This example is to show how to use the APIs of Hands-Free Client Component and the effects of them by providing a set of commands. You can use this example to communicate with a device that implements Hands-Free Audio Gateway (HF-AG) (e.g. a smartphone).
This example is designed to run on commonly available ESP32 development board, e.g. ESP32-DevKitC. To operate it should be connected to an AG running on a smartphone or on another ESP32 development board loaded with Hands Free Audio Gateway (hfp_ag) example from ESP-IDF.
-`PCM`: To use PCM, audio stream is directed from Bluetooth controller to the specific GPIO pins you set in the demo, and you should link these GPIO pins to a speaker via I2S port. The audio data will not go through the `Bluedroid`. In menuconfig, you should choose PCM in `menuconfig`:
-`vHCI`: To use vHCI, audio data stream will be directed from Bluetooth Controller through vHCI on ESP32 and go through the Bluedroid to the Application layer. In menuconfig, you should choose vHCI in `menuconfig`:
PCM Signal supports three configurations in menuconfig: PCM Role, PCM Polar and Channel Mode(Stereo/Mono).
- PCM Role: PCM role can be configured as PCM master or PCM slave. The default configuration is `Master`, you can change the PCM role in `menuconfig` path:
`Component config --> Bluetooth --> Controller Options --> PCM Signal Configurations --> PCM Signal Configurations: Role, Polar and Channel Mode(Stereo/Mono) --> PCM Role`
- PCM Polar: PCM polarity can be configured as Falling Edge or Rising Edge. The default configuration is `Falling Edge`, you can change the PCM polar in `menuconfig` path:
`Component config --> Bluetooth --> Controller Options --> PCM Signal Configurations --> PCM Signal Configurations: Role, Polar and Channel Mode(Stereo/Mono) --> PCM Polar`
- Channel Mode(Stereo/Mono): PCM frame synchronization signal can be configured as Stereo mode or Mono mode, where the Mono mode can be configured in two different forms(Mono mode 1 and Mono mode 2). As is shown in the figure 
- Stereo Mode(Dual channel): FSYNC and DOUT signals both change simultaneously on the edge of CLK. The FSYNC signal continues until the end of the current channel-data transmission.
- Mono Mode 1(Single channel): FSYNC signal starts to change a CLK clock cycle earlier than the DOUT signal, which means that the FSYNC signal takes effect a clock cycle earlier than the first bit of the current channel-data transmission. The FSYNC signal continues for one extra CLK clock cycle.
- Mono Mode 2(Single channel): FSYNC and DOUT signals both change simultaneously on the edge of CLK. The FSYNC signal continues for one extra CLK clock cycle.
- The default configuration is `Stereo Mode`, you can change the PCM Channel mode in `menuconfig` path:
`Component config --> Bluetooth --> Controller Options --> PCM Signal Configurations --> PCM Signal Configurations: Role, Polar and Channel Mode(Stereo/Mono) --> Channel Mode(Stereo/Mono)`
`CVSD` is the default setting and is also the widely used codec for voice audio. But, `mSBC` is designed to have a better voice quality through `HFP`. To select which one is in use, we provide `Wide Band Speech` item in the `menuconfig` path:
Switching on the `Wide Band Speech` means that the preferred codec is `mSBC`, but which one is actually being used also depends on the `Data Path` configuration.
- If you choose `PCM` for datapath, you can only use `CVSD` and hardware is responsible for the codec job. In the meanwhile, you cannot use `mSBC` by switching `Wide Band Speech` on, because the `mSBC` is implemented in the Bluedroid (Bluetooth Host Stack) by software.
See the [Getting Started Guide](https://docs.espressif.com/projects/esp-idf/en/latest/get-started/index.html) for full steps to configure and use ESP-IDF to build projects.
**Note: Only after Hands-Free Profile(HFP) service is initialized and a service level connection exists between an HF Unit and an AG device, could other commands be available.**
ESP32 supports both CVSD and mSBC codec. HF Unit and AG device determine which codec to use by exchanging features during service level connection. The choice of codec also depends on the your configuration in `menuconfig`.
- If you want to use `con` to establish a service level connection with specific AG device, you should add the MAC address of the AG device in `bt_app.c`, for example: `esp_bd_addr_t peer_addr = {0xb4, 0xe6, 0x2d, 0xeb, 0x09, 0x93};`
Due to the complexity of HFP, this example has more source files than other bluetooth examples. To show functions of HFP in a simple way, we use the Commands and Effects scheme to illustrate APIs of HFP in ESP-IDF.
- The example will respond to user command through UART console. Please go to `console_uart.c` for the configuration details.
- For voice interface, ESP32 has provided PCM input/output signals which can be mapped to GPIO pins. So, please go to `gpio_pcm_config.c` for the configuration details.
- If you want to update the command table, please refer to `app_hf_msg_set.c`.
- If you want to update the responses of HF Unit or want to update the log, please refer to `bt_app_hf.c`.
