Digimodes

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Working digital modes on amateur radio

Digimodes (digital modes) are a great way to make contacts.
You use your computer to turn text into sound (using various different algorithms), which is fed to your transmitter.
The returning audio is fed into your computer which decodes it back into text.

Benefits are:

  • Usually less power required than for voice
  • Uses less bandwidth than for voice, thus making your power go further
  • Good if you're newly licensed and/or are mike-shy.

Disadvantages:

  • Requires a computer or a phone app.
  • Often contacts are made with macros, leading to a rather impersonal-feeling QSO.
  • A bit tricky to get set up initially, and get used to.

Commonly used digital modes are:

  • PKS31
  • RTTY

Please note - this is not "packet radio" - that is something else.


In order to work digimodes, you'll need a computer, with some software to create and decode the sounds. The software I use is Fldigi, which is available for Linux and Windows. You will also need a soundcard, and the cables to connect to your radio. It is possible to simply put the radio microphone by your computer speaker, and the computer microphone by the radio speaker, but this isn't optimal. Most transceivers have a data socket on the back. You can often buy a dedicated data cable for these radios.

Some newer transceivers (Kenwood TS 590SG) emulate a soundcard. When you connect a USB cable from the radio to the computer, the computer sees a second soundcard.


I use Fldigi, which I find works very well, so the rest of this tutorial uses Fldigi. The principles still apply for other software, but you may need to look for the respective configuration options.
Windows and OSX users can download from here: http://www.w1hkj.com/
Linux users can install with yum install fldigi, apt-get install fldigi, or whatever other package manager your distro uses.

Getting started instructions for Fldigi

Start Fldigi, and follow the wizard for setting preferences.
The two tricky bits are configuring the audio settings, and the rig control settings.

Setting up your rig

All these instructions assume you have a correctly matched antenna connected or a dummy load.

Power

Important note: Ensure that you turn the power down to 20% of your radio's max power. Digital modes are 100% duty-cycle, and can sometimes take a minute or two to transmit an over. I have gone through two sets of finals (expensive!) by transmitting digimodes at full power. Don't be an idiot like me! (You could possible go up to 30 or 35% for a very quick QSO with a rare station, but be careful. Finals are expensive.)

Set the ALC correctly

Unlike voice where you want as much power going out as possible, too much audio will distort your digital signal and make it less readable.
On your radio, switch your meter to ALC.
On the bottom right of Fldigi, there is a value "TX level attenuator (dB)", given as a negative number, with arrows either side to alter it.
Click the "tune" button on the top right, and alter the TX level attenuator until it is just below where the ALC would start. This is the ideal setting. Click tune again to stop.
Please disable speech processing as well.

Setting up the audio

  • Select OpMode, BPSK31 from the menu.
  • Select Configure, Soundcard, Audio, Devices, and check that the correct soundcard is selected.
  • Click in the lower text box and press Ctrl T (transmit). If you hear sound from your speakers, this is good. Press Ctrl R (receive) to stop.

When Fldigi is in receive mode, there should be a waterfall at the bottom. This is what displays the received signals. If you are on a laptop, or have a microphone connected, you might see something, otherwise probably not.

  • Now connect the line-in socket on your PC to the audio out of your radio, and the line-out socket of your PC to the audio-in of your radio. You should hopefully start seeing some audio on the waterfall.

Setting up the rig control

Ideally, you want your radio to transmit automatically when you use Fldigi. You don't want to have to operate the radio as well as the computer. If your radio is old and has a serial (RS232) port, you may need a serial (RS232) cable to connect from your computer's serial port to your radio. Newer radios like the FT857 have this support built in to their cable. And very new radios like the TS 590SG do it all over the USB cable. (The TS 590SG emulates a soundcard as well as a serial port).

  • Quit Fldigi and turn the radio off
  • Connect the cable up
  • Turn the radio on
  • Start Fldigi
  • Choose Configure, Rig Control, Rig. Note there are several different methods of controlling your rig with Fldigi. I use Hamlib, but Rigcat is also common.

You will need trial and error to work out the settings. Your radio's manual will help. You might need to go into your radio's settings and see what baud rate (bps), stop bits, and other related settings are meant to be. I can't help here, as all radios are different. Eventually you should reach the stage that Fldigi shows your radio's frequency, and when you change it on the radio, it updates in Fldigi, and vice versa. Also, when you press Ctrl T, it should trigger the transmit on your radio.

Making a QSO

The easiest mode to start with is either RTTY or PSK31. I would say PSK31 is the most common, so select that in Op Mode. Find the part of the band you want to use that is dedicated for narrowband mode use. There are some regional variations. Google for your country's bandplan.

160M 1807 (USA) 1838 (Europe)
80M 3580
40M 7070 (most popular) 7040 (Europe) 7028 (Japan)
30M 10138-10142
20M 14070
17M 18100
15M 21070
12M 24920
10M 28120
6M 50290 (USA) 50250 (Europe)

Set your radio to USB (yes, even under 10MHz - the convention is USB for all (almost) digimode work), enable the data function if you have one, and set the dial frequency to the one listed in the table above.
Note: The actual frequency you will transmit on is your dial frequency PLUS the offset on the waterfall. This is important and is sometimes not understood by operators. If your dial frequency is 14070.000, and your red lines are on your waterfall at 1400, then the audio generated will be at 1400Hz, and thus will be transmitted at 14071.400. This is important for arranging skeds! View, Controls, Full in Fldigi will show you the actual frequency as well as the dial frequency.

If you have your radio set up correctly, and your PC set up correctly, you should be seeing activity on your waterfall.
9 PSK 31 signals are shown here: http://home.grandecom.net/~jchamberlain/qsl.net.files/digipan_screenshot.jpg
Click on the centre of a signal, and you should start seeing it decode in the top text window.
When you are ready (have you checked your power output is <= 20% of your maximum?!), you can reply by Ctrl T. Type your message, and click Ctrl R.


Other modes

Most modes have faster, wider-bandwidth versions. For example PSK31 sends roughly about as fast as someone can type, and uses 31Hz of bandwidth, thus maximising your power output, and making it great for DX.
PSK63, PSK125, PSK250, and PSK500 are the same, but take a lot more bandwidth (be nice to your fellow hams!), and send data much more quickly.

There are some other very interesting modes.
Olivia is a mode that is good at very low S/N ratios. It has a lot of different variations - RxID can help with this - see the advanced options. Olivia seems to be used a lot more for rag-chew contacts - macros don't seem to be used much on Olivia
FeldHell is reminiscent of some World War 2 ticker-tape messaging system. See if you can catch a FeldHell Sprint contest as it's not commonly used otherwise.

Advanced operation

Once you have your typing being translated into audio, and transmitted by your radio, and the returning signals being turned back into text, you have a working digimode setup.

Macros

You can use macros to help the "boring" bits of your operation. For instance, the CQ macro is configured like this:

<TX>
CQ CQ CQ de <MYCALL> <MYCALL> <MYCALL>
CQ CQ CQ de <MYCALL> <MYCALL> <MYCALL> pse k
<RX>

This transmits, and sends the text replacing <MYCALL> with whatever you've configured in the preferences, and then stops transmitting. You can edit macros, and there are a vast range of options you can use.
If you want to enter a contest, they become very handy. Once you hear a station, click once on their callsign. This will fill in the "Call" field. Then the ANS macro will use that to fill in the <CALL> field with that information:

<TX><CALL> <CALL> de <MYCALL> <MYCALL> <MYCALL> kn
<RX>