STFT
Purpose
Description
Tips
Macro Synopsis
Modules
Related Functions
Purpose
Short-time Fourier Transform
Description
"STFT" computes a time-frequency distribution of an input
signal x within a given frequency interval as a sequence of
short-time spectra of windowed signal segments with constant
length.
This function returns a 2D signal of the adaptive time-frequency
spectrum. It is displayed as time (x-axis) versus frequency (y-axis) with the
power or amplitude spectral values given on the z-axis.
Parameters of "STFT" are
- lower frequency f0 in reciprocal x-axis units,
- upper frequency f1 in reciprocal x-axis units,
- fres, which is the output resolution of the frequency
axis (number of points),
- tres, which is the output resolution of the time axis (number of points)
- window size N (length of a signal segment) in samples,
- spectrum type (spectype), which is either
- "Power" (0) or
- "Amplitude" (1)
and
- type of normalization (normtype), which is either
- "Absolute Power/Amplitude" (0),
- "Spectral Density" (1) or
- "Maximum Norm" (2).
Tips
There is always an inherent tradeoff between time resolution 
and frequency resolution 
with 
being the sampling period (scale) of the signal. It can be expressed by the uncertainty relationship 
. Thus, the appropriate choice of N strongly depends on the scale (i.e. the sampling period) and the characteristics of the signal to be analysed.
Macro Synopsis
y =
STFT(x,f0,f1,fres,tres,N,spectype,normtype);
signal x, y;
float f0, f1;
int fres, tres, N;
option spectype, normtype;
Note that y is a 2D signal.
Modules
Spectral
Related Functions
Momentary ARMA spectrum, Multiscale TFD.