Digital Signal Types Last Updated 4/2003 This is a poor imitation of the WUN Digital Signals FAQ, but it's short and good for beginners. In no way does it replace WUN's great document, however. Once you're comfortable with the general terminology of digital radio, go to the Worldwide Utility News web site, and get it. Digital radio dates to Marconi, whose original spark transmitters were either all the way on, blasting the whole low end of the radio spectrum, or all the way off. They had an on and an off state. While analog modulation of these radios came in pretty early on, Morse code remains the first digital mode. Dot, dash, and space timing is critical, and copy is usually by ear (a fuzzy mode), Morse in all other respects is an on-off-keyed, digital mode. It's often been said, but it's still worth repeating, that most of the digital signals encountered on HF won't decode no matter what you do. They're encrypted, scrambled, or just out of spec. Some respond to patience, autocorrelation measurement, or more money. One can always spend more money on digital radio. It's the best fund sink going. On-off keyed fuzzy modes A fuzzy mode is one generally using human intervention for the copy. While the gap closes yearly, humans are still better processors than even the most powerful computers, because with a little training they guess right more often. This allows a third logic state, known as "fuzzy logic" or "close enough." Tri-state logic is now a buzz word in the computer industry, especially in graphic imaging systems, but people did it first. Morse telegraphy (CW, WT) Don't bury Morse yet. As a fuzzy mode, it can go farther into the noise than any other, though some of the multitone things are coming up fast. Morse code has gotten a bad name because a lot of people don't understand why they have to learn it to get a ham radio license. Well, I don't understand why either, unless they want to communicate in it. Code-free licensing is the way of the future, though a case can be made that the Extra Class license, or its equivalents outside the US, should require knowledge of all popular ham radio modes, including at least a very basic exposure to the code. Original Morse was for machine copy on wire telegraphs, but people soon caught on that they could do better by ear. However, machine copy of Morse/CW never died out altogether, and it is still done today, allowing very high speeds on good circuits. This is undoubtedly the original teleprinting. The best way to learn Morse is completely by ear. Treat it like folk music. Get into the sound and the flow of the code, and learn to become the code. When you can nod off at the radio, wake up, and realize you haven't missed anything, you're fluent at Morse. Morse code is a bit of a misnomer, because the radio uses something more resembling "Continental Code," though it's informally known as the "International Morse Code." Old-style Morse ("American Morse Code") is clattery, and several characters contain spaces. International Morse has more of a flow to it. Other "Morse codes" exist for languages, such as Japanese and Russian, that need more characters. Everyone's seen a telegraph key. It's a switch. Morse is either all the way on or all the way off, hence the name "on-off keying" (OOK). If the transition between on and off state is too abrupt, severe off- frequency interference called "key clicks" is generated, so often the equipment uses a "shaping" filters to cut rise and fall times slightly. 5 milliseconds has long been considered a good shape, though some like to use a "harder" shape on very bad circuits. Morse is copied with the receiver BFO on, just like sideband, and most people seem to like a tone around 700 Hz. Many radios radios use this offset for code, instead of the single-sideband 1.4 - 1.5 kHz offset. Many operators like to use a lower pitch when pulling a signal out of interference. Of course, Morse can also be sent by FSK, which is heard sometimes, and by sending audio tones to single-sideband voice transmitters, as is quite common in ham radios. The result sounds exactly like true OOK. Morse was originally sent with spark transmitters, basically a step-up apparatus with a spark gap loosely coupled to the biggest wire antenna the financial budget and prevailing winds would allow. These rigs just buzzed, and caught fire regularly. Later on, rotary gaps were used to interrupt the signal and give a slightly musical tone more resembling the beeping associated with Morse today. When the newfangled gadgets called vacuum tubes replaced "King Spark," the resulting oscillation was a continuous alternating current rather than a damped wave. This gave the name still used for Morse telegraphy on the radio - Continuous Wave (CW). In the maritime service you'll also see it called Wireless Telegraphy (WT or W/T). Of course, there is very little Morse code left in this service, which is now more inclined to use computer networking modes or narrowband direct printing. The Morse information rate is in words per minute (WPM), which is more relevant than simple baud rates, due to the varying element lengths and emphasis on timing. For counting purposes, Morse words have five letters. Text WPM is computed by sending the word PARIS. This works out to 2.4 times the rate in dots/second, as can easily be counted on automatic keys. At any normal rate and shaping, CW is extremely narrowband, making it possible to use sharp filters or DSP processors to pull signals out of sheer cacophony. This and the fuzziness of Morse make it still the all- time DX