Bluetooth Classic Hands-Free Profile (HFP) — audio + vendor AT commands.

Use this for headsets, PTT-equipped earpieces (Hytera EHW02, etc), and any device that exposes an HFP control link plus an SCO audio link.

See Mob.Bt for pairing, discovery, and disconnect — those are device-level concerns. Profile-specific operations live here.

Typical flow

# 1. Pair (only needed once per device — Mob.Bt.pair/2)
socket = Mob.Bt.pair(socket, device)
# {:bt, :pair_succeeded, nil, device}

# 2. Connect HFP profile
socket = Mob.Bt.Hfp.connect(socket, device)
# {:bt, :hfp_connected, session_id, device}

# 3. (Optional) subscribe to vendor AT commands the headset emits.
#    Hytera EHW02 fires +CTXD on PTT press, +CUTXC on release.
socket = Mob.Bt.Hfp.subscribe_vendor_at(socket, session_id)
# {:bt, :vendor_at, session_id, %{cmd: "+CTXD", args: ""}}

# 4. (Optional) bring up the SCO audio link.
socket = Mob.Bt.Hfp.start_sco(socket, session_id)
# {:bt, :sco_started, session_id, %{sample_rate: 8000, ...}}
# then audio chunks stream as:
# {:bt, :sco_audio_in, session_id, pcm_bytes}

# 5. Send PCM audio out to the headset earpiece:
Mob.Bt.Hfp.send_audio(socket, session_id, pcm_bytes)

# 6. Disconnect (one canonical path — Mob.Bt.disconnect/2)
Mob.Bt.disconnect(socket, session_id)

Vendor AT commands

HFP defines a small core AT vocabulary (call control, volume, ring). Headset vendors extend with their own +-prefixed commands. Subscribing via subscribe_vendor_at/2 delivers any unrecognized AT command from the headset as {:bt, :vendor_at, session_id, %{cmd, args}} for your app to interpret.

Sending a vendor AT command to the headset is send_vendor_at/4. Standard responses (OK, ERROR) are emitted by Android automatically — use the response argument to override only when the AT spec demands custom payload.

SCO audio

SCO (Synchronous Connection-Oriented) is the real-time bidirectional voice channel HFP uses for call audio. start_sco/2 opens it; PCM bytes flow both ways until stop_sco/2 or disconnect.

Format is 8 kHz / 16-bit / mono PCM by default; modern devices may negotiate up to 16 kHz wideband (mSBC). The :sco_started event reports the negotiated parameters.