The Amateur Amateur: Creeping up on the Digital Modes

By Gary Hoffman, KB0H
Contributing Editor
June 8, 2005

Getting up and running on digital modes flitted into and out of my consciousness for a few years, but then I discovered PSK31. Even watching noise was interesting!

I became aware of Amateur Radio digital modes as soon as I began studying for my first Amateur Radio license. RTTY is mentioned in the same breath as AM, FM, and SSB in just about every book and pamphlet that describes our hobby. I never gave digital modes much thought, however, until my wife Nancy and I came across a ham radio demonstration held on the grounds of the St. Louis Science Museum. A local club had set up a couple of radios, and one of them was running packet.

LONGWAVE, shack computer

Longwave, a personal computer just for my shack.

I asked the ham at the packet station a few questions, and his answers interested me. But being a newly-minted no-code Technician, I wasn't quite prepared to jump into such a strange and exotic mode.

Digital modes flitted into and out of my consciousness for a few more years, mostly in the form of packet radio. Once in a while I would consider giving it a try, but I backed away very quickly every time I picked up a ham catalog and checked out TNCs--Terminal Node Controllers. The prices caused my baud rate to skyrocket.

I decided to put exploration of digital modes on the back burner for the time being--the very, very back burner.

One pivotal event brought digital modes back into focus for me. I decided to do a column about Field Day 2003, and visited the St Charles Amateur Radio Club's site (see Saturday in the Park with Ray, Part I and Saturday in the Park with Ray, Part II). The station running PSK31 garnered more spectators than anything else at the event. Not only was this an unusual setup for many of the onlookers, but its operator was an 11-year-old girl!

That image stuck in my mind and eventually prompted me to do a little research on PSK31.

The first thing I discovered was that radio amateurs have been experimenting with quite a few digital modes over the years. RTTY, AMTOR and PACTOR were all terms I'd come across while studying for my ham ticket, but PSK31 and something called MFSK16 seemed to be the hot new modes.

All of these modes seemed to work by sending tones over the air, each letter and number represented by some unique combination of those tones.

Rascal soundcard interface

The magic sound card/transceiver interface. But what was the extra connector supposed to do?

The second thing I discovered was that those particular digital modes seemed to be used primarily on the HF bands. Packet, while sometimes found on HF, tended to be more common on VHF and UHF.

Ah, but my third discovery really got me excited: One could run PSK31 using just the sound card of a personal computer. I could operate in that mode without buying an expensive TNC! Better still, I wouldn't have to build a sound card/transceiver interface, since those babies were already commercially available. Hey, I may like to experiment, but I'm still fundamentally lazy.

A quick check of my HF transceiver showed that it did indeed have a data port. More joy: a sound card/transceiver interface was available for my specific radio. I quickly ordered one. It arrived in short order, and I soon had my computer and HF rig linked together.

Hmmm. There seemed to be one wire and a connector left over, but I didn't worry about it. My main concern now was how to set up my computer.

PSK31 waterfall

A waterfall display. The vertical trace represents a signal.

Let me take a moment to express my extreme gratitude to all the folks who have developed today's digital mode software and made it available as freeware or shareware. I just can't say enough about these fine people. Maybe I would have one day gotten around to building my own adaptor, but I would never have taken on the task of trying to write a computer program. So, thanks guys!

There are a number of freeware and shareware programs for digital modes. Some of them are for one specific mode, such as PSK31, while others can run several modes. A couple of them are so user friendly and flexible that it's hard to believe they are free.

Some of these programs contain an eye-catching feature called the waterfall. This is a continually changing display which shows the activity on the frequency to which your transceiver is tuned. Now, at first this display had me confused. I finally figured out, however, that what it was showing me was the audio spectrum--the same thing I was hearing on my radio's speaker.

The waterfall was fun to watch, even when I wasn't tuned to digital mode frequencies. Actually, it was more than fun. It was instructive. Even watching noise was interesting. It became clear that voices took up a lot of space on the waterfall when compared to some sort of digital signal, which took up relatively little space--or bandwidth to use a more appropriate term.

I already knew that the typical human voice in radiocommunication systems like ham radio spans roughly 300 to 3000 Hz--or a bandwidth of approximately 2700 Hz. Individual tones obviously have a much narrower bandwidth. But it was cool to see this phenomenon displayed visually.

Next month I'll tell you what I saw and heard, and--better yet--what happened when I tried to send my own digital message. Stay tuned! And keep your eye on that waterfall display.

Editor's note: ARRL member Gary Hoffman, KB0H, lives in Florissant, Missouri. He's been a ham since 1995. Hoffman says his column's name -- "The Amateur Amateur" -- suggests the explorations of a rank amateur, not those of an experienced or knowledgeable ham. His wife, Nancy, is N0NJ. Hoffman has a ham-related Web page. Readers are invited to contact the author via e-mail, [email protected].

© 2005 American Radio Relay League


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