mode, working well below the noise floor. Hellschreiber People have a lot of fun with the name of this 70-year-old mode, which was invented in 1929 by a German named Dr. Hell. The name also means "Shiny Writing," from the silver drum once used. Originally, the transmitter resembled a crude sort of teletype unit, and the receiver was a primitive form of fax using paper tape. CW transmitters worked just fine, with the actual pixels of each character being sent using semi-synchronous, on-off keying. The receiver scanned each pixel onto a paper tape, printing each character twice for redundancy. This plus the fuzziness of a human-readable mode gave Hell a very early type of error-checking. The print becomes lighter with lower amplitude, making Hell even fuzzier. All this adds up to a darn good DX (distance/weak signal) mode. Hell got a good reputation for doing better than RTTY on badly degraded circuits, and it was used quite a bit by the German military in WW II. Receivers were built by the Allies for intercept. The most common mode is "Feld-Hell," "field Hell," which has a very distinctive, brrt-brrrt-brrrrrrrt sound. Today, this and some higher- tech variations are done with audio pulses fed to SSB transmitters, and received on a computer simulation of the moving tape. This is truly weird, but it's fun and it works very, very nicely. Hams raise a lot of Hell on 14060 - 14063 kHz. Continuous-tone fuzzy modes FAX FAX stands for facsimile, in this case by radio. It's a very old mode, dating from wire telegraphy of the 19th century. FAX lends itself well to computer technology. While the ubiquitous office fax machine uses digital modulation, HF radiofax is still analog. Radiofax machines can be formidable affairs, with large drums, but computers work just as well in most applications. Being an analog mode, FAX will become fuzzy (noise) or smeary (multipath and delay), but still be recognizable to people. HF radiofax is actually frequency-modulation (FM), though tuned in standard upper-sideband (USB). Like audio FSK, fax can be sent by a USB transmitter merely by feeding in the correct tones. Deviation is 400 hertz up or down from 1900 hertz, low (1500) being full black, and high (2300) being full white. For this reason, FAX is tuned 1.9 kHz lower on USB radios. Computer sound cards handle the tones quite well, making FAX a popular way to send weather charts to laptop computers aboard small vessels at sea. Significant parameters are "drum speed," in lines per minute, and a rather complex thing called "Index of Cooperation," which is usually 576. Weather charts, being mostly white, are sent at 120 LPM, but the few remaining newspapers on HF (mostly from Japan) can go at 60 LPM. A radiofax usually has a band at one edge, causing a beep which gives away the speed instantly (and which, at 60 LPM, can sound like a time signal). Automated FAX uses autostart and end tones, a system called Automatic Picture Transmission (APT). In this manner, a machine or computer can be set to grab hours of weather pictures while completely unattended. Slow-scan TV SSTV is kind of souped-up FAX with color capability. It sends one frame at a time, and on HF it is best described as VERY slow-scan TV, really more of a photograph or computer picture exchange mode than any kind of television. Many of the same programs can decode FAX and SSTV, and also dump files off to either for transmission, using only the computer sound card. This makes SSTV a lot more fun than when people only held up photos to fuzzy cameras. SSTV started out like HF FAX, with frequency modulation and analog tones between 1500 and 2300 hertz. Monochrome cameras and converters like the Robot were used. Today, however, we see a truly bewildering number of color SSTV systems, with names like Martin, Scottie, or color Robot. All use a sync pulse at 1200 Hz, and send one line at a time with sequential information for the RGB colors. Some use more sync than others. SSTV can recover from fades or interference - you just lose some of the picture. Like faxes, SSTV pictures get noisier or grungier before they become impossible to make out - another fuzzy mode. SSTV is also used on VHF and UHF, sometimes with repeaters, and even for automated pictures from spacecraft. All SSTV modes sound a lot beepier than FAX. Teleprinting schemes These are all slightly different means of doing the same thing, namely sending a message as letters instead of words, for instant hard copy or storage at the other end, skipping the step of writing or typing it down. Teleprinters used to do just that - print. They would type everything out. Some pretty serious computing was done with automatic electric typewriters for output devices, though these were quickly replaced by line printers. Today, "printing" is usually loosely used to refer to any output of text, to a computer screen, teleprinter, or computer printer. HF teleprinting has a special set of parameters, in that bandwidth is precious, and radio paths are prone to noise, fading and phase distortion. All the different funny noises you hear are means of dealing with these limitations in different ways. As we've seen, machine CW and Hellschreiber were teleprinting schemes from the start, but most others were and are variations on radio teletype (RTTY). RTTY-like teleprinting systems RTTY stands for Radio Teletype. "Teletype" was originally a proprietary tradename for mechanical teleprinters made by The Teletype Company. These were formidable machines, smelling of hot oil and requiring paper tape perforators for message storage. A scary