Lowpass filter characteristics used in the resample functions (Igor 6.37)

Share icon

Wed, 08/23/2017 - 12:51 pm

Hello,

I was wondering if someone could explain a bit more how does the LP filter work in the resample function of Igor Pro 6.37 when doing downsampling.

A bit more on my issue :

I'm working with an EEG (electrophysiologic) signal sampled at 3.52 Khz. I want to resample it exactly to 1 Khz. Therefore, I use the command "Resample/RATE=1 wavename".

According to what I understand from the manuel, this command automatically interpolate the number of points to get an integer number, then decimite it to 1 Khz, while applying a lowpass filter with an Hanning window beforehand. The problem is : when I look at the spectrum of my signal before and after downsampling, they kind of ... look the same up to 500 Hz (which is the maximal frequency i can look at according to nyquist). If a low-pass filter would be used with a low-pass set a 500 hz (half the sampling I want), I expect some difference before 500 hz - but I don't see it.

Thanks for any help regarding this issue !
Resample an impulse in a wave with the original sampling and then perform the FFT on the result to see the filter response.

--Jim Prouty
Software Engineer, WaveMetrics, Inc.

August 23, 2017 at 03:14 pm - Permalink

jaydeelive01 wrote:

Resample/RATE=1 wavename"


Pay attention to your wave's X scaling (sample rate).

/RATE=1 means resample to a 1Hz sampling rate, not a 1KHz rate.

jaydeelive01 wrote:
The problem is : when I look at the spectrum of my signal before and after downsampling, they kind of ... look the same up to 500 Hz (which is the maximal frequency i can look at according to nyquist)


Why would you want the response to be different for the 0-500Hz range?

Resample is working hard to make the response be the same over the valid frequency range.

If you want lowpass filtering, use FilterFIR, not Resample.

--Jim Prouty
Software Engineer, WaveMetrics, Inc.

August 23, 2017 at 03:20 pm - Permalink