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3. STT with Vosk (Offline)

Vosk is a lightweight speech-to-text (STT) engine that supports many languages and runs fully offline on Raspberry Pi. You only need internet access once to download a language model. After that, everything works without a network connection.

In this lesson, we will:

Check the microphone on Raspberry Pi.
Install and test Vosk with a chosen language model.

1. Check Your Microphone

Before using speech recognition, make sure your USB microphone works correctly.

List available recording devices:
```
arecord -l
```
Look for a line like card 1: ... device 0.

Record a short sample (replace 1,0 with the numbers you found):

arecord -D plughw:1,0 -f S16_LE -r 16000 -d 3 test.wav

Example: if your device is card 2, device 0, use:

arecord -D plughw:2,0 -f S16_LE -r 16000 -d 3 test.wav

Play it back to confirm the recording:
```
aplay test.wav
```
Adjust microphone volume if needed:
```
alsamixer
```
- Press F6 to select your USB microphone.
- Find the Mic or Capture channel.
- Make sure it is not muted ([MM] means mute, press M to unmute → should show [OO]).
- Use ↑ / ↓ arrow keys to change the recording volume.

2. Test Vosk

Run the program

cd ~/fusion-hat/examples
sudo python3 stt_vosk_stream.py

The first time you run this code with a new language, Vosk will:

Automatically download the language model (by default, the small version).
Print out the list of supported languages.
Start listening for audio input through the microphone.

You’ll see something like this in the terminal:

vosk-model-small-en-us-0.15.zip: 100%|███████████████████| 39.3M/39.3M [00:05<00:00, 7.85MB/s]
['ar', 'ar-tn', 'ca', 'cn', 'cs', 'de', 'en-gb', 'en-in', 'en-us', 'eo', 'es', 'fa', 'fr', 'gu', 'hi', 'it', 'ja', 'ko', 'kz', 'nl', 'pl', 'pt', 'ru', 'sv', 'te', 'tg', 'tr', 'ua', 'uz', 'vn']
Say something

This means:

The model file (vosk-model-small-en-us-0.15) has been downloaded.

The list of supported languages has been printed.

The system is now listening — say something into the Fusion HAT+ microphone, and the recognized text will appear in the terminal.

Tips:

Keep the microphone about 15–30 cm away for better accuracy.
Choose a model that matches your language and accent.
Use a quiet environment to improve recognition.

Code

from fusion_hat.stt import Vosk as STT

stt = STT(language="en-us")

while True:
   print("Say something")
   for result in stt.listen(stream=True):
      if result["done"]:
            print(f"final:   {result['final']}")
      else:
            print(f"partial: {result['partial']}", end="\r", flush=True)

Code explanation:

stt.listen(stream=True) — Starts streaming speech recognition and yields intermediate results as you speak.
result["partial"] — Displays the real-time recognized text (updated continuously).
result["final"] — Displays the final recognized sentence when you stop speaking.
The loop runs continuously, allowing hands-free real-time transcription.

Tip: This streaming mode is perfect for voice assistants, command control, or live transcription.

Troubleshooting

No such file or directory (when running `arecord`)

You may have used the wrong card/device number. Run:
```
arecord -l
```
and replace 1,0 with the numbers shown for your USB microphone.
Recorded file has no sound

Open the mixer and check the microphone volume:
```
alsamixer
```
- Press F6 to select your USB mic.
- Make sure Mic/Capture is not muted ([OO] instead of [MM]).
- Increase the level with ↑.
Vosk does not recognize speech
- Make sure the language code matches your model (e.g. en-us for English, zh-cn for Chinese).
- Keep the microphone 15–30 cm away and avoid background noise.
- Speak clearly and slowly.
High latency / slow recognition
- The default auto-download is a small model (faster, but less accurate).
- If it’s still slow, close other programs to free CPU.