Tech Arsenal 1

home *** CD-ROM | disk | FTP | other *** search

/ Tech Arsenal 1 / Tech Arsenal (Arsenal Computer).ISO / tek-20 / amt_apl.zip / CQ.DOC < prev next >

Wrap

Text File | 1991-01-14 | 33KB | 551 lines

----------------------------------------------------------------- The following article is a scanned copy from page 52-56 of the November 1990 issue of CQ magazine. ----------------------------------------------------------------- For many of us AMTOR is an abstraction., a simple click away on a mode switch. What is it? How does it work? How do I use it? These and many more questions are answered by K9GWT in simple, everyday terms. AMTOR For Beginners BY BILL HENRY*, K9GWT AMTOR is a relatively new and exciting "RTTY-type" mode. However, I find that many would-be "AMTORites" suffer from "chirp fright." At first, the alphabet- soup of acronyms, strange modes, and transmitter pulsing can be very intimidating. Not to worry. It's all quite simple once you get your feet wet. This is not the first-nor the last-beginner's article on AMTOR. The references list a number of other authors and articles on the same topic. You may find that their words are even more informative than mine. Multiple opinions always help. Take heart in the knowledge that AMTOR is actively used by many amateurs and is growing. You do not have to be a Ph.D. or computer-whiz to have fun on AMTOR! What Is AMTOR? Very simply, AMTOR is "modern RTTY." It is very similar in many ways to the plain old "teletype" we have used since the 1950s. AMTOR is also similar to packet. Some would say that AMTOR is the "technical bridge" between RTTY and packet. However you want to think of it, AMTOR is a mode which we use to send text back and forth between radio stations Like RTTY (and packet), AMTOR uses an FSK (Frequency Shift Keyed) modulation mode. Unlike RTTY, but similar to packet, AMTOR includes error correction. On poor signals, receiving RTTY can become tedious as the errors and misprints pile up. A good RTTY modem can help, but there will still be some errors when the other fellow's signal fades to "zip." Not generally so on AMTOR. It just keeps repeating until it "gets it right"! AMTOR Modes AMTOR is not just one mode to send text; it includes several related, but different modes. The major modes of AMTOR are: ARQ: In ARC mode the sending station sends three characters at a time. He then turns his transmitter OFF and the receiving station sends a one-character control signal to signify "OK, go ahead" or "Please Repeat Last Group. " I n ARQ mode both stations alternate between transmit and receive. The station sending text is called the ISS for "Information Sending Station," and the station re- ceiving text is called the IRS for "Information Receiving Sta- tion." ARQ mode is the normal mode used by amateurs when in QSO. ARQ may also be called "ModeA" on some controllers. This is the "chirp-chirp" stuff you hear between 14070 and 14080! ARQ mode is similar to packet in this respect. FEC: FEC (Forward Error Correction) mode is a lot like RTTY. One station turns on his transmitter, sends an entire message, and then stands-by for the other station to transmit his response. FEC does not have the "Acknowledge/Repeat" pulse exchange of ARQ mode for error correction. However, FEC corrects errors by the simple expedient of transmitting each character twice. The re- ceiving controller has the "smarts" to know when an error has occurred and to then look at the repeated character. FEC mode is primarily used for calling CQ to set up a QSO in ARQ mode, but it may also be used for RTTY-like QSOs and is especially useful when "broadcasting" bulletins to many stations. The W1AW bulletins are sent via FEC; commercial shore stations often transmit weather information using FEC mode. On some controllers FEC mode may be called "Mode B" or "Mode F." SFEC: SFEC (Selective FEC) mode is almost identical to FEC mode, but it includes a special call-up code so that it may be directed 10 selective stations, your controller won't lock onto SFEC unless you have coded the same Selective Call sign as that sent by the transmitting station. SFEC has to date seen little use in amateur radio. Its major use is for messages to a group of ships in the maritime service. SFEC may or may not be included in your AMTOR controller. SFEC is sometimes called "Mode S." STBY: STBY (Standby) Mode is just what it sounds like. This is the normal "rest" condition