Digital Modes (digimodes) with examples - G7SYW's amateur radio bits & bobs...
 

Digital Modes (digimodes) with examples

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Digital Modes

In the world of radio communications there are many varied methods of using digital signals, known as digimodes. In it’s simplest form, a digital mode is simply a digital signal transmitted over the air.

It must be noted that almost every digital mode is far more involved than this simple example. For communication to take place reliably, methods of frequency control, error correction and signal stability must be present or used.

Often argued as the earliest form of digital mode is Morse Code or CW as it is also known. In morse code an audible tone is either on or off. This equates to the digital (binary) states of 1 or 0.

Digimodes are becoming more and more popular on the amateur bands due to the widespread use of cheap computers within the amateur radio shack and also the low power levels needed to use these modes.

Digital modes are ideal for operators that have little or no room to erect large antennas as it is quite often possible to make contact with another digital user where a voice SSB/AM/FM contact would not.

A list of commonly used frequencies for the various digimodes can be found on my downloads page.


Examples of different digimodes

Jump to a specific section:

Morse Code

Morse code is named after Samuel F. B. Morse, inventor of the telegraph which used this system.

Morse is a very robust signalling system, it is possible for a skilled operator to decode the signal where most other communication modes would fail.

Letters and numbers are formed by dots and dashes created by turning an audio tone on and off in specific patterns. Download a copy of Morse code here – 120Kb PDF file.

Modern digital communication is simply sending data bits consisting of 1’s or 0’s, in other words, on or off as in CW.

CW Sample


Contestia

Contestia is a digital mode directly derived from Olivia but not as robust. It is a compromise between speed and performance.

Contestia has 40 formats which vary in bandwidth (125,250,500,1000, and 2000hz) and also the number of tones used (2,4,8,16,32,64,128, or 256).

The most commonly used formats are 250/8, 500/16, and 1000/32.

Here is a sample of Contestia 250-4


Domino / DominoEX

DominoEX is a digital mode using MFSK (Multi-Frequency Shift Keying), used to send data by
radio. MFSK sends the data by using different tones each sent one at a time. Each tone can carry several bits of data.

DominoEX uses new techniques to overcome the general limitations of MFSK in which the data is represented not by the frequency of each tone, but instead by the difference in frequency between sequential tones.

There are a few variations of the DominoEx protocol.

Listen to a sample of DominoEx4


Hellschreiber

Hellschreiber is a method of sending and receiving text using fax style technology. This mode has been around since before World War II. The most popular version (Feld-Hell) is the method of choice for HF operation.

It is an on-off keyed system with 122.5 dots/second, or about a 35 WPM text rate, with a narrow bandwidth (about 75 Hz).

As with telephone based fax machines the received text appears on the screen looking like a “rough” photocopy.


FT8

FT8 is the latest digital mode in the amateur radio operators arsenal of modes.

The mode employs an 8 frequency shift keying format devised by Steven Franke and Joe Taylor, hence the name of FT8. Tones are spaced at 6.25 Hz, and the signal occupies just 50 Hz. Unlike JT65 or JT9, transmit and receive cycles in FT8 each last about 15 seconds.

The software used for this mode can now take over control of the transmission sequence. If you send a CQ call and the receiving station replies to you the software takes over control and handles the rest of the contact for you automatically.

Likewise, when you answer a CQ call, if the calling station responds to your answer, the rest of the contact is handled by the software. All you need to do is initiate the contact and save it to the log when finished.

While it is not as reliable at weak signal levels as JT65, it is a much faster and almost automatic method of digital communication.

Like JT65 it requires accurate time synchronisation between the transmitting and receiving stations.


FT8 using WSJT-X

The software required to use this digital format is WSJT-X by Joe Taylor, and currently available for Windows, MAC and Linux from the WSJT website. Source code is also available to download.


JT65

In this mode, 65 tones spread out over 175Hz are sent and differs from other digital modes in that is
not a transmit-at-will or send-whatever-you-want mode.

It is highly structured and because the maximum number of characters you can send in free-form is 13 (that’s right, thirteen) messages sent contain just the necessary relevant information for the contact.

The sequence of a contact also depends on the computer clock being very accurate, just 2
seconds out is barely acceptable. 1 second out is usually OK but in practice it should be less than 0.5 seconds out.

JT65 is done in 60 second segments broken down into a 48 second transmission followed by 12 seconds of silence. During the 12 seconds of silence, the receiving station’s software uses this period to decode the received signals. The software is capable of decoding multiple signals received at the same time.


