Filter the data, slice by slice.
Calculate FFT of time segment consecutively.
Calculate Power Spectrum of each slice consecutively.
Convert spectrum into decibels.
Strip out the repeated data.
The frequencies go from 0 to half the sampling rate. Hence the frequencies are in steps of (in Hz):
Scale the spectra so that the values are between 0 and 255.
Consecutive rows represent slices in time, while columns contain the spectra at each time.