of your AMTOR controller when you are monitoring and waiting for someone to call you. AMTOR controllers are intelligent and can distinguish between ARQ, FEC, and SFEC signals. When a valid AMTOR signal is received (and you have a "SELCAL match" in ARQ or SFEC mode-more on this in a bit), the con troller automatically switches to the correct, mode and starts printing. The AMTOR controller should not respond or produce receive print when a RTTY, CW, or packet signal is re- ceived. MON: MON (Monitor) mode allows you to listen to ARQ, FEC, or SFEC signals. MONitor is also called "LISTEN" mode on many AMTOR controllers. This is a receive-only mode and therefore you do not get the error-correction feature when listening to ARQ signals. The controller does the best it can, but you will probably get some "hits" that would not be noticed if you were "connected" in ARQ mode. In some AMTOR controllers, MONitor mode automatically accepts and prints ARQ, FEC, and SFEC signals: in others, MON or LISTEN will only receiver ARQ mode signals and you must go back to STBY mode to listen to FEC or SFEC signals. Read your manuals on this one! Monitor is an amateur invention for AMTOR (credit G3PLX 1,2,3); it is not specified by CCIR-476 or CCIR-625, and is generally not available in commercial "SITOR" equipment. What Is This "SELCAL" Business? In ARQ mode (and SFEC mode) a special "station identifier" is sent at the start of the "link." This "identifier" specifies which station you wish to establish communications with. With out it, all AMTOR stations on your frequency might start "chirping- back" to you and your controller could not sort out the mess. The acronym "SELCAL" stands for SELective CALL. The SELCAL letters can actually be any letters we choose, but amateur convention is to use a contraction of your call sign to make up your own SEL- CAL. Eddie Schneider. (G0AZTP4) has the best table for figuring out your own SELCAL. 1 x2 Callsign-Drop the # and duplicate the first letter (example: K9CW SELCAL = KKCW). 1 x 3 Callsign-Drop the # and use all four letters (example K9GWT SELCAL = KGWT). 2x 1 Callsign-Drop the # and duplicate the first letter (example: KS91 SELCAL = KKSI). 2 x 2 Callsign-Drop the # and use all four letters (example: AB1CD SELCAL = ABCD). 2 x 3 Callsign-Drop the # and second letter (example: WA9YLB SELCAL = WYLB). This is a very convenient system first pro posed and used by Peter Martinez, G3PLX. It is logical, easy to figure out, and in use by practically all amateurs. However, it is not infallible 1. Consider the SELCAL for W9CD, "WWCD." The above formulas also say that "WWCD'; would be used by W9WCD, W1CD, W6WCD, and even "WW9CD" (if we ever get to that call sign). Fortunately, this happens rarely, but will occur more often as AMTOR grows in popularity. The "solution" is simply to pick another four-letter combination. Common Misconception: The FCC (or any other regulatory body) does not require that your AMTOR SELCAL be related to your call sign! The practice is simply a convenient way to construct a SELCAL. It is not required and a SELCAL does not relieve us from sending our full callsign for identification'. Recently, the FCC added a new version of AMTOR to Part 97 of our Rules and Regulations, the "CCIR-625" version. The major differ- ence between CCIR-625 and the original CCIR-476 version is that the new version has a seven-letter rather than a four-letter SELCAL. At this writing, CCIR-625 is brand new and only a few amateurs have made use of it. The seven-letter SELCAL does, however, solve the confusion caused by the four-letter algorithm discussed above. To date, there is no one strong recommendation for just how these seven letters are chosen. I suggested one idea in November 1989 CQ (K9GWT5), but there are many other schemes that will also work. CCIR-625 includes "downward compatibility", so don't fear, it will also recognize your ., older format" signal and communicate. CCIR-625 includes several other nice features and some much-needed clarifications of "gray-areas" of CCIR-476. Use of CCIR-625 will increase as our older AMTOR equip- ment is upgraded or replaced. I should also mention that this whole business of "making-up" letter-style SELCALs is strictly an amateur radio invention. The commercial SITOR systems are required to use number SELCAL codes that relate to a permanently assigned ship or land-based station identifier. In theory, each marine ship or