Filtering

[Robert Oostenveld]

Hi Vladimir,

On 31 Aug 2006, at 15:58, litvak at techunix.technion.ac.il wrote:
> Another question I have (you may post the answer to the list or put
> it in the
> FAQ if you want) is how do I translate from BESA filter settings
> (like 6 db/Oct
> or 12 db/Oct) to filter order in FT? I took a DSP course a couple
> of years ago
> but all memory of this faded by now.

The filter order for the Butterworth filter (filttype='but') is
different from that of the FIR filters. For the Butterworth, you can
look here (http://www.answers.com/topic/butterworth-filter#after_ad2)
for full details. Furthermore, in the Matlab signal processing
toolbox you can also find a filter design tool (a.o.). Type "help
butter" and "help butord" on the command line for more background.

Furthermore, on thefieldtrip website -> documentation -> example
scripts -> at the bottom you will see
http://www2.ru.nl/fcdonders/fieldtrip/doku.php?
id=fieldtrip:documentation:determine_the_filter_characteristics
which gives you some hands on tricks to determine it yourself. The
frequency response plots can be compared to the figs on the external
website: to translate the frequency response (expressed as power, not
amplitude) to decibel -> 10^-0.3 = -3 dB, and in general 10^-x =
-10*x dB.

best
Robert

P.S. note that a Nst order filter applied in both directions
(cfg.XXfiltdir='twopass') effectively corresponds to an 2xNth order
filter, since you filter twice.

P.P.S. to summarise: "4th order"+"twopass" (which are the defaults)
corresponds to 2x4x6 = 48dB/octave, so that is pretty steep.