pico-w-ble-midi-lib

This library contains a GATT server and a GATT client for the BLE-MIDI service, MIDI 1.0 packet encoder and decoder. The client and server can work by themselves, or a higher level application can switch between client and server modes. It implements the Specification for MIDI over Bluetooth Low Energy (BLE-MIDI) 1.0. Downloading this standard requires a free login from the midi.org website.

This library expects to be built using pico-sdk version 2.0 or later and it expects to execute on a Raspberry Pi Pico W board with a RP2040 processor and a WiFi/Bluetooth module that uses the Infineon CYW43439 WiFi/Bluetooth chip.

Before using this library in a commercial application, please read the LICENSE file and the comments in each source code header file. I have tried to structure this library to make as much code as possible MIT License.

For a simple example that uses the BLE-MIDI server to send and receive MIDI data, see the pico-w-ble-midi-server-demo project. For a more practical demo, see the ble-midi2usbhost project.

Before using any library

You must initialize the Bluetooth hardware by calling cyw43_arch_init() once before calling the init() function for either library.

BLE-MIDI server library

This library enables the user application to send and receive MIDI 1.0 data via standard MIDI 1.0 byte streams. The ble_midi_server_lib library provides all Bluetooth functions an application needs to send and receive MIDI 1.0 byte streams. It supports a simple profile that contains only the MIDI service and the device's GAP_DEVICE_NAME characteristic. It uses the ble_midi_service_lib library for the MIDI data transfers, and it handles all other control messages between Bluetooth stack and the application. If you want to make a more complicated profile than the one ble_midi_server_lib supports, you can use ble_midi_service_lib to implement it. For example, you might want to add a battery service to your profile if your project is battery powered. However, the code that replaces ble_midi_server_lib library will be more complicated.

The ble_midi_service_lib INTERFACE library uses the ble_midi_pkt_codec INTERFACE library to encode the MIDI byte stream to BLE-MIDI 1.0 packets time-stamped with the system time. It then sends the BLE-MIDI encoded data to the connected BLE-MIDI client.

The Accessory Guidelines for Apple Products specifies a minumum connection interval of 15ms and latency of 30ms (i.e., 2 connection intervals). The midi_service_stream_handler.c code specifies a minimum interval of 7.5ms and a maximum interval of 15ms with a maximum latency of 0. My iPad consistently connects at 15ms with 0 latency. It is probably worth experimenting with this because ideally the connection interval would be 7.5 ms with 0 latency to minimize timing jitter and latency.

When the BLE-MIDI client sends BLE-MIDI 1.0 data to this server, the ble_midi_service_lib works with the the ble_midi_pkt_codec to decode the packet to an array of time-stamped byte stream structures and buffers them for the application to process. It is up to the application to handle the timestamps for presentation order and jitter reduction. Currently, ble_midi_server_lib does not manage timestamps.

The ble_midi_server_init() function requires a profile_data argument. The definition of profile_data is normally automatically generated from a .gatt file that has this form

PRIMARY_SERVICE, GAP_SERVICE
CHARACTERISTIC, GAP_DEVICE_NAME, READ, "Your device name goes here"

#import <midi_service.gatt>

When your application builds the code, it should generate the GATT database .h using the Pico SDK cmake function pico_btstack_make_gatt_header. Be sure to add the path to this library to your call to pico_btstack_make_gatt_header or else the script won't be able to find the file referenced in the line #import <midi_service.gatt>. When the application includes this .h, it will have the definition of the profile_data variable. The

The file ble_midi_server.h contains the server API. The file midi_server_stream_handler.h contains the service API. The user application must call midi_service_stream_init() before it uses the other two functions. The user application must call this function with a non-NULL pointer to a btstack_packet_handler_t packet handler function. This packet handler function must, at a minimum, handle the BTSTACK_EVENT_STATE event to start advertising when the Bluetooth stack starts functioning. The callback must also process the HCI_EVENT_GATTSERVICE_META event and use the GATTSERVICE_SUBEVENT_SPP_SERVICE_CONNECTED event to capture the connection handle that is a required argument to midi_service_stream_read() and to midi_service_stream_write(). The callback should also handle security manager messages if encryption is required.

After your application calls midi_service_stream_init(), it must turn on the Bluetooth chip by calling the function hci_power_control(HCI_POWER_ON).

Once the main application's btstack_packet_handler_t packet handler function records a valid connection handle, the program's main loop may process MIDI data received by polling midi_service_stream_read(). It may also call midi_service_stream_write() to generate BLE-MIDI messages in response to button presses, control movement, incoming MIDI 1.0 stream from non-BLE sources, etc.

The files in this directory require the following libraries:

    pico_stdlib
    ble_midi_service_lib
    pico_btstack_cyw43
    pico_cyw43_arch_none
    ring_buffer_lib

The ring_buffer_lib library is custom code that can be found here. The CMakeLists.txt for this project expects the ring_buffer_lib source directory to be located at the same directory level as the source files for this project. For example, if this project is a subdirectory of proj/lib, then the ring_buffer_lib source files should be stored in proj/lib/ring_buffer_lib.

The ring_buffer_lib needs to have a configuration file called ring_buffer_lib_config.h stored somewhere in the project's include path. Because of the required ring buffer sizes to support this library, please define the RING_BUFFER_SIZE_TYPE macro to be a 16-bit data type or larger. For example

#define RING_BUFFER_SIZE_TYPE uint16_t

Note that the default definition is uint8_t, which is too small for Bluetooth MIDI ring buffers.

BLE-MIDI client library

This ble_midi_client_lib library implements the basic functions you need to implement a BLE-MIDI client. A client needs some sort of UI to manage scanning, connecting, and disconnecting. The library provides printf type console library output and is best suited for a command line interpreter-based application. The client also advertises the GAP_DEVICE_NAME charateristic.