I created this page because I wanted a reference for what different digital modes looked like in the waterfall of Fldigi. While writing this page, I came across a similar set of pages on the Fldigi website, found here. On this page, in each snapshot, a short test sequence was transmitted and received locally.
M1GEO TESTING 12345 M1GEO
The configuration used two computers, one for sending and one for receiving. The images where then printscreened and cropped to the maximum 2 kHz bandwidth – set by the largest mode, Olivia 64/2000. Where transmission times were longer than Fldigi’s receive waterfall would allow, I aimed to capture the start of the transmission. Where the transmission fits, I tried to capture the transmission in the middle of the receive waterfall. Please accept that this took some considerable time.
Also note that there is some receive noise in the background. There’s nothing I can do about that. I’m sure it will be obvious which are noise and which are not. Click on any image to be given more detail. There is also a “full resolution” link on the image information page.
And with that, in the order that Fldigi presents them in the Op Mode menu, here we go:
Contestia is a family of MFSK modes with a high redundancy Forward Error Correction system similar to Olivia. The mode works well on poor HF paths and has good sensitivity. It is a mode designed to be used for contesting and keyboard to keyboard contacts. The typing speed is varied by changing the number of tones used. The default mode is 8-500. Designed by UT2UZ. More here.
DominoEX is a family of offset incremental multi-frequency shift keyed modes with low symbol rate. A single carrier of constant amplitude is stepped between 18 tone frequencies in a constant phase manner. As a result, no unwanted sidebands are generated, and no special amplifier linearity requirements are necessary.
The tones change according to an offset algorithm which ensures that no sequential tones are the same or adjacent in frequency, considerably enhancing the inter symbol interference resistance to multi-path and Doppler effects. The default calling mode is DominoEX11. More here.
The name Hellschreiber means ‘Hell’s writer’. The technique was developed by Rudolf Hell in 1927, and was used as a successful radio mode first in 1937, well before RTTY. Use by Amateurs is relatively recent. The most popular mode is that used by the war-time Feld-Hellschreiber (field writer), which is on-off keyed, the same as Morse. In order to limit the bandwidth, the rise-time of dots is usually limited to about 1ms, and the best methods use raised cosine shaped dots. More here.
FSK Hell 105
These multi-frequency shift keyed (MFSK) modes with low symbol rate use a single carrier of constant amplitude is stepped (between 16 or 32 tone frequencies respectively) in a constant phase manner. As a result, no unwanted sidebands are generated, and no special amplifier linearity requirements are necessary. The tones selected are set by the transmitted (4 or 5 bit) bit pattern and a gray-code table.
The mode has full-time Forward Error Correction, so it is very robust. Tuning must be very accurate, and the software will not tolerate differences between transmit and receive frequency. The mode was designed for long path HF DX, and due to its great sensitivity is one of the best for long distance QSOs and skeds. MFSK8 has improved sensitivity, but is very difficult to tune, and suffers more from Doppler. It is useful as the band fades out. MFSK16 and MFSK8 were developed by Murray ZL1BPU and Nino IZ8BLY. More here.
MT63 is an Orthogonal Frequency Division Multiplexed (OFDM) mode consisting of 64 parallel carriers each carrying part of the transmitted signal. The tones are differential BPSK modulated. MT63 employs a unique highly redundant Forward Error Correction system which contributes to its legendary robustness in the face of interference and fading.
The tones have synchronous symbols, and are raised cosine modulated. The mode requires a very linear transmitter. Over-driving leads to excessive bandwidth and poorer reception. The mode is unpopular with many operators because of the excessive bandwidth used. The mode is very tolerant of tuning, as most software will handle 100Hz mistuning. The mode was designed by Pawel SP9VRC. More here.
Olivia is a family of MFSK modes with a high redundancy Forward Error Correction system similar to MT63. The family is very large, with 40 or more different options, which can make it very difficult to work out which is which. The mode works well on poor HF paths and has good sensitivity, and is best used for fixed skeds. The most widely used versions have a symbol rate of 31.25 baud.
The typing speed is varied by changing the number of tones used, but can also be changed by change of baud rate. RTTYM and Contestia are variants of Olivia. The MFSK is constant phase, and the transmission constant amplitude, so transmitter linearity is inimportant. The modes are moderately tolerant of mis-tuning. The typing rate is very modest, given the high bandwidth used. The mode was designed by Pawel SP9VRC. More here.