device called a "Terminal Unit" (TU) used a high voltage to interface your poor, unsuspecting radio, which usually had to be run at reduced power due to RTTY's continuous duty cycle. These mechanical printers were basically automated typewriters, and the tap-tap-tap sound of the news "wires" was as much a symbol of hard-news journalism as the green eyeshade. Whenever a new generation of Teletype machines was deployed, the old ones were given to amateurs who'd sign pledges never to use the devices commercially. There weren't a lot of takers. RTTY was the lunatic fringe of ham radio, as these huge machines, and their associated tape devices, scops, and general appratus took over one's life, not to mention their home, in a hurry. On HF, just about everyone came to swear by, and at, RTTY for teleprinting. Its many forms provided most of the funny noises heard on short wave radios in the pre-digital era. RTTY was pronounced dead when computers came in, but instead the inexpensive software and compact terminals gave the mode a new lease on life. Most of what sounds like RTTY will not print on simple sound-card computer programs or amateur-grade decoding units. There are a bewildering array of schemes for encrypting or enciphering RTTY, by inverting bits, making changes in the code, or adding masking. Some will print with more expensive hardware/software. Others won't, no matter what you try. Plain old RTTY has two bit-states, called mark and space. Normal polarity signals have mark as the lower of the two, and reverse polarity signals have space lower. Some RTTY stations idle on mark, while others send pips at the character rate. Tuning RTTY consists of centering two audio filters on the mark and space tones, which are usually changed via frequency-shift keying (FSK). The best hardware RTTY decoders usually have a scope with two lines. When these make a plus sign (+), the signal is correctly tuned. RTTY is traditionally tuned in lower sideband (LSB), though USB works just as well. USB, though, reverses the signal. Since normal and reverse polarities are both in common use, this is less a problem than another source of confusion in what can be a rather confusing mode. Transmitters designed for amateur service must be run at half or even quarter power due to FSK's continuous duty cycle. Here are some classic, RTTY-like systems: BAUDOT 5-bit asynchronous, aka ITA 2 (for International Telegraph Alphabet), oldest, still very widely used. A set of variations of ITA 2 exist for non-Latin characters, such as ATU-70 (Arabic). In common radio hobby usage, the terms "RTTY," "Baudot," and "ITA 2" are usually synonymous, with the other modes being described by name. This old digital code uses mark and space bits, with slightly longer pulses to frame characters. Originally, it was on-off keyed, but this was replaced by frequency-shift keying. This can be direct, by rapidly changing a constant carrier for each bit, but usually today we see audio FSK, where shifting tones are sent to SSB (typically LSB) transmitters. These two FSK modes sound the same to the ear, though certain measurable characteristics may differ slightly. Common shifts are 170 and 850 Hz, common speeds are 45 and 75 baud. Note that the "baud" is an information unit equal to one digital event per second. While it's named for Baudot, the inventor of this code, most other digital modes also measure physical data pump speed in baud. As one finds out pretty fast with digital communications, a baud and a bit per second can be very different things, and the same baud rate can give quite different word-per-minute speeds. Beginners are advised to try amateur RTTY signals at 45 baud and 170 kHz shift, because they are generally, by law and custom, unenciphered. The ARRL headquarters station W1AW has regular schedules in RTTY and some other digital modes, with good, strong signals perfect for setting up software. Ham RTTY uses two sets of standardized tones. These are the "low tones" (1275 Hz mark, 1445 Hz space, 1360 Hz center), and the "high tones" (2125 Hz mark, 2295 Hz space, 2210 Hz center). This sounds a lot more complex than it is, because the common receiving technique is to simply set the computer to the audio center. However, when you combine the offset of RTTY with the offset of single-sideband radios, which are usually how one tunes the mode, and then the mark frequency vs the center frequency, etc etc, the frequency shown on your radio can vary 2 or more kHz from the station's assigned frequency. 5 bits gives a restricted character set, so a FIGURES case is used for numbers, selected with a SHIFT IN and deselected with a SHIFT OUT code. Some languages such as Russian have a third, or even a fourth, shift for an expanded character set. If the SHIFT character is missed, the result looks rather strange indeed, and can result in loss of otherwise received data. Therefore, one can usually enable USOS (UnShift On Space) for tough situations. Baudot, like Morse code, has no lower case. Baudot is asynchronous, with a distinctive chattering sound. It idles on either a continous tone, or a series of short pips at the character rate. The character pair "RY" contains all the possible states of the Baudot code, so one still occasionally sees RY strings sent for circuit testing or channel marking. These can be useful for checking your polarity, because if it's "wrong" you'll get RY as SG. Baudot's biggest problem is its lack of an error check, meaning that noise and fading cause missed or wrong characters. Some of the most amazing gibberish in all