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App-device protocol

BLE Discovery​

The official app discovers the device by scanning for BLE devices with the name Omi.

BLE Services and Characteristics​

The Omi wearable device implements several services:

the standard BLE Battery Service​

The service uses the official UUID of 0x180F and exposes the standard Battery Level characteristic with UUID 0x2A19.
The characteristic supports notification to provide regular updates of the level (this does not work with firmware 1.0.x and requires at least v1.5).

the standard BLE Device Information Service​

Available since firmware version 1.0.3

The service uses the official UUID of 0x180A and exposes the following characteristics:

  • Manufacturer Name String (0x2A29) : "Based Hardware"
  • Model Number String (0x2A24) : "Omi"
  • Hardware Revision String (0x2A27): "Seeed Xiao BLE Sense"
  • Firmware Revision String (0x2A26) : the firmware version e.g. "1.0.3"

one BLE service to stream the audio data to the app​

The main service has UUID of 19B10000-E8F2-537E-4F6C-D104768A1214 and has two characteristics:

  • Audio data with UUID of 19B10001-E8F2-537E-4F6C-D104768A1214, used to send the audio data from the device to the app.
  • Codec type with UUID of 19B10002-E8F2-537E-4F6C-D104768A1214, determines what codec should be used to decode the audio data.

Codec Type​

The possible values for the codec type are:

  • 0: PCM 16-bit, 16kHz, mono
  • 1: PCM 16-bit, 8kHz, mono
  • 10: Mu-law 8-bit, 16kHz, mono
  • 11: Mu-law 8-bit, 8kHz, mono
  • 20: Opus 16-bit, 16kHz, mono

Starting with version 1.0.3 of the firmware, the device default is Opus. On earlier versions it was PCM 16-bit, 8kHz, mono.

Audio Data​

The audio data is sent as notifications on the audio characteristic. The format of the data depends on the codec type.
The data is split into audio packets, with each packet containing 160 samples.
A packet could be sent in multiple value updates if it is larger than (negotiated BLE MTU - 3 bytes).
Each value update has a three byte header:

  • The first two bytes are the overall packet number, starting from 0, incremented for each value sent and looped over to 0 after 65535.
  • The third byte is the index of value in the current packet, starting at 0.

For instance, 160 samples with a codec 0 (16-bit, 16kHz PCM) results in a packet size of 320 bytes.
On current iOS devices, this results in 2 value notifications:

  • packet number n, index 0, 251 bytes
  • packet number n + 1, index 1, 75 bytes

The data is sent in little-endian format.