shore station has a unique SELCAL that can be used to identify the station in an emergency. In practice, that wasn't the case for CCIR-476, since the available number pool has long since been exhausted. This was, in fact, the major reason for commercial adoption of CCIR- 625. CCIR 491 -1 defines how these SELCAL numbers are converted to letters that are actually sent on the radio link. CCIR-476 will support 4- and 5-digit numbers CCIR-625, 9-digit numbers. Amateurs are not required to use CCIR-491 procedures to define a SELCAL. Be glad,as it's not a simple translation! G3PLX's scheme makes a lot more sense. How Do I Use This SELCAL Thing? Simple! You "mash" the magic SELCAL but ton on your terminal, type in four letters, and away you go-hi! Actually, it's a little more involved than that, but not by much. In ARQ mode there are two SELCAL codes to worry about-yours and the other guy's. Your SELCAL identifies your station so that other stations may call you and "fink-up." Assuming that you do not change callsigns often, this is something you have to do when the controller is new and may never have to mess with again This SELCAL may be called "LOCAL SELCAL," "MYSELCAL," "MYS," or "MYA." Whatever it's called, read your manual, enter four letters, and store it ("perm" on some controllers; save "CONFIG" on the PCI- 3000). The second ARQ SELCAL is the one of the other station. This SELCAL will need to be changed each time you call a different station. This SELCAL may be called "REMOTE CALL," "HISSELCAL," or just be a prompt when you start to send in ARQ mode. Enter the four letters corresponding to the other station's SELCAL. There are actually two more SELCAL codes, both associated with SFEC (Selective FEC) mode. These are often called the "GROUP CALLs"-"local" and "remote." They will only be required when you are receiving or sending in SFEC mode. The "local" GROUPCALL corresponds to the SFEC SELCAL that your station will recognize when receiving SFEC messages. The "remote" GROUP CALL is the SFEC SELCAL you will send when transmitting in SFEC. Other stations must have this same SELCAL loaded as their "local GROUP CALL" to receive your SFEC transmission. How Do I Call CQ? We use FEC mode when calling CQ in AMTOR. This is a whole lot like RTTY, but with some special recommendations: 1. Keep your CQ short-2 or 3 lines. A short CQ is just common sense-and courteous. Two or three short CQs on a frequency will probably be a lot more productive than a long wordy call. I may have gone to sleep by the time you get done calling! 2. Include your SELCAL as part of the CQ text ' What you want is for the other guy to call you in ARQ mode. He can't do this if he doesn't know your SELCAL! 3. Start each CQ with a blank line. The "official" requirements of CCIR-476 and CCIR-625 say that FEC printing does not start until after receipt of a CR/LF. Not all AMTOR controllers require this, but it's best to be safe! 4. Do NOT include a string of "RYRYRY, RYRY ... " letters. AMTOR uses a special 7 bit data code. The combination "RYRYRY..." is the worst bit combination you can imagine for FEC synchroniza- tion. In fact, few (if any) AMTOR controllers will ever synchro- nize to a string of "RYRYRY - . . " and some will lose sync! " RYRYRYRYRY . . . " is a poor idea on RTTY and a disaster for FEC! I suggest the following AMTOR FEC CQ message: Transmitter ON, AMTOR controller in FEC mode (blank line) CQ CQ CQ DE K9GWT K9GWT K9GWT SEL- CAL: KGWT KGWT KGWT CQ CQ CQ DE K9GWT K9GWT K9GWT SEL- CAL: KGWT KGWT KGWT CQ CQ CQ DE K9GWT K9GWT K9GWT SEL- CAL: KGWT KGWT KGWT CQ DE K9GWT (KGWT), BILL IN URBANA, IL ZZZZ (HAL PCI-3000)-or-END command Transmitter OFF, AMTOR controller returns to STBY mode Paul Newland (AD716-7) also suggests that we delay between trans- mitter ON and the start of the CQ text, sending 5 to 30 seconds of ".FEC idles" at this start. This is a good idea, since "idles" are required for FEC receive synchronization. However, many AMTOR controllers do not include a way to delay text transmission. The CCIR "rules" require that synchronizing "idles" be sent at the end/start of each line, so the first CR/LF (blank line) gives you a headstart, and there are "idles" sent at the end of each line. Some AMTOR controllers also insert extra idle characters within each line. As G0AZT and AD7I both point out, this is a very good idea and one we all hope all manufacturers will soon adopt. How Do I Answer A