FreeDV: A digital voice mode for the HF bands

On the fourth Tuesday of every month, we have a local net on 70cms about digital radio. While its primary purpose is to generate some activity on the local DMR repeater and talk groups, the conversation this month turned to the subject of digital voice on the HF bands. It was then that the digital mode FreeDV was mentioned and I have to admit it was the first time I had heard of it.

Perhaps I had come across it before in a news item and I didn't take much notice but it sounds like an interesting mode. It seems that while it started back in 2012, it's only in more recent years that the performance has surpassed that of SSB when the signals are very weak and in the noise.

I thought it might be an idea to put up a blog post so that perhaps a few more people might be interested in trying out this new mode.

My first question I had about this digital mode was how could it possibly work on the HF bands with all of the multi-path interference and fading? Well, it seems as if it's not a case of all or nothing. The signal is broken up into separate sub-carriers and the information is modulated on these by phase shifting.

The modulation mode is OFDM which stands for Orthogonal Frequency Division Multiplexing. The spacing is chosen so that the subcarrier nulls coincide with the peaks of other subcarriers. By doing this, the subcarriers don't interfere with each other.

It works on the principle that even if there is some selective fading on some of the subcarriers, enough information is getting through on the others to allow a conversation to be held.

Waterfall display of a FreeDV signal showing selective fading

FreeDV... What it is and what it's not
First of all, what it's not. It's not for DX-ing, chasing DXCC countries, contesting, chasing squares or awards. It's not there to replace SSB or even your local net on HF.... (for the moment ;o)

COMREG, the Irish licencing authority defines Amateur Radio as follows..."The amateur service is defined by the International Telecommunication Union (ITU)1 as: A radiocommunication service for the purpose of self training, intercommunication and technical investigations carried out by amateurs, that is, by duly authorised persons interested in radio technique solely with a personal aim and without pecuniary interest"

The self training and technical investigation aspects are what FreeDV and digital voice are really about. It's a new technology that is evolving and will appeal to people who want to learn about new developments and how they might be implemented.

The big advantage of FreeDV is that you can use it with existing HF radios with SSB. It's not like DMR, C4FM or D-STAR which require a completely new radio to be bought. If someone is set up for digital modes like FT8 then they have the equipment already. As such, it can be used on any band where there is SSB... HF or VHF.

An interesting feature is that FreeDV demodulator can automatically acquire signals with a frequency offset of up to ±200 Hz. This means that you don't have to be the exact same frequency as the other station. You can see how this might be of value in say a net on SSB where everyone is trying to get on the correct frequency.... and there's always one who is off a bit :o)

Modes...
It's also worth mentioning that there are several FreeDV modes as well.
700D... This is for weak signal work on the HF bands and is claimed to work down as far as a signal to noise ratio of -2dB. The 700C version was introduced in 2017 and was replaced by the better 700D mode in 2018.
800XA... Similar to C4FM except over FM radio. Designed for VHF work.
1600... This is for a higher quality audio signal.
2020... 8 kHz of audio bandwidth is fitted in just 1.6 kHz of RF bandwidth. Delivers high quality audio when the signal are strong. Introduced in 2019.

Timeline...
2009 - Codec 2
2012/2013 - FreeDV 1600
2015 - SM1000 FreeDV hardware
2018 - FreeDV 700D low SNR > SSB
2019 - FreeDV 2020 8 kHz audio
2019 - SM1000 700D port

Video... The video below from the 2018 RSGB Convention gives a good outline of FreeDV and its use. Even if you have only a passing interest in digital voice, it's worth a watch.



Link...
1) FreeDV... https://freedv.org/
2) Demo of 2020 mode