Auto Caption Generator
Generate accurate captions from any video or audio file using on-device AI. No uploads to servers, no waiting in queues, no watermarks. Export as SRT subtitles instantly.
Upload a file · Boost, EQ, export · 100% in your browser
Captions are no longer optional. Every major social platform — YouTube, TikTok, Instagram, LinkedIn, Facebook — rewards captioned content with higher engagement, longer watch times, and broader reach. Studies consistently show that 80% of viewers are more likely to watch a video to completion when captions are available, and 85% of Facebook video is watched on mute. Whether you are a content creator, educator, marketer, or podcaster, adding accurate captions to your media is one of the highest-impact things you can do for your audience. The problem has always been the process: manual transcription is brutally time-consuming, and cloud-based auto caption services require you to upload your files to someone else's servers, wait in processing queues, and often pay per minute of audio.
Hearably Studio changes this equation entirely. Its auto caption generator runs 100% in your browser using a local instance of OpenAI's Whisper speech recognition model, compiled to WebAssembly for near-native performance. When you drop a video or audio file onto the tool, the audio is extracted and transcribed directly on your device. No bytes leave your machine. There is no upload step, no server queue, no cloud processing, and no third party ever sees your content. For creators working with confidential material — unreleased episodes, client projects, internal training videos, legal depositions — this privacy guarantee is not a nice-to-have, it is a requirement.
The transcription accuracy rivals cloud services like Otter.ai, Descript, and Rev's AI tier. Whisper was trained on 680,000 hours of multilingual audio data, giving it exceptional robustness against accents, background noise, overlapping speech, and domain-specific vocabulary. The model handles conversational speech, formal presentations, interviews with multiple speakers, and even music with lyrics. For content with technical jargon or proper nouns, you can review and correct the generated captions in the built-in editor before exporting — a workflow that takes a fraction of the time compared to transcribing from scratch.
Once captions are generated, you have full control over the output. The built-in caption editor lets you adjust timing, correct words, merge or split segments, and fine-tune the synchronization between text and audio. When you are satisfied, export as an SRT subtitle file — the universal format accepted by YouTube, Vimeo, Facebook, LinkedIn, and virtually every video editing application. The SRT file contains precisely timed text segments that video players overlay on your content, making it accessible to deaf and hard-of-hearing viewers, non-native speakers, and anyone watching in a sound-sensitive environment.
The auto caption generator online tool integrates seamlessly with Hearably Studio's other features. After generating captions, you can use Magic Cut to automatically remove silence and filler words from the audio, and the captions will adjust their timing to match the edited file. You can boost the audio volume up to 800% with the look-ahead limiter ensuring zero distortion, apply the 10-band EQ to enhance vocal clarity, and export the complete package — enhanced audio with perfectly synchronized captions — all from a single browser tab. No software to install, no subscription required for the core captioning workflow.
How Browser-Based AI Captioning Works — Whisper on WebAssembly
Hearably Studio's auto caption generator runs OpenAI's Whisper speech recognition model entirely in your browser. The model is compiled from its original PyTorch implementation to WebAssembly (WASM) using the whisper.cpp project as an intermediary — a high-performance C++ port that is then compiled to WASM via Emscripten. This produces a binary that executes at near-native speed inside the browser's sandboxed runtime, with no server communication required.
When you load an audio or video file, the browser first extracts the audio track using the AudioContext and decodeAudioData() API, converting it to mono 16 kHz PCM — the exact input format Whisper expects. The decoded samples are passed to the WASM module, which runs the model's encoder-decoder transformer architecture. The encoder processes the audio in 30-second chunks, generating a sequence of feature vectors that represent the acoustic content. The decoder then autoregressively generates text tokens, predicting each word (or subword) based on the acoustic features and all previously generated tokens. Timestamps are extracted from the decoder's cross-attention weights, which reveal which audio frames correspond to which text tokens — this is how the model produces word-level or segment-level timing data.
