There is a low pass filter . And my sampling frequency is fs.
the Matlab code of the lpf is as follows:
% y = LPFilter(x,fc),
% Second order zero-phase Lowpass filter
%
% inputs:
% x: vector or matrix of input data (channels x samples)
% fc: -3dB cut-off frequency normalized by the sampling frequency
%
% output:
% y: vector or matrix of filtered data (channels x samples)
In the code fc (cut-off frequency) is replaced by 0.7/fs . Can anyone explain why fc is replaced by 0.7/fs ?
Suppose there are three observations in centimeters: 10,20 and 40. If I divide these observations by the maximum of them, then the new data will be 0.25, 0.5, 1. Clearly the new range will be from 0 to 1 (If observations are positive). This is called normalization. Of course, there are also other methods to normalize.
There are signals of different frequency ranges. We need to apply LP,BP,BS,HP filters on any signal. To make the representation uniform, we divide the range by maximum frequency which is sampling frequency. We consider only positive part because the frequency response is symmetric.
The normalization use to:
1-take large frequency range.
2-to make the results more clear and simple to reader
It is because the coefficients of digital filters are calculated using the ratio of cut-off frequency and sampling frequency, that is, a low-pass of 100 Hz for a signal sampled at 500 Hz has the same coefficients as a low-pass filter of 200 Hz for signals sampled at 1 kHz.
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