WSQ
The result of these efforts is WSQ - a Weak Signal QSO mode for LF/MF. Like DominoEX and JASON, it uses Incremental Frequency Keying (IFK), making it moderately drift-proof and easy to tune. Unlike WSPR or THOR, it uses no error correction (DominoEX has already demonstrated clearly that error correction isn't necessary when using slow IFK), and while the baud rate is even slower than JASON, each symbol* carries much more information, taking the typing speed up to 5 WPM or better.
* SYMBOL: The smallest unique signalling entity which exists for a time in a digital transmission. (For example a Morse dot or dot-space).
A new sensitive waterfall display is used for tuning. Clearly, if you can't see to tune a signal, it makes contact fairly difficult. On the WSQ display you can easily see signals at -25dB SNR, making tuning reasonably straightforward, although some patience is required.
So what is WSQ
Uses phase coherent keying so you can transmit it using a typical LF/MF Class C, D or E (non-linear) amplifier without distortion.
WSQ uses 33 tones, spaced 1.953125Hz apart, resulting in a signal bandwidth of 64.4Hz, including the keying sidebands (bandwidth assessed according to ITU-R SM.1138). The modulation is constant amplitude, phase coherent MFSK with 2.048 second symbols (spacing 4/T), using IFK coding with 32 frequency differences. This means that each symbol carries enough information for all lower case letters to be expressed in just one symbol, which greatly enhances the speed.
WSQ is a decimal data system, so although 32 differences can be expressed exactly in five bits, this is actually not relevant. The way it works, there could indeed be any number of tones, which is a point of difference from most digital modes.
The next table shows, for several popular modes, the number of tones used and number of symbols required to express a letter of the alphabet:
Mode Tones Symbols per Letter
WSPR 4 Not applicable
JASON 16 2
DominoEX 19 1, 2 or 3
WSQ2 33 1 or 2
The symbol rate of WSQ is 0.512 baud, or about two seconds per symbol. We call this mode WSQ2. Synchronization is achieved through a voting process similar to JASON, which allows considerable tolerance of speed and sync timing. The unusual symbol rate helps simplify the receiving software achieve a low processor load (it will work on a 'Netbook').
Despite the very low symbol rate, the typing speed is remarkable - at least 5 WPM, and potentially up to 7 WPM if the user is cunning, making maximum use of lower case and 'CW-speak' abbreviations.
Varicode
Varicoding (where more commonly used characters are sent in fewer bits than those less often used) is an application of Huffman coding, a technique pioneered by Morse (well before Huffman described it!) and then used in PSK31 and DominoEX. WSQ achieves a very high coding efficiency because first of all there is more information per symbol; and secondly because it has available 28 single-symbol characters, which of course are used for the most frequently used letters, 'space', comma, and full-stop (period).
If you operate WSQ using standard Morse QSO protocol, using lower case text for callsigns, and use upper case sparingly for Q-codes and acronyms, you'll be able to operate at maximum efficiency. Here's an example:
ge om name hr Fred. ur rst 569. loc RF77ee. hw? VK7XYZ de ZL1ABC K
This complete over can be sent in just 88 symbols, and therefore takes under three minutes to send.
Comparison of Coding Efficiency
Any test message can be easily analysed to determine how many symbols are required to send the message. This information, combined with knowing the symbol rate, will allow prediction of the typing speed with reasonable accuracy. In this way we can compare the coding efficiencies and even the typing speeds of various modes, even when the symbol rates are different. Here are some examples:
Mode Text--> the quick brown fox jumps over the lazy dog Baud Duration WPM
WSQ2 1111111111111111111111111111111111111111111 = 43 symbols 0.488 88.11 6.1
Unusually, although WSQ2 has faster text than JASON (fast) mode, it actually has a slower symbol rate, which gives better sensitivity. It's also very obvious from an information theory point of view that Morse has a much higher symbol rate and therefore much less sensitivity that any of these other modes.
We can also use the same information to compare the number of symbols per word, the transmitted bandwidth and bandwidth efficiency (Hz used per WPM) for these modes:
Mode Symbols/word Bandwidth, Hz Hz/WPM
WSQ2 4.7 64.4 10.6
The LF and MF Amateur bands are characterized by relatively stable carrier phase on received signals, accompanied by low Doppler shift. These bands have very strong lightning interference, but brief and mostly local, unlike the background of random impulse noise encountered on lower HF. There can also be considerable man-made interference. While there is multi-path reception, especially on 160m, the path changes are slow. The slow fades can be very deep, and signals can be extremely weak, especially on 2200 and 630m, so in order to have a conversation at typing speed on these bands, we need a mode that is extremely sensitive, narrow band, has excellent impulse noise tolerance, but need not have strong phase change or Doppler tolerance. The way WSQ handles lightning noises is most impressive. This is due to the way that each symbol is received in a very narrow bandwidth (~0.5Hz) and integrated over about two seconds, much longer than the duration of a lightning burst.
Slow Tones
One good way to achieve robustness and sensitivity is to use very slow single-carrier signals, which can be integrated over a time frame which is much larger than the duration of interference bursts. With very slow signals the bandwidth can then be narrowed (in this case to 0.5Hz per carrier), further reducing the energy received from wide-band noise sources, and enhancing the sensitivity. Using a two-second 'integrate and dump' type signal detector, based on repeated Fast Fourier Transforms (FFTs), WSQ achieves excellent interference rejection, and the technique also gives incredible sensitivity. Tests have shown that the signal can be detected on the waterfall display at -30dB SNR in 2.4kHz bandwidth, and the software will print recognisable text from about -27dB, becoming 100% reliable at about -25dB. You won't actually hear the signal until it reaches about -10dB!
The WSQ2 Waterfall display with -25dB S/N signal
Any signal that you can receive with this software will also be seen clearly on the waterfall tuning display and tuned in easily. The picture above shows a real off-air signal at -25dB S/N. It is clearly visible on the spectrum display, standing out above the background band noise. The background noise is levelled automatically by AGC in the software, rather like ARGO. When tones are recognised, they can be 'tagged' with a yellow line to make them even more visible.