communication comes from degraded RTTY. Most later digital modes come from different attempts to solve this problem. SITOR (Simplex Telex Over Radio) 4/7 bit error correcting synchronous Sitor was developed as an improved narrowband direct printing system for the maritime radio service, where it is still widely used. Variations are Mode A (ARQ), automatic repeat request (3- character blocks, sounds chirpy), and Mode B Forward Error Correction (FEC), continuous, used for broadcasts. FEC sounds a bit like Baudot, except faster, higher pitched, and less chattery. ARQ has an extremely distinctive, rapid-fire, chirp-chirp-chirp sound. The standard tones are higher than those used in Baudot. Sitor represents an early attempt at error-correcting RTTY, and it also gives 100 baud and more characters. Its main problem is that the ARQ mode can slow down to nothing in degraded conditions, plus its timings are pretty tight for amateur level gear. The standard burst is only half a second, and character sync is so critical that long path skip is impossible. It remains the standard in the maritime service, however, where needs are considerably different. There is a lot of it on the air, and stations aboard ship are great utility catches. ARQ mode requires that the receiver achieve sync with the sender. This is usually done by sending a series of "blasts" made up of pulsed tones, to which a calling station can synchronize. There is usually also an identifier in standard Morse code. These are very, very common on the maritime commercial traffic frequencies. Once sync is achieved, the two stations take turns sending half- second bursts. This is what allows sync simplex (one-frequency, no break-in) operation, though simplex isn't used that much except by amateurs. The sending station gives a block of information, which the receiver error-checks and acknowledges received (ACK) or not received ("Negative acknowledgement," NAK). ACK/NAK signalling carried over into computer communication, and both signals are in the ASCII character set (See ASCII). ARQ mode resends NAK blocks, and if there are a lot of these, it will get slower and slower. Monitoring requires that the listener also achieve sync, which can be a bit tricky. Also, obviously, a listener cannot NAK a bad block. You just miss it, and probably a few subsequent ones getting back in sync. ARQ is altogether rather unforgiving, and it does not appear in low-end computer sound card programs, though FEC often does. A good Sitor feature for ute listeners is that if it's garbled by bad circuit conditions, it'll print a few nonsense characters, but only a few, before just losing sync. This is in conrast to Baudot, which will happily continue filling the screen with thousands of completely bogus characters, all looking as if someone is trying to send Klingon with a broken Japanese kana-kata machine, only worse. NAVTEX (Navigational Telex) is a special Sitor-B mode used to send Maritime Safety Information (MSI) bulletins on 518 kHz. It is an integral part of GMDSS, the Global Maritime Distress & Safety System, supplementing satellites in zones within range of coastal stations. Messages are given sequential headers, so that automated printers on board ship can skip ones previously copied. Most other teleprinting systems build on FEC and ARQ, so these are useful things to understand. AMTOR (Amateur Teleprinting Over Radio) Ham radio variation of SITOR AMTOR was once rather popular, but Pactor gives more bang for the buck, and it is a lot easier on equipment. Some ham radio programs now copy AMTOR, meaning they'll get SITOR too. A variation of AMTOR uses an extended character set based largely on ASCII (see below). This makes AMTOR useful for bulletin-board systems and mailboxes, though this is giving way to Pactor (also see below). VFT Voice Frequency Telegraphy Multiplex RTTY, often with orderwire Typically a multiplexed signal with 7 or more narrow FSK signals at once, using frequencies that fit into a voice channel. Sometimes the various subchannels send different messages, as in the old US military MULCAST, but more commonly nowadays they are used for time and frequency spreading, as in mil standard BR-6028, giving robust error correcting on fading circuits. It sounds like a collection of intermittent, discordant beeps. There used to be an awful lot of this stuff on, for news wires and other information, but now of course this has all gone to satellite. Other VFT types that you'll still hear today include multiple Piccolo or PSK signals. These are what give the real space ship sounds on HF. Other government/ commercial RTTY modes: There are hundreds of these, but here are some: ARQ-6/90 & 6/98 Single-channel 6-character block ARQ-E, ARQ-N 5-bit Baudot with framing and parity check ARQ-E3 Single-channel variant of ARQ-E ARQ-M Multichannel 4/7 bit ARQ DUP-ARQ Duplex ARQ used by Hungarian foreign service FEC-A 5-bit error correcting Baudot with parity HNG-FEC 15-bit scheme used by Hungary IRA-ARQ Duplex ARQ with the IRA (ITA 5) character set, used by Czech Republic and Slovakia. POL-ARQ Special ARQ with 5-character blocks, used by Polish foreign service RUM-FEC 16-bit scheme used by Rumanian foreign service SI-ARQx X=4 to 7 character blocks, different type of SITOR SWED-ARQ Variable block length ARQ scheme used by Swedish foreign service TWINPLEX 2-channel, 4-frequency-duplex, highly adaptive 100-300 baud mode used by UN, Interpol, and some foreign services. Seems to be giving way to Pactor in some of these places. Computer direct printing modes These are computer-oriented, usually