CQ? First we have to assume that you have successfully received an FEC CQ call and that it is formatted like the example. You must know the other guy's SELCAL to answer his CQ! Assuming that you did get his SELCAL, enter it as required and start sending in ARQ mode. (The procedure is different for every AMTOR controller. Read your manual!) If your answer is successful, the other sta- tion will soon start chirping back to you, and "away we go." I'm Linked-Now What? Ha! Wasn't that easy.? Now all you have to do is start typing text. Assuming that you have all the weird and wonderful computer buffer controls set correctly (read your manual), text will now be sent to the other station-three characters per "chirp." Even though our FCC regulations are now considerably relaxed concerning ID, I still like to start each major transmission with the standard "CW style" ID line-"K9CW DE K9GWT," for example. Old habits die hard for me! I also must admit that I send the ID line again at the " end of each of my transmissions. It's not neces- sary, but few have complained' Over ARQ mode has a special command to use when you have typed all you want and it's time for the other guy to beat on the keys. We call this the "OVER" command. The OVER command is actually the trans- mission of "plus" and "question mark" characters in sequence (+ ?). Once you send (+?), the AMTOR controllers take over and "turn the channel around"-you become the "IRS" and the other guy be- comes the "ISS." On some con trollers there is a special key to press or command to enter for OVER; on others you just type +?. Note: If you are still using an old Model 15 or 28 TTY machine as your AMTOR terminal, you must type "quote" and "question mark", the U.S. Baudot keyboard has a "quote" where the CCIR terminal has a .. plus." It's the same bit pattern, but different key-top labels! Master/Slave Note that I referred to an exchange of "ISS" and "IRS." This has nothing to do with "MASTER" or "SLAVE. " Since you were the original calling station in ARQ mode, we call you the MASTER station. In ARQ mode the MASTER station is simply the station that first started the link. The SLAVE station is the called station in ARQ mode. The terms "MASTER" and "SLAVE" have nothing to do with who is sending text and who is receiving text. The only real distinction between "MASTER" and "SLAVE" is that the "MASTER" sets the timing standards for the link and will be the station initiating a re-call if conditions get so bad that the link fails. In an ARQ QSO the ISS and IRS designations change with each OVER operation ' the MASTER/SLAVE designation does not change during an ARQ mode QSO. Forced Over The above describes how a "normal" OVER is you do it the other guy is long-winded ("long-fingered"?) and you want to interject a comment? Ha! We have a solution called "FORCED OVER." In this case the receiving station (IRS) can push another special button and take command-instantly (at least within three characters). Once you do this, you are again the ISS and what you type is sent to the other station. Here you had better hope that the "flow- control" between your terminal and AMTOR controller works right (and that of the other station as well). Properly configured, what should happen is that transmit text from the original ISS will be held at the FORCED OVER. When you again return control to the other station (by typing + ?), his transmit text should resume exactly where it was interrupted. Most computer systems can do this-, most mechanical machines cannot do this! A FORCED OVER can be rude and may result in loss of some text if flow-control is not working properly at both stations. Most of us now use "computer controlled terminals," so it is rarely a problem, However, beware if the other guy says he is using an old TTY machine as a terminal! End Command An ARQ link will go forever it you don't end it. ARQ has a spe- cial "END" command that 'is used to shut-down the chirping link. How you send the END command varies a lot between AMTOR control- lers. On some it's a special CTRL key; on others you type "Z777" to end. Read your manual! You may observe some of us saying quaint things like "I'LL LET You DOWN THE LINK" or simply "LINK DOWN." Obviously, we about to end the link! By the way, don't forget to send an ID line before dropping the link. The above has been just one example of how to use AMTOR. Eddie Schneider, (G0AZT4) Paul Newland, (AD7I6,7), and CQ's