FFT questions

Hi everyone,

I thank you for your help.

Here is another question : the goertzel algorithm uses float numbers
but the DSP i'm working with (TMS320VC5402) is a fixed-point one. How
will I be able to achieve this algorithm?

Thansk again.

Jerome.

> 
> > Jerry Avins <jya@ieee.org> wrote in message news:<40a25737$0$3039> 
> > 
> >>The Goertzel algorithm is very efficient. It can be looked on as an FFT
> >>that concentrates on one bin, but that confines one to using integers
> >>where a fractional value may be more efficient yet. Explore the links at
> >>http://www.google.com/search?q=goertzel%20algorithm and
> >>http://groups.google.com/groups?q=goertzel%20algorithm
> >>
> >>Jerry
> > 
> > 
> > It's me again! For the moment here are my first two questions about
> > Goertzel:
> > 
> > 1) In some papers k=0.5+(N*F)/Fs, and in other ones k=(N*fF)/Fs. Which
> > is the good one?
> 
> Probably both. I'll bet that the symbols are defined differently, and 
> that the corresponding formulas are correct for each definition.
> 
> > 2) With my DSP (TMS320VC5402) the samples x(n) are in the Q15 format.
> > I think I'll have problems when computing Q0=coeff*Q1-Q2+sample, won't
> > I?
> 
> Most samples that come from ADCs are integers, but you can construe them 
> any way you like. Q15 on a 16-bit machine is the equivalent of 
> construing integer shifted 15 bits (leaving the sign bit in place. The 
> values of such numbers lie between -1 and 1 - 2^-15. The product of two 
> such numbers is a double that lies between -1 and 1 - 2^-30. The more 
> significant word is the product you will store.
> 
> Note: When using an integer multiplier, you must shift the less 
> significant word left one place, putting its MSB into carry, then shift 
> the more significant word left to eliminate the "extra sign bit" and 
> shift carry into the LSB. A Q15 multiplier does these operations 
> automatically. When q15 multiplication is an option, it is usually 
> specified by a bit in a control register.
> 
> > Anyway thank you very much. The idea of using the Goertzel algorithm
> > seems to be, for the moment, the best i could find. Apparently it
> > exactly does what i need, so once again, thank you.
> 
> You're very welcome.
> 
> Jerry

french_student wrote:
> Hi everyone,
> 
> I thank you for your help.
> 
> Here is another question : the goertzel algorithm uses float numbers
> but the DSP i'm working with (TMS320VC5402) is a fixed-point one. How
> will I be able to achieve this algorithm?

Search TI's web site for their DTMF app note.  I believe it will have 
sample Goertzel code (I know ADI's DTMF app note does, but you're using 
a TI DSP).  They probably also have an app note describing fixed-point 
arithmetic - which they call the "Q15 format"  while ADI calls it "1.15 
format" or "1.31  format" (depending on word-size).


-- 
Jim Thomas            Principal Applications Engineer  Bittware, Inc
jthomas@bittware.com  http://www.bittware.com          (703) 779-7770
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