I'm analyzing speech signal for identifying voiced and unvoiced regions. Voiced regions are supposed to have a "pitch", which can be estimated using auto-correlation function (ACF). Basically, one estimates ACF for each frame of speech (say 20ms) and then finds the time lag between peaks in ACF output. If all the significant peaks in ACF output are equidistant, I can say that the signal is very much periodic. If the peaks have random spacing between the, that would indicate aperiodicity.
Based on this, what measure can I use to find out HOW MUCH periodic a signal is? If I denote time lag between successive peaks as lag1,lag2...lagN; the deviation from mean of these values can tell how much periodic the signal is. Any better ideas?