A TX-only [TNC](https://en.wikipedia.org/wiki/Terminal_node_controller) (Terminal Network Controller) to generate the [AFSK](https://en.wikipedia.org/wiki/Frequency-shift_keying#Audio_FSK) audio tones for [APRS](https://en.wikipedia.org/wiki/Automatic_Packet_Reporting_System) (Automatic Packet Reporting System) messages using a [Raspberry Pi Pico](https://en.wikipedia.org/wiki/Raspberry_Pi) microcontroller board.
An analog line-out audio signal will be generated by a filter connected to GPIO-pin 'GP0' which provides the binary PWM signal. You can probe it by a scope, listen to it by using an audio amp, or connect it to any RF transceiver to send it on the air (ham radio license required).
1. Install the Pico-SDK following the instructions given in the [Raspberry Pi 'Getting Started' Guide](https://datasheets.raspberrypi.org/pico/getting-started-with-pico.pdf) (pdf)
1. Set the `PICO_SDK_PATH` environment variable to point to your Pico-SDK installation directory
The analog AFSK audio signal will be available at the filter's line-out. You can probe it by a scope, listen to it by using an audio amp, or connect it to any RF transceiver to send it on the air (ham radio license required).
- For PCM => PWM conversion I'm using the audio lib from [pico-extras](https://github.com/raspberrypi/pico-extras) (WARNING: maturity seems to be alpha/beta)