What’s all this about PSK31, MT63, FEC, NBEMS and NVIS?

By Engineer Paul


In this article we take a quick look at PSK31 and MT63, two popular digital communications modes and what role each is best suited for with respect to emergency communications. Next, a short intro to ARRL’s NBEMS (Narrow Band Emergency Messaging Software) which features fldigi. We also touch on NVIS, FEC and a few other acronyms related to digital, emergency and disaster communications.

“When All Else Fails… Amateur Radio Works”

In a world of ever-advancing technology, our reliance on that technology for something as fundamental as communications leaves us all vulnerable should that technology fail us. As an illustration, we recently experienced county-wide communications failure when the main fiber-optic communications cable in our valley was severed during construction. Internet as well as land-line and cellular telephones went down. Until the line was bypassed and repaired, the only reliable form of communication was radio. Fortunately there were no emergencies during this time of vulnerability. It’s hard to imagine life without telephones and Internet until it happens.

In times such as our local communications failure, or a man-caused or natural disaster, amateur radio may provide the only means of civilian communication. Getting information into, out of, and throughout an affected area is vital. Voice communication works, but if available, digital communications provide a much faster and error resistant method for passing message traffic.

Granted, amateur radio digital communication relies on modern technology and is subject to its own failures. A solar flare or EMP could fry our computers and modern radios, but many hams have backup laptops and radios stored in Faraday cages. Though there is the possibility of such an event, the probability is much lower than other scenarios that lead to the loss of communications infrastructure, so prioritize your contingencies.

Digital Modes

Probably the most widely used digital mode (outside of Morse code) is PSK31. PSK31 (Phase Shift Keying, 31 baud) is a wonderfully efficient conversational digital mode – good for typing back and forth. PSK31 clocks in at about 50 Words Per Minute (wpm) and uses about 31Hz of bandwidth. It lacks error detection and correction, but that’s not usually necessary for a one-to-one conversation where the operator at the receiving station can simply type back “What did you just say?” if something gets garbled. This interaction is the human form of an error detection and correction protocol.

You can have the computer take care of error detection and retransmission for you. This is what ARQ protocols do: Automatically ReQuest retransmissions when necessary. This is fine for one station sending to a single receiving station, but when a station is sending a message to multiple recipients, it becomes impossible to manage multiple stations requesting retransmissions. Enough said about ARQ.

Let’s now look at FEC or Forward Error Correction, a fancy name for a simple concept. Simply send more than one copy of the information. Now, to make it more robust, let’s spread the data around in time and frequency so that no fading or noise bursts can corrupt all copies of that redundant data. MT63 (Multi Tone 63) does this. MT63-1000, for example uses 64 tones (numbered 0 to 63) spaced 15.625Hz apart, using 1kHz of bandwidth. That’s 64 parallel data streams on which to encode multiple copies of the data! MT63-1000 provides 100wpm throughput.

MT63 is a remarkably robust signaling mode in noisy conditions. Its encoding and FEC do much to ensure that a station receiving an MT63 message will have enough information to reconstitute the original message. This allows a sending station to “broadcast” a digital message to any and all stations that care to receive it and know that in all likelihood it will arrive error free. That’s something not possible with PSK31, nor practical with most other FEC modes.

Of the digital modes that implement FEC, MT63 has proven its worth in HF communications. MT63 is so robust that receiving it can be as simple as holding the microphone of your computer up to the speaker of your radio – even in a noisy room such as an Emergency Operations Center. It can even be copied with voice on the same channel. MT63 is recommended by the ARRL for EmComm and has been adopted by MARS Region One and others for message handling. Though developed for HF SSB, MT63 works great for VHF/UHF FM operation. We use it regularly on VHF, both simplex and through a repeater and are very impressed. As I write this article I’m testing MT63 with two laptops side-by-side using built-in speakers and mics in a noisy room and messages are getting through just fine!



