Firmware and hardware related resources for adding hands-free bluetooth support to a 746 british phone.
- esp32 devboard (v1)
- Raspberry PI Pico
- external DAC (TLC5615)
- GPO 746 telephone
- esp-idf (v5.1)
- pico-sdk ( >= v1.5.0)
- opencd (optional)
- picoprobe (optiona)
In a nutshell, the ESP32 is configured with SCO data path = HCI, this enables the improved mSBC codec on some AGs; the audio in/out is received/sent from/to a Raspberry Pi Pico I2C slave, acting as a recording/playback device.
The microphone is sampled using the Pico's 12bit ADC, while the earpiece speaker is driven by a buffered 10bit external DAC which interfaces with the Pico using SPI.
This is more complicated than it should mostly due to my lack of experience with the ESP32 and lack of a propper external ADC.
My initial plan was to use the ESP32's ADC & DAC simultaneous, but the I've run into the following problems:
- the ADC & DAC cannot be used at the same time in DMA Mode, since they both share the same DMA fifo of the I2S0 peripheral: ADC continuous mode Hardware Limitations - I couldn't find any proper way of making it work
- even if both ADC & DAC could work in continuous DMA mode, the ADC configuration doesn't allow sampling frequencies below 20khz (for mSBC & CVSD codecs a lower sampling rate is required: 16khz, 8khz respectively)
- the ESP32 doesn't have an analog ground which makes both the ADC & DAC really noisy
In a previous iteration of the project, both the Pico and the external DAC were connected to the ESP32 via SPI, but it struggled to keep a constant rate of sending/receiving PCM samples during an active call because each PCM sample required its own SPI transaction (both the Pico and the TLC5615 support only 2 byte transactions) - this caused horrible audio on both ends.
The Pico is configured as a slave I2C device receiving and transmitting audio data at the master's request (ESP32). Data is transmitted via a basic protocol built on top of I2C which consists of 5 commands. Each command is encoded as a 16 bit uint. The first 8 MSB encode the command code and the last 8 LSB encode any arguments the command might have.
Commands:
- AUDIO_ENABLE
Enable ADC/DAC. The sample rate is given as a parameter - AUDIO_DISABLE
Disable ADC/DAC - AUDIO_POLL
Poll the slave for new audio data sampled by the ADC. If slave returns
true, the AUDIO_RECEIVE command is sent to the slave to receive the PCM samples - AUDIO_TRANSMIT Sent by the master to transmit PCM samples for playback
- AUDIO_RECEIVE Sent by the master to receive new PCM samples from the ADC
Basic data flow between ESP32 and PICO:
1. (master) send AUDIO_ENABLE command
(slave) initialize ADC & DAC using the sample rate encoded in the command
(slave) respond with OK
2. (master) transmit any buffered PCM samples for playback (AUDIO_TRANSMIT)
(slave) receive PCM samples and transmit to DAC via DMA
3. (master) poll slave for new samples (AUDIO_POLL). If reply is OK, send AUDIO_RECEIVE command to receive new samples
Almost all the existing features of the GPO 746 are preserved once paired via Bluetooth HFP:
- Ringing works when an incoming call is detected. The ringer is driven by a AC coupled PWM 20hz signal amplified to 34V using a DC-DC voltage booster module.
- Earpiece / microphone
- Dialing via the rotary dialer works as expected
Visual indicator LEDs available for:
- power
- ringing
- bluetooth connection state
- handset on/off hook
A schematic on how to physically connect the phone to the board will be added soon.
The boards need to be separately flashed using the code in src.
cd src/esp32
source /path/to/esp-idf/export.sh
idf.py build flash
If using picoprobe and opencd:
# open pico-install.sh.template and change the path to opencd binary
# rename pico-install.sh.template to pico-install.sh
# make pico-install.sh executable
# copy pico-install.sh to a folder which is in your $PATH (I usually place this little scripts in ~/bin)
cd src/pico
make install
If not using picoprobe & opencd just build and flash using the manual steps provided in the getting started pico-sdk guide
The device is split into two PCBs, a main board and a control board. The main board houses all the relevant components for bluetooth and audio while the control board contains the indicator LEDs and a microswitch for toggling bluetooth pairing. The schematics and PCB designs were done using KiCad, all documents are inside the hw folder. I designed the main board to be single sided, but the pcb design file contains traces and vias for a two sided board only to pass DRC validation and given that I manually trace and etch my PCBs I cannot guarantee that the final designs are good enough for sending to a PCB manufacturer. Inspect the designs and tweak accordingly before ordering a set of boards. You can find some pictures of the boards in the img folder.
A 3d printed case model will be available in the future inside hw/3d-prints.