Receiving and Transmitting Radio Teletype (RTTY) With Your Macintosh

Introduction

Using MultiMode, you can send and receive RTTY with your Macintosh. Both Baudot and ASCII are supported. After connecting your Mac to your radio (see our interfacing notes for details on this), fire up your copy of MultiMode and set it for RTTY mode.

Next, tune in a station sending RTTY. When properly tuned in, the tuning display, which shows a spectrum of the received signal, should look something like this:

The peaks are the mark and space frequencies. When properly tuned in, they are equally space from the red line, which represents the center frequency. You can set the center frequency by typing it into the box marked Center. Most radios have an RTTY filter setting. If using it, you should consult your radio's documentation to see what the center frequency is, and set MultiMode up the same way, to get the best filtering of the signal possible.

Next you need to select the correct shift from the popup menu marked Shift. Usually this can be done by inspection, as there are only a few different shifts available. Most amateur radio operations use 170 Hz shift. Utility stations use other shifts, in this case the station is using 850 Hz shift.

Then, the correct baud rate must be chosen from the popup menu.

The Bit Rate, mode can also be used to determine the baud rate and shift of an unknown station.

The demodulation display looks like this:

The black and blue areas represent the mark and space tones. When properly tuned in, with the correct shift and center frequencies, the display should look like this one. If there are a lot of thin vertical lines, then the station is not properly tuned in, and/or the shift is not correctly set.

Also make sure that the audio level going into the Mac is adequate, without over-driving. We find that having it reach about the midway point on peaks works well.

You may have to click the INV (Invert) button to swap the mark and space tones, as some stations send inverted. This may also need to be done depending on whether you've tuned the station in using USB or LSB, as that can swap the tone pairs as well.

Most unencrypted RTTY communications take place using Baudot mode, so make sure that button is clicked, unless you know that the station is sending ASCII.

Decoded RTTY characters will now start to appear on your screen. If you have properly set up MultiMode and tuned in the station, you should be seeing readable text.

Note that other than amateur communications, and selected utility stations, such as those sending weather bulletins, the vast majority of RTTY transmissions are encrypted, and not readable.

The CYRILLIC button enables third shift Cyrillic transmissions to be copied, used by some Russian and other Slavic organizations.

The ABC button, when clicked, will reset the current shift back to letters mode, in case noise has left the program decoding in figures shift.

The UOS button, Unshift On Space, when clicked active, will cause MultiMode to automatically switch back to letters shift whenever a space character is received. Some transmissions require this mode, to be correctly copied.

The KG84 button will tell MultiMode to look for KG-84 bit sequences, and identify when they are detected. KG-84 is an encryption method. MultiMode doesn't decode KG-84 of course, but will tell you that's what it is. You can then move on to something more interesting ;-)

For transmitting RTTY, your Mac produces audio at the mark and space tones, as set by the center frequency and shift, at the selected baud rate. You can feed this into your transceiver's microphone input to transmit RTTY.

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Chris Smolinski