using standalone modems with standard PC computers, or even the sound card in the computer itself. Most of these are intended to improve the throughput of SITOR on HF when used in the amateur mode with weak signals or long propagation delays. Modems ("modulator-demodulator" units) can be single-tone, meaning they switch from one tone to the next in a sequential manner, or multiple-tone, meaning that a number of separately modulated FSK or PSK tones can be transmitted at once. Single-tone modems can have only two states, like RTTY, but more often there are 4, 8, or 16, using either FSK or PSK. The idea is to increase information density, and also redundancy, allowing better persormance on badly degraded HF circuits. Single-tone modems are serial, like the modem in a PC computer. Multi- tond modems are parallel. A great deal of experimentation is currently being done with use of parallel tones on HF, to create a more redundant signal less vulnerable to ionospheric fades and phase distortions, which tend to affect the different tones sequentially rather than all at once. "Orthogonal" tones are frequently mentioned. This simply means that each tone decodes independently of the other ones. The greater density of information allows the whole computer programmer's bag of tricks to be brought to bear on encoding and decoding the signals. The jargon gets complicated in a hurry. ASCII American Standard Code for Information Interchange A 7-bit, asynchronous, digital code, used mostly by computers on local area networks. The text mode of your PC uses ASCII. It offers a standard, 7-bit, set of 128-characters, derived roughly from ITA2, but with a lower case. There's a rather full set of control characters in the lower 32 values. Some of these come straight from old Teletypes, telling the terminal or computer to ring a bell (now more often a beep), or to feed a whole page. The most minimal ASCII terminal driver is still called a "TTY" in many places. SHIFT IN and SHIFT OUT also exist, allowing an alternate character set which is not in the ASCII standard. In the dark ages of text-only computer terminals, these were often used for graphic elements, allowing the drawing of boxes, windows, and lines on the standard 80x24 text screen. More advanced terminals implemented "escape codes," using the nonprinting ESCAPE character followed by various arcane sequences of alphanumerics. These could change screen colors, move the cursor around, cause text to blink or scroll sideways, and all the other effects that everyone associates with old computers. In this manner, straight terminals came to have a pretty slick look for the time, and most computers would use emulators or "termcaps" to duplicate the effects. One popular termcap was VT100, originally used on terminals made by Digital Equipment Company, and another was ADM3A, popular with packet radio systems. ASCII can be sent via any kind of modulation allowing the two logical states of computer bits. The effective bit rate can be greatly multiplied by various esoteric modulation schemes, especially on telephone modems, while keeping a far more modest physical "data pump" baud rate. This is why the terms "baud" and "bits per second" are so universally confused. HF ASCII started out as just FSK, at 110 or 300 baud, using telephone standards, but this did very poorly in bad band conditions. Today, ASCII is usually done with more robust modulation schemes, AMTOR/Sitor like parameters at a minimum. Various extensions to ASCII exist. Most common is 8-bit ASCII, which doubles the character set, allowing accented characters for such languages as Spanish and French, money symbols other than the dollar sign, and mathematical symbols. In the dark ages, some terminals (notably some early Apple computers) sent all ASCII with the high (eighth) bit set all the time, creating some rather impressive gibberish if the receiving computer was using the wrong circuit discipline. A newer standard, Unicode, builds on ASCII but adds many more characters. This standardizes all the "extra" character sets that existed, giving unique numbers to all the characters. Unicode is used quite a lot on the World Wide Web for exchange in different languages. ASCII characters are usually sent "framed," with extra bits to show start and stop, or for "parity," a crude error check. Common settings are 7E1 (7 bits, even parity, 1 stop bit) and 8N1 (8 bits, no parity, 1 stop bit). More complex framing schemes assemble characters into groups often called "datagrams" (see Packet Radio, below). MIL-188-110A A standard for single-tone modems, often 8-state serial phase or frequency shift keying, adapted for use in many other HF modem systems, including the high-frequency data link protocol used by aircraft (see HFDL, below). Other parts of the standard deal with parallel modems using 16 and 39 tones. PSK31 This is 31-bps keyboard-to-keyboard teleprinting using phase-shift keying on a very narrow bandwidth, creating a warble centered near 1000 Hz. Clever design allows a 50 WPM throughput, and good DX. The full, 8-bit, ASCII set is used. While PSK31 is synchronous at the baud rate, characters may be variable lengths, and the idle tone flips its polarity at this rate, maintaining sync and also reducing the duty cycle to something more suitable for amateur transmitters. It is possible to run more power than with RTTY, though there's no real reason to, since PSK31 is so efficient. PSK31 was originally described, in extremely glowing terms, in the May issue of QST, an amateur radio magazine. It sounded