No Subtitles? How to Auto-Generate AI Captions for Any Online Video with Zero Delay
How Do You Add Subtitles to a Video That Has None?
Install the CaptionGo or DualPiP browser extension to auto-generate AI captions for any online video — with zero delay between audio and subtitles. Both extensions use AI speech recognition (ASR) to convert spoken audio into text captions in real time. No subtitle file needed. No platform support required.
Countless online videos lack subtitles: TikTok clips, Twitter/X video posts, Vimeo creator content, niche course recordings, foreign-language lectures. For language learners, deaf or hard-of-hearing viewers, or anyone watching in a noisy environment, missing subtitles means missing the content.
Chrome's built-in Live Caption feature generates captions but only supports a handful of languages, shows a fixed-style bubble at the bottom of the browser, and disappears in picture-in-picture mode. Most importantly, Chrome's built-in captions always lag 1–2 seconds behind the audio. CaptionGo and DualPiP's preload mode eliminates this delay entirely.
Can a Browser Extension Generate Subtitles for Any Video?
Yes. CaptionGo and DualPiP use AI speech recognition to extract text from any video's audio and display it as captions. They share the same ASR engine but serve different use cases:
| Feature | CaptionGo | DualPiP |
|---|---|---|
| Caption display | Overlaid directly on the web page video | Inside a picture-in-picture floating window |
| Primary use | Bilingual subtitles, ASR captions | PiP player, bilingual subtitles, ASR captions |
| Mobile support | ✅ Android Chrome/Edge/Firefox | ❌ Desktop only |
| Fullscreen | ✅ | ✅ (inside PiP window) |
Both extensions automatically use preload mode on pre-recorded videos. No manual configuration needed.
What Is Caption Preloading and Why Does It Feel Better Than Live Captions?
Caption preloading means the extension downloads and transcribes audio ahead of your current playback position, caching the results. When the video reaches that timestamp, the caption is already waiting — it appears instantly with zero delay.
Traditional live captions work by capturing audio as it plays, sending it to a recognition server, waiting for the result, then displaying it. This process takes 1–3 seconds, so you always hear the words before seeing them on screen.
| Comparison | Live recognition | Preload mode |
|---|---|---|
| Caption delay | 1–3 seconds | Zero — perfectly synced |
| Seeking (scrubbing) | Must re-recognize, wait seconds | Captions appear instantly |
| Best for | Live streams, video calls | Pre-recorded videos |
| Caption completeness | May miss words | Full audio coverage |
| Network | Requires constant connection | One-time download |
Preload mode makes pre-recorded video captions feel like built-in embedded subtitles: frame-accurate timing, no lag, and instant response when you drag the progress bar.
Which Video Sites Support Preloaded Captions?
Preloaded captions work on most major video sites and streaming platforms. The extension discovers audio sources automatically — by intercepting browser network requests, monitoring media loading behavior on the page, and extracting audio URLs from page data.
| Category | Verified sites |
|---|---|
| Short video | TikTok, Twitter/X, Instagram (web) |
| Video platforms | Bilibili, Vimeo, Dailymotion |
| Learning platforms | Coursera, Udemy, TED, edX, Khan Academy |
| Other | Any site using HTML5 <video> |
Why Don't YouTube, Netflix, Disney+, Crunchyroll and Similar Sites Support Preloaded Captions?
YouTube uses a proprietary video delivery protocol (SABR/UMP) that differs entirely from standard HLS/DASH streaming. The extension cannot extract YouTube's audio stream through normal means. Netflix, Disney+, Crunchyroll, and similar streaming platforms use Widevine/PlayReady DRM hardware encryption, preventing the extension from accessing decryption keys to extract audio. We are actively researching a compatible solution for YouTube.
You won't be left without captions on these platforms: YouTube provides its own auto-generated captions, and both CaptionGo and DualPiP support bilingual subtitle overlay on top of YouTube's native captions. Netflix, Disney+, and similar platforms provide multi-language subtitles natively, and the extension can overlay bilingual translations on top of them. For videos without auto-captions, the extension automatically falls back to live recognition mode.
What Happens When Preloading Fails?
The extension has built-in smart fallback: if preloading isn't possible (live stream, DRM-encrypted content, or audio source blocked by the platform), it automatically and seamlessly switches to live recognition mode. You don't need to do anything, and you'll never be left without captions.
Does Caption Preloading Use a Lot of Data?
