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Introduction to the HF Digital Modes
 for the Ham Radio Operator

Plus my review of the KF5INZ "Easy Digi" Interface


How to get started the "Easy" way in HF digital communications using PSK 31.

This article is NOT intended as a technical description of how to us PSK 31 or any other digital mode but is intended to get you interested in starting the fun of using your computer sound card, simple interfacing to your hf transceiver and computer keyboard for real time keyboard to keyboard, over the air, QSO'S on the HF bands using digital modes.

You will learn the basics of what you need to get started in this fun method of using very low power in restricted antenna situations to communicate with fellow hams worldwide using a simple station setup. It's really easy when you know what you need and are not afraid of trying it.

Many hams are of the opinion that computers and ham radio do not mix! Well fellow hams, unless you live in the dark ages, I am here to say that you are not informed!

Can you imagine trying to copy a radio signal that is so weak and AT or IN THE NOISE LEVEL or BELOW that you can hardly hear it in your speaker? How is this possible? 

The answer is by using your computer and special software and an interface with your transceiver. With this simple setup the computer software can filter out most of, if not all of that noise that you normally hear with your ears, and you can get at or near 100% copy of that "signal" using many of the digital mode software programs that are FREE! What the station operator types on his keyboad is what you see on your computer screen, in real time. All of this is accomplished using a software program that is usually free, a simple radio to computer interface and your transceiver! Using your transceiver in USB mode, tune around 14.070Mhz carefully and you should hear many PSK31 stations on the air. You won't be able to "copy" them without some sort of interface and the software installed on your computer!

Ok, you have convinced me, maybe...., so what do I need and how is it used?

A little background first:

One of the most popular hf digital modes is PSK31 due to the simplicity of using it. It has been around for many years and is used all of the time by many hams worldwide on a daily basis and on many of the hf bands. It's ease of use and setup with your station makes it a top choice among most hams who enjoy going "digital" on the ham bands. So exactly what is it?

(A bit of technical stuff)

The mode, PSK31, is typically created by a software program, (many are free), that either decodes the received signal in the receive mode or generates an amplitude and phase modulated waveform that is converted to an audio frequency analog signal by a sound card which in turn is fed to the transceiver during transmit.

PSK31's bandwidth of 31.25 Hz was chosen because a normal typing speed of about 50 words per minute (if you can type that fast), requires a bit rate of about 32 bits per second, and specifically because 31.25 Hz could easily be derived from the 8 kHz sample rate used in many DSP systems, including those used in the computer sound cards commonly used for PSK31 operation (31.25 Hz is 8 kHz divided by 256, and so can be derived from 8 kHz by halving the frequency eight times).

Some History

PSK31 was developed and named by English amateur radio operator Peter Martinez (G3PLX) and introduced to the amateur radio community in December 1998.

PSK31 was enthusiastically received by a few hams, and its usage grew like wildfire worldwide after the word got out and it's popularity grew by leaps and bounds over the air waves. Due to the efficiency of PSK31, it became, and still remains, highly popular with ham radio operators whose circumstances do not permit the large antenna systems and/or the use of high power due to many location restrictions. In fact, high power is NOT needed with PSK31! Typically only 20 to 30 watts is all that is needed.

A PSK31 operator typically uses a single sideband transceiver connected to the sound card of a PC running PSK31 software. When the operator types in a message for transmission, the software produces an audio tone which sounds, to the human ear, like a continuous whistle with a slight warble. This is then fed through either a microphone jack (using an intermediate resistor to reduce the sound card's output power to microphone levels) or an auxiliary connection interface into the transceiver, where it is transmitted. Most operators use a commercially built interface between the transceiver and the computer designed specifically to do all of this which saves on lots of building time. 

From the perspective of the transmitter, a PSK signal amounts to little more than somebody whistling into the microphone at varing rates and tones. However, the software rapidly shifts the phase of the audio signal between two states (hence the name "phase-shift keying"), forming the character codes. These phase shifts serve the same function as the two tones used in traditional RTTY and similar systems.