too good to be true, working better than RTTY and with the bandwidth of good CW. Many people thought an April Fool article had run a month late. The mode really took off when computer sound cards were adapted as DSP filters, allowing hams to tune to 14070 kHz, set a wide receiver bandwidth, and click the mouse on the desired signal or transmit frequency. The warbling is getting pretty amazing on this frequency. CLOVER Phase-shift keyed, ARQ, duplex, 4-tone, radio modem proprietary to the HAL company for HF data transfer. Clover-I makes an intermittent bleeping sound. All things being equal, Clover, like G-TOR, will be 4 times faster than Pactor-I (see Pactor). Clover was originally invented for ham use, but it was always kind of a high-end protocol, like Pactor-II is today. It was for those who wanted the best and didn't mind paying for it. Clover has two newer, hotter versions, Clover-II and Clover-2000. Either of these two has a wide range of modulation schemes that will auto-select depending on conditions. In all cases, bandwidth is carefully limited to 500 Hz, no mean feat at the higher speeds. There's also a 400-Hz version of Clover, developed for an e-mail system being deployed worldwide by Globe Wireless on existing, 500-Hz, SITOR and CW channels. PICCOLO One of several 6 and 12 tone sequential synchronous systems used by British and Australian government stations, alone or in VFT. Sounds musical, hence the name. 12-tone Piccolo allows the full ASCII character set to be transmitted. COQ-8 Coquelet-8, a sequential 8-tone MFSK, mostly used on occasion by Algerian embassies. Its name refers to the crowing of a rooster, giving an idea of its sound, which is similar to MFSK8. MFSK16 The name says it all: this is 16-tone sequential frequency-shift keying, roughly similar to Piccolo. FSK has the interesting characteristic of decreasing its error rate as the number of states is increased, unlike phase-shift keying (PSK). The amateur system of MFSK16 is often called "Stream," from the name of a popular computer program. It uses narrow shifting to achieve a tight bandwidth, and it works much better on DX paths than straight FSK or PSK31. FEC error checking is used, so throughput is around 32 baud. There's also an 8-tone mode (MFSK8) for bad conditions. Decoders use convolutional algorithms, interleaving, and other fancy computer stuff. It also has a distinctive sound a bit like Piccolo. Crowd36 An old Soviet version of Piccolo, with 32 sequential tones. Makes a space-ship buzz. ALE Automatic Link Establishment (MIL-188-141A) An amazingly complex, 8-tone, orthogonal mode spelled out in several military standards, but also used on rare occasion by amateurs with free software. It's a multi-tone PSK, instantly recognizable from its cyclic gobbling. ALE allows short messages to be passed, but it is more of a networking protocol, enabling HF radios to choose the best frequencies and other parameters. Subsequent contact generally proceeds in another mode, such as voice or MIL-188-110A. 39-tone (AKA NATO, HARCO-39) Descriptively named military standard, now also available for civilian use, with 39 tones, allowing redundancy for error tolerance under fading conditions. It's seldom that fades hit all tones equally. The sound is very distinctive, with a sync tone followed by all 39 coming up, followed by what sounds like white noise. (It's an axiom that, as data density increases, any system will sound more like noise.) Variations include ANDVT, the military's Advanced Narrowband Digital Voice Terminal, which is not a scrambling scheme but can and usually does interface with secure voice modules. Other federal and military terminals can securely transfer files. The encrypted mode takes audibly longer to synchronize. MT63 An amateur variation of the multitone PSK modes seen in military comm, using 63 data tones and one reference tone. In its most common configuration, it's a full kHz wide, and makes a buzzy roar often confused with jamming. Its wide bandwidth and multiple tones allow extreme spreading of both time and frequency, and several error checks, meaning excellent DX performance on badly degraded circuits. It's common to watch good copy keep coming from weak MT63 signals, or transmissions with other modes right on top of them. POCSAG I believe this stands for "Post Office Code Standardisation Advisory Group." POCSAG is far more common on frequencies above 150 MHz than on HF, but a few such stations exist at the very high end of the band. Frequencies around 30 MHz are still being used for paging in South America, and in Canada a lot of the 30-45 MHz band seems to be going for paging. Both places will be heard for a few years around the 11-year solar peak, though this has just recently passed. POCSAG is primarily a mode for alphanumeric paging, as usually received on the ubiquitous "beepers" everyone seems to be carrying. It's basically framed, souped-up, RTTY, using direct FSK. Detection is best done straight from the discriminator stage of an FM radio. FSK at this speed is extremely splattery, and high-powered paging transmitters are probably the worst interference sources on the higher bands. They just tear everything up. POCSAG has a rather irritating sound, mixing beeps with buzzy high-speed data sounding more like the backscatter radar. Passing FSK of this speed through audio circuits creates some utterly remarkable intermodulations, which look like intricate webs on the typical FFT computer analysis program. Good art, perhaps, but a lousy way to decode a signal. Even so, POCSAG has been