Thank you to Wojciech Kaczmarski (SP5WWP) for sharing his recent blog post showing that a low-cost analog VHF/UHF US$50 Retevis C62 handheld radio can decode TETRA voice with some custom firmware. We note that the Retevis C62 uses the same BK4819 RF chip as the popular Quansheng UV-K5. 

Wojciech explains that because TETRA's pi/4-DQPSK encoding is differential, it can be treated as 4-level FSK after a plain FM demodulator, and a single register setting widens the BK4819 to 25 kHz channels. The TETRA stack and vocoder are then flashed onto the radio's CSK6011B microcontroller. This meant replacing osmo-tetra and ETSI's reference vocoder with Wojciech's libtetra and OpenACELP libraries, written for 32-bit MCUs.

The firmware is a fork of OpenRTX built on turnrye's C62 M17 port, and needs no hardware modifications. It is receive-only and plays only unencrypted speech. The demo below uses a sample from the HamTETRA group.

AI Disclaimer: On GitHub, many commits in the OpenRTX C62 port that this firmware builds on list Claude, and some list Codex, as co-authors. We found no AI disclosure in Wojciech's own TETRA code or libraries.