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Plot Filter (Analyze filter characteristics)

Type : Test
References : Posted by scanner598 at yahoo dot co dot uk

Notes :
As a newbe, and one that has very, very little mathematical background, I wanted to see what all the filters posted here were doing to get a feeling of what was going on here. So with what I picked up from this site, I wrote a little filter test program. Hope it is of any use to you.

Code :
// plotFilter.cpp
// Simple test program to plot filter characteristics of a particular
// filter to stdout. Nice to see how the filter behaves under various
// conditions (cutoff/resonance/samplerate/etc.).
// Should work on any platform that supports C++ and should work on C
// as well with a little rework. It justs prints text, so no graphical
// stuff is used.
// Filter input and filter output should be between -1 and 1 (floating point)
// Output is a plotted graph (as text) with a logarithmic scale
// (so half a plotted bar is half of what the human ear can hear).
// If you dont like the vertical output, just print it and turn the paper :-)


#define myPI 3.1415926535897932384626433832795

#define FP double
#define DWORD unsigned long
#define CUTOFF 5000
#define SAMPLERATE 48000

// take enough samples to test the 20 herz frequency 2 times

// Any filter can be tested, as long as it outputs
// between -1 and 1 (floating point). This filter
// can be replaced with any filter you would like
// to test.
class Filter {
FP sdm1; // sample data minus 1
FP a0; // multiply factor on current sample
FP b1; // multiply factor on sdm1
Filter (void) {
sdm1 = 0;
// init on no filtering
a0 = 1;
b1 = 0;
void init(FP freq, FP samplerate) {
FP x;
x = exp(-2.0 * myPI * freq / samplerate);
sdm1 = 0;
a0 = 1.0 - x;
b1 = -x;
FP getSample(FP sample) {
FP out;
out = (a0 * sample) - (b1 * sdm1);
sdm1 = out;
return out;

DWORD freq;
DWORD spos;
double sIn;
double sOut;
double tIn;
double tOut;
double dB;
DWORD tmp;

// define the test filter
Filter filter;

printf(" 9bB 6dB 3dB 0dB\n");
printf(" freq dB | | | | \n");

// test frequencies 20 - 20020 with 100 herz steps
for (freq=20; freq<20020; freq+=100) {

// (re)initialize the filter
filter.init(CUTOFF, SAMPLERATE);

// let the filter do it's thing here
tIn = tOut = 0;
for (spos=0; spos sIn = sin((2 * myPI * spos * freq) / SAMPLERATE);
sOut = filter.getSample(sIn);
if ((sOut>1) || (sOut<-1)) {
// If filter is no good, stop the test
printf("Error! Clipping!\n");
if (sIn >0) tIn += sIn;
if (sIn <0) tIn -= sIn;
if (sOut>0) tOut += sOut;
if (sOut<0) tOut -= sOut;

// analyse the results
dB = 20*log(tIn/tOut);
printf("%5d %5.1f ", freq, dB);
tmp = (DWORD)(60.0/pow(2, (dB/3)));
while (tmp) {
return 0;


Added on : 01/01/08 by niels m[ DOT ]
Comment :
You need to change the log() to log10() to get the correct answer in dB's.

You can also replace the if(sIn >0) .. -= sOut; by:

tIn += sIn*sIn;
tOut += sOut*sOut;

this will measure signal power instead of amplitude. If you do this, you also need to change 20*log10() to 10*log10().

Nice and useful tool for exploring filters. Thanks!

Added on : 18/09/11 by kiranpatel0305[ AT ]gmail[ DOT ]com
Comment :
Can anyone help me with C++ class composition to implement a single pole-filter design program. The program will allow the user to specify resistor and capacitor values and filter type. Once all the user parameters are specified, the program will return the cutoff frequency values for the filter. I need this as soon as possible and I am lost big time :(

Create a Multifile Project for the Composition Lab

1. Add three classes to the project: a Filter class, a Capacitor class, and a Resistor class.
2. The Capacitor class should be modeled after the Resistor class for class members and operation.              

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