Buck Rogers (K4ABT8) have also written excellent step-by-step beginner's operating instructions. You will learn other techniques and tips with a close reading of their articles. Won't All That Chirping Destroy My Radio? Baloney and poppy-cock! This is by far the , I greatest concern of every AMTOR beginner have met. It was my biggest concern when started. In spite of what you hear, AMTOR does not destroy rigs; at least it shouldn't. Modern transceivers will take all the click-click operation of the TX/RX relays I have been able to give them. I have never worn out a relay or lost a rig to ARQ chirping. In fact, since the duty cycle of ARQ mode is only 47% at worst, ARQ is a lot easier on the transmitter than RTTY or FEC mode(100% duty cycle). I do know of two cases where we thought we had wiped-out a TX/RX relay-both on ..? TS-830s. What actually happened is that multi- ple relay operations vibrated the relay out of its socket! The TS-830s TX/RX relay is mounted upside down; judicial placement of a 1/4 inch piece of foam rubber fixed both cases! Can I Use My Linear? Now I am going to "hedge my bet"! First of all, you do not need a linear for AMTOR. The error correction feature means that while you may get more repeats on weak signals, you can almost always get through with 100 watts or less. Second, with very few excep- tions, linears have large and slow relays. These relays are not suited for fast ON/OFF switching required in ARQ mode. The relays often switch high voltages and/or currents. Yes, you can get some pretty spectacular arcs and sparks in the linear, and they will cost big bucks to fix! So unless your linear manufacturer makes a big deal out of "QSK" operation of his linear, forget high power on AMTOR. You don't need it! Actually, we could all probably drop our output power to 10 watts or less and never notice the difference. If you have TVI, AMTOR with low power may be your solution! How About Switching Times? This is another supposed "problem" that has been blown out of proportion. Switching times used to be a concern. It really is not a major concern with the current and previous generation of transceiver models. They can all be used on AMTOR. However, switching time can be a big problem on older equipment. Some models are in fact unusable on AMTOR or may require substantial modifications to use ARQ mode. Many radio manufacturers now have data sheets that give switching times and suggested modifica- tions. Active AMTOR operators are also a good source for modifi- cation in formation-"Caveat Emptor"! By the way, if your rig has a "QSK" mode, use it! There is also a lot of on-the-air discussion about what values to use when setting the programmable delays in your AMTOR control- ler. Some amateurs say "set them as low as they will go." Good luck if you do! Here again, read your manual and start with the manufacturer's recommended settings. Messing with delays is a two-edged sword. Shorter delays extend your longest distance in ARQ mode, but may also increase your minimum distance. The number and purpose of programmable delays varies with each AMTOR con- troller read your manual! A "delay" time of 20 to 50 Ms will work with practically all radios-and cover the world. You do not need to set delays to "5" or " 10" to work DX! Chances are your radio is not that fast and you may actually be trashing some of your own data while the poor radio is trying to switch! Each control- ler/radio combination is slightly different. Do some experiment- ing and determine your own parameters. What About The Shift For AMTOR? In my opinion, there is only one shift that AMTOR stations should use, and that is 170 Hz SHIFT. This has been the amateur radio RTTY shift for years, is the shift used in all commercial SITOR stations,and is usually the only shift supported in the FSK mode of modern transceivers. 170 Hz shift is the standard, and you are only doing yourself a disfavor if you attempt to run AMTOR using another shift. A second shift, 200 Hz, has recently appeared on the amateur bands. 