decoded, with mixed results, on just a sound card and PC. Intercepting paging messages, however, is completely illegal in the United States, and is one of the few listening infractions that can actually get one into real trouble. POCSAG is a very sophisticated mode, with a header and various complex options for different levels of paging, up to and including short messages in straight ASCII. FLEX Another paging-oriented transmission mode with more performance than POCSAG. Flex can handle a much greater volume, which is important when everyone seems to be carrying a beeper. Computer data networking modes These use computer networking protocols similar to the ones seen in more familiar LAN and WAN technology. Most offer both automated and direct typing (keyboard-to-keyboard) modes, all using personal or laptop computers for terminals, most interfacing with radios through external controllers resembling more sophisticated modems. File transfers are often possible. Nearly all networking modes "packetize" data, which is usually 7-bit or 8-bit ASCII, but can also in some cases be raw binary - just ones and zeroes. This allows the network to handle different people's data at once by identifying, routing, and numbering packets, and it allows missed packets to be resent. The packets can be of varying lengths. They are exchanged, error checked, and processed by programs called "protocols," which provide standards for talking to other computers. The Internet is the most familiar packet-switched routing network, though it's hardly the only one. Internet needs an immense bandwidth, and it is usually implemented over telephone links and high-capacity fiber backbones (the "information superhighway"). However, there is no reason its underlying protocols can't be transmitted and received on the radio (see TCP/IP, below) Packet Packet-switched asynchronous data networking mode An ASCII protocol that sends data as little, framed "packets," one at a time, each a little message in itself. The framing has several devices that make the receiving station process and print the data better. Among these is an error check of sorts. Some packets are for control, others are for information. Information packets are numbered, and can be received out of order. Long packets are often compressed, being sent and received as 8-bit binary data, in packets of varying length File compression is great for receiving stations, especially on slow HF circuits. It's not so great for listeners, who see the compressed data broken up into 8-bit characters as gibberish. Each packet is answered copy/no copy (ACK/NAK) by the receiver, and missed ones are resent. Packet thus slows down under difficult conditions, making file transfers a potentially excruciating proposition on HF. This has not prevented packet from doing some rather useful things, allowing connection to remote "bulletin boards," and exchange of GPS positions over the air. Packet uses a well worked out "link-layer protocol." This basically means that packet communication passes data from layers of software that interact with the user, down through layers that do various things, finally to the transport layer (the radio signal), then back up again. The highest layers, which would implement the kind of slick user interfaces that people are used to seeing on computers, are only now being worked out. A 2-way exchange of packet data begins with a connection. X.25, AX.25 (amateur version of the standard), are the names of the most common transport used. AX.25 does badly on HF, with very slow throughput. It's way more common on VHF. Amateur packet network protocols are usually implemented in a terminal node controller (TNC), usually a standalone device that a computer talks to in text terminal mode. However, I have seen software TNCs. "MixW," a Russian sound-card program, has one of these. Protocols have names like Aloha (developed at University of Hawaii, where else?), NET/ROM, TAPR (Tuscon Amateur Packet Radio) and KISS (Keep It Simple, Stupid). Where VHF packet started out with the 1200 and 2200 Hz "Bell Tones," HF uses a 200-Hz FSK shift. Straight packet is buzzy sounding, in short bursts, with a sync pulse at the beginning. Most AX.25 stations can be configured to "digipeat," relaying packets from other stations for sometimes thousands of miles. Another use of VHF packet is by hams to keep tabs on DX stations appearing on the HF bands, using a software system called "DX Cluster." Straight packet was originally seen as the salvation of ham radio, giving young computer geeks some incentive to get their licenses. There was quite a bit of interest at one time in creating a global network of digipeaters, but this lost out to the Internet, and packet radio became old news, though still a fun and popular mode on VHF. Things have picked up for packet, though, with the coming of APRS, Automatic Packet Reporting System, which does some rather neat things in the way of vehicle tracking and mobile radio communication. APRS is a darling of Japanese radio makers, who are adding bells and whistles faster than I can write about them. Pactor, Pactor-I, Pactor-II, Pactor-III Packet Teleprinting Over Radio A composite mode which sounds a bit like Sitor Pactor is an evolving mode that was designed to combine the best features of packet radio (connections, file transfers, computer networking) with Sitor (robustness, simplicity). Pactor seems to add bells and whistles daily, becoming ever faster and more complicated. Pactor's development has three levels, using Roman numerals. Pactor-I is FSK, Pactor-II