Support iFFT (DSP-97)

[howard0su]

Current dsp lib supports only FFT, please add iFFT support. It is common to process signals in frequency domain which do fft, filter, ifft.

[dmitry1945]

Hi @howard0su

Yes, we will add.

Regards,
Dmitry

[drewbharris]

@dmitry1945 any updates on this? Thanks!

[copych]

I am not that good at math, but having FFT working it shouldn't be that hard to implement iFFT. But if we do that outside the ESP_DSP, that'll be waist of CPU cycles, cause we need to swap Re and Im arrays twice, while inside the library code you just change indexing instead. So please, add iFFT.

[crosswick]

I am also very interested in efficient iFFT.