Results

Notes

• Nayuki: in-place single-precision complex-complex
• Nockert: double-precision real-complex
• Nayuki: double-precision complex-complex. Forward transform is scaled and I've scaled it back again here, which may introduce rounding error.
• Cross: double-precision real-complex in C, compiled with Emscripten. This is considered a slow implementation amongst native code ones.
• KissFFT: single-precision real-complex in C, compiled with Emscripten. This should be faster than Cross. Despite its name, it is the most sophisticated implementation here.

Rationale

If 2150 iterations of real-to-complex FFT of size 2048 takes less than 10 seconds, then we may be able to make a high quality real-time phase vocoder (just).

A phase-vocoder of course must use overlapped windowed FFT (although you can choose the size, within limits), IFFT, and cartesian-polar conversion to calculate the phase for the instantaneous frequency.

A reasonable estimate of CPU cost for the whole thing is somewhere around 10x the cost of simple non-overlapping short-time forward Fourier transforms across the signal.

2150 iterations corresponds to 100 seconds of audio non-overlapped at 44.1kHz, so if that takes less than 10 seconds, then in theory we might be OK.