Digital Modes

One area of amateur radio that has really grown by a significant amount since the introduction of personal computers is the use of digital modes. These digimodes as they are sometimes called provide a very interesting way in which to communicate over ham radio, offering different challenges than common to the more traditional modes of transmission.

The first of the digimodes was RTTY. Originally this used mechanical teletypes. Eventually computers running terminal software replaced the noisy mechanical devices.

With further development in computer hardware and software it is possible to utilize many advanced transmission techniques and as a result quite a variety of digimodes have been invented and used. Each one of these digimodes has its own advantages and different types tend to be used for different applications.

The communications modes used on the HF ham bands must handle problems not encountered in other environments. HF signal paths tend to be noisy and distorted in ways that make the digital modes used successfully in other environments fail badly. A common voice grade land-line telephone circuit that many people used to use computer modems to transfer data will typically support 56 Kbits per second data rate. A HF SSB voice circuit can reliably support only a small fraction of that.

The variety of HF digital communication modes vary so widely in operating characteristics that measurements such as baud rate or bits per second are nearly meaningless. When comparing the various digimodes using different modulation methods and character encoding schemes it is often described as Words-Per-Minute (WPM) which comes from the common way of measuring of morse code (CW) transmission speeds.

Ham digital communication breaks down into two categories: keyboard-to-keyboard real time conversational and file transfer. File transfer can consist of sending things such as digital image files or text messages.

The basic way digital modes work is the HF transceiver’s microphone and speaker audio lines are connected to a computer’s sound card. And the transceiver is operated in SSB voice mode. Software decodes the receiver audio and generates transmit audio.

The program shown above provides an example of a current keyboard-to-keyboard digital communications program user interface. The main sections of the window above, starting at the top, are the receive text area, the (blank) transmit text area, and the control area. The control area includes a tuning audio spectrum display and a waterfall display. The display above shows several PSK31 signals in the receiver pass band, each digital conversation shows up as a vertical line. Selecting one of the signals for decoding is done by simply moving the mouse cursor and clicking on the desired signal.

Overview of popular digimodes:

CW/Morse Code – Yes, computers can be used to send and receive CW. It works but receiving is not terribly reliable in the presence of noise, interference, or when signals are weak. (5 – 60 WPM, -10 dB S/N with experience operator)
Hellschreiber or HELL – Nope, not that hot place with the sulfurous odor. This is the nickname of a communications mode invented in 1929 by a German whose last name happened to be Hell. There are variations of this mode but the most commonly used is FELD HELL that used simple On/Off keying to form visible characters by marking dots on a paper tape. PC programs simulate the paper strip as bands on the monitor screen. The PC program does not attempt to decode the characters as they are received. It merely makes light and dark marks on the simulated paper strip. It is up to the operator to make sense of what is shown. It is very much like watching a grainy strip of fax paper on one of the early fax machines scrolling by on the screen. If you hear something like high speed CW that you can’t make sense of, it is probably FELD HELL. (25 WPM, -10 dB S/N).

Frequency Shift Keying (FSK) type of digimodes.
FSK overcomes the decoding problem of CW by transmitting a continuous carrier and shifting its frequency for indicating On/Off conditions, known as Mark/Space in FSK terms. This allows the decoder active redundant information to work with so is more reliably decoded.

RTTY – This is the granddaddy of ham digital modes and is still very popular and used in large contests. Current practice is to use 170 Hz shift of the RF signal frequency with the higher frequency being the Mark and lower Space. The most common data rate used is 45.45 Baud. RTTY is typically used at high power: 100 watts to 1500 watts. (67 WPM upper case only, -5 dB S/N)
AMTOR – This is the ham version of a commercial protocol based upon standard RTTY. It uses an automatic retransmission request protocol to achieve reliable communications. It uses 200 Hz shift at 100 baud. There is also a Forward Error Correction (FEC) mode. AMTOR is normally used with hardware modems. (0 – 67 WPM, -5 dB S/N)
PACKET – Designed by hams in 1978 through 1984. It is used extensively on VHF, especially for APRS. HF Packet runs 200 Hz shift at 300 baud. It performs poorly on HF but is still used for automated messaging services. It operates in a dedicated connection mode that allows continuous transmission repeats until a message is finally delivered. It also has an unconnected mode that is used for general broadcast transmissions as with APRS position reports. (330 WPM, +20 dB S/N)

Phase Shift Keying (PSK) types of digimodes
Instead of shifting the carrier frequency as in FSK, PSK shifts the carrier’s phase, typically by 90 or 180 degrees. PSK modulation has proven to perform quite well on the HF bands, providing acceptable copy under conditions that would be difficult for FSK.

