Nonstationary signal analysis spectral estimates

Dear all,

I am analyzing a signal which does not show either the stationary mean nor stationary variance. Infact the signal has broadly two values of mean, mean1 & mean2.

If i divide my signal into segments of time frames out of 200millisec data.

mean1 one prevails for 1st 20ms, then mean2 for nest 2ms then again mean1 for next ~30sec, then mean2 for next ~1sec and so on till 200ms.

In this scenario what is the proper way to use sftf, as i can't define a definite "piece-wise" stationary time step.

Also i would like to know about details of stft process matlab does and How to interpret results ([s,f,t,p]=sftf(...))

Regards