|Type : LPF 24dB/Oct|
Type : Bessel Lowpass
References : Posted by Christian[AT]savioursofsoul[DOT]de
The filter tends to be unsable for low frequencies in the way, that it seems to flutter, but it never explode. At least here it doesn't.
First calculate the prewarped digital frequency:
K = tan(Pi * Frequency / Samplerate);
K2 = K*K; // speed improvement
Then calc the digital filter coefficients:
A0 = ((((105*K + 105)*K + 45)*K + 10)*K + 1);
A1 = -( ((420*K + 210)*K2 - 20)*K - 4)*t;
A2 = -( (630*K2 - 90)*K2 + 6)*t;
A3 = -( ((420*K - 210)*K2 + 20)*K - 4)*t;
A4 = -((((105*K - 105)*K + 45)*K - 10)*K + 1)*t;
B0 = 105*K2*K2;
B1 = 420*K2*K2;
B2 = 630*K2*K2;
B3 = 420*K2*K2;
B4 = 105*K2*K2;
Per sample calculate:
Output = B0*Input + State0;
State0 = B1*Input + A1/A0*Output + State1;
State1 = B2*Input + A2/A0*Output + State2;
State2 = B3*Input + A3/A0*Output + State3;
State3 = B4*Input + A4/A0*Output;
For high speed substitude A1/A0 with A1' = A1/A0...
Added on : 28/07/06 by Christian[ AT ]savioursofsoul[ DOT ]de
It turns out, that the filter is only unstable if the coefficient/state precision isn't high enough. Using double instead of single precision already makes it a lot more stable.
Added on : 15/10/06 by Christian[ AT ]savioursofsoul[ DOT ]de
Just found out, that I forgot to remove the temporary variable 't'. It was used in my code for the speedup. You can simply ignore and delete it.
Added on : 13/02/09 by karatecoffee[ AT ]yahoo[ DOT ]dk
Changing the frequency also seems to affect the volume quite a lot. That can't be right? Maybe you should re-post this (and remove the "t" this time)? ;)
Added on : 13/02/13 by dj_zef[ AT ]ngs[ DOT ]ru
Your filter does not work! Arise overload in the calculation of coefficients A and B in the cutoff frequency of approximately 10 khz. At low frequencies, the coefficient of the gain increases proportional to the frequency cutoff, which causes congestion. I also noticed that the filter on this basis, the package of ASIO/DSP - DDspBesselFilter.pas does not work, although Butterworth (DDspButterworthFilter.pas) and Chebyshev (DDspChebyshevFilter.pas) work perfectly!
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