What Are Those Pesky Modem Sounds on HF I Hear?


Hello everybody and welcome back to Everything Ham Radio! Over the past month, we have talked about the four major digital voice modes, both for amateur radio and for non amateur radio. This month we are going to be talking about the more traditional digital modes of amateur radio. In this post we will be talking about all the major HF digital modes. These topics will be covering things like RTTY, PSK31, HF Packet, and several others, including some that I have never heard of before doing research for this post.

One of the things that I find amusing, to a certain extent, is that in all my about 20 years in this hobby, and wife being a “computer person”, is that the only digital mode that I have used personally is VHF Packet. It is kind of sad really. I love computers! Before I had kids,  I was pretty much on them any time that I was awake. I have an associate degree in computers, I had several jobs where I worked in the computer field. Even after I got out of the computer field, I still worked on them because what I do now in 911 dispatching, relies heavily on computers. Of course, now that I have a wife and kids, my computer time basically consists of a few random hours at home, normally while she is watching her shows and when I am at work.


RTTY or RadioTeletype uses Frequency Shift Keying(FSK) to operate. Your radio will shift back and forth between two frequencies 170 Hz apart to communicate the different characters being sent. The higher frequency is known as the Mark frequency and the lower frequency is known as the Space frequency. RTTY uses the Baudot code which has five data bits plus a start and stop bit for each character.

This gives you a total character amount total to 32, but seeing how there is 26 letters in the alphabet, there wouldn’t be enough characters to include the numbers. So what Mr. Baudot did was include a Letter Shift and Figure Shift character built into the code. When sending the code for the number 3, it is the same as the letter E but with a Figure Shift character included before the letter code.

Further Reading


So what is AMTOR? AMTOR stands for Amateur Teleprinting Over Radio. It is one of the oldest of digital amateur modes and is basically a specialized form of an older format RTTY which we will talk about in a little bit. Amtor basically takes RTTY and gives it built-in error correction. AMTOR only transmit at about 100 baud.

Further Reading:


Pactor is the next step in the evolution of HF Digital radio. It is also the standard now built into modern multi-mode terminal node controllers(TNC). Pactor is a combination between packet and Amtor. It has double the baud rate of Amtor at 200 baud and is mainly used for sending a receiving emails over the radio. Since Pactor’s conception, Pactor II and Pactor III has been developed and is much more robust and faster than Pactor is. There is even a Pactor IV, but it is not available in the US yet.

Further Reading:


Next in the TOR family is the G-TOR system. G-TOR is only available in Kantronics TNC’s therefore it isn’t as widely used and didn’t really take off like the other modes did. It does, however, have error correction and automatic repeat requesting built into it. It also can be used up to 300 baud, 100 baud over Pactor. It has a feature where it starts at 300 baud, and if it gets some kind of interference or something, it will drop down to 200 baud and then 100 baud. It also has a built into data inter-leaving system that assists in minimizing the effects of atmospheric noise and has the ability to fix garbles data.

Further Reading:


CLOVER is a digital communications mode that conveys 8-bit digital data over narrow-band high-frequency radio. It can also transfer ASCII text and executable computer files without using the additional control characters required in other digital modes, which decrease throughput. It measures signal conditions, and automatically changes modulation format and data throughput to match current link quality. Reed-Solomon data encoding provides forward error correction (FEC) within each data block to repair many errors without the need for retransmission.

Further Reading:


PSK31 is the first digital mode to find popularity on HF bands in a while. PSK31 is a digital communications mode which is intended for live keyboard-to-keyboard conversations, similar to radioteletype. Its data rate is 31.25 bauds (about 50 word-per-minute), and its narrow bandwidth (approximately 60 Hz at -26 dB) reduces its susceptibility to noise. PSK31 enjoys great popularity on the HF bands today and is presently the standard for live keyboard communications. Most of the ASCII characters are supported. A second version having four (quad) phase shifts (QPSK) is available that provides Forward Error Correction (FEC) at the cost of reduced Signal to Noise ratio. Since PSK31 was one of the first new digital sound card modes to be developed and introduced, there are numerous programs available that support this mode – most of the programs available as “freeware”.

Further Reading:


HF Packet, is much like its VHF/UHF counterpart. HF Packet isn’t as reliable as other forms of HF digital communications but it does have the functions of VHF/UHF Packet in that maintain the same protocols and has the ability to use several stations on the same frequency as “nodes”. It only has a baud rate of 300 baud, due to the noise level of HF, unlike the 1200 or 9600 baud that can be used on VHF and UHF. It is mainly used to fill in the gaps where VHF/UHF packet can not be used. Both HF and VHF/UHF packet has seen a revitalization effect since it is the protocol that is used by APRS.

Further Reading:


Olivia was developed by Pawel Jalocha and is a ham radio digital mode designed to work in difficult (low s/n ratios plus multipath propagation) conditions on HF bands. The signal can be decoded even when it is 10-14 db below the noise floor (i.e. when the amplitude of the noise is slightly over 3 times that of the signal). It can also decode well under other noise, QSB, QRM, flutter caused by polar path propagation and even auroral conditions. Currently the only other digital modes that match or exceed Olivia in sensitivity are some of the WSJT program modes that include JT65A and JT65-HF which are certainly limited in usage and definitely not true conversation capable.

The standard Olivia formats (bandwidth/tones) are 125/4, 250/8, 500/16, 1000/32, and 2000/64. However the most commonly used formats in order of use are 500/16, 500/8, 1000/32, 250/8, and 1000/16. This can cause some confusion and problems with so many formats and so many other digital modes. After getting used to the sound and look of Olivia in the waterfall, though, it becomes easier to identify the format when you encounter it. About 90% of all current Olivia activity on the air is one of the 2 formats : 500/16 and 1000/32.

Further Reading:


DominoEX is a digital mode using MFSK (Multi-Frequency Shift Keying), used to send data (for example, hand-typed text) by radio. MFSK sends data using many different tones, sent one at a time. Each tone element (‘symbol’) can carry several bits of data. Most other digital modes uses each tone to represent only one bit. Thus the symbol rate is much lower for the same data rate when MFSK is used. This is beneficial, since it leads to high sensitivity with good data rate and modest bandwidth. More importantly, low symbol rates are less effected by multi-path reception timing effects.

DominoEX uses a series of new techniques to counter the general limitations of MFSK. To avoid tuning problems, IFK (Incremental Frequency Keying) is used, where the data is represented not by the frequency of each tone, but by the frequency difference between one tone and the next, an equivalent idea to differential PSK. An additional technique, called Offset Incremental Keying (IFK+) is used to manage the tone sequence in order to counter inter-symbol interference caused by multi-path reception. This gives the mode a great improvement in robustness.

Further Reading:



These are just some of the digital modes that are out there. I found several pages out there that talk about the different modes. Some with very extensive information and some with just a little bit. I hope that you found this article interesting and that maybe you might just try one of these modes. Like I said earlier, some of these modes I had heard of before but others I had no idea what they are and am going to have to do Further Reading myself, and maybe even try some of them out when I get my home station back up here in a few months. In the next post we will be talking about Packet radio on VHF/UHF. This post will be a companion post to the next episode in my podcast so, check out both of them on Tuesday.

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If you haven’t yet checked out my podcast, I would like to invite you to do so. I have decided to move it to a weekly podcast now rather than a bi-weekly one. In the first four episodes, we have covered the four major digital voice modes both in amateur radio and out, so please check them out. If you like what you hear, please head on over to ITunes and give me a star rating and a honest review. It really helps out to spread the word and helps me in the ITunes ratings.

Until next time…

73 de Curtis, K5CLM

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