# CallPipe Relay (Raspberry Pi)

The Pi pairs to the phone as a **Bluetooth HFP hands-free unit**. During a call the
phone routes call audio to it over SCO; `bluez-alsa` exposes that as ALSA PCMs.
`callpipe-relay.py` is a dumb audio pipe: Bluetooth SCO (via `arecord`/`aplay`) ⇄
the phone's audio TCP server over WiFi. Call control lives on the phone.

See `../docs/relay-setup.md` for the one-time Bluetooth stack setup.

## Install

```sh
# on the Pi (root):
apt-get install -y bluez bluez-firmware bluez-alsa-utils alsa-utils python3 python-is-python3
install -D callpipe-relay.py /opt/callpipe/callpipe-relay.py
install -D callpipe-relay.service /etc/systemd/system/callpipe-relay.service
# set the phone IP:
systemctl edit callpipe-relay        # or edit Environment=CALLPIPE_PHONE=... in the unit
systemctl enable --now callpipe-relay
```

## Config (env)

| var | default | meaning |
|-----|---------|---------|
| `CALLPIPE_PHONE` | `10.0.1.239` | phone (hub) IP |
| `CALLPIPE_PORT`  | `9010`       | phone audio TCP port |
| `CALLPIPE_BT`    | auto         | HFP device MAC (auto-detected from bluez-alsa) |

## Wire protocol (TCP, both directions)

`[4B big-endian len N][1B type][payload]`  where `N = 1 + len(payload)`
- `A` (0x41): PCM S16LE mono 16 kHz, 20 ms (640 B) frames
- `C` (0x43): control JSON — phone sends `{"cmd":"start"|"stop"}`; relay sends
  `{"event":"hello|started|stopped|error", ...}`

## Manual test

```sh
CALLPIPE_PHONE=<dev-ip> python /opt/callpipe/callpipe-relay.py
# with tools/phone_stub.py running on <dev-ip>, place a call routed to BT
```