The model weights are downloaded once and cached in the browser's Cache Storage API, so subsequent uses load instantly without re-downloading. Hearably uses the whisper-small model (244M parameters), which balances accuracy and speed: it transcribes roughly 1 minute of audio per 3-5 seconds on modern hardware with a capable GPU. The WebAssembly runtime leverages SIMD (Single Instruction, Multiple Data) instructions when the browser supports them, accelerating the matrix multiplications that dominate transformer inference. For browsers with WebGPU support, computation can be offloaded to the GPU for even faster throughput. The entire pipeline — audio decoding, feature extraction, transformer inference, and timestamp alignment — runs in a dedicated Web Worker thread, keeping the main UI responsive during transcription.
How to get the best audio on Auto Caption Generator
Use clean audio for the best transcription accuracy
Whisper handles background noise well, but cleaner audio produces more accurate captions with fewer corrections needed. Before captioning, consider using Hearably Studio's volume boost and EQ to enhance vocal clarity — boost the 2-4 kHz speech presence band and apply the high-pass filter to cut low-frequency rumble. Cleaner input means fewer manual corrections in the editor afterward.
Review and correct proper nouns after generation
AI speech recognition excels at common vocabulary but can struggle with brand names, technical terms, and uncommon proper nouns. After generating captions, use the built-in editor to search for and correct these terms. This targeted review is dramatically faster than full manual transcription — you are editing, not writing from scratch.
Adjust segment timing for natural reading speed
The auto-generated timing is based on speech boundaries detected by the model. Occasionally, segments may be too long or too short for comfortable reading. The caption editor lets you split long segments into shorter ones (aim for 2 lines, under 42 characters per line) and merge very short segments that flash by too quickly.
Export SRT for maximum platform compatibility
SRT (SubRip Text) is the most widely supported subtitle format across video platforms and editing software. YouTube, Vimeo, Facebook, LinkedIn, TikTok (via CapCut), Premiere Pro, Final Cut Pro, and DaVinci Resolve all accept SRT files natively. Always export as SRT unless a specific platform requires a different format.
Caption podcast episodes to create show notes and transcripts
The auto caption generator is not just for video. Drop a podcast MP3 or M4A file, generate captions, and export as SRT or copy the plain text. You now have a complete episode transcript for show notes, blog posts, and SEO. Many successful podcasters use AI transcription to repurpose every episode into written content.
Process long files in the background while you work
Transcription runs in a Web Worker thread, so the browser tab remains responsive during processing. Drop a 60-minute podcast or lecture recording and continue working in other tabs while the model processes. A progress indicator shows estimated time remaining. On modern hardware, even hour-long files complete in minutes.
Combine with Magic Cut for social media clips
After generating captions, use Magic Cut to remove silence and filler words. The captions automatically adjust their timing to match the shortened audio. Export the tight, captioned clip for TikTok, Reels, or YouTube Shorts — the combination of polished audio and accurate captions maximizes engagement on every platform.
Use captions to identify filler words before removing them
The generated transcript makes filler words like "um," "uh," "you know," and "like" visually obvious in text form. Review the captions to understand your filler word patterns, then use the filler word remover to automatically strip them. This workflow gives you both clean audio and clean text for repurposing.
Built for this exact use case
AI-Powered Transcription
OpenAI Whisper model runs locally via WebAssembly. Handles accents, background noise, multiple speakers, and technical vocabulary. No cloud processing — your audio stays on your device.
Built-In Caption Editor
Review, correct, and refine generated captions in a synchronized editor. Adjust timing, fix words, split or merge segments. See captions alongside the waveform for precise alignment.
SRT Subtitle Export
Export perfectly timed SRT files accepted by YouTube, Vimeo, TikTok, LinkedIn, and every major video editor. Universal subtitle format with segment timestamps and text.
100% Browser-Based Privacy
The Whisper model runs entirely in your browser. Zero server uploads, zero cloud processing. Confidential content, unreleased episodes, and client projects never leave your machine.
Choose your method
Different situations call for different tools. Hearably gives you both.
Chrome Extension
Enhance audio live while you stream. The extension intercepts your tab's audio and processes it in real-time — volume boost, EQ, presets — without downloading anything.
