Digital Communications – A Practical Exercise

Wait!  Don’t run away!

If the thought of digital communications makes your brain hurt and seems complicated, IT’S NOT.  And it’s not a scary, intimidating thing that’s only for experienced Ham operators dedicated to the task.  We’re just talking about RECEIVING.  That’s a whole lot more simple and if you stay with me you’ll be up and running in just minutes.  It’s a lot like texting on your phone, versus making a voice call, but only using radios instead of phones.  You do not need a special radio to do this.  Besides a radio receiver, you’ll need a laptop, a patch cable, and a free downloadable software program.  We’ll walk you through every step here.

Why is this important?  Because AmRRON operators use digital communications often, for a number of reasons.  They use specialized equipment and software settings to transmit, which aren’t necessary if you’re only receiving.  This keeps you in the loop for information flowing among the AmRRON operators.   Text (digital) can often get through when voice communications cannot.  A message can be pre-formatted (typed) on the laptop, and then sent in a burst that is sometimes faster than voice communications, depending on the mode used.  And also, very few bad guys (roving bands of marauders) will possess the ability or the desire to achieve digital communicationsWe will be demonstrating several modes for this exercise, and will explain some of the ‘what’ and ‘why’ of each.

Once you’re set up, decoding digital communications is a simple matter of plugging an audio patch cable from the headphone jack of your radio receiver to the microphone ‘In’ jack on your computer.

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For this practical exercise, we will not be using a radio, as pictured above.  You will plug the audio patch cable from the headphone out jack (OUT) of your computer, and the other end into the Microphone (IN) jack on your computer.

–3.5mm Stereo Audio Patch Cable for Radio

–2.5mm to 3.5mm Audio Patch Cable for Radio (Baofeng, Wouxun, Kenwood)

If you don’t have an audio patch cable, you can simple hold the radio speaker close to the internal microphone, or tape a set of earbuds over the microphone.  Popeye wrote a great article about this at Receiving Ham Radio Digital Communications.

There are smart phone apps for decoding some digital modes, but we’re not going to get into that here.

The first step is downloading the software.  Do that by going here: http://www.w1hkj.com/download.html

Then select the fldigi/flarq download that matches your computer’s OS (Windows, MAC, or Linux).  After downloading the fldigi program, open it and follow the setup instructions.

Next step:  Open the fldigi program (Now a little bit of setup).  Adjust the settings so the fldigi program uses the correct microphone input.  To do this, click on the ‘Sound Card’ in the drop-down menu from the ‘Configure’ tab at the top-left of the window.

Be sure that the box next to ‘Port Audio’ is checked and that you have ‘External Mic’ selected in the ‘Capture’ portion.

Select the ‘External Mic’ from your ‘Capture’ options.  This may be different from one operating system to another, so you might have to play around with it until you find the mic input that the fldigi program will recognize.

The ‘Playback’ options won’t matter at this time, because you’re only receiving, not sending.

Also, in your computer’s ‘Control Panel’ in ‘Sounds’ settings make sure your microphone is ‘Enabled.’ 

 

NOTE TO PARTISANS:  You can type a message and then hit ‘transmit’ by right clicking your mouse.  Your message isn’t really being sent anywhere because you’re not set up for that, but it will still create the sound which, when recorded, can be played back and decoded simply by playing the audio file on any computer, iPod, CD, etc..  This could be a great way to transfer a message to a disk, iPod, or thumb drive (or attached to an email).  It is much more secure than simply passing a hand-written note.  If intercepted it can be decoded using the right hardware and software, but if it is an encrypted message that is then recorded as an audio file, you’ve just added another layer of security.

Next Step:  Connect one end of your audio patch cable to the headphone (OUT) jack in your computer and the other end into the microphone (IN) jack on the same computer.  This is just a practical exercise!

Next step:  Prepare to decode an audio file of a digital mode.  From the ‘Op Mode’ tab, select the mode you are trying to decode.  Some popular modes are:

– CW:  Morse Code

– PSK-31:  The most commonly-used digital mode (using Phase Shift Keying)

– Contestia 4/250:  Used for AmRRON HF (shortwave) digital

– MT63 1kL:  Used for AmRRON 2m digital communications

Once the audio file of the digital message begins to stream into your fldigi program, a line will appear in the lower portion of your fldigi screen, called the “Waterfall” window.  Click on that message in the waterfall and it should begin decoding in the yellow field in the upper portion of your fldigi window.

Once you decode the following messages, you should be confident enough to connect to a shortwave radio, scanner, or any other audio source from where digital communications are coming.

 

 

TIP:  To avoid your yellow screen filling up with gibberish from ambient noises, you can click on the ‘SQL’ button in the lower right-hand corner of the fldigi screen.  The vertical slider to the right of it will adjust the squelch level until you’re only decoding messages.  You may have to adjust this when listening to real ham digital traffic, because some signals can be so weak that fldigi won’t decode with the SQL clicked on

TIP:  To avoid excessive bandwidth ‘bleed’ on the waterfall, turn the volume of your radio, computer, or audio source down until the bandwidth of the signal is just about as wide as the lines on each side of your center point where you clicked your mouse.

And now for the practical exercise (CW, PSK-31, Contestia 4/250, and MT-63 100L):

CW (aka ‘Morse Code’)

This is a Morse Code message.  Don’t forget to select ‘CW’ from the ‘Op Mode’ dropdown menu.  This is a great way to receive Morse Code messages generated from another software program.  However, when receiving CW from a human-generated source, there are often many errors and inaccuracies in the message because the tempo and rhythm is not as precise or consistent as software-generated CW.  CW is the most robust mode since it can be transmitted many hundreds (or thousands) of miles in the noisiest and/or poorest atmospheric conditions with 10 watts, 5 watts, and even as little as 1 watt of power.

 

PSK-31

Now change your ‘Op Mode’ to –> PSK and then select –> BPSK31

 

Contestia 4/250 

 

MT-63 1000L