zephyr-ble-midi

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zephyr-ble-midi

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A Zephyr implementation of the BLE-MIDI (MIDI over Bluetooth low energy) service specification.

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# zephyr-ble-midi

This is a [Zephyr](https://www.zephyrproject.org/) implementation of the [BLE-MIDI service specification](BLE-MIDI-spec.pdf), which allows for wireless transfer of [MIDI](https://en.wikipedia.org/wiki/MIDI) data over [Bluetooth low energy](https://en.wikipedia.org/wiki/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](CMakeLists.txt) file shows one way of adding this module to an app.

The public API is defined in [ble_midi.h](ble_midi/include/ble_midi/ble_midi.h).

## Sample app

The [sample app](src/main.c) 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](https://docs.zephyrproject.org/latest/boards/arm/nrf52840dk_nrf52840/doc/index.html).

* __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`.

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