What use is an F-call?

Recently I talked about a digital mode that had the ability to give you an estimated gain of 13dB over Analogue FM. It's the equivalent of gaining more than 2 S-points or like turning up your transmitter power from 10 Watts to 200 Watts.

Of course, the receiver at the other end doesn't all of a sudden see their S-meter go up and neither does the power draw from your transmitter spike. The actual transmitted power is still the same and the actual received power is also the same.

What's going on for this magic to happen?

If you've ever listened to Morse code, not to understand it, I'm not there yet either, but just to hear it, you'll notice that you can detect individual dits and dahs at a very low signal level, much lower than it would be possible to hear an SSB signal in the same environment.

The reason that happens is because your ear only needs to detect the presence or absence of a tone. Once you can hear the tone, you can work out how long each tone is and then your brain can decode a dit or a dah. Do that enough and you can decode a letter, then a word, then a sentence.

So, under Morse conditions there are two basic variables, a tone or not and the length of that tone.

If you had a great filter you could make it possible to filter out all but the wanted signal, making it possible to hear even weaker signals.

What we're really talking about here is something called a signal to noise ratio. That is, the difference between the background noise, coming from the atmosphere, the neighbours and the radio itself, and the signal, or the Morse code you're trying to detect.

The simpler the signal, the easier it is to hear. Of course there are limitations. You can only key so fast, your radio can only key on and off so fast, etc.

What if you could key your radio differently? What if you used multiple tones, could you get the same effect?

If you look at JT65, a weak signal digital mode, originally designed to do Earth-Moon-Earth communications, but now widely in use on HF, it does exactly that. Instead of on and off, it uses 65 tones to encode information. It uses a whole lot of mathematics, error correction and the like to ensure that each of these tones is decoded correctly and the message is either conveyed entirely, or ignored.

Doing this allows JT65 to work in an environment where the noise is higher than the signal.

And get this, the performance is entirely dependent on the software decoder in the receiver.

What that means is that as we figure out how to improve software signal processing, the performance of JT65 will get better.

The rabbit hole goes deep when you start digging and I can assure you, this just scratches the surface.

I'm Onno VK6FLAB