Resistance to interference(1)

PSK31 can often overcome interference and poor propagation conditions in situations where voice (ssb) or other data methods of communication fail. However, PSK31 was only designed for leisure use by amateurs, and due to its relatively slow speed and minimal or no error control, is not suitable for transmitting large blocks of data or text, or critical data requiring high immunity from errors. It's a FUN mode that is very enjoyable on the air and many hams spend hours just "looking" rather than "talking". Even when the higher frequency bands like 12 and 10 meters "appear" to be "dead", you tune up on the most used PSK or digital mode frequencies...and watch the "waterfall" display ...you may be surprised at what is "alive" there!

Digital modes work!
When writing this article, I took a break and checked the 10 meter band on 28.120Mhz using PSK31....there were a couple of stations in Chile and Brazil that were carring on QSO'S with other stations I could not hear (see). Also heard (saw) a Costa Rica station well. I did not try to get back to him...this article awaited and my break was over! I changed to the ssb mode on the voice portions of these bands but there was noting except background noise in the receiver.  

To decode PSK31, the received audio whistle (the actual tones and warbles from over the air that you may hear at times), from the transceiver's headphone output, is fed into the sound card's audio input along with all of the noise that the receiver hears), and the software decodes the PSK mode digitally. Even many of the very weak signals seen on the screen of the program display that your ears may not hear are decoded. This is what is so remarkable. You are decoding signals you may not hear well from the radio speaker! All you hear from your radio speaker is noise from it and very weak whistling sounds unless the transmitting station has a strong signal.

The PSK software also includes a user interface and control screen on the PC monitor, which is used to display the decoded text and manage the software configuration. You simply see the tracks of the signals on a "waterfall" screen and pick one with your mouse and the program does the rest. (See the screen shot below). You look in another window and see the decoded text that the other operator is sending. So basically you may usually see many signal tracks side by side on the "waterfall" display (a slowly moving background that looks like a colored waterfall) and pick from one with your mouse. The program decodes it an you see what he is sending as it is being typed at the speed of light (real time) by him and then you can reply back to him using your keyboard over the air! Your keyboard or mouse keys your transmitter and you are having a QSO in real time as you type your comments while they are being transmitted back to the other station!

The use of PSK31 does not require exclusive use of a dedicated computer. When it is not running the PSK31 program, such as DigiPan (reported to be the most popular PSK program due to ease of use), the station computer can still be used for normal use as needed. Because PSK31 was developed for use through a computer's sound card, many programs have since been created to use the same technology for other modes, such as RTTY, Hellschreiber, Olivia, MFSK etc. So once it has been set up to run PSK31, a computer and interface can be used to explore a variety of digital modes using the program or others, and the interface, which is a MOST important part of the setup for digital modes. Using a poorly home brewed interface will cause you more problems than QSO'S!

DigiPan program description:

DigiPan stands for "Digital Panoramic Tuning". The program brings a panoramic display on your computer screen and allows the use of your mouse to pick and choose from the stations displayed for receiving and transmitting for the band frequency you are monitoring. Point and click and you are ready to go!  DigiPan provides a panoramic display of the frequency spectrum nearby your VFO frequency in the form of an active horizontal dial scale extending the full width of the computer screen. Depending upon your transceivers IF bandwidth, it is possible to "see" and choose from many stations at one time using the panoramic display depending on band conditions.

Screen shot above (scaled down for web page) of the DigiPan panoramic viewer in action on the 20 meter band. The green Flag over the red diamond symbol shows the station selected! You willl notice the frequency coverage is from 14.069Mhz to over 14.073Mhz (receiver set to "wide") as viewed from left to right in the panoramic viewer above. Not shown is the actual text that is being received in another window in the program.

OK, I'm hooked! I want to try this out! What do I need for the setup to get me started and on the air quickly?

Aside from a standard hf radio ssb transceiver, very little equipment or skill is required to use PSK31 or other digital modes for that matter. Good typing skill is a help but many operators use the hunt and peck method. Normally, an older PC and a few cables will suffice and many PSK31 software applications are free out there on the internet. You simply download the free programs and install them which is a simple process. Hook up the interface (see info below) between the radio and the computer sound card and get on the air.

To save loads of time, and in many cases some frustration, and some foul language, many operators now use commercially available products such as interface/modem devices between their computers and radios as they save much "getting started" time in getting set up and operating because...... you have to find the plans for the interface, understand them, acquire the parts needed, shipping, find the correct cables to interconnect it all, build it, test it and if it does not work, you have to figure out why and on and on. If you don't want to build your setup from scratch or can't for some reason then read on......

