Real-time Voice
Mode Control
| Operation | payload |
|---|---|
| Enable off-network session | 00 02 04 07 01 |
| Disable off-network session | 00 02 04 07 00 |
| Press PTT | 00 02 04 02 01 |
| Release PTT | 00 02 04 02 00 |
| PTT status acknowledgment | 00 82 04 02 |
Voice Encoding
| Parameter | Value |
|---|---|
| Codec | AMR-NB MR475 |
| Sample rate | 8000 Hz |
| PCM | mono PCM16LE |
| PCM frame | 320 bytes |
| Frame duration | 20 ms |
| Encoded frame | 12 bytes |
An AMR-NB MR475 IETF storage frame consists of a 1-byte ToC and a 12-byte payload. AT2 transmits only the 12-byte payload. Prepend 04 when decoding it as an IETF storage frame.
Transmit Frames
Real-time voice uses family 02, command 04, and subcommand 03.
| Type | payload | Duration |
|---|---|---|
| Normal packet | 00 02 04 03 00 00 [60-byte voice] | 100 ms |
| Tail packet | 00 02 04 03 00 00 [48-byte voice] | 80 ms |
A normal packet concatenates five encoded frames and is transmitted approximately every 100 ms. A tail packet concatenates four encoded frames.
Receive Frames
The device sends family 02, command 04, and subcommand 03. Remove leading zeroes after the subcommand, then split the remaining data into 12-byte AMR-NB MR475 encoded frames.