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Digital Modes

For Information on digital modes click on links below.

 

[PSK Information]     [Cwing]     [APRS]     [RTTY]

 

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APRS Information 

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APRS Introduction The DOS version of APRS (Automatic Position Reporting System) was created by Bob Bruninga. Later on, versions for other operating systems came along written by Mark Sproul, Keith Sproul, Brent Hildebrand, and Steve Dimse to facilitate:
  • Tracking of weather events
  • Tracking Automobiles, Airplanes, Balloons, and Satellites
  • Relaying of real time data such as races, emergency service, public events
  • RDF Locating of Problematic Transmitters
  • Observing HF Band Conditions
This page favors the Windows APRS version and is meant to serve as a semi-technical overview of WinAPRS

Until now, most packet applications have been text based. These applications have been accessed by typing, and displayed information in a textual manner. They also required the user to have knowledge of the local network, and actually connect to a server or bbs to get information.

APRS is different. With this system, users can operate entirely hands off, and the information is presented in an easy to understand format, overlayed onto maps of local areas. Maps on APRS do not have to represent geographic areas. Maps can be created that define charts and tables - even chessboards. Any user in the network can take control of an object and update it's position. Imagine a chart listing people in a shelter, supplies, or resources. Any of these resources can be moved graphically to any other location by any station in the net. The local EOC supply truck doesn't have a GPS? No problem, just create one on the map, and manually move it as you talk to him on the radio. His position will be shown on every APRS screen in the area in a matter of seconds.

APRS has all the symbols for weather conditions, and each weather condition can be placed on the map for all to see. Information is transmitted instantly to all other APRS stations. In the event that a station does not hear this information, it will be transmitted 3 times in the first 60 seconds, 3 times in the next 10 minutes, and 3 more times the next hour. New stations coming online can query other stations for known information, such as the locations of shelters, and hospitals.

Another way that differentiates APRS from other packet programs is it's ability to pick information off the air passively. A very few stations can get information out to several hundred stations with only one or two transmissions. Other packet messaging systems require that each station receive it's own unique copy of any given message. This could lead to 100's of transmissions to distribute the same information aprs could accomplish in one single transmission!

The APRS network infrastructure is generic by nature... all stations are on the same frequency, all relay points are named identically. This universal generic network allows operators unfamiliar with the "lay of the land" to hit the ground running. The maps available vary in detail from the entire world right down to the tables at a hamfest.

In the past several years, specialized hardware has been created just for APRS. There are special APRS microphones which transmit your position at the end of each transmission like a "roger beep", specialized TNCs just for transmitting weather, and most importantly, APRS is so popular that it has reduced the cost of hardware for the average user. Whereas a "one size fits all" TNC used to cost $120 to $150, you can now buy a transmit only TNC for $25! 

 

Special thanks to Rick Irvine KC9CJJ for the APRS map below. Please go add check out his site on APRS.

RTTY Information

    Whether you desire to operate RTTY for DX'ing, contesting or ragchewing, you have to start somewhere.  It's my hope that I can point you in the right direction.  From there you will be able to make your own decisions on how you want to operate RTTY from your station.

     RTTY is baudot code.  More specifically, on the Amateur HF bands, it's 5 bit baudot meaning that every character consists of five bits, either mark or space.  In general, a baud rate of 45.45 baud is used on HF.  45.45 baud is equivalent to 60 wpm.  Even though 45.45 is standard, you will occasionally come across a RTTY signal at a different speed.  Ed, P5/4L4FN, prefers to run 50 baud rate (66 wpm).  75 baud (100 wpm) can also be used on the Amateur HF bands.

     The standard mark and space tones are 2125 hz and 2295 hz respectively.  Although these standard tones are used by most Amateurs, it's possible to operate RTTY using other frequency tones. This is fine as long as you maintain the standard 170 hz shift (2295-2125 = 170 hz).  Some commercial TNC's such as the KAM and PK232 use a 200 hz shift when running AFSK.   Although 200 hz shift will work OK,  170 hz is standard on HF.  Sound card programs will sometimes change the frequency of the mark and space tones when transmitting AFSK while using the NET feature to line up your transmit signal with that of a received station.  Although that's important to know.AFSK vs. FSK (Audio Frequency Shift Keying vs. Frequency Shift Keying)

The biggest decision you will make when you begin to set up for RTTY, is whether you want to use AFSK or FSK to transmit RTTY.  Either way is acceptable.

There is an excellent explanation of AFSK and FSK in the MMTTY help file so we won't go into a lot of technical detail here.  For simplicity sakes, AFSK and FSK are terms to describe how RTTY is transmitted.  AFSK is when you send audio from a TNC or Sound Card to the audio input of your transmitter either via the mic input or accessory jack.  FSK is when you send on/off keying from a TNC or Serial COM port to the FSK input of your transmitter.  Most modern transceivers today have an FSK input.  By using the FSK input to your transceiver, you can then operate the radio in the RTTY or FSK position and make use of filters available for receiving RTTY, such as a 250hz or 500hz IF filter.  In most cases, when using AFSK, your radio will be placed in the LSB position (although some, especially in Europe, prefer using USB).

    FSK prefered over AFSK all of the time.  There are advantages and disadvantages to both.  When operating AFSK, you must make sure the audio coming from your TNC or Sound Card is at the correct level and maintain this level.  If the audio is too high it will overdrive your transmitter and more than likely result in a distorted RF signal or cause your radio to put out "image" signals across the band.  You also must make sure you do not have any speech processor turned on when running RTTY.  This will also cause problems on your RF signal.  The only real advantages to running AFSK are that you can get started rather quickly using this method because it's simple audio to a Sound Card input for receive and audio from a Sound Card output back to your transceiver for transmit and you can also make use of the NET features of sound card programs.  But personally, I think AFC and NET are nice features, but over-rated.  You can get in trouble quickly by misusing AFC and NET on RTTY.  Another disadvantage of using AFSK is that most transceivers will not allow you to use the narrow IF filters in your radio when operating in the LSB or USB positions.  Some recently manufactured radios allow you to change the characteristics of your IF filters so you can effectively receive RTTY in the SSB position.  Check your manual to see if this is the case for your radio. 

    FSK prefered because it's straight on/off keying into the FSK input of my radio.  We don't have to worry about the audio level or whether we forgot to turn the speech processor off.  We can then use the FSK mode on my radio along with the 250 and 500 hz filters.  When I switch from SSB to RTTY, only have to change the mode on my radio.  There is a little more work involved when using FSK.  If you use a sound card, you must have a spare serial COM port available on your PC in order to key both FSK and PTT, whereas if you use AFSK, you don't need this extra COM port and you can use VOX to key the radio.  However, if you use MMTTY or any other sound card program to generate AFSK and use VOX to key your radio, any other sound generated by Windows could possibly key your radio and send that sound out on the air.  That would not be good.  There are ways to keep this from happening.  OK, enough said about AFSK and FSK. We won't get mad at you if you run AFSK as long as you put out a clean signal! Hope you have fun running rtty.

Copyright 2001-2005 All Right Reserved E-mail Problems to Shannon Davis KB9WNM