Continuous FFT Filter Development
I have high frequency hearing loss and the frequency response of my ears drop off rapidly after about 4.5kHz. A device, SongFinder, used to be available which divided frequencies by 2, 3, or 4 as well as offering a set of starting frequencies [high, medium, low?]. This is a hardware device implementing the transformation in chips (with embedded code?), it was expensive [~$500], and is no longer made [I have a used one from eBay!].
My idea was to recreate this hardware device in software [Python] -- I knew FFT could do the filtering. I had an RPi4+ with enough CPU power. What I didn't realise was the RPi audio was very limited and had no audio input at all. Also, NOBODY -- it seems, has created a practical realtime/continuous FFT filter in software. [I suspect some implementations exist for Ham Radio.]
After much research, I found the Overlap/Add technique [OLA] but found it was not applicable to continuous processing. I decided to implement a continuous OLA so that I could do FFT filtering with no glitches. My wireFFT.py started-out as the continuous input/output example for SoundDevice as example wire.py. I have added code in its Callback routine to handle OLA and rfft/irfft. [My feeling is that my approach is not elegant -- perhaps make it a class?] No filtering so far, but that is next, and I already have various divide-by and compression schemes implemented.
I bought an addon board for audio I/O -- Audio Injector Stereo SoundCard, -- with stereo I/O and an on-board electret microphone for monoral input. It can improve sampling rates to 96000Hz but even at the default 44100 sampling delays are minimal. [USB devices are cheap but introduce intolerable delays unrelated to sampling rate and are OK for development but useless in practice!] My SongFinder has fantastic stereo electret mics and Sony earphones -- the SoundCard doesn't support external electret mics [it has one on-board]. Thus, I'm a fair way from a portable software version of my 'hearing assist,' but I do have a proof of concept!
One day, perhaps a pocketable, inexpensive, and configurable PySongFinder will be a realization?