Head and Torso Simulator

Description

The Head and Torso Simulator (HATS) is a critical piece of test equipment defined within 3GPP specifications for the objective evaluation of speech and audio quality in telecommunications terminals. It is not a network function but a physical measurement apparatus designed to emulate the acoustic characteristics of an average adult human head and torso. This includes the shape, size, and acoustic impedance of the head, pinnae (outer ears), ear canals, and mouth simulator. The HATS incorporates high-fidelity microphones placed at the ear reference points (simulating the listener's ears) and a loudspeaker or artificial mouth at the mouth reference point (simulating the talker's mouth). These transducers are calibrated to known standards, allowing for precise measurement of acoustic signals.

The primary role of HATS in the 3GPP ecosystem is to enable standardized testing methodologies for Terminal Acoustic Characteristics (TAC) for speech transmission, as defined in specifications like 3GPP TS 26.131 and 26.132. During testing, a device under test (DUT), such as a handset, is positioned in a standardized manner against the HATS's artificial ear and mouth. Test signals are played through the artificial mouth, received by the DUT's microphone, processed by the DUT's codec and network, then returned and played out through the DUT's receiver or loudspeaker into the HATS's artificial ears. The HATS's microphones capture these signals, allowing for analysis of key parameters like Send and Receive Loudness Ratings (SLR, RLR), frequency response, distortion, and background noise performance.

This setup creates a controlled, repeatable laboratory environment that eliminates the variability introduced by human test subjects. The HATS's anthropomorphic design ensures that acoustic coupling, diffraction, and reflection effects approximate those of a real human user, making the measurements relevant to real-world usage. The data collected is used to verify compliance with 3GPP minimum performance requirements, ensuring a baseline level of speech quality and interoperability across different manufacturers' devices and network equipment. Its use extends from traditional circuit-switched voice calls to Voice over LTE (VoLTE) and Voice over NR (VoNR) testing, ensuring consistent audio quality assessment as core network technologies evolve.

Purpose & Motivation

The HATS was introduced to solve the fundamental problem of subjective and inconsistent audio quality testing in the telecommunications industry. Prior to its standardization, manufacturers and network operators relied heavily on subjective listening tests with human panels. These tests were time-consuming, expensive, and produced results that varied based on the listeners, their hearing acuity, and environmental conditions. This made it difficult to objectively compare the acoustic performance of different terminals or to enforce consistent quality standards across the industry.

The creation of the HATS, standardized in 3GPP Release 8, was motivated by the need for an objective, repeatable, and quantifiable method to assess the electro-acoustic performance of handsets and other terminals. It provides a reliable reference model of a human user, allowing engineers to measure key transmission parameters in a laboratory with high precision. This shift from subjective to objective testing was crucial for the mass production and certification of devices, enabling faster development cycles, reliable compliance testing, and the assurance of a minimum acceptable speech quality for end-users, regardless of the handset or network they use.