So what is it that frogs, humans and the radio have in common? I'm sure you're eager to know! :-)

Chinese torrent frogs (Amolops tormotus) have evolved a curious way to communicate over the din of noisy waterfalls in their natural habitat. Instead of croaking in the normal lower frequencies, they croak in ultrasonic! Essentially, they have shifted their frequency up to the ultrasonic bands to minimize interference¹.

Now, we humans cannot exactly boost our voice to ultrasonic frequencies just because we want a minimal noise channel to communicate over. But, since when have such things ever held us back? Humans don't have big sharp teeth to compete with or fend off predators. But that's what we invented the knife for - our very own customized sharp teeth! We didn't have all cozy and comfortable fur to shelter us from the cold. But then, we invented clothes! Likewise, what do we do when we cannot exactly shout loud enough for our voices to be heard from miles away? Invent the radio, of course!

Yes, basically, amplitude modulation is nothing more than capturing audio signals with a microphone, upshifting the frequency by beating it with a high frequency "carrier" signal and then transmitting it!

Now, to those with an eye for technical detail, yes, there are quite a few variations of AM radio such as the double side band full carrier (DSB-FC), double side band suppressed carrier (DSB-SC), single side band suppressed carrier (SSB-SC), etc. But, essentially they are all the same idea of boosting the human voice to higher frequencies suitable for long distance transmission. But, if you still want to be mathematically precise, of all the variations of AM radio, single side band (SSB) radio is the one which is exactly and precisely nothing more than upshifting the frequency like I mentioned earlier.

If you're interested in the QTH and the QTR of this idea, I saw this connection in my head when I was at the local amateur radio club last weekend. I own a VHF FM receiver (the homebrew LRR200), and usually listen on the VHF bands at night every day. But at the radio club, it's usually SSB modulation on the HF bands.

What immediately struck me was the stark difference in noise levels between the VHF FM and the HF SSB. FM with its capture effect simply locks onto the frequency variation of the strong incoming carrier and almost completely rejects the atmospheric noise. SSB on the other hand, having to depend on amplitude variations, has to demodulate both the transmitted voice and the atmospheric noise, and put them both out together on the speaker as one composite noisy audio signal.

So, in effect, SSB radio is like shouting above the din of the noise existing in the band. The noise always exists in the band, and in order to be heard, one must shout above the noise. This is much like being on a very noisy road with all sorts of vehicles going bong, bong with their horns, and you are trying to talk to your friend nearby. In order for your voice to be heard and understood by your friend, you need to talk loud enough for your friend to be able to distinguish your voice from all the surrounding noise.

The FM capture effect also means that when two stations transmit at the same time at the same frequency, the FM receiver simply locks onto the stronger signal and ignores the other. However, in SSB (and other AM modes), the signals of both the stations would be simultaneously demodulated and superimposed on one another. Thus SSB would sound like two people shouting at the same time.

Thusly, FM radio, though very good for voice transmissions due to its noise immunity, may not be so good for listening to atmospheric phenomena and other such "noise" sources. So, if you, like me, are also interested in listening to these "noise" sources, better equip yourself with an AM radio, preferably SSB. For, just as SSB transmission is all about upshifting your voice to a higher frequency, SSB reception is all about simply downshifting high frequency noise to the audio frequency range and listening to it.

So, humans may not naturally be equipped with vocal cords capable of transmitting over a wide frequency range, or with ears that can sense and listen to a wide frequency range, but we sure more than make up for it with our radios! Amazing how much our brain power makes up for and extends the functionality of, isn't it?