Get to the details already!

You should already have much of the needed equipment, your computer and your hf transceiver, so you can mark that off your list. The most important part of getting on the air with hf digital modes like PSK31 is the radio to computer interface!
Don't skimp with it! Check out my review of it below!

The "EasyDigi" interface by KF5INZ.....makes it "Easy".

My Review of the "Easy Digi" KF5INZ digital modes interface.

I love it! Plain and simple! You can take this as my recommendation for this radio to computer interface for the digital modes or not. This is up to you but when I heard about it, I wanted to give it a try. I was introduced to the KF5INZ products at a local ham radio club meeting by a fellow ham who had built one from scratch in kit form! I was impressed with it at once and the price was very "right"!

I did a bit of research on these interfaces on the web and contacted Clifford, KF5INZ, at his company where they are made and sold wanting to get one for my Icom IC-718 so I could try it out on PSK31. (He offers it in kit form or ready to use custom built for most rigs.  I chose the ready made model but you won't believe how inexpensive the kit is! This is what caught my eye at first having been introduced to it in kit form at my local ham radio club by another ham operator.

I did some quick math in my head after looking at the kit and parts, quality, etc and decided that there was no way I could come up with all of the parts needed, plus the board to mount them on, a case for it, shipping charges, etc at the price my fellow ham paid for his!

Plus....I simply just did not have the time to build one.

So....shortly after contacting Clifford and getting one on order, he emailed me wanting to know if I would do a simple test for him on my rig so he could make sure that his interface would work properly on it. The test turned out fine and he said "good to go" for the Icom IC-718 to be used with it.

A few days later I received it well packed and I opened the shipping box and was thrilled at how small it was and so well made. All of the "electronics" are in an enclosure that measures only about 3 inches by 1 1/2 inches and is made of heavy duty ABS. The enclosure even comes with mounting tabs or "brackets" on the ends if you want to mount it out of the way!

Photo showing typical (non kit) K5INZ Easy Digi Interface (Labeling will depend on your radio)

All of the needed jacks are installed and the enclosure is fully labeled appropriate to your radio and ready to go. It came with all the needed cables and even schematics, an internal circuit board parts layout diagram, hookup diagrams, and a mic plug on a long cable all wired for my transceiver. There where urls included where I could go to get the driver for it to work with my Windows XP computer. And the best part, Made in the U.S.A. in the great state of Texas!

Getting it going and hookup:

I will be honest.....hookup and getting it to work correctly, was a problem at first...dummy me! In my haste to get it hooked up to the computer and radio and to get on the air quickly, I had reversed two cables and the power output was way down regardless of my sound card or transmitter settings! I found my "dummy" mistake, reversed the cables and all was well! This was like, "you can't see the forest for the trees", reversed! Repeat...dummy me! After I discovered my mistake and reversed the cables, it worked fine.

On the air with the KJ5INZ Interface!

PSK31 operation was simple using the KF5INZ interface and the DigiPan program worked correctly with it displaying those beautiful yellow colored streams of "paint" falling down the blue "waterfall" background. 20 meters was in fairly good shape at the time of my testing and I made several contacts back to back using only about 10 to 20 watts output fed to an old "junker" converted base station CB antenna converted loosly to 20 meters and a tuner. The interface pumped out the PSK31 signal without a problem!

I did, after a while, test it using FLDIGI, but not being familar with that program, it did take some reading of the Help files. The FLDIGI probram, at least to me, is a much more complicated program with a high angle learning curve. After my self help tutoring, the interface worked fine on it also and more good contacts were made using it on 20 meters. I also used the KF5INZ interface with Ham Radio Deluxe and the Airlink software...all work great with it.

How Would I Score the KF5INZ Interface on a scale of 10, with 10 being fantastic?
My impression of this interface and how well it works would have to be scored a 10 out of 10! You will be very pleased with yours!
(One minor "problem" I encountered was the fact that when using the external speaker jack on the rear of my Icom IC-718 to connect to the interface, was that the internal speaker audio was muted along with the headphone jack. I like to hear the stations I am working. ) This is NOT a fault with the interface!. But in the operation of DigiPan, you don't need to hear the other station because you "see" his signal on the waterfall in the program diaplay! So this is not really a problem after all.)
Hookup of the supplied cables was a snap. Very simple hookup. Don't guess like I did when I did the "dummy" mistake thing! My "Easy Digi" interface was clearly labled so my fault it did not work properly.