is PSK or FSK, Pactor-III is a hot-rod version of II adding some more extra modulation modes, giving more speed on optimum circuits. All levels incorporate robust error-checking to overcome the problems of HF packet. Pactor is becoming the mode of choice for the many amateur and commercial HF e-mail systems which are giving short wave a whole new lease on life. These include such networks as SailMail and Bushmail. Pactor-I is half duplex, synchronous, linked, and heard in one- second bursts (an ARQ error correcting mode) or continuously (FEC). It uses a 200-Hz shift, and its modulation is fully symmetrical, meaning it can be tuned either in USB or LSB with no difference. Speed is 200 baud, dropping to 100 if conditions require. The two speeds really sound different. Timing is considerably relaxed from AMTOR, allowing long path skip. While Pactor uses a direct-connection scheme like packet, the keyboard- to-keyboard mode is popular for RTTY-style QSOs. Simplex Pactor sounds like a slowed-down version of Sitor, with the stations alternating bursts. The slower switching is much easier on the T/R circuits of ham and USB maritime radios. Pactor-II is very proprietary to SCS, a European firm started by hams, and it is being licensed by them to a handful of other companies. It's up to 8x faster, and like Clover it changes modulation schemes on the fly for changing band conditions. These different modulations can hiss, warble, or slow down into a beep like Pactor-I. Pactor-II is a nice mode, but the TNC controllers tend to be very expensive. Right now (2003), Pactor-III is limited to the newest SCS units, or relatively recent ones with an upgrade. Since Pactor uses connections, it's great for HF networking, and wireless e-mail systems, which are becoming common. Despite its expense, Pactor is a well thought out system and it seems to be winning out in the marketplace. HF Pactor is used a lot by hams, relief agencies, e-mail programs, and quasi-amateur military support networks such as the US MARS. G-TOR Golay Teleprinting Over Radio, named from a type of data coding used. It's a PSK refinement of Pactor-II, adding the more sophisticated error checks and data compressions common in military standards to amateur-grade equipment. It was developed by Kantronics, a maker of packet gear, and remains proprietary to them. All things equal, G-TOR will be four times faster than Pactor-I, though admittedly Pactor I is pretty slow. G-TOR bursts are 2.4 seconds long, which is one way to tell them from the 1-second bursts of Pactor-I, or the half-second Sitor. Golay is also a Motorola paging system used on VHF, but this is a completely different mode that uses the same general data structure and processing. TCP/IP Transfer Control Protocol/Internet Protocol Packet radio use of the same protocol suite used on the Internet, as opposed to X.25. Once everything's working, the user interface is quite a bit like the old dialup unix days, with use of such programs as telnet and ftp. It is even possible to use Windows. It is also possible to use TCP/IP via AX.25, using "datagram" mode. As with the Net, TCP/IP is something of a black art, as practiced by masters with years of initiation in its strange syntax and arcane protocols. If you like hacking the TCP/IP stacks that come with computers, you're going to love this mode. TCP/IP is useful for "gateway" stations, which allow HF radios to connect to the Internet, or to e-mail services that can work with the Internet and other packet protocols besides. ACARS/HFDL HF ACARS is a modified version of the Aircraft Communications, Addressing, and Reporting System deployed on VHF some years back by ARINC, the communication giant originally started by the airlines. The HF version resembles packet radio, but with a single-tone 8PSK modem based on MIL-188-110A. Networking protocols are like ACARS and resemble HF packet, with aircraft logging into "slots" and exchanging data with a global network of numbered ground stations. The HF variation of ACARS is a key part in a complex protocol called HFDL (High-Frequency Data Link). It can be decoded with personal computers, and it's a fun mode. Ground stations often identify every 32 seconds with a "squitter" somewhat similar in concept to the VHF ACARS. These can contain quite a bit of information. Link-11 The famous US Navy/NATO "gator," an intermittent system with 16 tones, one for sync, one for doppler lock, the rest for data. The sync tone can be heard as that distinctive bong just before the gator hiss. Link-11 is standard for passing tracking data between participating units in a military operation. Makes a rhythmic sort of sound mixing long and short bursts, sometimes in pleasing, percussive beats. The "alligator" name comes from the hissing sound of the actual data transmission, which greatly resembles a noise that gators emit when they are happily making little gators. While by far the most common on HF, Link-11 is only one of a comprehensive suite of protocols and modes used to network military units together in complex tracking and data arrangements. These are sometimes called TADILs (Tactical Data Information Links). Some TADILs are as simple as variations on basic RTTY, while others are too complex to be reliably propagated on HF. In the aggregate, all this information transfer is what enables the kind of fast-tempo, coordinated attack integral to today's "high-tech warfare." There is a also a single-tone variant of Link-11 which holds up in skip conditions on HF, while the parallel-tone version is strictly groundwave. # # #