A MATLAB toolbox for Computation of Speech Transmission Index (STI)
Pavel Rajmic, Jiří Schimmel, Šimon Cieslar
TL;DR
This work addresses the need for accessible, standards-compliant STI implementations by delivering an open-source MATLAB toolbox that implements both direct (Full STI and STIPA) and indirect STI per sti_norm2020. It provides detailed signal-generation and measurement-chain methodologies, along with ambient-noise and auditory-effect corrections, and uses the Schroeder framework for indirect STI from impulse responses. The toolbox is numerically verified and benchmarked against a professional device in a real environment, demonstrating practical viability and highlighting nuances between direct and indirect estimates, as well as STIPA limitations in reverberant conditions. By enabling hardware-independent STI computation and rich visualization, the work supports education, research, and device testing, and invites community contributions to broaden applicability.
Abstract
The speech transmission index (STI) is a popular simple metric for the prediction of speech intelligibility when speech is passed through a transmission channel. Computation of STI from acoustic measurements is described in the IEC 60268-16:2020 standard. Though, reliable implementations of STI are not publicly accessible and are frequently limited to the use with a proprietary measurement hardware. We present a Matlab STI implementation of both the direct and indirect approaches according to the standard, including the shortened STIPA protocol. The suggested implementation meets prescribed requirements, as evidenced by tests on reference signals. Additionally, we conducted a verification measurement in comparison to a commercial measurement device. Our software comes with open source code.