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Discussion Groups | Audio Signal Processing | FFT on a narrow band

Technical discussions related to Audio Signal Processing (digital effects, acoustics, noise reduction, musical signal processing, etc).

  

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FFT on a narrow band - Jean Viljoen - Nov 3 5:50:00 2003

Does anybody know if it is possible to calculate a FFT on a certain
bandwith, less that the sampling frequency?
 
For example:  I only have the resources to do a 64 point FFT, but I know
the energy I want to measure is between 1kHz and 2kHz (fs = 8 kHz). Can
I do a 64 point FFT on the band 1kHz – 2kHz ?
 
Rgds
JEAN
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Re: FFT on a narrow band - Al Clark - Nov 3 10:26:00 2003

At 03:50 AM 11/3/2003, Jean Viljoen wrote:
>Does anybody know if it is possible to calculate a FFT on a certain
>bandwith, less that the sampling frequency?
>
>For example:  I only have the resources to do a 64 point FFT, but I know
>the energy I want to measure is between 1kHz and 2kHz (fs == 8 kHz). Can
>I do a 64 point FFT on the band 1kHz ­ 2kHz ?
>
>Rgds
>JEAN
>
>  Yes, Its called a zoom FFT. Try a Google search on this topic and I 
> think you will find your answer.
>
>Here is a link to the basic idea: http://www.numerix-dsp.com/zoomfft.html
	Al Clark
Danville Signal Processing, Inc.
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Re: FFT on a narrow band - Tony Zampini - Nov 3 11:27:00 2003

Jean,
Check out the Chirp z-transform. I'm not sure, but I think
this can do what you want.

Regards,
Tony
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www.dspglobal.com

----- Original Message -----
From: "Jean Viljoen" <jean.viljoen@jean...>
To: "Audio DSP Group" <audiodsp@audi...>
Sent: Monday, November 03, 2003 4:50 AM
Subject: [audiodsp] FFT on a narrow band
	> Does anybody know if it is possible to calculate a FFT on a certain
> bandwith, less that the sampling frequency?
>
> For example:  I only have the resources to do a 64 point FFT, but I know
> the energy I want to measure is between 1kHz and 2kHz (fs == 8 kHz). Can
> I do a 64 point FFT on the band 1kHz - 2kHz ?
>
> Rgds
> JEAN
>
>
>
>
>
>
>
> _____________________________________
> 
>
>
> 
>
>
>
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Re: FFT on a narrow band - alex...@philips.com - Nov 3 12:45:00 2003

If you want full spectral resolution in the band from 1kHz to 2kHz (32
frequency
bins for a 64 point FFT) then do the following...

What you could try is first bandpass filter the signal between 1kHz and 2kHz*,
then decimate by 4 to give a sample rate of 2kHz (now the spectrum will be
re-mapped: 1kHz to 2kHz becomes 0 to 1kHz).
Performing a FFT will give the spectrum from 2kHz to 1kHz.

* Note: you should suppress all components below 1kHz and above 2kHz
to avoid aliasing, so your pass band will be less than 1kHz.

Alex Young
DSP software Engineer
Consultant for Philips Digital Systems Laboratories
	                                                   To:   "Audio DSP
Group" <audiodsp@audi...>                                            
                     
                                                   cc:   (bcc: Alex
Young/LEU/PDSL/PHILIPS)                                                         
                  
                                                   Subject:    [audiodsp] FFT on
a narrow band                                                                   
     
                                                                                
                                                                                
     
               "Jean Viljoen"                      Classification:    
                                                                                
               
               <jean.viljoen@jean...                                         
                                                                                
     
               >                                                             
                                                                                
        
                                                                                
                                                                                
     
               03/11/03 10:50
	Does anybody know if it is possible to calculate a FFT on a certain
bandwith, less that the sampling frequency?

For example:  I only have the resources to do a 64 point FFT, but I know
the energy I want to measure is between 1kHz and 2kHz (fs == 8 kHz). Can
I do a 64 point FFT on the band 1kHz ? 2kHz ?

Rgds
JEAN
	_____________________________________
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