// Compute the spectrogram of a signal with window width nfft and overlap
// percentage overlap.
func Compute(signal []float64, nfft int, overlap float64) Spectrogram {
spectra := make([][]complex128, 0)
start := 0
end := start + nfft
off := int((1 - overlap) * float64(nfft))
// pre-compute the window function
window := window.Hamming(nfft)
// pre-allocate buffer to hold window * signal
windowed := make([]float64, nfft)
for end < len(signal) {
for i, s := range signal[start:end] {
windowed[i] = s * window[i]
}
spectrum := fft.FFTReal(windowed)
// FIXME: memory is still used for the entire spectrum since the GC doesn't
// understand and can't free internal and partial pointers, to free it must
// do a copy. If this spectrum persists for a while then we should do a
// copy().
spectra = append(spectra, spectrum[0:len(spectrum)/2])
start += off
end += off
}
spectrogram := Spectrogram{spectra, nil}
return spectrogram
}
// ApplyHamming applies a hamming filter to each column
func ApplyHamming(x [][]float64) {
segments, size := len(x), len(x[0])
hamming := window.Hamming(size)
for i := 0; i < segments; i++ {
for j := 0; j < size; j++ {
x[i][j] = x[i][j] * hamming[j]
}
}
}