Setup3 min read
How to add subtitles to NotebookLM Video Overviews (step by step)
This is the full procedure for getting synchronized, exportable subtitles onto a Google NotebookLM Video Overview, including the exact clicks needed for a free API key and the settings that matter most once captions are live.
What you need before you start
- Google Chrome, Edge, Brave, or any Chromium browser that supports Manifest V3 extensions.
- A free Groq or Mistral API key. Both have permanent free tiers and neither asks for a credit card.
- About two minutes.
Step 1: Install LM Subs
Download the release zip from the interactive setup page, unzip it into a permanent folder such as Documents, open chrome://extensions, turn on Developer mode, and click Load unpacked. Pin LM Subs from the Access requested section at the top of Chrome's puzzle menu so the toolbar icon stays visible.
Step 2: Create a free Groq API key
- Open console.groq.com/keys.
- Log in with a Google account, GitHub account, or email address. No credit card is required.
- Click API Keys in the left navigation, then click Create API Key.
- Name the key LM Subs, click Submit, and copy the
gsk_...value immediately. Groq shows it only once.
Step 3: Paste the key into LM Subs
Click the LM Subs toolbar icon, open Providers and API keys, confirm Groq is the primary provider, paste the key, and click Save key. Then click Test. A successful test reports the provider name, model, and response latency.
Step 4: Play a Video Overview
Open any notebook with a finished Video Overview and press play. LM Subs pulls the audio-only stream, transcribes it, and renders captions inside the video frame. The CC button appears inside NotebookLM's own bottom play bar and fades in and out together with the native controls, so it never blinks between sentences.
Step 5: Export the transcript
Open the caption options from the play bar or the toolbar popup and export in whichever format you need. Every export uses the current timing offset and includes speaker labels when the provider detects them.
| Format | Best for | Keeps timestamps |
|---|---|---|
| .SRT | Video editors, YouTube, VLC | Yes |
| .VTT | Web players, HTML5 caption tracks | Yes |
| .TXT | Reading, printing, pasting into notes | Optional |
| .MD | Study notes and Obsidian vaults | Yes |
| .CSV | Spreadsheets and Anki decks | Yes |
| .JSON | Scripts and downstream tooling | Yes |
Settings worth changing on day one
- Word highlight delay: if the highlight runs ahead of the narration, raise it toward +300 ms. If it lags, drag it negative.
- Timing offset: press the left and right bracket keys to shift every cue by 0.25 seconds if the whole track feels early or late.
- Caption size: captions scale automatically with player size, so set the base size once and let fullscreen handle itself.
- Fine-grained controls: turn this on in the settings page only if you want pixel-level typography and playback controls in the player.
Frequently asked questions
How long does caption generation take?
LM Subs uses a sub-five-second fast path on most Video Overviews by extracting NotebookLM's audio-only stream instead of the full MP4. Very long overviews are chunked and transcribed in parallel.
Do I have to pay for a speech-to-text provider?
No. Groq offers roughly eight hours of transcription per day on its permanent free tier and Mistral offers a free Studio tier. Neither requires a credit card. You can also point LM Subs at a local Whisper server and pay nothing at all.
Does LM Subs upload my NotebookLM video anywhere?
No video is uploaded. Audio extraction happens in your browser and only the compact audio track is sent to the provider whose key you entered. Captions stay in local browser storage unless you sign in and enable sync.