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hi....
 i'm currently working on Analog Devices Dsp processor ADAU 1701..
Sigma studio is the software we are now on,in that i have some queries in the
way they calculate  filter coefficient.

- main specification i needed to design a filter 
1)Type[LPF,HPF,  etc]
2)frequency
3)Q
4)Gain

eg:
I want a 2nd order LPF with
Frequency= 1000
Q=0.5
Gain=1
 
The given below derivation is the one which i got from sigma studio help
window.

->ω0 = 2*pi*f0/Fs
->gainLinear = 10^(gain/20)

->Lowpass

->Transfer Function
->H(s)=1/(s^2+(s/Q)+1)  

->Coefficients
->alpha = sin(ω0)/(2*Q)
->a0 =   1 + alpha
->a1 =  -2*cos(ω0)
->a2 =   1 - alpha
->b0 =  (1 - cos(ω0)) * gainLinear / 2
->b1 =   1 - cos(ω0)  * gainLinear
->b2 =  (1 - cos(ω0)) * gainLinear / 2
After compiling we'll get coefficients a1,a2,b0,b1,b2 in hex format in CAPTURE
WINDOW of software.
i did manual calculation using the expression provided above and compare with
that of coeffients of software, but it differs.
Please help me to get detail calculation with the specification i given above.

 We know ω0 = 2*pi*f0/Fs
-what is the value of pi(180 0r 3.14)
-ω0 is the angular representation of requency i think it will be 180
-then in above value of alpha will be zero always.
-it means Q doesnt have any importantce in filter design
-please suggest a good book which describe in detail about filter design

with regards 
rammya

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