I'm trying to plot a proper spectrogram for sound data, in Qt. And now i can plot a nice looking spectrogram with a fft window length 1024 and overlapping data length 976. But it is usual to use small window length to plot spectrogram such as 256, and that ll give me a better time resolution. But when I reduce the window lenght to 256 I get empty rows in the spectrogram. I know that reducing the window length causes low frequency resolution on FFT, but why isn't there any empty rows in spectrograms of other programs. Am I doing something wrong or is this result normal?
Here are the main steps that i use to get the spectrogram: -Get the data from the sound file. -FFT on first window(hanning). -convert the complex fft output into magnitude array( magnitude = sqrt(re^2 + im^2) ) -convert magnitude to log scale(dB) magnitude_dB = 20*log10(magnitude) -plot the first column of the spectrogram by scaling the spectrum data to fix it into the column -move the window 48 samples, for next operation..
You can see the problem on these screen shots: window length 1024 window length 256
(btw my goal is to make a program which can recognize the phonems from speech sound data. Using FFTW library)
I really appreciate any help you can provide.
