FreeDV

FreeDV is open-source digital voice for HF. The program runs on a Windows, macOS or Linux computer connected to an ordinary SSB transceiver through a sound card interface, so the radio you already own can carry it. Unlike most digital voice systems, it was designed for HF from the start, and anyone may study and improve the code.

The project grew up around Codec 2, an open speech codec developed by David Rowe. Its current flagship mode is RADE, short for Radio Autoencoder, which mixes machine learning with conventional signal processing. RADE needs about 1.5 kHz of spectrum, can work at signal-to-noise ratios as low as about −2 dB, and is the mode the program selects by default.

The older modes, 700D, 700E and 1600, are still in the program as legacy modes. They stay hidden until you turn on Enable Legacy Modes in the options. Both stations must use the same mode, so if you hear nothing but noise, check that first.

On a regular SSB receiver, FreeDV sounds like a steady hiss or rumble. Decoded, the voice comes through with no background static. When the signal gets too weak it drops out instead of fading slowly into the noise, which takes some getting used to.

To transmit you need two sound devices: one connected to the radio and one for your microphone and speakers. The computer’s own audio works for the second. Set the transmit level so ALC barely moves, just as you would for any sound card mode.

To find someone to talk to, check FreeDV Reporter, a live list of stations on the air. The FreeDV website also lists regular activity frequencies and nets, and FreeDV Activity Day runs on the third weekend of each month.

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