Written by N1UZ   

RTTY

Introduction

Everyday more and more Amateur Radio operators are operating on the HF digital modes, in particular, RTTY.  There are several reasons for wanting to be a RTTY op.   No matter what that reason might be, it is the purpose of these pages to assist getting you started on RTTY.

Anyone can operate RTTY.  You don't have to know how to touch type to run RTTY.  Hunt & peck works fine.  Every program for RTTY includes special "buffers" which hold pre-typed messages that can be sent by pressing a function key or clicking a button on your screen with a mouse.

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 for excellent information on Mr. Baudot and his 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 actuality Baudot is 8 bits because a start bit and two stop bits are added for synchronization see Getting Started in RTTY.  In general, a baud rate of 45.45 baud is used on HF.  45.45 baud is the equivalent of 60 wpm.

The standard mark and space tones are 2125 hz and 2295 hz respectively.  These frequency tones are also referred to as “high” tones.  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).

For the purpose of getting you started, MMTTY will be used here as the example program to use for running RTTY.  Why?  For several reasons.  MMTTY is a simple program to use. The MMTTY help file has an abundance of information about RTTY in general.

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.  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. 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 narrow 250hz or 500hz IF filter.  In most cases, when using AFSK, your radio will be placed in the LSB position.

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 the speech processor turned on when transmitting AFSK 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-soundcard input for receive and audio-from-a-soundcard output to your transceiver for transmit and you can also make use of the NET features of sound card programs.  But personally. 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. 

FSK is prefered because it's straight on/off keying into the FSK input of my radio.  IYou can then use the FSK mode on your radio along with the narrow 250 and 500 hz filters.  When you switch from SSB to RTTY, You 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 (or parallel LPT 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 or LPT 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. 

Source: http://wiki.contesting.com/index.php/Getting_started_on_RTTY_-_Introduction

Last Updated on Monday, 21 February 2011 21:44
 

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