comparing filtered data3: filtData (bandpass) vs filtfilt with filter object vs filfilt with sos matrix

问题

new attempt to understand this.

I'm trying to get what bandpass matlab function does.

So digging into the m-files, the filter object is constructed with:

opts=signal.internal.filteringfcns.parseAndValidateInputs(x,'bandpass',var);
opts = designFilter(opts);

Then, the filtering is done with:

y = signal.internal.filteringfcns.filterData(x,opts);

Then going into the filterData function, the actual filtering is done with:

y = filtfilt(opts.FilterObject,x);

because it is an IIR filter. Now, if I look into the filtfilt function, it doesn't have a way to deal with the filter object, only the b-a, or the sos-g representation are possible. b-a representation is really bad, so I tried with the sos-g representation, and the results are not the same.

I plotted three signals, filtered with: filterData, filtfilt with filter object and filtfilt with sos-g representation. Only the first two are equal.

Next are the PSD of each signal: filtData: filtfilt filter object: filtfilt sos-g: Notice the greater psd on the lower frequencies on the sos-g filter.

So, what does matlab does different when a filter object is used int the filtfilt function?

Here there is an example (download data.txt):

load('data.txt')
srate=64; 
freqrange=[0.4 3.5];

x=data(60*64:64*90);

var{1}=freqrange; var{2}=srate;

%% append at the beginning and at the end m=numel(x);
R=0.1;%10% of signal 
Nr=50; 
NR=min(round(m*R),Nr);%At most 50 points 
x1=2*x(1)-flipud(x(2:NR+1));%maintain continuity in level and slope
x2=2*x(end)-flipud(x(end-NR:end-1)); 
x=[x1;x;x2];

%% design filter
opts=signal.internal.filteringfcns.parseAndValidateInputs(x,'bandpass',var); 

opts = designFilter(opts);

%% default filtering 
xx = signal.internal.filteringfcns.filterData(x,opts); 
x_fil=xx(NR+1:end-NR);

%% what default filtering is supposed to do 
xx = filtfilt(opts.FilterObject,x); 
x_fil2=xx(NR+1:end-NR);

[z,p,k]=opts.FilterObject.zpk; 
[sos, g] = zp2sos(z, p, k);

%% default filtering with sos-g method 
xx = filtfilt(sos,g,x); 
x_fil3=xx(NR+1:end-NR);

f=figure;
plot([x_fil x_fil2 x_fil3])
title('default vs ff_D vs ff_{sos}')
legend('filterData','filtfilt_D','filtfilt_{sos}')
grid('on')

来源：https://stackoverflow.com/questions/58958392/comparing-filtered-data3-filtdata-bandpass-vs-filtfilt-with-filter-object-vs

标签

matlab

filtering

signal-processing

bandpass-filter