Shut Up And Say Something: Amateur Radio Digital Modes
IN recent article, I complained about my disgust with having a classic amateur radio call – calling CQ, calling someone from far or around the block, exchanging call signs and signal reports, and maybe a few nice things. I think empty chat is a big exception for many people who would otherwise be interested in the biggest hobby in the world ™, but fortunately there are many ways to get the microphone on the air. So as a public service I would like to look at some of the many digital modes offered by amateur radio as a way to avoid talking while still communicating.
The modes and modulations
The hams speak in terms of modes and modulation when describing their radio broadcasts. The difference between the two terms in most cases is not important for our discussion and in practice many thighs use the terms interchangeably. But for completeness, modulation is a way to impress radio wave information and mode is a way to use modulation for communication. Modulation schemes include amplitude modulation (AM), frequency modulation (FM) and single sideband modulation (SSB). Modes include continuous wave (CW), analog voice, digital voice, images and data.
The digital modes I want to discuss are those in which you can easily sit down at a keyboard and your message magically appear on the terminal of another ham around the world. I will not cover the CW as a data mode here, although it was obviously the first and perhaps the most successful digital data mode to date. The International Morse Code has been in place for 140 years, and with the many benefits of CW modulation, it is likely to remain a powerful tool as people take care to learn their skills. Yes, there are applications that will translate Morse keystrokes and vice versa, but it just feels like a scam.
Those of us with long enough memories will remember the early days of interwebz, when dial-up connections were the only way to get online. The sound from the modem dialing Compuserve or AOL and negotiating a connection was the soundtrack from the days before the Internet. The modem was mod– extending the data signals from your computer in audio tones that would match the analogue telephone line, and demodhandling of returning audio signals. All radio ham modes come down to the same process – with the addition of a little outboard, the data from your computer is converted into audio tones that are fed to your transmitter, and the audio from your receiver is decoded back into data.
In many cases, the additional equipment needed to turn on most data modes these days is minimal. There was a time when special converters were needed, but with a powerful DSP built into any computer sound card, almost any computer will do it. Many ham transceivers already have built-in sound cards, so sometimes all you need is a USB cable and the right software. FL-Digi is a popular package that supports most of the popular digital modes, provides a waterfall display that allows you to easily visualize a huge range of bands and even controls your platform by adjusting it to the selected frequency and pressing the transmitter when needed.
There are a puzzling number of data modes out there, and crossing the HF lanes at night may sound a bit spooky. The flickering tones that seem to float in the bands as the ionosphere performs its night dance are a little eerie. The sound signature of each data mode is quite distinctive and experienced professionals can choose the mode by sound only or perhaps with a little help from its appearance on the display of the waterfall. Noobs can get help identifying modes from any number of websites or rely on its automatic mode detection software package.
Which mode to choose is largely a function of what will work best under the given conditions. Unlike telephone line modems, the physical medium in ham data modes is subject to much greater potential for interference, both natural and man-made. The signals can be interrupted by breakdowns of static electrical storms, two signals can arrive in different ways and suffer from phase problems, or the signal strength can be so low that it is just above the bottom of the noise. Any useful data mode should take these whims into account, and some do a better job of coping with one set of conditions than another.
Here is a list of the main data modes you will run and the relative benefits of each:
The radio teletype, or “ritty” as the buttons call it, is the original digital data mode. It dates back almost as long as commercial radio, with the first RTTY service created between San Francisco and Honolulu in 1932. Then, as now, RTTY uses the 5-bit Baudot code to encode each character. The simplest modulation scheme for RTTY is audio frequency switching (ASFK) with a difference of 170 Hz between the brand and the intervals. This results in an incredible 45-stitch connection (you’ll notice that most digital ham modes tend to be on the low side in terms of bandwidth, thanks to the limited bandwidth available at the relatively low frequencies needed to take advantage of the required ionospheric pass for long-distance contacts.) As slow as it sounds, it’s still about 60 words per minute, which is fast enough to keep up with most typists.
RTTY has been joined by many other data modes, but there are still RTTY fans moving on the air. The lower end of the 20-meter strip is a good place to find RTTY operators.
One of the advantages of RTTY is that it is technically easy to implement. But it does not perform very well at very low signal levels. To fix this, [Peter Martinez (G3PLX)] decided to come up with a better RTTY. PSK31 was introduced in 1998 and has since become quite popular.
[Martinez] adopted a two-way approach: first, it developed a new method for encoding alphanumeric characters called Varicode. Instead of a fixed word length like Baudot used in RTTY, Varicode’s word length was more Morse code, with commonly used letters represented by shorter codes than rarer letters. Then, to modulate the code, [Martinez] uses the DSP in the computer’s sound card to shift the phase of the sound signal by 180 ° to represent zero in the varicode, while the unchanged sound is logical.
This phase shift of the keyboard (PSK) results in a bit rate of 31 – slower than RTTY, but designed to handle the average typist. The PSK31 is more efficient than RTTY in terms of bandwidth – only 31Hz wide – and combined with the fact that the receiver and transmitter need to be synchronized and the DSP algorithm can predict when to expect phase transitions that transmit signal data, PSK31 excels when downloading data from weak signals.
The user experience for RTTY and PSK31 is quite simple – the sending party dials a short, full of abbreviations message on the keyboard, and the receiving party reads the message on an alphanumeric display. But lest you think that amateur data modes are just for sending straight text messages like those sent by Model 33 teletypes during the day, there are many packet modes for sending more complex messages, including email.
PACTOR is a set of modes that are based on 200Hz shift frequency manipulation (FSK). Unlike RTTY and PSK31, PACTOR encodes data as 96- or 192-bit packets, allowing the use of the Automatic Repeat Error Control Protocol (ARQ) to request packets that fail to be resent by the CRC. PACTOR is measured at about 200 stitches.
Unfortunately, PACTOR requires expensive equipment called a terminal node controller (TNC) between the radio and the computer. To address this, the WINMOR protocol was developed. Like PACTOR, since it is a packet mode with error correction, WINMOR removes the TNC using a cheap USB audio connection or using a sound card built into many modern transceivers.
Both WINMOR and PACTOR are gateway protocols to the Winlink 2000 network, which provides e-mail service via HF radio. Winlink is an extremely diverse hybrid network of HF and VHF radio links in Internet messaging servers. Emails can be composed with the full-featured RMS Express client, which looks and feels almost like any other email client. Emails can include attachments and can be sent equally or over the network to any other Winlink user.
As useful as the Winlink network is – it has been a huge boon to emergency communications in natural and man-made disasters that disrupt local Internet service – using it is as exciting as sending an email because that’s what you do. For my money, digging a one-to-one contact from noise with a few watts on the PSK31 sounds a lot more fun. I’m glad that the Winlink network is there and it’s worth practicing with it from time to time, but there are much more challenging data modes to research, at least in my opinion.
I have only scratched the surface of the digital modes available for the micro-shy ham. Here we hope that this will attract a few more new people to the hobby, or maybe even get these licensed but largely inactive broadcasts on the air. After all, we all need more people not to talk to.