App-device protocol
BLE Discovery
The official app discovers the device by scanning for BLE devices with the name Friend.
BLE Services and Characteristics
The Friend wearable device implements 2 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 regulat updates of the level (this does not work with firmware 1.0 and requires at least v1.5).
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
The device default is 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.