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espressif / esp-dsp

DSP library for ESP-IDF
Apache License 2.0
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Fixed-point FFT is slower than floating point FFT #13

Closed wzli closed 4 years ago

wzli commented 4 years ago

Hi, Is there any official benchmarks for the speed of fixed point FFTs?https://docs.espressif.com/projects/esp-dsp/en/latest/esp-dsp-benchmarks.html does not seem to be updated.

I've been testing myself on a AI-Thinker ESP32-CAM board, and my results for N=64 show that the sc16 FFT is slower than fc32 FFT by about 40%. This hold true for N=1024 as well, where it was 130k cycles for floating point, and 230k cycles for fixed point. Is that expected performance or did I do something wrong?

dmitry1945 commented 4 years ago

Hi wzli,

Yes, it's expected performance. And the reason for that is next: In Esp32 we have floating point unit. And for floating point operation and for fixed point operation we will spend the same amount of add and multiply operations. But, for fixed point, to have acceptable SFDR, you have to make additional job like rounding. You can look to the implementation of fixed point FFT in ansi C. You will see the difference.

Regards, Dmitry

wzli commented 4 years ago

I see. I was under the impression that there would be SIMD instructions optimized for short integers (like the ARM Cortex-M DSP extentions). Otherwise there seems to be no reason to support 16-bit fixed point in the first place, compared to the float implementation which is better in general. In any-case, please posts the benchmarks for future reference. Thanks.