Saturday, September 23, 2017

Digital Amateur Radio

I wanted to talk a little today about the "digital" modes of transmitting information over the radio waves.  A digital mode involve signals that transmit some combination of discrete frequencies by one station that can then be decoded by another station to reveal the information being transmitted.

The oldest "digital" mode is what is called Continuous Wave (CW) that transmits Morse code.  The transmitter is either on or off.  The length of the elements are either long or short.  These long, often called "dah" and the short, called "dit" thus make up a binary code, the differing combinations of which make up all the letters, digits, punctuation and pro-signs of Morse code.  CW is still practiced in Amateur Radio, and can be used anywhere in the amateur bands.

Radio Teletype, abbreviated RTTY, is a radio implementation of a land line technology that was an early form of sending text over the phone lines.  The wire services such as UPI, used teletype extensively.  RTTY uses two tones, usually 170 Hz apart, to transmit the letters, digits, punctuation, and again certain pro-signs to transmit text as opposed to voice.  RTTY proceeds at 45.45 baud, or about 60 words per minute.  Once upon a time, to operate RTTY one had to make a substantial investment in converting old teletype machines to transmit over the air.  But with the advent of personal computers, software took over this function, and with a simple sound card as the interface between the computer and the transmitter, one could be in the RTTY business.   Today, the American Radio Relay League (ARRL) promotes RTTY as a digital mode that is very popular.  It is also one of the easiest to understand, and to set up the software to run. But in fact, only a few experts with RTTY operate during contests, and there seems to be little other activity.  So I have to question the ARRL's notion of the term "popular."  As a result, a new practitioner can not get any experience themselves, because RTTY operators only use their equipment at certain times when a new person will be reluctant to try out the mode.

Since 2000, a number of digital modes using inexpensive sound cards as the interface between your computer and a transmitter have popped up like mushrooms.  One of these, PSK31, is a digital mode that uses phase shift keying to key the transmitter at 31 baud.  The mode was designed to be used as a conversational mode for "rag chewing" as opposed to simple contesting.  As late as 2012, ARRL was promoting PSK31.  The advantage of PSK31 is that 25 or 30 conversations can be going on simultaneously in the same bandwidth as a voice conversation over Single Side Band (SSB).  Unfortunately, PSK31 has fallen out of favor as well.

The latest bright shiny object in the digital amateur radio constellation is a product called FT8.  I have not looked into FT8 yet, but it comes out of the same developers as JT9 and JT65.  These modes were designed to explore the common problem of digging a signal out of the noise.  The high frequency bands, where Amateur Radio operates are plagued by varying degrees of static noise.  As more and more electrically powered devices come on the market, the static noise has become stronger and stronger.  The JT programs are capable of digging a readable signal that is buried deep in the dirt.  They are great for contesting, for DXing (radio talk for contacts between two countries) and for QRP (radio talk for low power operation).   However, these programs are not designed for conversing.   A typical exchange involving call sign, grid locator, and signal report takes 6 minutes!   Radio, if it is to be more than an expensive toy, must be able to convey more that the person calling, and a location and signal report.  In times of disaster, we must be able to convey what we need to survive, health and welfare traffic, and other emergency communications.  In good times, Amateur Radio shows that people, wherever they are, are fundamentally the same.

While it is interesting to get signals out of static, and amateurs should pursue that, I would like to see more activity on modes that operate at or just above the noise threshold such as RTTY and PSK31.  These modes actually can convey useful information in a timely fashion.  Using PSK31 signals, I have noted that my CQs have been heard as far as Europe, South America, Africa, and across the U. S, at only 25 watts of power.  Imagine that a signal with the power of a 25 watt light bulb can be decoded at half way around the world, in high static conditions.  Yet no one is responding.  I guess they are all chasing after the latest shiny object.  

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