Notch filter¶
- Author or source: Olli Niemitalo
- Type: 2 poles 2 zeros IIR
- Created: 2002-01-17 02:11:07
Creates a muted spot in the spectrum with adjustable steepness. A complex conjugate pair
of zeros on the z- plane unit circle and neutralizing poles approaching at the same angles
from inside the unit circle.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 | Parameters:
0 =< freq =< samplerate/2
0 =< q < 1 (The higher, the narrower)
AlgoAlgo=double pi = 3.141592654;
double sqrt2 = sqrt(2.0);
double freq = 2050; // Change! (zero & pole angle)
double q = 0.4; // Change! (pole magnitude)
double z1x = cos(2*pi*freq/samplerate);
double a0a2 = (1-q)*(1-q)/(2*(fabs(z1x)+1)) + q;
double a1 = -2*z1x*a0a2;
double b1 = -2*z1x*q;
double b2 = q*q;
double reg0, reg1, reg2;
unsigned int streamofs;
reg1 = 0;
reg2 = 0;
/* Main loop */
for (streamofs = 0; streamofs < streamsize; streamofs++)
{
reg0 = a0a2 * ((double)fromstream[streamofs]
+ fromstream[streamofs+2])
+ a1 * fromstream[streamofs+1]
- b1 * reg1
- b2 * reg2;
reg2 = reg1;
reg1 = reg0;
int temp = reg0;
/* Check clipping */
if (temp > 32767) {
temp = 32767;
} else if (temp < -32768) temp = -32768;
/* Store new value */
tostream[streamofs] = temp;
}
|
Comments¶
- Date: 2006-09-27 09:53:49
- By: moc.liamtoh@18recnuor
i tried implementing it, failed,
and its wierd how it looks further in time
instead of backwards, you cant use it in
a running rendered stream cause the end of
it stuffs up....
- Date: 2006-09-27 11:43:49
- By: musicdsp@Nospam dsparsons.co.uk
I think it's a type, and should be
=-=-=
reg0 = a0a2 * ((double)fromstream[streamofs]
+ fromstream[streamofs-2])
+ a1 * fromstream[streamofs-1]
- b1 * reg1
- b2 * reg2;
=-=-=
In which case there is some rangechecking to be done when streamofs<2
You could just use the coeffs, and stick them into whatever biquad code you have hanging around :)
DSP
- Date: 2007-02-17 10:44:00
- By: moc.loa@561kluafla
Still doesn't work. Either way its looking one each side of a centre value, so it doesn't change the maths. Not sure what the code should be. Ideas???
- Date: 2009-04-24 16:34:32
- By: moc.liamg@naecohsife
does this code work with float output?
I really need a notch filter, i will try the code.
- Date: 2011-05-12 11:22:57
- By: moc.oohay@skinyela
yes, there are some errors in this source.
Here is the correct version:
// (C) Sergey Aleynik. aleyniks@yahoo.com
// BW_Coeff is changing from 0.0 to 1.0 (excluded) and the more the narrow:
// | BW_Coeff | Real BandWidth (approxim.) |
// | 0.975 | 0.00907 * Sampling_Frequency |
// | 0.95 | 0.01814 * Sampling_Frequency |
// | 0.9 | 0.03628 * Sampling_Frequency |
void Notch_Filter_Test(short int *Data_In,
short int *Data_Out,
long nData_Length,
double Sampling_Frequency,
double CutOff_Frequency,
double BW_Coeff)
{
// If wrong parameters:
if((NULL == Data_In)||(Data_Out)||(nData_Length <= 0)) return;
if((Sampling_Frequency < 0.0)||(CutOff_Frequency < 0.0)) return;
if(CutOff_Frequency > (Sampling_Frequency/2)) return;
if((BW_Coeff <= 0.0)||(BW_Coeff >= 1.0)) return;
static const double pi = 3.141592654;
// Filter coefficients:
double z1x = cos(2*pi*CutOff_Frequency/Sampling_Frequency);
double b0 = (1-BW_Coeff)*(1-BW_Coeff)/(2*(fabs(z1x)+1)) + BW_Coeff;
double b2 = b0;
double b1 = -2*z1x*b0;
double a1 = -2*z1x*BW_Coeff;
double a2 = BW_Coeff*BW_Coeff;
// Filter internal vars:
double y=0, y1=0, y2=0;
double x0=0, x1=0, x2=0;
long i;
for(i=0; i<nData_Length; i++)
{
y = b0 * x0 + b1 * x1 + b2 * x2 - a1 * y1 - a2 * y2;
y2 = y1;
y1 = y;
x2 = x1;
x1 = x0;
x0 = Data_In[i];
if( y > 32767) y = 32767;
else if (y < -32768) y = -32768;
Data_Out[i] = (short int)y;
}
}
- Date: 2011-11-11 03:30:26
- By: rf.liamtoh@abe.yrduabreivilo
Just an idea on coef notch :
a0 = 1;
a1 = -2 * cs;
a2 = 1;
b0 = 1 + alpha;
b1 = -2 * cs;
b2 = 1 - alpha;
notch: H(s) = (s^2 + 1) / (s^2 + s/Q + 1);
omega = 2*PI*cf/sample_rate;
sn = sin(omega);
cs = cos(omega);
alpha = sn/(2*Q);