UNDERSTANDING DIGITAL: Part Two
As an introduction, allow me to review the basic premise of the part one article, “Understanding Digital”, as published in last month’s newsletter. We learned that digital communications in amateur radio (and elsewhere) is carried over an analog carrier wave. In review, also recall that this series of articles is meant to help you understand digital modulation and how it works rather than to familiarize you with any of the myriad of digital modulation operating modes that are used on the amateur bands. Perhaps we can cover that in future articles. If you can’t wait, you can learn how to use one or more of the digital modes of operating on the amateur radio bands from an “Elmer”, or as a last resort an operating or user manual. Chances are however that what you won’t learn from these sources is how digital communications really works! We’ll keep our lessons in understanding digital as simple as possible, and so we’ll avoid formulas and some mathematics involved in learning exactly how digital modulation is applied to an analog carrier.
In order to better understand digital communications, you should know a bit of its history. Most people associate digital with computers, and assume that’s where it comes from. Well, it doesn’t! Prior to the start of the Second World War, the United States and Britain exchanged voice communications between senior leaders by using primitive voice encoding techniques, according to a publication released by the National Security Agency (The Start of the Digital Revolution by Boone and Peterson.) Attempts by both countries to “scramble” trans-Atlantic communications were easily decoded. What was needed was a modulation method that would allow voice encoding to be “invisible” to the listener and sound the same as noise.
The answer was in a device developed by Bell Telephone Laboratories about 80 years ago called a “Vocoder”. The Vocoder allowed voice to be transformed into digital data. It was demonstrated to the public at the 1939 World’s Fair in New York, and formed the basis of a contract for the development of a system that would be secure enough to be used for military communications and be undetectable by the enemy. Such a system was developed and placed in service in 1943. It was used for over 3,000 conferences during the Second World War, all of which could not be understood by the enemy. There were 12 stations in the system, with major terminals in New York, Washington and London. It was primitive by today’s standards, with each station needing 40 racks of equipment weighing 55 tons each in order to make it work! The system was removed from service in 1946, and remained a secret for twenty years thereafter! Basically, it was a digitally modulated FSK (Frequency Shift Keying) signal requiring a synchronized encryption key to decode. More importantly, it was the beginning of the digital communications revolution!
In the next article in this series, we’ll learn exactly how analog information is converted to digital, step by step. Once you’ve mastered that information, you’ll know how digital really works and how it gets “from here to there” regardless of what amateur mode (D-Star, C4FM, PSK31, etc.) is used. Stay tuned!
Rick Fearns
K6VE
