STI (Speech Transmission Index). We may approximately consider speech signal as broadband signal modulated by low-frequency signal. Articulation speed determines modulation frequency. When modulation depth decreases, speech signal becomes similar to noise and its intelligibility decreases. Accordingly, intelligibility decrease can be estimated according to modulation depth decrease also. Whole speech range is divided into 7octave bands. An octave noise signal is the input. The test signal intensity distribution agrees with the distribution of speech signal intensities. The modulating signal frequencies vary from 0.5 to 12.5 Hz with one-third-octave interval (14 frequencies in all). The STI measuring method is stated in the International standard IEC 268-16.

RATSI/STIPA (Rapid Speech Transmission Index). The STI method needs a lot of measuring procedures and calculations. A simplified method was developed, which provides for measuring only in 2 bands with 5 modulation frequencies and reduces the number of measuring procedures and calculations. For good intelligibility RASTI values must be not less than 0.6.

As these are analogous methods they have common disadvantages: speech transmission index as well as rapid speech transmission index imitates speech production process by means of noise model, but to take into account the properties of speech production and hearing in such a way is far from optimum.

Anticipating why we are posting short methods description and their disadvantages? That will be discovered in the next post about C50 – factor of clearence method.

The speech intelligibility index takes into account only the properties of hearing, not speech production and that is an obvious disadvantage of the method..

More methods to automate voice quality analysis are about to follow in the next posts!