Although I see a lot of wireless projects, I’m always surprised at the lack of variety in the radio parts. I’m a radio amateur (WD5GNR; I was licensed in 1977) and Ham use a variety of radio techniques. If you think Ham only uses Morse Code and voice communication, think about your grandfather’s amateur radio. Modern hams have gone digital, communicating via satellite, video, and many different digital techniques that could easily be applied to various wireless projects.
Of course, Morse Code could have been one of the first digital modes. But Ham has used teletype, fax, and other digital modes for years. With personal computers and sound cards in common use, Hams led the way in developing sophisticated digital wireless technologies.
The motivation for developing unique digital modes is twofold. First, hams do not have unlimited bandwidth, especially in the radio frequency (RF) bands that allow distance communication. Anything that requires less bandwidth is welcome. Second, RF bands have broad characteristics. Signals fade, atmospheric noise causes static electricity and crashes, and other stations cause interference. There are other special cases too, such as signals bouncing off meteors, where the right digital strategy can greatly improve the likelihood of receiving messages.
One of the first of these sound card methods was PSK31. This mode uses a very low symbol rate and encodes data using phase transitions. In order to avoid high-frequency harmonics, the phase shifts only occur at zero crossings. The 31 in the name refers to the baud rate of 31.25 (this is not K-Baud, this is baud; only more than 30 bits per second). To compensate for the slow baud rate, the system uses a variable length code so that common letters use shorter bit patterns. Of course, the bandwidth used is also around 31 Hz, so that’s an advantage.
Hams use software to simultaneously decode all of the signals in their receiver’s audio. In the past, you wanted to filter out all signals except what you are hearing. With PSK31, you want a wide filter and leave the DSP all of the signals at home, each of which is digitally filtered. For example, the following figure shows a waterfall display with several PSK31 signals that are incoming at the same time. The waterfall is a special graph where the X-axis (left to right) is frequency and the Y-axis (vertical) is time. The “worms” crawling down the waterfall are signals (you can also see the snow from random sounds). The PC can easily read data from all signals at the same time.
The digital sound card modes have become even more sophisticated from then on. A ham [WB8NUT]has an interesting side to the combines many modes and contains sound clips. Different modes have different purposes. For example, packet is an adaptation of X.25 over radio frequencies and can be used as a physical layer for TCP / IP. JTMS is useful for meteorite scattering and JT65 is useful for moonbounce (yes, that’s what it sounds like) and can enable slow data transmission with signals inaudible to the human ear. Many of these protocols use forward error correction and use sophisticated codes such as Reed-Solomon for error detection and correction.
Even Voice has gone digital. We’ve covered this before, and the linked YouTube video (and the image on the right) shows a voice contact (a QSO) that is difficult to understand in normal analog transmissions but perfectly clear when using digital techniques. Better performance and reduced bandwidth thanks to PC-based DSP. Then again, Not all solutions require DSPHowever, its widespread adoption is due to its ease of development and use with a shared PC.
However, my point doesn’t apply to any specific mode (if you really want to see a demo of PSK31, check out the video below). In the non-ham community, we see people building new computer languages, operating systems, and even CPUs. However, it is quite rare for new radio modulation schemes to appear. With a few exceptions, hackers buy radios and take what they get.
If you want to experiment with little or no investment, Take a look at WebSDR (see picture below). With the right web browser, you can borrow someone else’s radio (which is cool on its own) and then feed audio into one of the many programs in digital mode (e.g. fldigi, for example). First, find a radio in any part of the world that is daytime, tune in the 20 meter band, and check out 14,070 for PSK31 activities. Note that in the screenshot I was one of 147 users listening to the radio at the same time and that we didn’t all have to hear the same thing.
Perhaps the hacking community could orient itself on the ham. Radio experiments have a lot of potential. Granted, it’s a little easier because hams have radio frequencies on which they are allowed to experiment. However, digital and voice bands are available to everyone, and (depending on your local laws) it may be interesting to see how much data you can send over such an audio channel. Or you could bite the bullet and become a ham yourself (This is a lot easier than it used to be in most countries, including the US).
Challenging? You bet. But really no more demanding than a scanning tunneling microscope or building a custom CPU. Who is ready for this?