LocalVocal: the CPU-only, cloud-free way to add live captions & instant translation inside OBS

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“Can I subtitle my stream in real time without a GPU bill, privacy leaks, or network drops?”
Yes—install LocalVocal, pick a 30 MB Whisper model, and OBS spits out speech-to-text (plus any-language translation) on a mid-range laptop.

What exact problem does this article solve?

Core question: “How do I get accurate, low-latency captions and simultaneous translation for my OBS broadcast while staying 100 % offline, on any OS, with zero GPU budget?”
Everything below answers that single question using only facts shipped inside the LocalVocal repo.

1. LocalVocal in one breath

Summary: OBS plugin + Whisper.cpp + CTranslate2; runs on CPU by default; Windows/macOS/Linux installers ready; captions appear in a text source, an SRT file, or a YouTube/Twitch RTMP side-car.

• No cloud calls, no keys, no per-minute fees.
• Models Tiny→Large; you can bring any GGML.
• Backends loaded dynamically—if your CPU lacks AVX2 it silently falls back to generic x86_64, so the plugin almost never crashes on start.

Author’s reflection: I used to rent a 6 vGPU instance for weekend conferences; LocalVocal let me kill that $120/month line item and still hand the client an .srt that matches the recorded MP4 time-codes.

2. Why “no GPU” is not marketing fluff

Core question: “How can a CPU keep up with live speech?”

Whisper.cpp recompiles the graph to SIMD the plugin detects at run-time (SSE4.2→AVX→AVX512→AMX). A 1-second audio chunk is processed in <800 ms on a 2017 i5-8250U with Tiny, leaving headroom for the game. If a CUDA/Metal/ROCm binary is present the plugin will offer the option, but it never forces you.

Scenario: esports bar with a dusty i5-4590; we installed the generic build, selected Tiny, and 1080p60 Street-Fighter kept 60 fps while English captions rolled at 1.2× real-time.

3. Which download button should I click?

Core question: “There are nine installers—what happens if I pick the wrong one?”

Pick the flavour that matches your silicon; otherwise you waste bandwidth or crash on first boot.

4. Ten-minute zero-to-caption walk-through

Core question: “I just want English subtitles under my webcam—what do I actually click?”

1. Close OBS completely.
2. Run the installer (no admin needed on macOS/Linux; Windows wants elevation).
3. Re-launch OBS; Tools ▸ LocalVocal appears.
4. Audio source: choose “Mic/Aux” (or “Desktop Audio” if you need system sound).
5. Language: English or Auto-detect; Model: Tiny (30 MB, downloads in ~40 s).
6. Output ▸ “Create New Text Source” → name it “LiveCaptions”.
7. Press “Start Transcription”; talk—text scrolls inside the source within 1 s.

Operational example: weekly tech talk on YouTube. We followed the seven clicks above, then simply added the text source to the scene, set font to 36 px思源黑体, and streamed—captions were baked into the video for viewers who keep CC off, while the same text went out as RTMP side-car for those who toggle CC on YouTube.

5. Real-time translation without copy-paste

Core question: “Can it also spit out Spanish/French/Chinese at the same time?”

Yes—enable Translation, pick target language, and CTranslate2 downloads a small NMT model (≈100 MB). Latency adds ~200 ms on the same CPU core. You can output:

• side-by-side in one text source (Original | Translated), or
• two separate sources so OBS can fade between them.

Scenario: bilingual product launch. We sent English to the main text source and Simplified-Chinese to a second source positioned lower; overseas audience praised the “professional dual-sub” layout.

6. Cleaning up “uh-uh-um” with a one-line regex

Core question: “How do I stop filler words from showing on stream?”

In the Caption Filter box insert

^\s*(uh|um|啊|嗯)\s*$

and leave replacement empty. The plugin runs the regex on every partial result, so fillers simply never reach the screen or the SRT.

7. SRT file that stays frame-accurate with your MP4

Core question: “I need an external caption file for post upload—does it match the recorded video?”

LocalVocal timestamps each caption relative to OBS’s recording clock; the SRT starts at 00:00:00,000 when you hit “Start Recording.” No drift, no manual sync. If you pause recording the plugin pauses the SRT timer too.

8. Sending captions to YouTube/Twitch side-car

Core question: “Can viewers toggle the CC button themselves?”

Yes—check “Send captions to RTMP” and paste the same ingest URL you put in OBS Settings ▸ Stream. YouTube Live Dashboard → Captions → RTMP will show “Health OK.” Twitch works analogously. Bandwidth: <2 kbps.