No. Caption preloading downloads only the audio data, never the video. A 100 MB MP4 video typically contains about 10% audio — the extension downloads only that 10%.
| Video format | Optimization | Data downloaded |
|---|---|---|
| HLS stream | Auto-selects lowest bitrate audio track | Audio track only |
| DASH stream | Auto-selects lowest bitrate audio | Audio track only |
| MP4 file | Parses file structure, downloads only audio byte ranges | ~10% of file size |
The extension also detects silent segments (intro music, scenes with no speech) and skips them automatically, further reducing speech recognition API usage.
Why Do Some Videos Still Show Delayed Captions?
Preload mode cannot be used in these situations, and the extension automatically switches to live recognition (1–3 second delay):
- Live streams: No future audio to preload
- Hardware-encrypted DRM videos (Netflix, Disney+, Crunchyroll, etc.): Widevine/PlayReady encryption prevents the extension from accessing decryption keys (standard AES-128 and ClearKey encryption are handled automatically)
- YouTube: Proprietary SABR/UMP protocol, not standard HLS/DASH
- Some anti-hotlinking platforms: CaptionGo v1.1 and DualPiP v1.9 already solved TikTok and other major platforms, but new restrictions may appear
- DRM platforms still offer captions: Netflix, Disney+, etc. cannot use preloaded ASR captions, but these platforms provide native multi-language subtitles that the extension can enhance with bilingual translation overlay
In every case, the extension never leaves you without captions. Live recognition mode is always available as a fallback.
What Are the Known Limitations of Preloaded Captions?
Caption preloading is a new feature under active development. Known limitations:
| Limitation | Reason | Current workaround |
|---|---|---|
| YouTube not supported | Proprietary SABR/UMP protocol | Researching solutions; auto-uses live mode |
| Netflix, Disney+, Crunchyroll, etc. | Widevine/PlayReady DRM encryption | Use platform's native subtitles + bilingual translation |
| Some DRM videos can't preload | Widevine/PlayReady hardware encryption | Auto-falls back to live mode |
| Not available for live streams | No future audio to download | Auto-uses live recognition |
| Rare site download failures | CDN anti-scraping updates | Remote config updates, no extension update needed |
| Accuracy depends on audio quality | Background music, overlapping speakers | Adjust language setting or silence filter |
All limitations have automatic fallback — when preloading isn't available, the extension seamlessly switches to live recognition. Site compatibility is continuously updated via remote configuration, no extension update required.
How Do You Enable Caption Preloading?
No configuration needed. After installing the extension and setting up your speech recognition provider, the extension automatically chooses the best mode:
- Pre-recorded videos → Preload mode, zero delay
- Live streams → Live recognition mode
- Preload fails → Auto-switches to live recognition
The extension auto-detects the video's language (35+ languages supported) without manual selection. If detection is inaccurate, it re-detects and switches within seconds.
Install CaptionGo (in-page bilingual subtitles, mobile support): Chrome Web Store | Edge Add-ons
Install DualPiP (picture-in-picture player, desktop): Chrome Web Store | Edge Add-ons
Frequently Asked Questions
Does preloading work with live streams?
No. Preloading requires downloading audio that hasn't been played yet, which isn't possible with live content. Live streams automatically use live recognition mode with approximately 1–3 seconds of delay.
How is this different from YouTube's built-in auto-captions?
YouTube's auto-captions are generated by YouTube's own backend. CaptionGo and DualPiP use high-accuracy ASR models like Deepgram nova-3 and work on any video site, not just YouTube. On YouTube, we recommend using YouTube's native captions with the extension's bilingual translation feature.
Does it work on mobile browsers?
CaptionGo supports Android Chrome, Edge, and Firefox, including fullscreen mode. DualPiP's picture-in-picture feature is desktop-only.
Can it generate bilingual subtitles?
Yes. Preloaded captions integrate fully with the AI translation engine. After ASR generates the original-language captions, the AI engine translates them in real time. Both languages display simultaneously. We recommend AI models like DeepSeek or GPT for translation quality that far exceeds traditional machine translation.
How accurate are the captions?
Preload mode uses the same speech recognition engine as live mode. With Deepgram's nova-3 model, English word error rate (WER) is below 8% — one of the most accurate real-time ASR models available in 2026. Accuracy depends on audio quality, speaker accent, and background noise. Because preload mode processes complete audio segments, it can actually be more accurate than live streaming in some scenarios.