**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?