Mouth Reference Point

Description

The Mouth Reference Point (MRP) is a critical component of the 3GPP's standardized acoustic test framework, specifically defined for terminal equipment like mobile phones and headsets. It is not a physical component within the network but a reference model used in laboratory environments. The MRP is part of an artificial head and torso simulator (HATS) that mimics the acoustic properties of a human speaker. Its primary function is to provide a geometrically and acoustically defined point from which test speech signals are emitted, simulating a human mouth's position and acoustic characteristics. This ensures that all devices under test receive the same acoustic stimulus, allowing for fair and comparable assessments of their transmission performance.

The technical implementation of the MRP is detailed across multiple 3GPP specifications, which define its exact spatial location relative to the artificial head's reference point (HRP) and the ear reference point (ERP). The specifications, such as TS 26.131 and TS 26.132, provide stringent requirements for the acoustic source used at the MRP, including its frequency response, directivity, and sound pressure level. During testing, a standardized speech signal is played through a loudspeaker or simulator positioned at the MRP. The device under test, placed in a standardized position relative to the artificial head, captures this signal. The received audio is then analyzed to measure key performance indicators like sending loudness ratings, frequency responses, and background noise transmission.

The role of the MRP extends beyond simple signal generation; it is foundational for ensuring interoperability and quality of service (QoS) for voice services. By providing a controlled and repeatable acoustic source, it enables network operators and device manufacturers to verify that terminals comply with 3GPP's transmission requirements. These requirements are essential for maintaining consistent voice quality across the network, regardless of the handset model. The MRP is used in conjunction with other reference points, like the ERP for listening tests, to form a complete end-to-end acoustic test setup. This holistic approach validates both the sending (talker) and receiving (listener) paths of a voice call.

Furthermore, the MRP's definition supports the evolution of voice codecs and advanced services like Voice over LTE (VoLTE) and Voice over NR (VoNR). As new codecs (e.g., EVS, AMR-WB) are introduced, the MRP ensures that acoustic testing remains consistent, allowing for the evaluation of enhanced voice quality features. Its specifications are periodically reviewed and updated to accommodate new testing methodologies and device form factors, such as wearable devices and smart speakers, ensuring the reference point remains relevant for modern communication scenarios.

Purpose & Motivation

The Mouth Reference Point was created to solve a fundamental problem in telecommunications testing: the lack of reproducibility and consistency in acoustic measurements of voice terminals. Before its standardization, different manufacturers and test houses might use varying setups for simulating a human talker, leading to inconsistent test results. This made it difficult to objectively compare the transmission quality of different devices or to enforce network quality standards. The MRP provides a unified, objective reference, ensuring that all parties measure acoustic performance under identical conditions.

Historically, the need for such standardization became acute with the proliferation of mobile phones and the globalization of the telecommunications market. Network operators needed guarantees that any certified device would provide an acceptable user experience on their network. The MRP, introduced in 3GPP Release 5, was part of a broader effort to define comprehensive terminal testing standards. It addressed the limitations of ad-hoc testing by providing a scientifically defined model of the human vocal system's acoustic output, based on extensive research into speech acoustics and hearing.

The MRP continues to be essential because voice quality remains a key performance indicator for mobile networks, even as data services grow. It solves the problem of ensuring that advancements in device design (smaller microphones, noise cancellation) do not degrade the fundamental voice transmission parameters. By defining a stable reference, it allows for the evolution of test standards to cover new technologies like wideband audio, hands-free operation, and voice over IP, while maintaining backward compatibility and a consistent baseline for quality assessment.