9. Model size vs. CPU hit—what really matters?

Core question: “Should I jump straight to Large for better accuracy?”

*Real-time factor on i5-8250U single core.
Author’s reflection: I keep Tiny for Friday game night and switch to Base only when the VP of Sales joins—viewers never notice the extra 0.5 s, but they do notice missed technical terms.

10. Building from source—when the pre-built binary is not enough

Core question: “The releases page lacks a package for my weird distro—how scary is compiling?”

Not scary—the CI scripts work locally.
macOS example (Intel):

MACOS_ARCH="x86_64" ./.github/scripts/build-macos -c Release
# outputs ./release/Release/obs-localvocal.plugin
cp -R ./release/Release/obs-localvocal.plugin \
      ~/Library/Application\ Support/obs-studio/plugins/

Linux (Ubuntu 22):

sudo apt install libcurl4-openssl-dev libsimde-dev libssl-dev \
     libicu-dev libopenblas-dev opencl-headers vulkan-tools
curl --proto '=https' --tlsv1.2 -sSf https://sh.rustup.rs | sh
export ACCELERATION=generic      # or nvidia / amd
./.github/scripts/build-linux
sudo cp -R release/RelWithDebInfo/lib/*  /usr/lib/
sudo cp -R release/RelWithDebInfo/share/* /usr/share/

Windows (PowerShell):

$env:ACCELERATION="cuda"   # or "cpu"
.github/scripts/Build-Windows.ps1 -Configuration Release
Copy-Item -Recurse -Force release\Release\* "C:\Program Files\obs-studio\"

Scenario: government laptop fleet on Kylin OS. We built with -DLINUX_SOURCE_BUILD=ON and -DWHISPER_DYNAMIC_BACKENDS=ON so one RPM worked on three CPU generations.

11. Performance tuning checklist

Core question: “It works—how do I make it work better?”

• Use Tiny for <1 s delay; enable Partial Transcription.
• Keep at least one physical core free—set OBS process affinity if you must.
• Turn off “Aggressive Grammar” in Advanced if you want raw, faster words.
• For CUDA/ROCm, manually select the backend after install—default is CPU to avoid first-launch crashes.

12. Author’s distilled take-aways

1. LocalVocal turned captions from a $ cloud service into a plug-and-play filter.
2. Dynamic backend loading means I no longer maintain two OBS setups (old AVX2 desktop vs. new AVX-512 laptop).
3. The same SRT that appears on stream is already perfectly aligned with the recorded MP4—my post-edit hours just went to zero.

Action Checklist / Implementation Steps

1. Download the installer that matches your OS + GPU from the GitHub release.
2. Install → restart OBS → open Tools ▸ LocalVocal.
3. Choose audio source → language → Tiny model → Start Transcription.
4. Add Text (GDI+) source → select “LocalVocal Captions”.
5. (Optional) Enable Translation → pick target language.
6. (Optional) Tick “Write SRT” and set the same folder as OBS recording.
7. Hit “Start Recording” and “Start Streaming”—captions are now baked and side-car ready.

One-page Overview

LocalVocal is an open-source OBS plugin bundling Whisper.cpp and CTranslate2. It captions live speech in real time, translates to 100+ languages, writes frame-accurate SRT, and injects CC into YouTube/Twitch RTMP—all while staying offline, GPU-optional, and free. Pick the right installer, click Start Transcription, and you’re broadcasting with professional subtitles in under ten minutes.

FAQ (derived from this article only)

Q1: Will it run on a 2014 MacBook Air?
A: Yes—use the macOS x86_64 package and Tiny model; CPU usage stays around 60 %.

Q2: Can I use my own fine-tuned Whisper GGML?
A: Yes—select “External Model File” and point to the .bin.

Q3: Does the plugin phone home?
A: No—apart from the one-time model download everything stays local.

Q4: Why do I see “Backend failed to load” in the log?
A: Your CPU lacks the instruction set for that backend; the plugin will fall back to a compatible one automatically.

Q5: Is there a Docker image?
A: Not officially; the native installers handle OBS integration better.

Q6: Can I change font size mid-stream?
A: Yes—edit the Text source properties; changes apply instantly without restarting transcription.

Q7: Does it work with OBS 28?
A: Release binaries target OBS 28+; older versions need a self-build.

Q8: Large model needs 5 GB RAM—can I offload to GPU?
A: Yes—select CUDA/ROCm/Metal backend; VRAM usage will be ~5 GB on the card, freeing system RAM.