Overall Quality of the interface...great. Should last for years!

Ease of use...after you hook it up, set your sound card settings and com port. Your ready to go!

Could I build it for the same or cheaper price? Absolutly not, unless I already had everything in my "junk boxes" including all of the cables, jacks, plugs, computer connections, enclosure, circuit board, soldering skills, etc.

Check out all of the digital interface options and kits by clicking
 on any "Easy Digi" banner on this page, like the one above!

-----This is the end of my review but read on for more good info-----

These devices incorporate the necessary impedance matching and sound level adjustment to permit the sound card output to be injected into the microphone input, or the accessory jack on the rear of your rig in many cases and the radio audio output to be sent to the sound card input, and also handle the radio's transmit-receive switching via your keyboard. Recently introduced interfaces also incorporate their own sound card, and can therefore be powered and run from the PC via a single USB connection. The latter are much more expensive than the more simple interfaces like the "Easy Digi" from K5INZ and usually perform much the same function. So these expensive interfaces are usually not needed and are overkill in many cases in my opinion!

Spectrum efficiency compared to other modes

PSK31's efficiency and very narrow bandwidth make it highly suitable for low-power and crowded-band operation. PSK31 contacts can be conducted at less than 100 Hz separation so in theory, you can get about 10 stations operating within a 1KHz wide space on the band and usually using as little as 20-30 watts or LESS, so with disciplined operation at least twenty simultaneous PSK31 contacts can be carried out side-by-side in the bandwidth required for just one SSB voice contact! Do not use audio compression...turn it off! Use of it will cause splatter and poor use of bandwidth, distortion of your signal, etc.

Common PSK 31 Frequencies

The following amateur radio frequencies chart shows the bands and frequencies that are commonly used for transmitting and receiving PSK31 signals. There are many others also. They normally occupy the lower edge of each band's digital modes section and are not etched in stone. PSK31 operators generally use upper sideband (USB) even on frequencies below 10 MHz where the convention normally calls for lower sideband. This is because (a) signals then spread upwards in frequency into the digimode section from the "base" frequency, and (b) using QPSK requires both stations to use the same sideband.

Common PSK31 Frequencies in use

Frequency Amateur Band

1.838 MHz

160 meters

3.580 MHz

80 meters

*7.040 MHz

40 meters (region 1)

*7.035 MHz - 7.070Mhz

40 meters (regions 2,3)

10.138 - 10.142 MHz

30 meters

14.070 MHz (very popular)

20 meters

18.100 MHz

17 meters

*21.080 MHz

15 meters

24.920 MHz

12 meters

28.120 MHz

10 meters See note below

50.290 MHz

6 meters See note below

NOTES: The frequencies listed above are typical but not fixed in stone. Tune as needed around them.
In the U.S., Novice and Technician class license holders are only authorized portions of 10 meters and 6 meters on the hf bands for data signals like PSK31. No other hf bands can be used for data type transmissions by them. Consult FCC Part 95 regulations.

* Current usage based on observation, is centered on 7.070.15 and 21.070.15. 7.035.15 is commonly used in Region 2 as of 2012. There is no authoritative list as the frequencies are determined by common useage and band crowding.

Summary, Footnotes and other good info.

If you have never tried the "digi" modes on HF, I encourage you to get started very simply using PSK31, a great interface like the ones from K5INZ, and your ssb rig. Just because you are limited by antenna restrictions, lack of high power, or the space for outdoor antennas does not mean that you can not make contacts on the air. Very low power and a simple indoor antenna will get you into the exciting world of digi modes! Give it a try! 73 de N4UJW

Portions of this article are referenced from Wikipedia and re-edited for use on this page and are assumed to be accurate.

Download the free DigiPan program here.

If you consider using DigiPan for PSK 31, very good help (other than the Digipan Help file that comes with the program) can be seen at the links below:

Good help info for DigiPan can be seen here!

Another good help article for DigiPan here! (pdf format)


Good Reading From ARRL Publications at Amazon.com about HF Digital!

Look for the many yearly editions of the ARRL HF Digital Handbook including the latest edition at Amazon!



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