Codec 2 HF Data Modes Part 2

Over the past few months I’ve been working on HF data modes, in particular building up a new burst acquisition system for our OFDM modem. As usual, what seemed like a small project turned out to be a lot of work! I’ve now integrated all the changes into the FreeDV API and started testing over the air, sending frames of data from a Tx at my home to remote SDRs all over Australia.


  • Importantly – this work is open source – filling a gap in the HF data world. HF is used for Ham radio, emergency communications and in the developing world where no other infrastructure exists. It needs to be open.
  • High performance waveforms designed for fast fading channels with modern FEC (thanks Bill, VK5DSP).
  • Implemented as a C library that can be cross compiled on many machines, and called from other programs (C and Python examples). You don’t need to be tied to one operating system or expensive, proprietary hardware.
  • Further development is supported by a suite of automated tests.

I’m not aiming to build a full blown TNC myself, just the layer that can move data frames over HF Radio channels. This seems to be where the real need lies, and the best use of my skills. I have however been working with TNC developers like Simon, DJ2LS. Together we have written a set of use cases that we have been developing against. This has been very useful, and a fun learning experience for both of us.

I’ve documented the Codec 2 HF data modes in README_data, which includes simple examples of how to use the API, and simulated/real world results.

Further work:

  • Automated testing over real world channels
  • Tuning performance
  • Port a higher bit rate QAM16 mode to C
  • Working with TNC developers
  • Prototype very simple low cost HF Data links using RTLSDRs and RpiTx transmitters

Reading Further

HF Acquisition Pull Request – journal of the recent development
README_data – Codec 2 data mode documentation (HF OFDM raw data section)
Codec2 HF Data Modes Part 1

One thought on “Codec 2 HF Data Modes Part 2”

  1. That’s a very important work!

    Even though not the same frequency range. But in the UHF and up to 2.4G bands LORA is really taking over the market from all the other chipsets like nrf24l01 and similar.

    Really good to see you are steadily working on progressing digital modes inline with open source philosophy, as hamradio was always meant to be.

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