Line 21
Line 21 is an AI-supported software designed to facilitate the creation, enhancement, translation, and delivery of live captions. The tool is uniquely designed ...
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What is Line 21?
Line 21 is a UK‑based live captioning platform that combines human captioners with AI agents to deliver real‑time closed captions for online streaming, in‑venue events, and broadcast. It supports over 100 languages and integrates with YouTube, Twitch, Facebook, Vimeo, Microsoft Teams, Zoom, Microsoft Skycap, and custom RTMP or HLS streams, making it suitable for creators, broadcasters, event organizers, and agencies that need accurate, accessible captions at scale.
The platform lets users choose between fully manual captioning by expert captioners, automatic speech recognition (ASR) from multiple providers, or a hybrid mix where AI captions are polished by humans. It includes an AI Proofreader that continuously monitors the context of an event and corrects captions in real time to reduce misnaming and errors, while also enabling real‑time translations and synthetic voices for multilingual audiences.
Line 21 is designed for high‑stake events that demand low latency and high accuracy, such as webinars, conferences, TV broadcasts, and corporate broadcasts, as well as for agencies that serve multiple clients with different destinations and language requirements. Through its API, overlay tools, and encoder options (including CEA 608/708), it can plug into existing broadcast and streaming workflows so teams can manage captions from a single interface without rebuilding their pipeline.
Line 21 pricing
Pricing model: Paid
Line 21 does not show a fixed public pricing grid; instead it offers custom quotes based on events, streams, and agency needs. Pricing typically depends on the mix of human captioning versus AI ASR, number of hours streamed, number of languages, and complexity of integrations (e.g., RTMP/HLS, multiple platforms, or multi‑destination outputs). Agencies and large‑scale broadcasters can request tailored packages that bundle captioning, AI Proofreader, translations, and technical support, while smaller events may be quoted per‑hour or per‑event based on requirements.
Line 21 pros
- Supports over 100 languages for captioning and translation
- Hybrid captioning with both human and ASR options
- AI Proofreader that corrects live captions in real time
- Real‑time AI‑powered translations for multilingual viewers
- Integrates with YouTube, Twitch, Facebook, and Vimeo
- Supports RTMP and HLS streaming for custom setups
- Works with Microsoft Teams, Microsoft Skycap, and Zoom
- Embeddable player for browser‑based caption views
- Meeting Bot for automated captioning in meetings
- Custom caption encoders with CEA 608/708 support
- Low‑latency delivery to multiple output destinations
- API for programmatic control and integration
- Live flow monitor to track caption quality and status
- Human captioner interface optimized for professional captioners
- Flexible pricing and usage tailored to events and agencies
Line 21 cons
- No clearly listed free tier; pricing is custom‑quoted
- Requires external ASR integration rather than built‑in proprietary engine
- Complex setup may require technical support for large deployments
- Limited details on latency figures for all integration paths
- Global language support does not guarantee equal quality for every language
- No self‑serve public pricing table visible on the main site
- Support and account management may be agency‑led rather than fully automated
- Some advanced features may require coordination with their team
Frequently asked questions about Line 21
How does Line 21 handle live captioning?
Line 21 routes live audio through either human captioners or external ASR engines, then processes the captions through its platform before sending them to your chosen destinations. It can inject captions into RTMP or HLS streams, embed them into browser links, or push them to platforms like YouTube, Twitch, Facebook, Vimeo, Microsoft Teams, and Zoom, all with minimal added latency.
What is the AI Proofreader and how does it work?
The AI Proofreader is a feature that learns the context of your live event in real time and applies corrections to captions before they appear on screen. It uses large‑language models such as GPT‑4o, GPT‑5, and Claude 3.5 to fix misnamed terms, typos, and inconsistent phrasing, acting as a final accuracy layer without delaying the live feed.
Which platforms can Line 21 caption?
Line 21 supports captioning for YouTube, Twitch, Facebook, Vimeo, Microsoft Teams, Microsoft Skycap, Zoom, and custom RTMP or HLS streams, as well as browser‑based caption views and venue displays. You can configure one source feed and route captions to multiple platforms simultaneously from the same Line 21 setup.
Can Line 21 translate captions into other languages?
Yes, Line 21 can translate live captions into over 100 languages in real time, either starting from source‑language captions generated by ASR or human captioners. Translated subtitles can be sent to different outputs or embedded into the same stream with multiple language options for viewers.
Is Line 21 suitable for broadcast TV and in‑venue events?
Yes, Line 21 is designed for broadcast TV and in‑venue captioning via RTMP, HLS, and CEA 608/708 encoding, allowing TV channels and AV teams to insert closed captions into feeds shown on screens, set‑top boxes, and broadcast systems. It provides low‑latency caption delivery and supports mandatory caption standards for regulatory compliance.
How do I integrate Line 21 into my existing streaming workflow?
You integrate Line 21 by configuring your encoder or streaming software to send an audio or video feed to Line 21, then routing the processed captioned stream back to your destination platforms or displays. The platform provides API, encoder settings, and documentation to connect RTMP/HLS pipelines, meeting bots, and external services such as AWS, chat platforms, and ASR providers.
What is the difference between human captioning and ASR on Line 21?
Human captioning uses trained captioners who type in real time, typically yielding higher accuracy on complex or high‑stakes events, while ASR uses external speech‑recognition engines for faster, scalable captioning at lower cost. Line 21 lets you run either or a mix, with the AI Proofreader improving both.
Does Line 21 support offline or VOD captioning?
Line 21 focuses primarily on live captioning but can also generate caption files for playback and archives through its HLS player and download assets features. Post‑event you can export captions and time‑aligned text for on‑demand videos or repurposed content.
How do I manage multiple languages and feeds?
Line 21 lets you configure multiple language tracks and feeds from a single dashboard, including separate captioners or ASR sources for each language, plus AI‑driven translations. You can then route different language outputs to specific platforms, browser links, or encoders while monitoring everything in the live flow monitor.
What support and documentation are available?
Line 21 offers documentation that walks through setup, integrations, and use cases, plus a Discord community where you can get real‑time support, updates, and guidance. The team also provides direct support and onboarding for agencies and large broadcasters to ensure smooth deployment for complex events.