[FieldTrip] mtmconvol with hanning window (Jochem Rieger)

Fri Aug 4 18:08:56 CEST 2023

Dear Jochem,

Thank you for this detailed discussion.  I wasn’t referring the Welch’s method, because I understand that method collapses across the time dimension and produces a power spectrum. I am interested in the other methods that produce time-frequency maps.

My understanding is that when using mtmconvol, the multitaper method is only applied if the Slepian windows (dpss) are used in the taper field.  With the hanning window, it is a single taper that is applied to overlapping segments of data (that’s what I meant by sliding window).  

I do get the advantages and disadvantages of using a rectangular window.  In Mike X. Cohen’s textbook, “Analyzing Neural Time Series Data,” he has a chapter on Short-Time FFT where he recommends using an overlapping taper such as a Hanning window.  So, at least according to him, the STFFT doesn’t need to use strictly rectangular windows.  

In any case, how would you describe an analysis done with mtmconvol and a hanning window in the methods section of a paper, if not short-time fft? It seems to me that it would be incorrect to refer to it as a multitaper method.

Thanks again.


> ------------------------------
> Message: 2
> Date: Mon, 31 Jul 2023 16:32:28 +0200
> From: Jochem Rieger <jochem.rieger at uni-oldenburg.de>
> To: "Weisholtz, Daniel S.,MD via fieldtrip" <fieldtrip at science.ru.nl>
> Subject: Re: [FieldTrip] mtmconvol with hanning window
> Message-ID: <596f1ab4-03b0-40c2-92c8-a37597aac43a at uni-oldenburg.de>
> Content-Type: text/plain; charset="windows-1252"; Format="flowed"
> Hi Dan,
> think of short term fft as windowing with a rectangular window. This has 
> rather bad frequency resolution (wide central lobe, many side lobes in 
> the amplitude spectrum). It produces "veridical" values for the 
> amplitude estimate over the analysis  interval (your window length)
> The other methods underestimate the amplitudes systematically. If you 
> are only interested in relative amplitudes (most cases) that doesn't 
> matter.
> The Hanning window improves the cross talk and is typically applied 
> once  the whole intervall and then an FFT is calculated to get the 
> amplitude spectrum. The cost for that improvement is that the amplitude 
> estimate are biased towards the center of the window and the amplitude 
> values are underestimated.
> Multitaper analysis applies multiple orthogonal windows and obtains by 
> that approach multilple amplitude spectra, which are then combined for 
> the final estimate. Approach reduces the noise in the amplitude 
> spectrum  lets  you trade-off frequency resolution against noise.
> When you write multiple sliding windows you likely refer to the Welch 
> method which applies multiple shorter and partly overlapping tapers to 
> the analysis interval and then combines those correlated amplitude 
> estimates. This method reduces the noise in the amplitude spectrum. In 
> principle, you could use different types of windows.
> Hope that helps,
>     Jochem
> On 27.07.23 18:43, Weisholtz, Daniel S.,MD via fieldtrip wrote:
>> ACHTUNG!Diese E-Mail kommt von Extern! WARNING! This email originated 
>> off-campus.
>> Dear FieldTrip community,
>> I understand that using the hanning window with this function will 
>> make this function use a single sliding taper window rather than 
>> multitaper. Does that make this analysis equivalent to a short-time 
>> fft analysis? If you do this, will FieldTrip ignore the tapsmofrq 
>> parameter?
>> Thanks for your help!
>> Best,
>> Dan

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