LPF 24dB/Oct

Type : Chebyshev
References : Posted by Christian[AT]savioursofsoul[DOT]de
Code :
First calculate the prewarped digital frequency:

K = tan(Pi * Frequency / Samplerate);

Now we calc some Coefficients:

sg = Sinh(PassbandRipple);
cg = Cosh(PassbandRipple);
cg *= cg;

Coeff[0] = 1 / (cg-0.85355339059327376220042218105097);
Coeff[1] = K * Coeff[0]*sg*1.847759065022573512256366378792;
Coeff[2] = 1 / (cg-0.14644660940672623779957781894758);
Coeff[3] = K * Coeff[2]*sg*0.76536686473017954345691996806;

K *= K; // (just to optimize it a little bit)

Calculate the first biquad:

A0 =   (Coeff[1]+K+Coeff[0]);
A1 = 2*(Coeff[0]-K)*t;
A2 =   (Coeff[1]-K-Coeff[0])*t;
B0 = t*K;
B1 = 2*B0;
B2 = B0;

Calculate the second biquad:

A3 =   (Coeff[3]+K+Coeff[2]);
A4 = 2*(Coeff[2]-K)*t;
A5 =   (Coeff[3]-K-Coeff[2])*t;
B3 = t*K;
B4 = 2*B3;
B5 = B3;

Then calculate the output as follows:

Stage1 = B0*Input + State0;
State0 = B1*Input + A1/A0*Stage1 + State1;
State1 = B2*Input + A2/A0*Stage1;

Output = B3*Stage1 + State2;
State2 = B4*Stage1 + A4/A3*Output + State2;
State3 = B5*Stage1 + A5/A3*Output;

Comments
from : musicdsp[at] Nospam dsparsons[dot]co[dot]uk
comment : You've used two notations here (as admitted on KVR!).. Updated calculation code reads: =-=-=-= Start =-=-=-= Calculate the first biquad: //A0 = (Coeff[1]+K+Coeff[0]); t = 1/(Coeff[1]+K+Coeff[0]); A1 = 2*(Coeff[0]-K)*t; A2 = (Coeff[1]-K-Coeff[0])*t; B0 = t*K; B1 = 2*B0; B2 = B0; Calculate the second biquad: //A3 = (Coeff[3]+K+Coeff[2]); t = 1/(Coeff[3]+K+Coeff[2]); A4 = 2*(Coeff[2]-K)*t; A5 = (Coeff[3]-K-Coeff[2])*t; B3 = t*K; B4 = 2*B3; B5 = B3; Then calculate the output as follows: Stage1 = B0*Input + State0; State0 = B1*Input + A1*Stage1 + State1; State1 = B2*Input + A2*Stage1; Output = B3*Stage1 + State2; State2 = B4*Stage1 + A4*Output + State2; State3 = B5*Stage1 + A5*Output; =-=-=-= End =-=-=-= Hope that clears up any confusion for future readers :-)

from : neolit123[AT]gmail[DOT]com
comment : Just ported this into Reaper's native JesuSonic. There are errors in both of the codes above :D Use this: //start A0 = 1/(Coeff[1]+K+Coeff[0]); A1 = 2*(Coeff[0]-K)*A0; A2 = (Coeff[1]-K-Coeff[0])*A0; B0 = A0*K; B1 = 2*B0; B2 = B0; A3 = 1/(Coeff[3]+K+Coeff[2]); A4 = 2*(Coeff[2]-K)*A3; A5 = (Coeff[3]-K-Coeff[2])*A3; B3 = A3*K; B4 = 2*B3; B5 = B3; Stage1 = B0*Input + State0; State0 = B1*Input + A1*Stage1 + State1; State1 = B2*Input + A2*Stage1; Output = B3*Stage1 + State2; State2 = B4*Stage1 + A4*Output + State3; State3 = B5*Stage1 + A5*Output; //end @RossClement[ AT ]gmail[ DOT ]com 'State3' should be added in this line -> State2 = B4*Stage1 + A4*Output + State3;

from : RossClement[AT]gmail[DOT]com
comment : The variable State3 is assigned a value, but is never used anywhere. Is there a reason for this?