How Not to Reduce DFT Leakage

• Tips and Tricks

This blog describes a technique to reduce the effects of spectral leakage when using the discrete Fourier transform (DFT).

In late April 2012 there was a thread on the comp.dsp newsgroup discussing ways to reduce the spectral leakage problem encountered when using the DFT. One post in that thread caught my eye [1]. That post referred to a website presenting a paper describing a DFT leakage method that I'd never heard of before [2]. (Of course, not that I've heard everything there is to hear about DFT leakage.)

The paper's author presents a method for reducing spectral leakage when an r(n) input sequence is applied to the DFT. In that method, a left-to-right time-flipped version of r(n) is appended to the original r(n) sequence creating a new r_e(n) sequence with twice as many samples as r(n). As I understand the idea, the below Figure 1(a) shows an example of a 32-sample r(n) time sequence and Figure 1(b) shows the 'leakage reduction' method's resulting 'data-flipped' 64-sample r_e(n) time sequence.

According to the paper's author, when r_e(n) is applied to the DFT, its DFT results will exhibit reduced spectral leakage compared to the DFT results of the original r(n). In addition, the author stated that the DFT of the r_e(n) sequence will have the same spectrum as the DFT of the original r(n) sequence. Reading those later words made me suspicious. As it turned out, many things in that paper bothered me—bothered me a lot—but I won't bore you with the details.

Wait a second, ... I will mention one thing that is truly important. The paper's author stated that the purpose of windowing is to ensure that the first and last samples of a windowed time sequence are zero-valued. That statement is not correct. The fact that the first and last samples of a windowed time sequence are zero-valued is an effect of windowing, not the goal of windowing. The goal of windowing is to reduce the amplitude discontinuities in a circular version of a DFT's input sequence. Stated in different words, if you created a repetitive version of the windowed time sequence, by repeating the windowed sequence over and over, the goal of windowing is to reduce the amplitude discontinuities in that repetitive sequence. Now in fairness, I'll admit that the author's mistaken conclusion about the purpose of windowing is an easy mistake to make.

What troubled me most was that in my Matlab simulations of that data-flipping spectral leakage reduction method is that I found a case where it does not work properly! For example, when a 183 Hz sinusoidal r(n) was used as the input to the data-flipping leakage reduction method the resulting frequency response is that shown by the solid curve in Figure 2. (The f_s sample rate of r(n) was 1000 Hz.) For reference purposes, the dashed curve in Figure 2 is the spectrum of r(n) windowed by a Hanning window. Notice two things: (1) the sidelobe levels of the data-flipping scheme are far higher than the sidelobes in the Hanning windowed spectrum; (2) the troubling dip in the mainlobe of the data-flipping method's frequency response.

I believe the problem with the paper's data-flipping leakage reduction method is as follows: Where the frequency transforms of the standard Hanning and Hanning windows are not dependent on the frequency of a time sequence to which they're applied, the paper's data-flipping leakage reduction method DOES depend on the frequency of an input r(n) sequence.

So, ...if I understand the paper's technique correctly, I'd never recommend that it be used in practice.

Postscript:
One of the subscribers here, nairen, posted a comment giving the frequency response of the 'data-flipping' process. A PDF file of nairen's very clever derivation can be downloaded by clicking here.

References
[1] From: "Lightbulb85", Subject: Re: Reducing Spectral Leakage of Rectangular Window?, Date: Thu, 26 Apr 2012.
[2] http://www.slac.stanford.edu/cgi-wrap/getdoc/slac-pub-6221.pdf

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