MFSK

This mode is an advancement to the THROB mode and encodes 16 tones.

Continuous Forward Error Correction (FEC) sends all data twice USING technique to reduce errors from impulse and static noise.

MFSK is very good under poor radio conditions.


MT63

This mode is a DSP based mode for sending keyboard text over paths that experience fading and
interference from other signals.

MT63 is not commonly used by amateurs because of its wide bandwidth and the difficulty in tuning in an MT63 transmission.

MT63 being very robust allows almost 100% certain copy when most other modes would fail.


Olivia

Olivia was developed to be a digital mode designed to work in difficult conditions on the noisy
HF bands.

The signal can be decoded even when it is only just above the noise.

Olivia has many different formats all of which can sound the same to the inexperienced operator.

The two most common formats for Olivia are 500/16 and 1000/32 – where the first number denotes the audio
bandwidth in Hz and the second number is the number of tones used.

Here is a sample of 4-250 mode olivia


Packet

Packet operates at 1200 baud and is used mainly on the VHF bands using FM.

This provides the reduced noise levels that seriously degrade the signal to such a point that communication would be impossible.

The most common use for packet is for APRS – Automatic Position Reporting System mostly on 2m (144.800MHz) VHF.

This system simply sends short bursts of data giving information on the sending stations identity, location, heading, speed and any extra short length of information the operator may wish to add.

Much like the old dial-up BBS services that were popular a few years ago, this has got left behind from modern technology. That’s not to say it’s no longer used.


PSK

PSK31 was one of the first digital sound card modes to be developed and introduced.
This mode is designed for “real time” keyboard operation and at a 31 baud rate is only fast
enough to keep up with the typical amateur typist.

PSK31 enjoys great popularity on the HF bands today and is presently the standard for live keyboard communications.

Since it’s introduction the baud rate has been increased to add a further two sub-modes of PSK63 and
PSK125, having baud rates of 63 and 125 respectively.

PSK31 Sample

PSK63 Sample

PSK125 Sample


RTTY

RTTY is a very simple technique which uses a five-bit code to represent all the letters of
the alphabet, the numbers, some punctuation and some control characters.

The 5 data bits allow for only 32 different codes, which cannot accommodate the 26 letters, 10 figures, space, a few punctuation marks and the required control codes, such as carriage return, new line, bell, etc.

To overcome this limitation, the teleprinter has two states, the unshifted or letters state and the shifted or numbers or figures state.

The change from one state to the other takes place when the special control codes LETTERS and FIGURES are sent from the keyboard or received from the line.

In the letters state the teleprinter prints the letters and space while in the shifted state it prints the numerals and punctuation marks.

There is no form of error correction provided in RTTY thus noise and interference can make communication
difficult.

Lots of information for RTTY can be found on RTTY.com


SSTV

Although called Slow Scan TV, it’s not TV in the normal sense of the term.

SSTV is simply the sending and reception of “still” images or photographs etc.

Originally photographs etc were converted to audio frequency signals for transmission, of course, with the PC now being common and powerful enough to manipulate images the majority of images are now computer
generated.

As with other digimodes SSTV has various formats, the most common being Martin and Scottie.

The images are transmitted line by line over a period of around minute which goes to explain why a single image rather than motion pictures are sent.

Of course, technology and modes are advancing all the time with this analogue style of SSTV slowly making way for digital versions.


Throb

THROB is based on tone pairs with several characters being represented by single tones.

As with many of the other digital modes, there are various variations of Throb.

Throb is quite a slow mode and is therefore probably quite resilient to the effects of signal fading
and noise etc.

The downside is that it takes quite a time to complete a transmission.


Thor

A single carrier of constant amplitude is stepped between 18 tone frequencies in a constant
phase manner.

Thor, like other similar modes has a variety of speeds and tones to choose from, dependent on band conditions and signal levels.

The modes are Thor 4, 5, 8, 11, 16 and 22.

Thor is a relatively new mode and is very closely related to DominoEx.


FSQ

FSQ is a Fast Simple QSO mode designed specifically for HF. This mode has been developed by Con Wassilieff ZL2AFP with the assistance of Murray Greenman ZL1BPU. This is possibly the very latest mode in use across the amateur bands, being originally released in it’s current form in 2015.

More in depth reading and history can be found here on the official web page

A sample of FSQ.



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