EEP - cntdown

cntdown

Synopsis


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EEP 3.1  Max-Planck-Institute of Cognitive Neuroscience 1996-99
cntdown 3.8   (OSF1 V4.0 alpha)               Wed Sep 15 13:30:20 1999
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  cntdown <source cnt> [<dest cnt>] -s <skip>|-m <skip>|-f <freq>

options:
  -s <skip>  write each <skip>th source sample to dest
  -m <skip>  write each mean of <skip> source samples to dest
  -f <freq>  resample to <freq> Hz via linear interpolation

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

Description

cntdown changes (decreases) the sampling rate of .cnt-files. This allows to unify data which were recorded at different sampling rates. Another useful application is just to reduce the amount of stored data (and thus all evaluation runtimes). For example, a 1000Hz EEG/MEG could be downsampled to 250Hz or slower if you look for (10..20)Hz components only.

There are three algorithms implemented:

pull mode: each skip-th sample point is loaded from input and copied to output
mean mode: the mean of each skip sample points is calculated from input and stored in output
interpolation mode: output points comes from linear interpolation between the previous and the next input point (this mode could "upsample" too, but such a process makes no sense to me)

According to the recommended EEP usage rules, a cntdown run should be one of the first steps in an evaluation. One can avoid a lot of trouble with invalid or inaccurate control data files (.trg, .rej) this way...

Consider also Shannon's theorem, filtering and other signal processing fundamentals.