This minimalistic Android application is a Walkie-Talkie style digital voice frontend for your radio, which uses open source [Codec2](https://github.com/drowe67/codec2) for speech audio frame encoding/decoding.
It is mainly intended for DV experimentation with ultra low cost 3-8$ radio modems, such as LoRa and 15-25$ esp32 board flavours with built-in LoRa module: T-Beam,
LoPy, TTGO, Heltec and others, but could also be used with custom modems + external transceivers or as a test harness for Codec2 frames generation and their playback.
Application connects to your radio KISS Bluetooth/USB modem, records speech from the phone microphone on transmit, encodes audio into codec2 format, encapsulates into KISS frames and sends to your modem.
It does not deal with radio management, modulation, etc, it is up to your modem and radio, it could be just AFSK1200, GMSK 9600, LoRa, FSK, FreeDV or any other modulation scheme. Radio just needs to expose KISS Bluetooth interface for speech frames.
- Application could also be used with your Android network radio, such as Inrico TM-7, apk just needs to be installed over USB, see [Discussion](https://github.com/sh123/codec2_talkie/issues/4)
- Modem, radio module or transceiver which supports [KISS protocol](https://en.wikipedia.org/wiki/KISS_(TNC)) or can process raw Codec2 audio frames over serial Bluetooth or USB
- **PTT hardware button**, `KEYCODE_TV_DATA SERVICE` (230 key code) hardware buttons are used for PTT (latter is used on some Android network radios), `KEYCODE_HEADSETHOOK` tx on/off on click, volume up/down buttons could be used for ptt if enabled in Preferences
- **USB serial connectivity** (default 115200 bps, 8 data bits, 1 stop bit, no parity), just select this app after connecting to USB and it will use given connection, baud rate could be changed from Preferences
- **Bluetooth connectivity** on startup, lists paired devices, so you can choose your modem and connect, you need to pair with your Bluetooth device first from Android Bluetooth Settings, default Bluetooth device could be set from Preferences
- **Voice codec2 mode selection**, which allows you to select various codec2 modes from 450 up to 3200 bps on the fly, sender and receiver should agree on the codec mode and use the same codec mode on both ends as codec2 mode negotiation between clients is not implemented at the moment
- **Codec2 loopback mode**, which records and plays your recorded voice back to test and evaluate different Codec2 modes and speech quality, could be enabled or disabled from Preferences, this mode is activated if no USB or Bluetooth connection were made
- **KISS buffered mode**, non-real time, playback will start only after all speech is received, use when modem bit rate is lower than codec2 bit rate to avoid gaps during playback at the cost of longer receiving delay before playback
- (BT) custom AFSK1200 LibAPRS based modem with increased TXTail parameter and Baofeng handheld transceiver: 450 works fine, 700 works with small gaps, probably LibAPRS needs some tweaks: https://github.com/markqvist/LibAPRS
- (USB) HC-12 modules: works, but application needs to use lower USB serial bit rate (change from Preferences), because module RF bit rate is hardwired to its serial bit rate, also module needs to be preconfigured with AT commands first
- most likely you do not have enough bandwidth, use higher throughput modem mode, for LoRa modems you need approximately 100-150% of data rate for the given codec2 bitrate, AFSK1200 modems should work at 450 bps codec2 mode, 700 bps might work, but not on all modems, higher speed codec2 modes won't work on AFSK1200 for real time, need to enable KISS non-realtime buffering mode
- modem goes into RX state too fast after sending first packet, increase TxTail parameter for your modem if it does not handle corresponding KISS command, so it will stay longer in TX before switching to RX
- if you are using LibAPRS based modem, set TxTail and TxPreamble programatically by using `APRS_setPreamble` and `APRS_setTail` methods as it does not seem to set these values from `CMD_TXDELAY` and `CMD_TXTAIL` KISS commands
- enable KISS buffering mode in Preferences, this will help with gaps at the cost of larger delay on receive before playback
KISS command extensions are used for radio module control and signal report events on port 0, command for radio control is defined as 6 (KISS SetHardware) and signal report command as 7. Radio modules/modems can implement these commands, so they will be controllable from the application and application will be able to show signal levels on S-meter.
- QSO log and non real time voicemail style communcation, where incoming transmissions are recorded, stored and could be played back later if recipient cannot reply immediately in real time
- [M17 Project](https://m17project.org) protocol support, including possible gating to [M17 reflectors](https://m17project.org/reflector/) on modem side
