EEP - cntfilter

cntfilter

Synopsis


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EEP 3.1  Max-Planck-Institute of Cognitive Neuroscience 1996-99
cntfilter 3.16   (OSF1 V4.0 alpha)            Wed Sep 15 13:30:22 1999
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  cntfilter <source cnt> [<dest cnt>] [<trg in>] <fir in> [-c <regex>]

options:
  -c <regex>    filter selected channels

files:
  foo.cnt  ->  foof.cnt
  foo.trg

Description

cntfilter reads a precalculated finite impulse response from a textfile (*.fir) and applies this Finite Impulse Response Filter to the cnt file. Filters can be designed with the xfir module. Some filters are available in the fir directory.

There is a parallelized version (cntfilter_mp) installed at the origin. It can increase the performance by a factor 2..3 for long filters (> 500 points) but should not be used for short filters. If you have to filter multiple files, it is faster to start, say, 4 sequential cntfilter jobs in parallel than 4 parallel cntfilter_mp jobs in sequence.

Discontinuity Handling

The filtering window (N_coeffs samples) does not "overrun" any signal discontinuities to reduce unwanted edge effects. The record is filtered in independent epochs instead, where record start, record stop, discontinuity("__") and DC reset("Rs") are the epoch limits.

At each epoch limit, the signal is virtually extendend by a constant signal of (N_coeffs - 1) / 2 samples. This means that the edges of a filtered signal epoch are not influenced by the signal values of the previous/next epoch. Of course, the edges are influenced by the artificial constant signal.

In the case of record start, record stop and discontinuity("__" trigger), the first, respectively the last, sample value is used to extend the signal. The signal values near DC reset positions are normally trash and a bad choice to estimate the past/future of the recorded signal. Therefore, the signal value of reset_time -/+ 15 ms is used to extend the signal. There are options -l <ms> and -r <ms> to override the 15 ms builtin.