Sign language service in HbbTV with WebAssembly

3Cat is developing a new approach to sign language service for DTT. The current model burns the interpreter’s video into the broadcast, with no option to customize it. The new approach streams the interpreter via broadband and shows it as a personalized PiP in an HbbTV app, synchronized with the broadcast. WebAssembly enables real-time software decoding on HbbTV TVs, proving feasible even on entry-level models. The result: a flexible, user-driven accessibility service ready for standardization

Description

3Cat has explored a new approach to delivering sign language on DTT while overcoming the limits of the current model, where the interpreter’s video is burned into the broadcast and cannot be disabled or customized.

The proposal delivers the interpreter’s video as a separate broadband stream, displayed through a personalized Picture-in-Picture window within an HbbTV app synchronized with the broadcast. Synchronization relies on TEMI timelines and low-latency DASH, ensuring precise alignment between interpreter and live content. This hybrid architecture keeps the broadcast untouched while adding accessibility via broadband.

To prove feasibility, 3Cat developed a WebAssembly (Wasm) module that decodes the sign language video entirely by software within a standard HbbTV environment. The app manages the full service, letting users enable, resize, move, or hide the interpreter window according to their preferences. A data injector was also created for the DTT headend to insert TEMI timeline metadata, while the low-latency DASH stream was generated using commercial off-the-shelf encoders, ensuring compatibility with existing infrastructures.

The main challenge was that most HbbTV TVs cannot natively decode two video streams simultaneously. Software decoding was once considered unfeasible—until Wasm brought near-native performance to browser-like environments, with 50–80% efficiency gains.

A complete proof of concept demonstrated smooth playback and precise sync even on entry-level TVs. The prototype was showcased at IBC Amsterdam, the HbbTV Symposium in London, and the EBU Experts Meeting in Paris, attracting strong interest from broadcasters and accessibility experts. Interoperability trials across Europe confirmed that TVs supporting Wasm can deliver the service reliably, paving the way for scalable deployment and future standardization.

Impact

The adoption of WebAssembly for sign language services in DTT has an impact that goes far beyond solving a technical limitation. For viewers, it represents a qualitative leap: the interpreter’s window can be enabled, resized, moved, or hidden, giving Deaf audiences real autonomy and placing sign language on equal footing with other accessibility services—optional, parallel to the main content, and configurable to user preferences. This flexibility enhances the viewing experience while reducing frustration for those who do not require the service.

For broadcasters, the model streamlines production. Instead of burning the interpreter’s video into the broadcast, they can deliver it as a separate stream for live, pre-recorded, or on-demand content, avoiding duplicate assets for VoD SL service, reducing costs and enabling scalable deployment. It also supports compliance with accessibility regulations, which increasingly recognize connected TV as a valid delivery channel.

At the industry level, incorporating WebAssembly into HbbTV strengthens the ecosystem by enabling performance-intensive applications beyond accessibility—such as interactive services, advanced graphics, and real-time visualizations. Tests confirming smooth operation even on entry-level TVs extend inclusion without requiring new hardware.

Socially, the initiative promotes equality and cultural recognition, supporting multiple sign languages and proving that accessibility can drive innovation across the entire broadcast landscape.