Tuesday, May 29, 2018

Sydney - Adelaide via FreeDV 700D. Better than SSB

It's 4:30pm Sydney time and I've just had a digital voice contact using David, VK5DGR's new 700D FreeDV mode. Sydney to Adelaide is about 1,100km so a good distance for 7MHz. My local noise level here is not great at about S6 and yet we were able to have a pretty good contact on 40m.

Version 1.3, which includes the new 700D mode, is available as a pre-built binary for macOS, Linux and Windows. I grabbed the latest development source and it builds according to the README on Ubuntu 18.04 just fine.

My set up is a USB headset mic and a Signalink USB radio interface. All works well although David suggests that decoded digital voice might be more readable via a speaker than via headphones.

We chatted back and forth in digital mode pretty well and then switched to SSB to compare. I was certainly putting out more power on SSB and I suspect David was too but to me the digital copy was as good as or slightly better than SSB. This is a fantastic milestone to reach!

Signal notification service

As there isn't a huge amount of FreeDV activity at the moment, there is a fantastic feature that can monitor and send an email if there is five seconds of sync. You run a python script included in the source and give it your email credentials. This script listens on UDP port 3000 and if you enable this in FreeDV's options screen it will email you when someone's on the channel. (It only sends one email a minute so you don't get swamped).

This is a great idea and should be a standard for other digital mode clients.

How I sound

We had a three way contact today on 7.177 between Peter, VK3RV in Sunbury Victoria, David, VK5DGR in Adelaide and myself, VK2TPM in Sydney. David recorded one of my transmissions and sent me the off air recording which can be decoded in the FreeDV app. We could barely hear each other in SSB. Here's how it looks and sounds:

Note that there are command line tools distributed with the FreeDV source code including freedv_tx and freedv_rx which will encode and decode all the way from voice wav through to modem audio.

Here's how you decode a captured file:

freedv_rx 700D vk2tpm.wav - | sox -t raw -r 8000 -e signed-integer -b 16 - decoded.wav

Note that the output file is a 16 bit mono file with 8kHz sample rate.

Check out the FreeDV site for much more information and give me a call on 7.177Mhz.

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