PSK31 – The most popular keyboard-to-keyboard modes for new digital operators. PSK31 enjoys great popularity on the HF bands today and is presently the standard for live keyboard communications. PSK31 derives its name from the modulation format and rate used. The modulation used is phase shift keying (PSK) and it transmits data at a rate of 31.25 bits per second - the rate being chosen to enable the rate to be easily derived from the 8 kHz sampling used in many digital signal processors. The scheme is widely used on HF and is resilient to interference. It allows real-time "chat" style contacts to be made and in view of all its advantages it has gained widespread acceptance, especially for low power and DX. PSK31 uses 180-degree phase shifts at 31.25 baud with carefully controlled waveforms to produce a very narrow transmitted signal. It is a very efficient signal and occupies very little signal bandwidth and it is not unusual to see a dozen or more QSOs in progress in a 2 KHz chunk of radio spectrum. A variable length coding for characters is used with the most commonly used characters having shorter codes. Lower case letters are shorter than upper case letters. There are several variation with higher baud rates and some with FEC. (50 WPM lower case, 37 upper case, -11.5 dB S/N).
PSK63 has become more and more popular among hams because the data transfer is fast enough to have a more dynamic real time chat and is often used for those that are more interested in having a 'rag chew'. (60 WPM).
PACTOR –This is proprietary set of protocols that perform quite well but are protected from general implementation by patents. Dedicated TNC’s are needed for PACTOR operation. The current price is about $1300 for a Pactor III TNC. (Performance is very good but it should be for the price)

Multi-Frequency Shift Keying (MFSK)
MFSK differs from FSK in how it shifts the carrier frequency. FSK shifts between two tones. MFSK shifts between more than two, with 4 to 32 being common. The advantage of MFSK over simple FSK is that each frequency shift can signal more than one bit of information. As a simplistic MFSK example, consider a four tone scheme. No shift could indicate a sequence of two zeros, or ‘00’. The second tone could indicate zero followed by a one, or ‘01’, the third, ‘10’, and the fourth, ‘11’. Each shift would transmit two data bits. Our theoretical 4 tone MFSK signal could shift its tones at half the speed as a two tone FSK signal for the same transmitted data rate. This gives a decoder twice as long to detect each tone, improving its reliability by a factor of two or better. The current popular MFSK modes use sophisticated bit coding schemes to produce performance that is quite amazing in weak signal HF communications. The coding schemes are particularly good at ignoring lightning static. Most even manage to do a good job of ignoring non-MFSK signals such as CW on top of the MFSK signal.

MFSK16 – This mode has been in use for only about 4 years now. It was the first successful ham implementation of MFSK. It uses 16 tones, spaced 15.625 Hz, at 15.625 baud. It works quite well as a QRP mode, occupying only about 350 Hz bandwidth. There is a version, MFSK8, which operates at 7.8125 baud for better weak signal performance but at half the typing speed. (42 WPM, -13.5 dB S/N)
OLIVIA – This is a newer mode that expands upon concepts implemented in MFSK16. It has 40 sub-modes that are combinations of 4 to 32 tones and with bandwidth ranging from 250 Hz to 2 KHz. The most commonly used are 32 or 16 tones, 1 KHz wide for DX contacts. 8 or 16 tones, 500 Hz wide for domestic contacts. (Varies with number of tones and bandwidth: 20 WPM @ -14 dB S/N, 39 WPM @ -10 dB S/N)
DominoEX – This mode is still experimental but is becoming popular. Its claim to fame is that narrow, 140 Hz to 388 Hz, and that it uses a scheme that allows a wide tuning latitude of 60 Hz or more. That is a big plus as most modes require 3 or 4 Hz tuning accuracy. In its current form, it does not seem to handle QRN and QRM as well as OLIVIA. (Varies with speed, 27 WPM @ -14.5 dB S/N, 154 WPM @ -9 dB S/N)

JT65

FSTV and SSTV
Fast scan TV (FSTV) and slow scan TV (SSTV) are modes used by hams to send pictures or images over the radio. SSTV is generally used on the HF bands and can only send a single still picture at a time due to its low data rate and bandwidth. FSTV on the other hand is generally used on the UHF bands and can send a moving picture just like a traditional broadcast TV signal. Recently several HT manufacturers have produced handheld radios with built-in cameras and screens for use in this mode.

Digimode Examples

Here is a website that has good examples of what each of these types of digimodes sounds like on the air.
W1HKJ also has some good webpages that show examples and sounds of the various digimodes.
K2NCC also has videos of many popular digimode in use on his youtube channel: http://www.youtube.com/k2ncc

Popular Software

Here are some of the popular software used for running amateur radio digital mode transmissions.

Fldigi - Very Popular open source program that can do most all of the popular digital modes. Considered by some to be the most accurate and reliable digital signal decoder (free, Windows, Linux, Mac).
DM780 - Included as part of Ham Radio Deluxe suite, also very popular among the large group of hams using the HRD Suite. Uses the Fldigi decoding engine with a slightly more modern wrapped gui around it (free, Windows).
DigiPan - A very simple program strictly used for PSK. One of the first available and still popular. (free, Windows)
WinWarbler - Included as part of the DXLab suite of ham programs. (free, Windows)
WSJT - A very new program focused on weak signal digital modes such as JT65 (free, Windows, Linux).
WSPR - Growing in popularity, this program is focused on the WSPR(whisper) protocol which is designed to be used for very low power beaconing and the study of global radio propagation (free, Windows, Linux).
MultiMode - Commercial program which runs on Mac OSX and supports many different digimodes including.
iPad PSK31 - Psk31 app for the iPad (decode only)
iPhone I-Psk31 - App for iPhone that both decodes and sends PSK-31 by connecting the iPhone to the radio through the mic/headset jack.

While it may seem that there is very wide variety of digimodes that can be used, this comes from the fact that people are experimenting and trying to improve the effectiveness of digital modes of transmission as technology moves forward.

While there are many digimodes to choose from, the situation may not be as difficult as many may imagine because it is often possible to receive and send a large number of types of transmission using a single interface. Most digital ham software supports many different modes so it is quite easy to send and receive many different transmission modes.