SideTone Masking Rating

Description

The SideTone Masking Rating (STMR) is a psychoacoustic parameter defined within 3GPP specifications to objectively measure the performance of the sidetone path in a terminal, such as a mobile phone or a VoIP handset. Sidetone is the deliberate introduction of a user's transmitted speech signal, attenuated and possibly processed, back into the receiver (earpiece). This acoustic feedback is essential because it provides users with auditory confirmation that their voice is being transmitted and helps them regulate their speaking volume in varying ambient noise conditions. Without sidetone, users tend to speak too loudly in quiet environments or too softly in noisy ones, leading to unnatural conversation dynamics and potential listener discomfort.

Technically, STMR is derived from measurements of the terminal's electro-acoustic characteristics. The process involves applying a standardized test signal (typically speech-like noise) to the terminal's microphone and measuring the resulting acoustic output at the receiver. The STMR value, expressed in decibels (dB), quantifies the effective loss or attenuation of the sidetone path. A higher STMR value indicates a lower level of sidetone (more attenuation), while a lower value indicates a higher level of sidetone. The measurement accounts for the terminal's overall frequency response, including the microphone, digital signal processing (DSP) algorithms for noise suppression and echo control, and the receiver's acoustic properties. The goal is to ensure the sidetone level is within a range that feels natural to the user—present enough for conversational comfort but not so loud as to cause instability (howling) or mask the far-end speaker's voice.

The role of STMR in the network ecosystem is one of quality assurance and interoperability. While sidetone generation is a function of the terminal itself, 3GPP standards define target STMR ranges to ensure a consistent user experience regardless of the handset manufacturer or network technology (e.g., 2G, 3G, VoLTE). Network operators and terminal manufacturers use these specifications during device type approval and conformance testing. Compliance with STMR requirements helps prevent user complaints related to voice quality, such as the sensation of talking into a 'dead' phone or experiencing uncomfortable echo. Furthermore, in the context of Voice over LTE (VoLTE) and Voice over NR (VoNR), where voice is packetized, the terminal's DSP plays a critical role in managing sidetone in conjunction with acoustic echo cancellation (AEC) algorithms. The STMR metric ensures that these complex digital processing chains still deliver a fundamental, natural telephony experience.

Purpose & Motivation

The STMR metric was created to address a fundamental challenge in telephony: replicating the natural acoustic feedback present in face-to-face conversation. In early telephony systems, sidetone occurred naturally due to imperfect isolation between the microphone and receiver in the carbon-granule telephone handset. As technology evolved towards electronic and later digital terminals, this acoustic coupling was eliminated to prevent instability and improve voice clarity. However, its complete removal made the telephone feel unnatural and disconnected, leading users to speak unnaturally. The purpose of standardizing STMR was to provide an objective, repeatable method for terminal designers to engineer a controlled, optimal amount of electronic sidetone back into the system.

Prior to standardization, sidetone implementation was largely at the discretion of equipment manufacturers, leading to significant variability in user experience. A user switching between different phone models could encounter jarring differences in how 'live' the connection felt, potentially impacting perceived call quality and user satisfaction. The introduction of STMR in 3GPP Release 5, alongside other voice quality metrics, formalized this aspect of terminal performance. It solved the problem of inconsistent subjective quality by defining a measurable target. This was particularly important with the globalization of mobile communications, ensuring that a phone certified in one region would provide an acceptable basic voice experience in another. It addresses the core problem of balancing the need for clear, echo-free transmission with the human factors requirement for conversational comfort and natural interaction.

Key Features

• Objective psychoacoustic measurement of sidetone path loss
• Standardized test signals and measurement procedures defined in 3GPP TS 26.131/132
• Expressed as a single-figure rating in decibels (dB)
• Ensures consistent conversational feel across diverse terminal designs
• Integral part of terminal acoustic conformance testing
• Supports quality of experience (QoE) for voice services

Evolution Across Releases

Defining Specifications