200 Hz shift originated with the Bell 103 telephone-line modem and was adopted for inclusion in packet controllers. When packet moved from VHF to HF, the 200 Hz shift PLL modems came "for free." I think all who run HF packet would have to admit that 200 Hz shift really does not work well for packet. There is an active and much needed drive for improvement in HF packet modem standards. Changing the shift (and modulation type) is a recognized requirement for improved HF packet operation. It 200 Hz shift doesn't work for HF packet, why use it on AMTOR? It your controller has 200 Hz shift, you face a double disadvan- tage when talking to the rest of us. First of all, your 200 Hz shift transmit signal does not match our 170 Hz spaced receive filters. This makes our receiver tuning much more critical and increases the odds that we will get "hits," many repeats, and eventual link failure. Second, your receive filters do not match our 170 Hz transmitted signal. You also have to "straddle-tune," face tuning errors, and accept less than optimum demodulator performance. A 30 Hz difference does not sound like a lot, but it can make a big difference on marginal links. If you have an AMTOR controller with a 200 Hz shift modem, l suggest that you consider retuning the receive filters and trans- mit tones for 170 Hz shift. Your AMTOR will be much better for the effort! By the way, retune for 2125 Hz MARK and 2295 Hz SPACE. That's the standard and what your transceiver FSK circuit is set up for! You will not be "true transceive" with any other tone frequencies. What Is APLink? APLink is a very nice message-forwarding computer program written by Vic Poor, W5SMM- APLink serves as a link between HF AMTOR and VHF packet networks. APLink is run in an "IBM-Compatible" Person- al Computer (PC) and provides two data ports-one for the AMTOR controller and one for the packet controller (TNC). The two ports share a common message storage; each can access the other's message files (with some guidance from the control operator). Vic has written APLink drivers for the AEA PK-232, AMT-1, and HAL PCI-3000 AMTOR controllers. Only a "Gateway" or message relay station needs to run APLink software, so it's a program that many of us will never need. However, we may all want to use the long- haul message relay feature of an APLink Gateway station, Many Gateway stations also include frequency scanning to monitor 8 to 16 different bands and/or frequencies. These stations there- fore avoid many of the problems associated with skip distance and changing band conditions with time. It's also an efficient use of radio spectrum, since the control circuitry automatically skips frequencies which are already in use. The best way to find out more about APLink is to tune around 20 meters, find an APLink station, and watch the fun in MONITOR or LISTEN mode. You will get the idea after a couple of listening sessions and then be able to call and request HELP files for more data. APLink is an exciting new use of RTTY, one that makes full use of the error correcting and robust nature of AMTOR. Conclusions AMTOR is great mode. It is really not hard to run. AMTOR has fewer cryptic commands and is actually simpler to use than pack- et. HF AMTOR is a whole lot more reliable than HF packet or "straight RTTY." AMTOR does not require high transmitter power. Practically all modern transceivers can run all AMTOR modes. There are now a lot of amateurs all over the world running AMTOR. Whether you like to rag-chew or run traffic, AMTOR is an ideal mode. I must admit that running ARQ mode in a RTTY contest is a lot of work, but it can also be very rewarding! All around, AMTOR is the HF data mode for many of us long time RTTY fanatics. References 1 -Peter Martinez, G3PLX, "Amtor, an improved radioteleprinter system, using a microprocessor," Radio Communications, August 1979, RSBG, Cheimstord, Essex, England, pp. 714 719. 2. Peter Martinez, G3PLX, "Amtor, the easy way," Radio Communica- tions, June/July 1980, RSBG, Chelmsford, Essex, England, pp. 610- 615, 3. Peter Martinez, G3PLX, "Amtor, an Im proved Error-Free RTTY System," QST, June 1981, ARRL, Newington, CT 061 1 1, pp. 25-27. 4. Eddie Schneider, W6/GOAZT, "AMTOR" (monthly column), RTTY Journal, April, May/ June, July/August 1990, 9085 La Casita Ave., Fountain Valley, CA 92708. 5. Bill Henry, K9GWT, "New AMTOR Mode! CQ, The Radio Amateur's Journal, November 1989, CQ Communications, Hicksville, NY 11801,pp.36-40. 6. Paul Newland, AD7], "An Introduction to Amtor," QST, July 1983, ARRL, Newington, CT 06111,pp.11-13. 7. Paul Newland, AD71, "A User's Guide to AMTOR Operation," QST, October 1985, ARRL, Newington, CT 0611 1, pp. 31-34. 8. Buck Rogers, K4ABT, "The Newcomer's Guide to AMTOR," CQ, The Radio Amateur's Journal, November 1989, CQ Communications, Hicks- ville, NY 1 1 801, pp. 51 -55.