Wow and Flutter measurements visualizer

I stumbled on the Wow and Flutter visualizer today and realized it would be a good tool to use to check the performance of a modulation pedal I am testing. I can see that my LFO is working, and my pitch is actually varying, but it would be helpful if the visualizer display showed the modulation frequency value. I can compute it by looking at the data points in the waveform, but to have it displayed numerically would be helpful.

I’d also like to have the max and min frequencies displayed.

As the vast majority of my audio experience has been with amplifiers, preamps, and mixing consoles W&F is not my area of expertise - other than that I know what it is. I’m trying to understand the 2sigma value better. Any more info you can provide on this would be great.

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Hi @voltist, It’s not intuitive because it’s hard to see, but in the upper left of the wow and flutter tool there’s a help button that will open in your browser a small HTML file of help on the tool. I’ve pasted that below. The two-sigma value is twice the standard deviation.

Wiki notes:

“Physical scientists often use the term root mean square as a synonym for standard deviation when it can be assumed the input signal has zero mean, that is, referring to the square root of the mean squared deviation of a signal from a given baseline or fit.[8][9] This is useful for electrical engineers in calculating the “AC only” RMS of a signal. Standard deviation being the RMS of a signal’s variation about the mean, rather than about 0, the DC component is removed (that is, RMS(signal) = stdev(signal) if the mean signal is 0).”

So, let’s say you are looking at a 10 Hz carrier that is modulated by a 1 Hz low frequency modulator. That would give a resultant that spans from 9 to 11 Hz. In the W&F tool, that would show as a sine centered on 10 Hz with peaks +/-1 Hz. The RMS of that would be ~10. But the standard deviation would give the AC only, which would be 0.707.

In the picture pasted below, you can see this math in Excel. The second column is a sin() with an amplitude of 1 added to a fixed value of 10. On the right, you’ll see that value squared. At the bottom of the right hand column, you can see the mean and root and we arrive at an RMS of 10 as expected. And at the bottom of the second column you can see the standard deviation is 0.707, which is what we’d expect for a sin with an amplitude of 1.

We’ll add a way to see max and min, as that is good info to have that’s hard to pull from a plot. But roughly, just take a look at the stddev value and divide it in half and that is your RMS (with the carrier removed) if the waveform is a sine.

(Below is pasted from html help)

Wow and Flutter

This visualizer will measure the unweighted wow and flutter of a playback device. The range of tones expected falls between 1 KHz to 3 KHz, but other frequencies will work.

The Sample Length determines the lower bound low-frequency variations that can be detected. A good starting point is around 32K to 64K size FFT and a 48K sample rate.

The input signal needs to be fairly strong and free of noise. A good target is around -30 dBV or higher, with the attenuator off. Noisy signals will be more prone to variation.

Measurement Computation

The incoming signal is demodulated, and a graph is displayed showing frequency versus time. Ideally there would be no variation in playback and the line would be horizontal with no variation or noise.

The indicated Frequency is the frequency averaged over the entire sample interval (eg 32K points).

In computing the 2-Sigma value, the demodulated signal standard deviation is computed. The displayed value is the peak-to-peak detected and twice the computed sigma, in accordance with AES6-2008. Currently, however, the reading isn’t weighted.

PS. Can you share some plots of what you are seeing? It’d be cool to see.

The confusing part is that the 2sigma value is in Hz. So to verify my 10Hz LFO, I take the 2Sigma value (here about 30Hz) divide by two then compute the RMS? I then get 10.6 Hz which is close. (am I understanding that?) The readings bounce around quite a bit - I know the fluctuation is not in my device. Again, it would be helpful of the visualizer could display the modulation frequency as well.

BTW - I’ve got two outputs, basically stereo, that’s why there are two signals out of phase. That’s intentional.

Yes, good plot, I think you got it

Again, it would be helpful of the visualizer could display the modulation frequency as well.

You should be able to see that on the main display. Set your FFT size to max, set your sample rate to min, set your x-axis to linear, and then you should see a classic FM spectrum. In your case, you’d see a carrier frequency at 1.025K and you’d see sidebands +/- 10 Hz on either size of that. You can put a marker on the center (carrier) and one of the sidebands and read off the carrier freq and also the modulation frequency. And (bonus points!) you can also know the modulation index via the amplitude of the sideband relative to the carrier.

cool, I’ll try that today. Thanks for the help!