Voice Carry Over

Description

Voice Carry Over (VCO) is an assistive telecommunication service standardized by 3GPP to support users who are deaf or hard of hearing but can speak. In a VCO call, the user with the hearing impairment uses their voice to speak directly to the called party. However, because they cannot hear the audio response, the response from the called party is converted into text and delivered to the user's device. This text is typically displayed on a text terminal, which could be a specialized Teletypewriter (TTY), a smartphone with a TTY software application, or a web-based relay interface. The conversion of speech to text for the hearing-impaired user is often performed by a human-operated or automated relay service.

The architecture for VCO involves several components. The user initiates a call, often dialing into a relay service first. The relay service operator (or an automated system) then places a call to the desired destination party. Once both connections are established, the VCO user speaks directly to the destination party. The destination party's speech is routed to the relay service, where it is transcribed into text. This text is then sent back over the network to the VCO user's text terminal. The underlying network must support the simultaneous or alternating transmission of voice and text data streams. This is often facilitated through in-band modem tones (for traditional TTY over voice channels) or more modern IP-based real-time text (RTT) protocols, which can be carried concurrently with voice in an IMS/VoLTE environment.

VCO is a critical component of Total Conversation services, which aim to provide equivalent telecommunications access by combining real-time video, voice, and text. 3GPP specifications such as TS 22.226 and TS 26.226 define the service requirements and codecs for VCO and other relay services. The service works in conjunction with other modes like Hearing Carry Over (HCO), where a user can hear but types their response. The implementation ensures low latency in text delivery to maintain conversational flow, making telecommunications accessible and practical for users with specific disabilities.

Purpose & Motivation

Voice Carry Over was created to address a significant accessibility gap in telecommunications for individuals who are deaf or hard of hearing but retain the ability to speak. Traditional telephone services were entirely audio-based, completely excluding this user group from direct conversation. While text telephones (TTYs) existed, they required both parties to have and use a TTY, which was not practical for communicating with the general public. VCO solves this by allowing the hearing-impaired user to leverage their own voice while relying on text for reception, enabling them to communicate with anyone via a relay service.

Its standardization in 3GPP, starting from Release 8, was motivated by legal and regulatory mandates in many countries requiring telecommunications services to be accessible. It addressed the limitations of previous assistive services by providing a more natural and efficient conversational model compared to a full text-to-text relay. By integrating VCO into the core network service specifications, 3GPP ensured that mobile and fixed network operators could deploy interoperable, reliable accessibility features, promoting social inclusion and compliance with disability rights legislation.

Key Features

• Enables users with hearing impairments to speak directly in a call
• Delivers the other party's speech as real-time text to the user
• Typically operates with the assistance of a telecommunications relay service
• Supports transmission via traditional TTY tones or modern Real-Time Text (RTT)
• Can be integrated into IMS/VoLTE networks for concurrent voice and text streams
• Facilitates more natural conversation flow compared to text-only relay

Evolution Across Releases

Defining Specifications