Of the programs available that support MT63, a free program by the name of fldigi (pronounced “F-L-Digi”) is one that stands out. Go to http://www.w1hkj.com/ and click on the download page link. The ARRL has endorsed fldigi and its companion programs and dubbed them NBEMS or “Narrow Band Emergency Messaging Software.” Another plus is that fldigi et al. run on Windows, Mac and Linux. Dust off that old PC and get it running fast again with Linux! Oh, and did I mention that the fldigi suite of programs is available for free?

What NBEMS gives you is not only an abundance digital modes, but also standard message formatting and message integrity assurance. The latter two are quite useful in ARES where operators are responsible for making sure that the proper message forms, like ICS-213 and the ARRL Radiogram, are used with the various organizations and agencies. Messages are error free and can be logged on disk and/or neatly printed.

Fldigi is the main stand-alone program for digital modes. Flmsg adds message forms support. Flwrap can convert binary files to text for transmission and can add a checksum to ensure file integrity. Flarq is used to add an ARQ protocol to many of the digital modes. Use flwrap if you need to send a binary file such as a zip, image or Word doc.

NBEMS Setup Notes

To enable automatic transmission and reception of form-based messages, simply let fldigi know where the flmsg executable file is located on your system. To do this, click on fldigi’s Configure|Modems|Misc|NBEMS to get to the configuration tab so you can locate flmsg. Also make sure the “NBEMS data file interface|Enable” and the “Reception of flmsg file|Open with flmsg” checkboxes are checked. Then click Save, then Close. Now when you receive an NBEMS wrapped message, it will automatically open in flmsg.

If you wish to compose and then send an NBEMS message, simply run flmsg, select and fill out the desired form, then click the AUTOSEND button and it will be sent via fldigi (which is already running) using the currently selected mode. I hope you selected MT63-1000.

One important software configuration for MT63 has to do with the accuracy of your sound card’s sample rate. You should calibrate your sound card against the one-second tics from WWV. If you don’t have HF receive capability you can calibrate with the help of another station that has been calibrated against WWV. Fldigi includes a special mode for receiving and sending calibration signals. Follow the documentation on this one. Being very busy (and lazy) I haven’t yet done this on my newest sound card interface (RigExpert Std), but MT63 seems to be happy anyway.



VHF/UHF is line-of-sight communications and will get you to the local repeaters. HF is generally for long-haul world-wide communications. So how do you communicate with someone just over the mountain range, or in that 50 to 300 mile dead zone known as the skip zone? That’s where NVIS or Near Vertical Incidence Skywave propagation comes in.

Unlike normal HF propagation that is either in the form of ground-wave (like your local AM radio station) or skywave (bouncing off the ionosphere and landing hundreds or thousands of miles away), NVIS uses a  unique property of the 1.8 to 15 MHz range of HF. These frequencies can be shot almost straight up and will bounce off the ionosphere and come almost straight down again. The analogy is if someone is hiding behind the living room couch, the only way you can hit him with the garden hose is to spray the ceiling. Don’t try this at home.

Simply put, NVIS allows communications in the range of 50 to 300 miles radius, even over mountain ranges. What it takes is the right frequency and the right antenna. The lower frequencies (around 3.5 MHz) are used at night while the higher ones (around 7 MHz) work best during the day. The right antenna is one that radiates much of your signal almost straight up. Good choices are the horizontal dipole mounted around 30 feet high or lower, the inverted-V and the horizontal loop or “loop skywire” as its called. Some of us hams, myself included, inadvertently put up NVIS antennas because the trees our dipoles are hanging from are a little too short!


In an emergency or loss-of-infrastructure situation, reliable communications are an imperative. Digital modes, especially MT63 provide robust message communication under rough conditions. In the aftermath of a disaster our survival could depend on reliable regional communications. NVIS techniques fill the gap between short-range VHF/UHF line-of-sight and global HF skywave communications. Digital modes are fast, fun and reliable. Practice emergency communications techniques now so you’ll have them the next time things fall apart. Getting important messages through will help save lives and property. Remember, “when all else fails, amateur radio works! “

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