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Low-distortion Audio-range Oscillator
Low-distortion Audio-range Oscillator
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Old 23rd May 2020, 10:26 PM   #9141
ARailsback is offline ARailsback
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REW

Coherent averaging
This option is only applicable to harmonic distortion measurements. If it is selected the FFT data is phase aligned according to the phase of the fundamental before averaging, this can lower the noise floor substantially compared to magnitude averaging without needing very long FFTs - in fact shorter FFTs (e.g. 64k) will allow faster averaging and more quickly lower the noise floor. The noise level drops by about 3 dB for each doubling of the number of averages. Mains frequency components will be suppressed along with any other noise not harmonically related to the fundamental, so this option is only suitable for examining harmonic levels. The various noise-related values in the distortion panel are not relevant when using coherent averaging due to the way the averaging suppresses noise. Note that if the harmonic levels are varying coherent averaging will converge to their average level whilst magnitude averaging will converge to their rms level.

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Old 24th May 2020, 08:49 AM   #9142
Sigurd Ruschkow is offline Sigurd Ruschkow  Sweden
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I am always skeptical.


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Originally Posted by electron-ic View Post
It is accurate?
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Old 24th May 2020, 11:06 AM   #9143
Edmond Stuart is offline Edmond Stuart  Netherlands
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Default Coherent averaging

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Originally Posted by electron-ic View Post
It is accurate?
Quote:
Originally Posted by Sigurd Ruschkow View Post
I am always skeptical.
It works on the premise that the phase of the fundamental has been exactly determined for each FFT record. Not that easy when the records don't contain an integer number of cycles. Moreover, it doesn't for work for frequencies not equal to one of the harmonics.

Cheers,
E.
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Old 24th May 2020, 11:41 AM   #9144
phofman is offline phofman  Czech Republic
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Edmond: But params of the fundamental within the samples block fed to FFT do not have to be measured by the FFT itself - you use that, I do, and apparently REW does too.

Generating 1555.293Hz@-1dB signal with SoX

Code:
sox -V -n -t alsa arta_loop synth 10000 sine 1555.293 gain -1
REW rounds the measured value to 1555.29Hz in the distortion display - 8k FFT with bin width 6Hz. I would assume the phase is measured the same way. We can ask John how the fundamental is actually measured - bin interpolation, sine fitting, etc.

I use sine fitting (amplitude and phase) and it is very precise using just a few incomplete periods of samples.
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Last edited by phofman; 24th May 2020 at 11:43 AM.
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Old 24th May 2020, 12:39 PM   #9145
Edmond Stuart is offline Edmond Stuart  Netherlands
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Quote:
Originally Posted by phofman View Post
Edmond: But params of the fundamental within the samples block fed to FFT do not have to be measured by the FFT itself - you use that, I do, and apparently REW does too.
Hi Pavel,

I don't know how John has done it, but I have calculated the fundamental phase outside the FFT routines, i.e. by means of a separate algorithm. It's mathematically correct (provided that the signal is not too much contaminated by noise etc.).

Quote:
Generating 1555.293Hz@-1dB signal with SoX

Code:
sox -V -n -t alsa arta_loop synth 10000 sine 1555.293 gain -1
REW rounds the measured value to 1555.29Hz in the distortion display - 8k FFT with bin width 6Hz. I would assume the phase is measured the same way. We can ask John how the fundamental is actually measured - bin interpolation, sine fitting, etc.


I use sine fitting (amplitude and phase) and it is very precise using just a few incomplete periods of samples.
That's a clever solution, congratulations!

edit: So it is still accurate, even when the records don't contain an integer number of cycles?

Cheers, E.
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Last edited by Edmond Stuart; 24th May 2020 at 12:44 PM.
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Old 24th May 2020, 06:35 PM   #9146
JohnPM is offline JohnPM  United Kingdom
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Using info from the FFT works fine for me. Surprisingly enough even just using the phase of the nearest bin to the fundamental is correct to within a few millionths of a degree, typically. For centre frequency I use the centroid of the bin values around the peak, sum(bin freq * bin magn sq)/sum(bin magn sq).
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Old 24th May 2020, 09:17 PM   #9147
KSTR is offline KSTR  Germany
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I've played a bit with this new REW feature today, using some synthesized test cases and it works like a charm with the correct setup! Could even trick it to work with two-tone IMD, or when the fundamental is suppressed heavily.
Thank you, @JohnPM
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Old 25th May 2020, 12:27 PM   #9148
Edmond Stuart is offline Edmond Stuart  Netherlands
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Quote:
Originally Posted by JohnPM View Post
Using info from the FFT works fine for me. Surprisingly enough even just using the phase of the nearest bin to the fundamental is correct to within a few millionths of a degree, typically. For centre frequency I use the centroid of the bin values around the peak, sum(bin freq * bin magn sq)/sum(bin magn sq).
Sure, it works fine. With this method I got even lower errors: a few billionth of a degree (depending on the record length, of course)
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