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Thursday
Sep262013

Hilbert Transform and Windowing

We recently fixed a bug in GNU Radio where the rectangular window actually didn't work. It would fall through to the Hamming window because of a missing 'break' statement. Embarrassing bug, but not all that detrimental since the rectangular window is so rarely used.

The one place it was being used was in the Hilbert transform block. We can build Hilbert transform filters using the filter.firdes.hilbert function. In this way, we've been able to set the window type to anything we'd like (Hamming, Hann, Blackman, etc.). But with the Hilbert transform block (that is, filter.hilbert_fc), we could only ever specify the number of taps to use in the transform and the window would default to a rectangular window -- except, as noted above, it would actually default to the Hamming window.

So we decided to add another argument when creating a Hilbert transform block that allows us to set whatever window we wanted and not force a decision on the users. To keep the API sane, though, I wanted to make sure there was a good default for this, which brought up the question: what is the right default window for a Hilbert transform?

That question depends a bit on who you ask. Johnathan Corgan mostly uses the Hilbert as a way to suppress sidebands while I tend to use it to convert real signals to analytic signals. These have slightly different properties, and the window used can help determine this (a few dB here and there). And while we have a decent understanding of these effects, I didn't really have a gut feel for which window was the right for any given course of action. So I played with it.

I created a GNU Radio flowgraph that runs a noisy (real) sine wave through Hilbert transforms with different windows and plotted the resulting PSD on the same graph. The first image you see below is the full two-sided spectrum. You can easily see how each of the windows has different attenuation in the negative frequencies. If we're trying to make an analytic signal, we want to remove as much of the negative frequency as possible to make the signal's real and imaginary parts as near orthogonal as we can. Looks like the Blackman-harris window is the best for this.

 But for sideband suppression, we want to minimize signals near 0 Hz as much and as quickly as possible. Below are two zoomed-in looks at this graph. The first is right around 0 Hz and the second is offset a little so we can see the negative image of the 1 kHz sine wave.

From these images, the Hamming window produces the fastest overall roll-off, even if it doesn't get quite as low as the Blackman-harris window does for full negative frequency rejection. On the other hand, the Hann window looks like it's the best compromise solution.

I find these plots to be generally helpful in giving me a basic understanding for what I'm trading off with the different windows, and so I wanted to provide them here, too.

As for the right default? Well, we have always been using the Hamming window, and it provides the best sideband suppression results. So we're going to stick with it as the default window so we don't change the behavior of existing GNU Radio projects that use the Hilbert transform block. The upside is that now we can select the right window for our needs if the default isn't suitable.

Reader Comments (3)

Hey Tom -

Awesome post! Thanks for sharing!

Quick question:

You say that the second and third graphs are zoomed-in versions of the first, but it looks like the plotted Power (dB) is different between the 1st and the other two - especially with the Kaiser window. Can you clarify this at all?

Thanks!

Cheers,
Ben

September 26, 2013 | Unregistered CommenterBen Hilburn

Thanks, Ben!

I wasn't being very accurate when I zoomed in on these two plots. The y-axis were not kept consistent, even between the two zooms. I just dragged a zoomer rectangle over the region I wanted to look into and saved the figures from there.

But I don't think that there's any inconsistency between the plots. And there shouldn't be since they are the same graph. The Kaiser window is pretty much on top of everything here, so you can follow the magenta line to see where it all fits. Just notice that the second graph spans 0 Hz (from -1 to 1 kHz to see the sine wave and it's negative frequency component) and the third graph is just looking at the negative frequency component of that sine wave. In each graph, this peak is just above -40 dBm.

Tom

September 26, 2013 | Registered CommenterTom Rondeau

I found the same "falling through" bug just yesterday. and a more serious problem with the coefficients returned by the Kaiser window function.
Posted an article (with patch!) about it on discuss-gnuradio mailing list today but it seems the domain has either been hijacked or registration expired and it's now a parked domain.

October 3, 2013 | Unregistered CommenterChristopher Sylvain

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