With conditions being dismal over the last few years coverage of Utility Listening, especially Digital Utilities, has been dropped from most of the main stream shortwave magazines. But in and of itself, Digital Utility listening is inherently more difficult than regular shortwave listening. The possibility of decoding the signal received adds another level of complexity. This FAQ is an attempt to let those beginners who are thinking of or wondering what digital signals can be received and decoded and maybe provide the more advanced listener with a little more information to identify those unknowns.
Here in lies the basic problem with digital utility listening - lack of information. Many systems are used by Military or Diplomatic Services and information on the specifics of a particular mode are impossible to find, even from the manufacturer. Many are considered proprietary, but that doesn't mean that a signal can not be identified! With the proper tools a given signal can be identified via the way it sounds (aurally) or how it looks (visually). Most decoders that include some kind of signal analysis can ID a signal by bit-pattern or baud rate. Many signals have a unique baud, i.e. 300 baud packet, 240 baud HC-ARQ or 164.5/218.3 ROU-FEC. Once a signal is identified there are many decoders that can print the traffic for you but keep in mind various kinds of encryption are commonly found in use with these signals. Encryption types include figure group or letter group messages and even random bit-masking or bitstream encryption, which looks like a continuous stream of random characters. You may often read the term "on-line" and "off- line" used in conjunction with various encryption schemes. Generally, off-line encryption is taken to mean groups of letters or numbers (most usually groups of five), whereas on-line schemes just appear as a continuous stream of random characters.
And now a word about decoders...
There are many kinds of data decoders available ranging from public domain packages to professional dedicated units. Prices vary from free up to very expensive and price is dependent on how much you want to be able to decode and what tools are available for signal analysis and identification. Public domain packages, while good, can not compete with the capability provided by the more expensive dedicated data decoder unit. It is safe to say that price goes up with increased capability in this market - be prepared to spend some big money if you want to cover a lot of modes. A good rule of thumb is that a top-spec decoder will cost as much as a top-spec radio, i.e. upwards of 2,000 dollars. You'll also need to decide upon whether to buy a stand-alone decoder or one that requires a computer to run. The latter option will of course increase the cost if you don't already possess a machine, but does add flexibility to a decoder. See the Decoders reference for unit specifics on capability and pricing.
What should you look for in a decoder? Some useful features include:
You can't beat a good Signal Identification Mode - both the Wavecom units and Hoka units include this option. A good Signal Identification mode simplifies the task of figuring out what mode is currently tuned, but keep in mind that even the best Identification mode is not always 100% correct. A common problem is that some keying systems share common idle characteristics (for example: SWED-ARQ, SITOR-A and TWINPLEX or SITOR-B and POL-ARQ) and active traffic is needed to correctly identify the exact mode. Also the presence of local interference, various propagation effects, or a noisy signal can make it difficult to correctly identify. Universal decoders do not include an Identification mode.
Accurate measurement of baud rate is another vital capability. Many modes can be accurately identified on baud rate alone because many rates are unique to a keying system. It also provides the opportunity to "fingerprint" a signal, system or the user. For example, the Hoka decoders can measure baudrate accurately to 3 decimal places in the presence of a quality signal but also do well on marginal signals, eventually settling down on a reasonable measurement. If your signal is full of noise you might not see 3 decimal places but at least on Hoka decoders you will have displayed those decimal places that make sense - a very nice feature. Universal decoders have trouble with accurate baud rate measurement on the faster keying systems (for example: 192 ARQ-E) and noisy signals can be particularly confusing resulting in some very odd numbers. I don't have any direct experience with the Wavecom line of decoders so I will not offer a comparison here. See the Baud Rate Summary Table for further information.
Auto-correlation Bit is a technique that samples the incoming digitized bit stream and presents the data as a graph of bit occurrences plotted against time. This will show when patterns occur within a signal giving you another piece of information when working out an unidentified system. This kind of analysis tool can reveal cycle period and show when there are NO patterns in a signal indicating an encrypted or random bit-masked signal, allowing you to move quickly onto more productive signals. Hoka and Wavecom decoders include auto-correlation bit modes.
Mode variety is a personal preference. I would like to have a module for any mode I can receive in the spectrum! While not possible or realistic I will take as many as I can get. I find there is nothing more frustrating than being able to receive a clean signal and then not being able to identify or decode it (ignoring the problem of encrypted signals for the moment). As of this writing it seems that Hoka offers the largest variety of modes, followed by Wavecom and finally Universal. See the manufacturers listing for the modes decoded by various units.
The ability to save decoded output to a file and/or the printer should be considered a very important feature of any decoder. Having some form of hard copy, on disk preferably, allows for archiving for later reference or later analysis and independent printing and editing. Hoka decoders have the ability to save decoded text to disk or output to the printer. I believe Wavecom units have a similar ability. Universal decoders support direct output to a printer and with some software can capture to disk.
I also like a responsive manufacturer who regularly updates their decoder in line with developments "on the air". Variations on existing systems and completely new systems are still appearing today.