zephyr-ble-midi

This is a Zephyr implementation of the BLE-MIDI service specification, which allows for wireless transfer of MIDI data over Bluetooth low energy. The implementation covers the entire BLE MIDI packet encoding specification, including running status, timestamp wrapping and single and multi packet system exclusive messages.

To minimize latency, a connection interval of 7.5 ms is requested, which is the smallest interval allowed by the Bluetooth low energy specification. Note that a central may or may not accept this value.

By default, outgoing MIDI messages are sent in separate BLE packets. When building for nRF SoCs with nRF Connect SDK, outgoing messages can optionally be buffered and transmitted in a single BLE packet just before the next connection event to reduce latency, see CONFIG_BLE_MIDI_TX_MODE_CONN_EVENT.

Usage

The BLE MIDI service implementation is contained in a Zephyr module. The sample app's CMakeLists.txt file shows one way of adding this module to an app.

The public API is defined in ble_midi.h.

Sample app

The sample app shows how to send and receive MIDI data. The app requires a board with at least four buttons and four LEDs, for example nrf52840dk_nrf52840.

• Button 1 - Send three simultaneous note on/off messages
• Button 2 - Send a short sysex message
• Button 3 - Send a long, streaming sysex message
• Button 4 - Send enqueued messages (only if CONFIG_BLE_MIDI_TX_MODE_MANUAL is set)
• LED 1 - On when connected but not ready to communicate
• LED 2 - On when ready
• LED 3 - Toggles on/off when receiving sysex messages
• LED 4 - Toggles on/off when receiving non-sysex messages

Configuration options

• CONFIG_BLE_MIDI_SEND_RUNNING_STATUS - Set to y to enable running status (omission of repeated channel message status bytes) in transmitted packets. Defaults to n.
• CONFIG_BLE_MIDI_SEND_NOTE_OFF_AS_NOTE_ON - Determines if transmitted note off messages should be represented as note on messages with zero velocity, which increases running status efficiency. Defaults to n.
• CONFIG_BLE_MIDI_TX_PACKET_MAX_SIZE - Determines the maximum size of transmitted BLE MIDI packets (clamped to the MTU - 3).
• Use one of the following options to control how transmission of outgoing BLE packets is triggered:
  • CONFIG_BLE_MIDI_TX_MODE_SINGLE_MSG - Each utgoing MIDI message is submitted for transmission immediately, meaning that each BLE packet contains one MIDI message. This is the default option. May have a negative impact on latency but does not rely on nRF Connect SDK specific APIs and should work out of the box on nRF multi core SoCs.
  • CONFIG_BLE_MIDI_TX_MODE_CONN_EVENT - Buffer outgoing MIDI messages and send them in a single BLE packet just before the next connection event to reduce latency. Use with nRF Connect SDK v2.6.0 and above. Relies on the Event Trigger API added in v2.6.0.
  • CONFIG_BLE_MIDI_TX_MODE_CONN_EVENT_LEGACY - Use with nRF Connect SDK versions older than v2.6.0. The same as CONFIG_BLE_MIDI_TX_MODE_CONN_EVENT but relies on the MPSL radio notifications API that was removed in v2.6.0.
  • CONFIG_BLE_MIDI_TX_MODE_MANUAL - Buffer outgoing MIDI messages and leave it up to the caller to trigger transmission. Can be useful in combination with a custom connection event notification mechanism.

nRF multi-core considerations

When CONFIG_BLE_MIDI_TX_MODE_CONN_EVENT or CONFIG_BLE_MIDI_TX_MODE_CONN_EVENT_LEGACY is enabled, it is assumed that the BLE controller runs on the same core as the BLE MIDI service. This is true for a single core SoC like the nRF52840 or when running the entire application on the network core of a multi core SoC like the nRF5340. When running the BLE controller and host on separate cores, you currently have these options:

• Send one MIDI message per BLE packet using CONFIG_BLE_MIDI_TX_MODE_SINGLE_MSG.
• Roll your own mechanism for relaying the appropriate triggers from the network core to the application core to achieve BLE packet transmission synchronized to the start of connection events. See CONFIG_BLE_MIDI_TX_MODE_MANUAL.