New low-level routines for mtmconvol and mtmfft implemented

[Vladimir Litvak]

Dear Roemer,

Why did you decide to drop the support for variable number of tapers in
mtmconvol? That was actually quite a handy feature that I'm using to get TFs
with more or less uniform smoothness across frequency. I don't know how many
other people are using it but if there are many perhaps we can campaign for
retaining it in the new implementation ;-)

Best,

Vladimir

On Tue, Jul 27, 2010 at 3:48 PM, Roemer van der Meij <
r.vandermeij at donders.ru.nl> wrote:

>  Hi everybody,
>
> I just finished fully implementing the new low-level frequency analysis
> routines for cfg.method = mtmconvol and mtmfft. They are not yet used by
> default, and I would appreciate if it can be tested more before optimizing
> computation speeds (should be relatively equal now, but many optimizations
> possible).
>
> The new routines can be used by adding a third input argument to your
> ft_freqanalysis calls, as:
> New implementation:
> freq = ft_freqanalysis(cfg,data,1)
> Old implementation (default):
> freq = ft_freqanalysis(cfg,data,0)
>
> *Things that are different:*
> - all mtmconvol output is now phase-shifted such that an angle of 0 of any
> fourier-coefficient *always *means a peak of an oscillation in the data,
> and an angle of pi will *always* mean the trough of an oscillation
> (wavelet wise angle = 0 is implemented as cosine at peak, and sine in
> up-going flank)
> - all mtmfft output is now phase-shifted such that any angle from any
> fourier-coefficient is from the perspective of the oscillation in the data
> being at its peak at time = 0
> - mtmconvol now uses an accurate frequency vector for building its wavelets
> (determined by fsample and nsample, with cfg.foi as starting point), instead
> of an *uncorrected* cfg.foi (mtmfft already used an accurate one) (note:
> this causes slight differences between the old and new implementation)
> - mtmfft can now take a cfg.foi input-vector as well, instead of the usual
> cfg.foilim (backwards compatible)
> - because of the above, nearly *all* fourier-output is different, but e.g.
> phase differences between coefficients should be the same
> - we decided to drop the support for variable number of tapers in
> mtmconvol, cfg.tapsmofrq can only be a scalar now (warning is given, and
> first element of vector is selected)
>  - in the new format, many things can be upgraded/implemented etc. in the
> future
>
>
> As freqanalysis is a rather fundamental pilar of FieldTrip, I would really
> appreciate if some of you can do some testing based on the above differences
> and anything else you can think of. Different types of data, different
> approaches, etc. to see if both implementations give comparable results and
> such (do note raw fourier-coefficients will almost always be different). It
> could be I missed some fields from the old functions that I diligently
> checked, or made some errors that did not affect my current testing
> environment.
> I can be reached at the e-mail address below, via the mailing-list, or via
> bug-reports. Thanks in advance.
>
> All the best!
>
> Roemer
>
>
> PS: the changes will be on the ft-server tonight, but are of course already
> in the svn-version
>
> --
> Roemer van der Meij MSc
> PhD student
> Donders Institute for Brain, Cognition and Behaviour
> Centre for Cognition
> P.O. Box 9104
> 6500 HE Nijmegen
> The Netherlands
> Tel: +31(0)24 3655932
> E-mail: r.vandermeij at donders.ru.nl
>
>  ----------------------------------
>
> The aim of this list is to facilitate the discussion between users of the
> FieldTrip toolbox, to share experiences and to discuss new ideas for MEG and
> EEG analysis.
>
> http://listserv.surfnet.nl/archives/fieldtrip.html
>
> http://www.ru.nl/fcdonders/fieldtrip/
>

----------------------------------
The aim of this list is to facilitate the discussion between users of the FieldTrip  toolbox, to share experiences and to discuss new ideas for MEG and EEG analysis. See also http://listserv.surfnet.nl/archives/fieldtrip.html and http://www.ru.nl/neuroimaging/fieldtrip.
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