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Intelligibility - Methods for measuring STI according to the standard ( IEC 60268-16)

Intelligibility - Methods for measuring STI according to the standard ( IEC 60268-16). Lars H. Morset Morset Sound Development Trondheim, Norway. Standard IEC 60268-16: Objective rating of speech intelligibility by speech transmission index

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Intelligibility - Methods for measuring STI according to the standard ( IEC 60268-16)

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  1. Intelligibility - Methods for measuring STI according to the standard (IEC 60268-16) Lars H. Morset Morset Sound Development Trondheim, Norway

  2. Standard IEC 60268-16: Objective rating of speech intelligibility by speech transmission index ISO/TR 4870:1991, ITU-T Rec. P.50:1994 , ITU-T Rec. P.51:1994 3 methods defined: STI - Speech Transmission Index RASTI - RApid STI STITEL - STI for TELecommunication systems You can use STI and forget about RASTI and STITEL Why? RASTI and STITEL are condensed versions of the STI and have limitations (distortion and noise interference) that the STI does not have. The computation time is not a problem on todays PCs What does the standard say?

  3. Based on MTF (Modulation Transfer Function) MTF should be evaluated Reduction in the modulation index STI is today found from impulse response must also consider noise If microphone and control room are to be included, then: loudspeaker directivity is important (mouth simulator) Correcting for reverberation changes due to occupancy is difficult but the STI should not get worse, so for a verification it only needs to be considered in questionable cases You do not have to include background noise in measurement easier to do it in the post-processing can later see what the improvement is by reducing background noise More about the Standard

  4. 2 Factors: Room impulse response (early reflections and reverberation) Speech level compared to background noise in octave bands If low background noise, it can be omitted Problem when measuring it in one operation: need correct for background noise and source Solution: Measure impulse response Measure noise (e.g. using software or SPL-meter) or predict noise Measure speech level (or use standard values) Correct for speech and noise as post-processing If using MLS, must use emphasis (Gaussian amplitude distribution) if not, distortion will not be treated as noise (spurious peaks) Our approach for measuring STI

  5. This method is operator-independent. “Impulse response”-part is automatically computed No fiddling around with setting of variables Speech and noise is measured You can easily find the STI for various speech or noise levels Useful for estimating how to meet the STI recommendations This can be done as post-processing, not during the measurements You can play the excitation signal from CD or .wav-file Using a single sine sweep this should be no problem Good repeatability You will get exactly the same answers when repeating the measurement You do not need to add external noise during the measurement Noise can be added as post-processing Some advantages with this method

  6. Load proper setup file Perform imp. res. measurements Perform noise measurement Using software Alternativ: Predict noise (NR, NC) C50, RASTI, STI, % ALcons (derived from STI) Table / Plot of C50, STI See effects on changing Noise and Speech Measurement example

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