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FFT and IFFT using CMSIS DSP library, q15 fractional format

jeremy plantier
Hello,

I am currently trying to use the CMSIS DSP library to implement DSP functionality, by doing FFT or IFFT.

I don't have an embedded Floating point Unit, thus I would like to use the fixed q15 fractional format.
By reading the CMSIS documentation html page, it is specified that the FFT/IFFT output format is different from the input one...
For example, for a FFT length of 256, the input format is q15 (1.15) and the output format is 9.7. With 8 bits to upscale.
If I want to perform an FFT and directly fllowed by an IFFT, I have to convert output FFT data  which are in 9.7 into a q15 for IFFT Input format...

My question is, from my understanding, the conversion from q15 to 9.7 or the reverse, would be just done by a simple shift of 8 bits...?
But, I guess that data may be lost between each conversion...?
It is very confused to me...

Could someone help me ...?

Thanks!

PjEmY
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  • Note: This was originally posted on 18th June 2012 at http://forums.arm.com

    Thank you isogen74.

    Actually, I performed what I wanted latter during the last week.

    My problem was just to perform the right up scalling before sending data to the DAC. I am currently using the Q15 fractional format because it is the only SIMD instructions optimized format (without FPU use) available in the DSP CMSIS library.

    Now I have to implement Filtering (FIR) functions to avoid the resulting additional noise which appears after FFT/IFFT.

    Thank you again for your help!



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Reply
  • Note: This was originally posted on 18th June 2012 at http://forums.arm.com

    Thank you isogen74.

    Actually, I performed what I wanted latter during the last week.

    My problem was just to perform the right up scalling before sending data to the DAC. I am currently using the Q15 fractional format because it is the only SIMD instructions optimized format (without FPU use) available in the DSP CMSIS library.

    Now I have to implement Filtering (FIR) functions to avoid the resulting additional noise which appears after FFT/IFFT.

    Thank you again for your help!



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