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Megha-

> I had one question. In functions like butter (for filters) we declare a variable Wn
> (the cut off frequency). ex: [B,A] = BUTTER(N,Wn). In this Wn value is between 0
> and 1, where 1 means half of sampling rate. What I am not understanding is where is
> it getting sample rate value from. I saw some codes and in that the sample rate
> value is not declared before using the command. Directly the command is used. Where
> is it getting sample rate value from?
> What I understand is suppose I have to run that filter design on an AIC. And AIC is
> set at some sampling rate ex 48KHz. Then the code is understanding that sample rate
> and setting Wn accordingly. Kindly correct me if I am wrong.

If your actual sampling rate is 48 kHz, and you want lowpass Butterworth filter with
2 kHz cutoff, then set Wn = 0.0833.

Digital filters don't "know" the actual analog sampling rate; they only know the
Z-plane unit circle, or 0 to 1 as you mention.  In this example, if you vary the
sampling rate (reprogram the AIC) then the filter cutoff is always 0.0833*fs/2.

-Jeff
> Jeff Brower <j...@signalogic.com> wrote:
>
>       Megha-
>     > I tried getting more material on maxflat but I am not getting much. I
>     > am not understanding what exactly SOS and G stand for.
>     > If I want coeff for IIRCAS51 (cascaded IIR filter design with biquads
>     > qith 5 coefficients) I want 3 coeff for zero (numerator) and 2 for
>     > poles (denominator). I am not getting how to get those. i am not
>     > understanding what does what stand for.
>      Maxflat() is for Butterworth IIR filter design.  It's one option -- you
>      also might want Chebyshev or elliptic design.Another MATLAB function for
>      converting direct-form transfer function to cascade of biquad sections is
>      tf2sos().  Here are some pages that might give you some clues:
>      http://www.math.psu.edu/local_doc/matlab/toolbox/signal/tf2sos.html
>      http://www.ee.ic.ac.uk/pcheung/teaching/ee3_Study_Project/iir_lab2.pdf-Jeff
>
>     > Jeff Brower <j...@signalogic.com> wrote:
>     >
>     >       Megha-
>     >       > This is in continuation to my previous mail. I also tried
>     >      with a breakpoint at my
>     >      > next statement after fltoq15. If I run that. It stops in
>     >      between and gives the
>     >      > error:
>     >      > Trouble running Target CPU: Attempted write to ROM at
>     >      Addr:0x00ffff, Page:0
>     >      > I guess it is because of the infinite loop thing. Kindly
>     >      reply and suggest
>     >      > something.Waiting for your reply.Your code is trying to
>     >      write somwhere in mem locations 0x8000-0xffff, which are
>     >      external Flash as you mentioned before. Writing directly to
>     >      Flash area won't work
>     >      (although there is a way, which involves a series of Flash
>     >      chip-specific commands).
>     >      You have to check the C code ptr and array address values
>     >      just before the breakpoint
>     >      to see why they are not pointing at onchip or offchip
>     >      SRAM.Also, did you see the c55x group thread about IIR
>     >      cascade fixed-point filters? I've
>     >      included a copy of the latest message below. If you're not
>     >      subscribed to the c55x
>     >      group, you should. C55xx devices are the next generation
>     >      after C54xx -- both 16-bit
>     >      fixed-point.-Jeff
>     >       -------- Original Message --------
>     >      Subject: Re: [c55x] IIR filter coefficients in iircas5 dsplib
>     >
>     >      Date: Tue, 16 May 2006 08:37:25 +0200
>     >      From: Christian Narvaez
>     >      To: c...@yahoogroups.com thanks Misan, thanks RK:
>     >      indeed this was the problem... I have successfully
>     >       implemented the
>     >      filter now.CNnasim ahmed ha scritto:
>     >      > hi christian,
>     >      > u r trying to get q14 fixed point format but dsplib
>     >      > has q15 format, i suggest u to use both in same format . u
>     >      can
>     >      > multiply with 2^15 . And two q15 multiplication results
>     >      q2.30. here 2
>     >      > sign bit presents so u hav to modify that according to ur
>     >      desired
>     >      > output format. i hope it will help u , if u hav any prob u
>     >      can send me
>     >      > message.
>     >      >
>     >      > regards misan
>     >      >
>     >      > */Christian Narvaez /* wrote:
>     >      >
>     >      > Hi,
>     >      > I am having a problem trying to implement an IIR filter on
>     >      an TI c55x.
>     >      > I have calculated the filter on matlab getting the
>     >      following
>     >      > coefficients for 2 second order sections:
>     >      >
>     >      > SOS
>     >      > 1.0000 1.6854 1.0000 1.0000 -1.7197 0.8565
>     >      > 1.0000 -1.9999 1.0000 1.0000 -1.8899 0.9250
>     >      > G
>     >      > 1.0000
>     >      > 0.1041
>     >      > 1.0000
>     >      >
>     >      > Simulations perform as expected.
>     >      > Since the coefficients are between ]-2..2[ I transform the
>     >      SOS matrix
>     >      > as SOS_fix = SOS*2^14:
>     >      >
>     >      > SOS_fix
>     >      > 16384 27614 16384 16384 -28175 14033
>     >      > 16384 -32766 16384 16384 -30964 15155
>     >      >
>     >      > The question is: can I run this coefficients on iircas5 of
>     >      the TI
>     >      > dsplib? I know dsplib is Q.15 while my coefficients are
>     >      Q.14, but I
>     >      > suppose this is not a problem if the filter's input is
>     >      small
>     >      > enough not
>     >      > to cause any overflow. Unfortunately this does not work,
>     >      the
>     >      > output is
>     >      > completely wrong.
>     >      >
>     >      > The second think I tried was to multiply the numerator
>     >      > coefficients of
>     >      > the second section of the filter by 0.1041 (according to
>     >      G), before
>     >      > converting them to Q.14, getting this:
>     >      >
>     >      > SOS_fix_G
>     >      > 16384 27614 16384 16384 -28175 14033
>     >      > 1706 -3412 1706 16384 -30964 15155
>     >      >
>     >      > But this also does not work.
>     >      >
>     >      > Does anyone have any idea how I should convert my
>     >      coefficients in
>     >      > order
>     >      > to work with iircas5?
>     >      >
>     >      > Thanks.
>     >      >
>     >      > Christian
>     >
>     >

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