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Altera_Forum's avatar
Altera_Forum
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16 years ago

Band Pass filter output data

Hi everyone,

I use FIR compiler v8.1 to generate a band pass filter 50hz

Configuration :

- Input Data unsigned width : 8 bits

- I use a coef file generated by Matlab

- coefficients scaling : Auto ... Bit Width : 8

--- Quote Start ---

-0.03758602

-0.04600525

-0.05416903

-0.05998012

-0.05842221

-0.04470215

-0.01824851

0.01576782

0.04887938

0.07399496

0.08870862

0.09481480

0.09481480

0.08870862

0.07399496

0.04887938

0.01576782

-0.01824851

-0.04470215

-0.05842221

-0.05998012

-0.05416903

-0.04600525

-0.03758602

--- Quote End ---

and I try to undestand why i have a signed data out ( see image )

fir data in/out (http://img190.imageshack.us/img190/9161/firbp.jpg)

21 Replies

  • Altera_Forum's avatar
    Altera_Forum
    Icon for Honored Contributor rankHonored Contributor
    16 years ago

    Your coefficients are signed but your input is declared unsigned. I believe this will cause trouble. You can declare your input as signed adding the sign bit = '0' always and see.

  • Altera_Forum's avatar
    Altera_Forum
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    16 years ago

    one more issue.

    You should expect that your output will revert back to signed. The reason is your filter doesn't pass dc. if your input is unsigned it implies dc offset which is filtered off.
  • Altera_Forum's avatar
    Altera_Forum
    Icon for Honored Contributor rankHonored Contributor
    16 years ago

    I can't use signed data in because it's provided from unsigned FIR LP.

    Sorry but I don't understand the relation with DC... In this case, what should i do
  • Altera_Forum's avatar
    Altera_Forum
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    16 years ago

    In that case your input is always treated as positive only i.e. there is dc offset in it (dc equals the mean of the signal). You can remove this by subtraction but better leave it and your bandpass filter will remove the mean. Hence your output will be signed and tending to around zero. There is nothing wrong in this case.

  • Altera_Forum's avatar
    Altera_Forum
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    16 years ago

    You mean your output gets stuck for many samples on same value before it moves to next...

    Can you post your input data and output data values and I will get output data through matlab
  • Altera_Forum's avatar
    Altera_Forum
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    16 years ago

    Your output doesn't look right.

    your input swings between 0 and 243, yet your output is of +160,000 order with repeated close values.It doesn't look you have done any bit truncations.

    Go to matlab and type:

    h = < your above coefficients>;

    x = <your input vector>;

    now apply filter:

    y = filter(h,1,x); you should check your results against this as reference

    I got an initial y values of +80 followed by +4
  • Altera_Forum's avatar
    Altera_Forum
    Icon for Honored Contributor rankHonored Contributor
    16 years ago

    I got the same thing and i don't know how to resolve the problem!!!!