Digital Operating Modes


In the meantime  computer and the internet found  the way into the ham shack. 
Therefore  digital operating modes becomes more and more popular beside the well known
modes FM, AM, SSB and  CW.

One advantage for the digital  modes versus FM, SSB and AM is the small bandwidth and therefore the insensitive against QRM and QRN.
Due to low bandwidth and decoding possibilities robust connections are possible with low transmitting
power.
If your transceiver has a small CW filter it reduce even more the QRM and the readability of the signals.
Some weak digital signals are not audible out of the noise but the programs decode it nearly perfect. 
PSK31, OLIVIA  are some of this modes.

With a soundcard, interface and a program it is very easy be a part of the digital operating communities.  DX is possible even with antenna and transmitting power restrictions.

 

For the IARU Region 1 digital transmissions should be in the following band segments.
The permissible bandwidth are max. 500 Hz
 

10 Meter Band: 28.070 - 28.190 MHz

20 Meter Band: 14.070 - 14.099 MHz

12 Meter Band: 24.915 - 24.929 MHz

30 Meter Band: 10.140 - 10.150 MHz

15 Meter Band: 21.070 - 21.110 MHz
                         21.120 - 21.149 MHz

40 Meter Band: 7.040 - 7.060 MHz

17 Meter Band: 18.095 - 18.109 MHz

80 Meter Band: 3.580 - 3.600 MHz

 

Each mode has its own unique characteristics advantages and disadvantages.
You will find here some short descriptions of the most popular digital modes with sound
samples (wav-Files).
 

  AMTOR
is a specialized form of RTTY. The term is an acronym for Amateur Teleprinting Over Radio and is derived from the commercial SITOR system (Simplex Telex Over radio) developed primarily for Maritime use in the 1970s.
AMTOR improves on RTTY by incorporating a simple Error Detection technique.

It is an FSK mode that has been fading into history. While a robust mode, it only has 5 bits (as did its predecessor RTTY) and can not transfer extended ASCII or any binary data. With a set operating rate of 100 baud, it does not effectively compete with the speed and error correction of more modern ARQ modes.

The system remains relatively uncomplicated but AMTOR performs well even in poor HF conditions. While there can still be many errors in AMTOR data, the Error Detection helps a lot and the result is quite tolerable for normal text mode conversations because of the high redundancy in plain language text. Certainly much better than RTTY. But for more critical data such as program code, or even some technical information messages, NO errors can be tolerated. There are two modes used in Amtor: The non-ARQ version of this mode is known as FEC, and known as SITOR-B by the Marine Information services.

   RTTY
or "Radio Teletype" is a FSK mode that is older than any other digital mode (except for Morse code). With a five-bit code all the letters, numbers, punctuation and control characters are covered.
At 45 baud (typically for amateur radio) and a tone shift of 170 Hz each bit is 1/45.45 seconds long, or 22 ms and corresponds to a typing speed of 60 WPM.
There is no error correction provided in RTTY; noise and interference can have a seriously detrimental effect.  But beside this disadvantages RTTY is very popular.
Bandwidth is 170 Hz and fits perfectly in a 250 Hz CW-Filter.

  PSK31
combines the advantages of a simple variable length text code with a narrow bandwidth
(about 70 Hz)  phase-shift keying (PSK) signal using DSP techniques. 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.  There are some other PSK modes available, but the most popular is PSK31.

OLIVIA
was developed  for weak signal QSO. For this reason the MFSK (Multi-Shift Frequency Keying) modulation was chosen, as it is a good FEC code in itself and its waveform has an almost constant envelope, so that the radio transmitter can work at its maximum power. As well MFSK passes well through the ionosphere made distortions.

The default settings for the “Olivia” mode are to send 32 tones spaced by 31.25 Hz at the rate of 31.25 baud. This results in 1000 Hz of total bandwidth. However, the user can chose to send 2, 4, 8, 16, 32, 64, 128 or 256 tones and the total bandwidth can be set to 125, 250, 500, 1000 or 2000 Hz. The correct baud rate is calculated according to the number of tones and the bandwidth.

The overall arrangement results in 5 characters being sent every 2 seconds, thus the typing speed is 2.5 characters per second for the default settings. This corresponds to about 15 words per minute (WPM). For the signal to noise performance, the simulation shows that the transmission can be still decoded when the signal is 10 dB below the noise, where the noise power is measured within the 1000 Hz bandwidth.


  HELLSCHREIBER
is a method of sending and receiving text using facsimile technology. This mode has been around along time.  The single-tone 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). Text characters are "painted" on the screen, as apposed to being decoded and printed.

Feld-HELL Club  web page



  MT63
is also new digital mode. It is accomplished by a complex scheme to encode text in a matrix of 64 tones over time and frequency.
This overkill method provides a "cushion" of error correction at the receiving end while still providing a 100 WPM rate. The wide bandwidth (1Khz for the standard method) makes this mode less desirable on crowded ham bands.


 THROB
is  another MFSK (Multiple Frequency Shift Keying) sound card mode that attempts to use Fast Fourier Transform technology  to decode a 5 tone signal. The THROB program is an attempt to push DSP into the area where other methods fail because of sensitivity or propagation difficulties and at the same time work at a reasonable speed. The text speed is slower than other modes.

 
  MFSK16
 is an advancement to the THROB mode and encodes 16 tones. Continuous Forward Error Correction (FEC) sends all data twice with an interleaving technique to reduce errors from impulse noise and static crashes. A new improved Varicode is used to increase the efficiency of sending extended ASCII characters.
 The relatively wide bandwidth (316 Hz) for this mode allows faster baud rates (typing is about 42 WPM) and greater immunity to multi path phase shift. This mode is becoming a standard for reliable keyboard to keyboard operation.

  CLOVER
is a PSK mode which provides a full duplex simulation. It is well suited for HF operation (especially under good conditions), however, there are differences between CLOVER modems. The original modem was named CLOVER-I, the latest DSP based modem is named CLOVER-II. Clovers key characteristics are band-width efficiency with high error-corrected data rates. Clover adapts to conditions by constantly monitoring the received signal. Based on this monitoring, Clover determines the best modulation scheme to use.

  SSTV
Slow Scan Television allows the user to send fixed pictures, in colors or in grey scales, with a transmission band similar to the one of HF Fax . Different SSTV modes exist.
SSTV is, as Fax mode, an analog mode, not a digital mode.
 

  PACTOR
is an FSK mode and is a standard on modern TNCs. It is designed with a combination of packet and Amtor Techniques. It is the most popular ARQ digital mode on amateur HF today. This mode is a major advancement over AMTOR, with its 200 baud operating rate, Huffman compression technique and true binary data transfer capability.

  G-TOR
(Golay -TOR) is an FSK mode that offers a fast transfer rate compared to Pactor. It incorporates a data inter-leaving system that assists in minimizing the effects of atmospheric noise and has the ability to fix garbled data. G-Tor tries to perform all transmissions at 300 baud but drops to 200 baud if difficulties are encountered and finally to 100 baud.