Narrow band modes such as PSK31 are low symbol rate, single carrier differential Binary PSK (called 2-PSK or BPSK) or Quadrature PSK (4-PSK or QPSK). With digital phase modulation the phase changes abruptly, and without additional measures wide sidebands would be created. To prevent this, all these modes also include 100% raised-cosine amplitude modulation (ASK) at the symbol rate, which reduces the power to zero at the phase change.
Because of this amplitude modulation, the signal bandwidth is relatively narrow. Synchronization at the receiver is straightforward because it can be recovered from the amplitude information. Differential PSK is used to provide continuous phase changes when idle (to maintain sync), and by allowing the receiver to measure phase difference from symbol to symbol, to reduce the effects of ionospheric Doppler phase changes which modulate the signal. The slower modes are more affected by Doppler, and the QPSK modes are particularly affected.
With no interleaver and limited coding length, the QPSK mode Forward Error Correction coding gain is limited, and under burst noise conditions (HF) the performance is usually worse than the BPSK option at the same baud rate. In general the narrow-band BPSK modes work well on a quiet single-hop path, but give poor performance in most other conditions.
To counter this lack of robustness under adverse conditions, the PSKR (“R” for “robust”), series of modes has been developed. Using a similar design as the MFSK modes with a convolutional encoder and an interleaver these modes provide a much more robust link at the expense of the data speed which is divided by half when compared to the standard BPSK mode. Soft bits decoding was also added to maximize the probabilities of decoding the right sequence.
With these modes, a very linear transmitter is required. Over-driven operation results in excessive bandwidth, poorer reception and difficult tuning. However, the sensitivity is such that very little power is usually required.
RTTY has been used by radio amateurs since the 1950s. Initially an electromechanical system designed for use on telephone wires, it was not conceived as a radio system, and could not be used by radio until the development of the Ratio Detector during the 1939-1945 war. RTTY (the name means simply Radio Teletype) uses FSK to avoid noise on the transmission path, but requires high power and is still prone to propagation effects, especially selective fading and multi-path timing.
Early RTTY equipment used separate oscillators for each of the tones, and so could produce very wide key clicks, requiring extra filters. Modern software uses phase coherent switching between tones, which somewhat improves the signal bandwidth.
With no error correction, and a start-stop system that is prone to false starts on noise, RTTY is not the best mode for amateur use. However, it is easy to use, easy to tune, fast, tolerant of drift, and is widely used for contesting for these reasons alone. A linear transmitter is not required.
THOR is a family of offset incremental multi-frequency shift keyed modes with low symbol rate, closely related to DominoEX. A single carrier of constant amplitude is stepped between 18 tone frequencies in a constant phase manner. As a result, no unwanted sidebands are generated, and no special amplifier linearity requirements are necessary. The tones change according to an offset algorithm which ensures that no sequential tones are the same or adjacent in frequency, considerably enhancing the inter-symbol interference resistance to multi-path and Doppler effects.
The mode has full-time Forward Error Correction, so is extremely robust. The default speed (11 baud) was designed for NVIS conditions (80m at night), and other speeds suit weak signal LF, and high speed HF use. The use of incremental keying gives the mode complete immunity to transmitter-receiver frequency offset, drift and excellent rejection of propagation induced Doppler.
The THROB family of modes are MFSK in nature, but are unusual in that (like DTMF) they use two tones at a time. Unique among other MFSK modes, the THROB family also uses amplitude modulation and sometimes sends just one tone!
The mode has no Forward Error Correction, is difficult to tune, but reasonably sensitive and moderately robust. Because of the limited character set however, the typing speed is reasonably good, one character per symbol. Tuning must be very accurate, and the software will not tolerate differences between transmit and receive frequency. THROB is essentially a curiosity mode.
Another unusual feature (which led to the name), THROB also includes raised cosine AM modulation of each symbol. This combined with two tones transmitted at the same time, means that a very linear transmitter is required. For THROB, nine tones are used, spaced 8 or 16 Hz. For THROBX, 11 tones are used, spaced 7.8125 or 15.625 Hz. The THROB family was developed by Lionel G3PPT.
End of message. SK SK.