows have even lower window side lobe heights. The results automatically compensate for window characteristics.