HFP Audio Gateway

import { Aside } from ‘@astrojs/starlight/components’;

mcbluetooth can act as an HFP Audio Gateway (the “phone” role), allowing you to test Bluetooth headsets and hands-free devices by simulating calls and controlling audio indicators.

HFP Roles

HFP (Hands-Free Profile) defines two roles:

Role	Description	Example Device
AG (Audio Gateway)	The phone side — originates calls, sends ring signals	Phone, Linux (with mcbluetooth)
HF (Hands-Free)	The headset side — answers calls, controls volume	Headset, car kit, ESP32 test device

mcbluetooth implements the AG role, so headsets connect to Linux as if it were a phone.

Enable the AG Profile

bt_hfp_ag_enable

This registers a custom HFP AG profile with BlueZ via the ProfileManager1 D-Bus interface. Headsets can now discover and connect to Linux.

Connection Flow

When a headset connects, the following happens automatically:

1. HF connects RFCOMM    → BlueZ calls NewConnection
2. AT+BRSF exchange      → Both sides share feature flags
3. AT+CIND=? / AT+CIND?  → Indicator mapping and values
4. AT+CMER=3,0,0,1       → Enable indicator reporting → SLC established
5. AT+BAC / +BCS         → Codec negotiation (CVSD or mSBC)

Check the connection status:

bt_hfp_ag_status

Simulate an Incoming Call

Once a headset is connected with SLC established:

bt_hfp_ag_simulate_call address="AA:BB:CC:DD:EE:FF" number="+15551234567"

The headset receives:

RING alerts every 3 seconds
+CLIP with caller ID information

The headset can:

Answer with ATA (call becomes active)
Reject with AT+CHUP (call is ended)

End a Call

From the AG (Linux) side:

bt_hfp_ag_end_call address="AA:BB:CC:DD:EE:FF"

This terminates both ringing and active calls, updating the headset’s indicators.

Control Volume

HFP volume uses a 0-15 scale:

# Set speaker volume
bt_hfp_ag_set_volume address="AA:BB:CC:DD:EE:FF" type="speaker" level=12

# Set microphone gain
bt_hfp_ag_set_volume address="AA:BB:CC:DD:EE:FF" type="microphone" level=10

Update Status Indicators

Simulate phone status changes shown on the headset display:

# Signal strength (0-5)
bt_hfp_ag_set_signal address="AA:BB:CC:DD:EE:FF" level=4

# Battery level (0-5)
bt_hfp_ag_set_battery address="AA:BB:CC:DD:EE:FF" level=3

These send +CIEV indicator updates to the headset.

HFP Indicators

The AG maintains 7 standard indicators, reported to the HF device:

Index	Name	Range	Description
1	service	0-1	Network service available
2	call	0-1	Active call exists
3	callsetup	0-3	0=none, 1=incoming, 2=outgoing, 3=alerting
4	callheld	0-2	0=none, 1=held+active, 2=held only
5	signal	0-5	Signal strength
6	roam	0-1	Roaming active
7	battchg	0-5	Battery charge level

E2E Testing with ESP32

The HFP AG tools are designed for end-to-end testing with the mcbluetooth-esp32 test harness, where the ESP32 acts as the HF (headset) device.

Typical Test Flow

# 1. Enable AG on Linux
bt_hfp_ag_enable

# 2. ESP32 connects as HF (from mcbluetooth-esp32)
#    SLC auto-negotiated

# 3. Linux simulates incoming call
bt_hfp_ag_simulate_call address="<esp32>" number="5551234567"

# 4. ESP32 answers (sends ATA)
#    Call becomes active

# 5. Linux ends call
bt_hfp_ag_end_call address="<esp32>"

# 6. Check status
bt_hfp_ag_status

AT Commands Handled

The AG automatically handles these AT commands from the HF device:

Command	Purpose
AT+BRSF	Feature exchange
AT+CIND	Indicator mapping/values
AT+CMER	Enable indicator reporting
AT+CHLD	Call hold/multiparty support
AT+BAC / AT+BCS	Codec negotiation
ATA	Answer call
AT+CHUP	Hang up / reject
ATD	Outgoing call (from HF)
AT+VGS / AT+VGM	Volume control
AT+CLCC	List current calls
AT+COPS	Operator name query
AT+CNUM	Subscriber number
AT+BVRA	Voice recognition
AT+NREC	Noise reduction

Disable the AG Profile

bt_hfp_ag_disable

This unregisters the profile, disconnecting any active HFP sessions.