问题
I am trying to use FFT to decode morse code, but I'm finding that when I examine the resulting frequency bin/bucket I'm interested in, the absolute value is varying quite significantly even when a constant tone is presented. This makes it impossible for me to use the rise and fall around a threshold and therefore decode audio morse. I've even tried the simple example that seems to be copied everywhere, but it also varies... I can't work out what I'm doing wrong, and my maths is not clever enough to understand all the formulas associated with FFT. I now it must be possible, but I can't find out how... can anyone help please?
回答1:
Make sure you are using the magnitude of the FFT result, not just the real or imaginary component of a complex result.
In general, when a longer constant amplitude sinusoid is fed to a series of shorter FFTs (windowed STFT), the magnitude result will only be constant if the period of the sinusoid is exactly integer periodic in the FFT length. e.g.
f_tone modulo (f_sampling_rate / FFT_length) == 0
If you are only interested in the magnitude of one selected tone frequency, the Goertzel algorithm would serve as a more efficient filter than a full FFT. And, depending on the setup and length restrictions required by your chosen FFT library, it may be easier to vary the length of a Goertzel to match the requirements for your target tone frequency, as well as the time/frequency resolution trade-off needed.
来源：https://stackoverflow.com/questions/50049437/fft-frequency-bucket-amplitude-varies-even-with-constant-tone-applied