Raspberry Pi 5 + USB mic → AssemblyAI live speech-to-text

Streams 16-bit PCM audio from a USB microphone straight to AssemblyAI's Universal-Streaming WebSocket API and prints text as you speak.

1. Install

chmod +x setup.sh
./setup.sh

This installs PortAudio (libportaudio2, needed by sounddevice), creates a venv, and installs the assemblyai + sounddevice packages.

2. API key

Get a key from https://www.assemblyai.com/dashboard/home, then:

echo 'export ASSEMBLYAI_API_KEY=<your-key>' >> ~/.bashrc
source ~/.bashrc

3. Find the mic

arecord -l                                       # ALSA card/device numbers
./.venv/bin/python transcribe_mic.py --list-devices

4. Run

./.venv/bin/python transcribe_mic.py --device USB

Partial results update in place on one line (… hello wor), finalized turns print on their own line. Ctrl+C ends the session cleanly (important: AssemblyAI bills streaming per second of open connection, not per second of audio).

Useful flags:

Flag	Purpose
--device 2 / --device USB	pick input by index or name substring
--sample-rate 48000	force a rate if auto-detection picks a bad one
--keyterms GPIO Raspberry	bias recognition toward domain words
--save transcript.txt	append timestamped final turns to a file
--speech-model universal-streaming-multilingual --language-code de	non-English
--wav sample.wav	smoke-test the API/network with a 16-bit mono WAV, no mic needed

Pi-specific notes

• Sample rate: the script tries 16 kHz first and falls back to 48/44.1 kHz if the mic doesn't support it, then tells AssemblyAI the rate actually used. No resampling, so there's no quality loss or CPU cost.
• Make the USB mic the default (optional) — create ~/.asoundrc with card N from arecord -l:

  pcm.!default { type asym capture.pcm "hw:N,0" }
  

• Input too quiet: alsamixer → F6 (pick the USB device) → F4 (capture) → raise gain.
• Overflow warnings (input overflow): raise BLOCK_MS in the script to 100.
• Headless/boot autostart — /etc/systemd/system/stt.service:

  [Unit]
  Description=AssemblyAI live transcription
  After=network-online.target sound.target
  
  [Service]
  User=pi
  WorkingDirectory=/home/pi/pi5-assemblyai-stt
  Environment=ASSEMBLYAI_API_KEY=<your-key>
  ExecStart=/home/pi/pi5-assemblyai-stt/.venv/bin/python transcribe_mic.py --device USB --save /home/pi/transcript.txt
  Restart=always
  
  [Install]
  WantedBy=multi-user.target

  Then sudo systemctl enable --now stt.
• Network: streaming needs a stable outbound WSS connection to streaming.assemblyai.com; prefer Ethernet or 5 GHz Wi-Fi. A Pi 5 uses <5% CPU for capture since all recognition happens server-side.