The Speech Transmission index is based on the Modulation Transfer Function. The MTF can either be measured directly in the modulation domain, as suggested by T. Houtgast and H.J.M. Steeneken, or calculated from the impulse response. The method described in this paper measured the noise-free impulse response and calculates the MTF according to the definition by M.R. Schoeder. The noise-free MTF is then corrected for the influence of noise by a measurement of the background noise.
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