Controlled FreeDV Testing

This post presents some results from a controlled experiment to test FreeDV against SSB over a range of real world HF channels.

As described in [2] I take a 10 second speech sample and transmit it using SSB then FreeDV. By transmitting them more or less at the same time, we get to test them over the same channel. The peak power of the SSB and FreeDV signals are adjusted to be the same. The SSB is compressed [3] so that we are generating as much SSB talk power as possible.

Over the past few weeks we’ve collected 1158 samples of FreeDV and SSB signals, using a network of KiwSDRs and some scripts to automate the collection of off-air samples. Every 30 minutes my IC7200 would click and whir into life as the laptop connected to it transmitted a test signal in various FreeDV modes. Simultaneously, the signals would be received by a remote KiwiSDR and recorded as a wave file. This was then decoded to produce a side by side SSB versus FreeDV audio signal.

Jose, LU5DKI, also collected some samples for me using his station. Jose is very experienced at using FreeDV with SDRs over international DX paths.

I’ll present some of the more interesting samples here. If you open the spectrogram in a new tab you should be able to see a larger version. The spectrograms are like a waterfall but time flows left to right. The tests were conducted with FreeDV 700C/D/E [4].

Serial Comment SSB & FreeDV Spectrogram
0036 Lower limit of 700D, which can handle lower SNRs than other modes. SSB very difficult copy. Listen
0105 FreeDV 700C with some co-channel SSB, results in a few errors in the decoded voice, SSB affected more. Listen
0106 Fade at the start of SSB and 700D, however 700D takes a few seconds to sync up Listen
0119 Impulse noise, e.g. atmospherics, lightning, largely suppressed in decoded FreeDV. Listen
0146 Slow fade, occasionally nulling out entire signal Listen
0500 700D falling over on a weak fast fading/NVIS channel, note the moth-eaten spectrogram Listen
0501 700E coping with the same NVIS channel as Serial 0500 – 700E is designed to handle fast fading Listen
0629 “Barber pole” frequency selective fading carving a notch out of the 700D signal, but no errors at 5dB SNR. SSB improves only slowly with increasing SNR – still quite noisy. Listen
j01 DX 700D from Argentina to Rotorua, New Zealand. Both SSB and FreeDV pretty good. Listen
j03 700D DX sample from Argentina to Ireland, SSB a weak copy but 700D not getting sync Listen
j03 700E As above but with 700E, which is decoding with some errors. 700E was designed for long distance paths Listen


  1. The initial “hello” sounds buzzy as the microphone equaliser [5] is still kicking in.
  2. Most of these samples are low SNR as that’s an interesting area of operation for me. Also – the experiment didn’t collect many high SNR samples, perhaps due to the limitations of my station (50W into a simple dipole) and band conditions during the experiment. I would have liked to have collected some high SNR/fast fading examples.
  3. Unfortunately the output audio levels aren’t normalised, so the FreeDV part of the sample will sound louder. I didn’t apply any noise cancellation to the SSB samples, but if you have access to such software please feel free to download the samples and see what you can do.


Special thanks to Jose LU5DKI, Mooneer K6AQ, Peter VK3RV for their help in manually collecting samples and and discussion around these experiments. Thanks also to the KiwiSDR community, a very useful resource for experimental radio.


[1] Codec 2 HF Data Modes Part 3 Similar script based automated testing for Codec 2 HF data modes.
[2] Automated Voice Testing Pull Request, containing more details of the experiment and test software.
[3] FreeDV 700E and Compression, including a description of the SSB compressor used here.
[4] Summary of various modes in FreeDV manual.
[5] Codec 2 700C Equaliser Part 2

One thought on “Controlled FreeDV Testing”

  1. Thanks for these tests.
    We are slowly getting there. Are there any plans for the 450 mode?
    How can we help to achieve this, even for comparison?

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