- Streaming on Auto Caption Generator, Netflix, Spotify
- Video calls on Zoom, Meet, Teams
- Any website with audio
- When you want instant, always-on enhancement
Free Online Studio
Upload an audio or video file, apply volume boost + 10-band EQ, preview in real-time, then download the enhanced WAV. Your file never leaves your browser.
- Downloaded videos or music files
- Podcast episodes you want to boost before sharing
- Voice recordings, lectures, interviews
- When you need a permanently enhanced file
Pro tip: Use a YouTube-to-MP3 tool to download the audio, then enhance it in Hearably Studio with EQ + volume boost. Perfect for offline listening, DJ sets, or sharing on social media.
Three clicks to better audio
Install
Add Hearably from the Chrome Web Store. Under 300KB, installs in seconds.
Enhance
Click the Hearably icon and tap "Enhance." Boost kicks in instantly.
Enjoy
Adjust volume, EQ, and presets. Works on any website with audio.
Frequently asked questions
How accurate is the auto caption generator?
Hearably Studio uses OpenAI's Whisper model, which achieves word error rates competitive with professional human transcription on clean speech. Accuracy depends on audio quality, speaker clarity, and background noise. Clear recordings with a single speaker typically produce 95%+ accuracy. Noisy environments or heavy accents may require more manual corrections, but the generated output is always a faster starting point than transcribing from scratch.
Do my files get uploaded to a server for captioning?
No. The Whisper speech recognition model runs entirely in your browser via WebAssembly. Your audio and video files are decoded, processed, and transcribed locally on your device. No data is sent to any server. You can verify this by disconnecting from the internet after the page loads — the tool works fully offline once the model weights are cached.
What languages does the auto caption generator support?
Whisper was trained on multilingual data and supports over 90 languages including English, Spanish, French, German, Portuguese, Japanese, Chinese, Korean, Arabic, Hindi, and many more. Accuracy is highest for English and major European languages, but the model handles most widely spoken languages with practical accuracy for captioning purposes.
How long does it take to generate captions?
On modern hardware with a capable processor, the tool transcribes approximately 1 minute of audio in 3-5 seconds. A 10-minute video generates captions in under a minute. Longer files take proportionally more time but run in a background thread, so your browser remains responsive. GPU acceleration via WebGPU further reduces processing time when available.
Can I edit the generated captions before exporting?
Yes. The built-in caption editor displays all generated segments with their timestamps synchronized to the audio waveform. You can click any segment to play that portion of the audio, correct text errors, adjust start and end times, split long segments into shorter ones, and merge fragments. This edit-then-export workflow is how professional subtitlers work.
What subtitle formats can I export?
The primary export format is SRT (SubRip Text), which is the most universally accepted subtitle format across video platforms and editing software. SRT files contain numbered segments with timestamps and text, and are accepted by YouTube, Vimeo, Facebook, LinkedIn, TikTok (via CapCut), Premiere Pro, Final Cut Pro, DaVinci Resolve, and virtually every other tool that handles subtitles.
Does the auto caption generator work with video files?
Yes. Drop any video file — MP4, MOV, WebM, MKV — and the tool extracts the audio track automatically for transcription. The video itself is not re-encoded or modified. You receive captions timed to the original video, which you can then burn in or attach as a separate SRT track in your video editor.
Is the auto caption generator online tool free?
Yes. The core captioning workflow — AI transcription, caption editing, and SRT export — is completely free with no account required. Pro unlocks additional features including batch captioning for multiple files, enhanced model accuracy, and integration with Magic Cut for combined audio cleanup and captioning workflows.
How is this different from YouTube's auto-captions?
YouTube generates captions only after you upload your video to their servers, and the results are tied to YouTube's platform. Hearably Studio generates captions locally before you upload anywhere, giving you a portable SRT file you can use on any platform. You also get a full editing interface to correct errors before publishing — YouTube's editor is more limited and corrections are not portable.
Can I use this for meeting recordings and lectures?
Absolutely. The auto caption generator handles any spoken audio — podcasts, lectures, meetings, webinars, interviews, and presentations. For multi-speaker recordings, the model detects natural speech boundaries and creates appropriately segmented captions. The resulting transcript doubles as searchable meeting notes or lecture documentation.