Hi!
I wonder how to interpret the data after applying a Short Time FFT (fftw3
for real data) on audio samples (sinus tone 1400Hz).
My goal is to visualize the Energy in the frequency bands.
First, I do a 512 point FFT on the whole audio file (frame shift without
overlap) and the highest value after computing
[1] re*re + imag * imag
is about 9997 in a fft bin.
Second, I do a 1024 point FFT and now the highest value after computing [1]
is '228802'.
Why there is such a difference?
I mean, my 16Bit audio file is a sinus tone at 1400Hz, normalized to 100%,
means minimum sample value is -32768, maximum 32767.
I think, the highest value of energy does not depend on the fft size I use
to find this value?
Frequency analysis in CoolEdit show that the highest peak (0dB) of my sinus
tone is at 1400Hz. The more FFT coefficients I use, the more spiky it is.
But there is still a maximum of 0dB every time. And this maximum could be
found at 1400Hz.
So, do I have to apply a certain normalization after computing [1]?
I want to map the results of [1] to a color map for displaying the
spectrogram. But how to map it proper when the highest value differs with
the number of fft-coefficients.
Thanks in advance, Michael