This section is part of DSP-10 User's Manual, Chapter 2 - Weak Signal Operation
When one looks at a waterfall spectral display it is practical to copy very slow Morse code by looking where the trace is lighter. W7LHL has come up with a code that is based on this principle, but is faster for a given level of performance and lends itself to easier manual and automatic decoding. This code is of a fixed length and therefore does not adapt to the signal strengths as can be done with PUA43. But due to the arrangement of the frequencies it is much more robust to frequency errors. It likewise is less subject to disruption by airplane and rain scattering.
LHL-7 uses 7 FSK tones arranged as
End of Character 600 Hz
Dot 750 Hz
Dash 900 Hz
Times 2 1050 Hz
Times 3 1200 Hz
Times 4 1350 Hz
Times 5 1500 Hz
For instance, the letter 'E', a single dot, would be two tones, 750 and 600 Hz, one after the other. The number '7' dash-dash-dot-dot-dot would be five tone bursts, 900, 1050, 750, 1200, 600 Hz.
The lengths of the tones are preset by agreement between the sending and receiving ends and can be 2, 4, 10, 20 or 60 seconds per tone. The top of the minute is always a break between tones. Thus if the tones were 20 seconds each, they would start at 0, 20 and 40 seconds after the start of the minute. To gain the maximum advantage, the clocks at each end should be set to a small part of a second.
The demodulator is flexible in the frequency tolerance it will accept. The best performance comes from a frequency accuracy of a Hz or better, but automatic demodulation/decoding can be set to accept up to about 40 Hz of frequency error. Manual decoding can be more tolerant by watching the wavy lines!
Provision is made for adding an identification in Morse code. This is entered as a ampersand '&' and the preset ID phrase is sent at 500 Hz offset.
A Test Pattern consisting of each of the seven tones, in order of low to high, is sent if the plus sign '+' is entered. This is very useful for manual decoding.
DSP-10 SETTINGS FOR LHL-7 - Both ends of a path must agree on the base frequency, displayed in the Transmit Frequency box. The only SpecAnl width available is 4800 Hz, but the screen only displays half of this as experience has shown this works best for manual copy. The SpecAve values are adjustable to all values that are consistent with the length of the tones. The power should be adjusted to not overheat any following power amplifiers with the nearly 100% duty cycle.
The windowing function should be set to None (Non) or Tukey25 (Tuk) if a single center bin is being used for the signal. This implies that very good frequency accuracy is available on both ends. If multiple bins are assigned to the signal, the windowing function should be set to Hamming (Ham).
Clock Set - The software clock must be set for LHL-7. See the separate section on Clock Setting below.
EME Corrections - This is toggled on and off by the Alt-L or Alt-l key and enables the Doppler correction and the 2.6 receive timing offset. The path must be selected on the bottom line as "MOON:Local Stn, Other Stn..." by use of the Srcl-F3, '<', and '' commands. This requires that the latitude and longitude for both stations has been entered into the .CFG file,
ALT-B BOX FOR LHL-7 - This box is used to select options for the LHL-7 code. The details of the options are explained below, but the Alt-B Box entries are as follows:
1, 3, 5, 7 or 9 bins to be included
Peak or Average Levels
2, 4, 10, 20 or 60 seconds per tone
The ID string, such as "DE W7XYZ"
Single or 2 Line decoder output
The last item refers to the very top of the
upper spectral display
that is used for input and output of symbols for the LHL-7 transmission
and reception. The selection is either a single line of twice
high characters, or two lines of standard characters. Either way,
when the line() get full they will scroll to make more room.
ALT-A DATA BOX FOR LHL-7 - After you get to LHL-7 mode (Alt M) use Alt A to bring up the informational box on the right. This can be left open. It shows the operating parameters and the measured S+N/N.
The following items are shown in the box:
"Tag" is the seconds after the top of the minute that was attached to the last measurement. It is not the current time, as it takes about 3 seconds to process the data, and if the seconds per tone is greater than 2, the tone length must be added in.
"bigi" is the strongest of the 7 tones for the last measurement period. They are numbered 0 for 600 Hz, 1 for 750 Hz, ...
"mcbyte" is the sequence of tones that have been decoded. This is compared with the Morse code table. Any time it is found not possible to decode a character in the future, the code is reset. A reset code is a single 1, as this is a prefix character to know where the number begins. For instance, all characters must start with a dot or a dash, and we will not accept a x3 at the beginning. This process continues as the mcbyte is built up. If you watch it, the incorrect byte will show up and it will then disappear and be followed by either "1", "10", or "11". Due to the changing of tones, over the months, and my desire to not redo the character table, the mcbyte numbers are not quite the same as the bigi values. For mcbyte, 0=dot, 1=dash, 2=x2, 3=x3, 4=x4, 5=x5 and the EOC does not show up in mcbyte.
"Bins per tone" refers to the 9.375 Hz spaced filter bins. 1 means that only the bin at the center frequency will be examined. 3 means that the center and one on either side of it will be examined. This continues up to 9. If these are averaged, it is fully equivalent to having a wider FFT bin. There are guard bins on either side of the tone bins, since the windowing causes widening of the signal. Outside of the guard bins are four noise measurement bins that are averaged together to estimate the noise level. Thus the frequency bins for "3 bins per tone", for the 600 Hz tone would be:
Bin 60 562.5 Hz Noise
Bin 61 571.9 Hz Noise
Bin 62 581.2 Hz Guard
Bin 63 590.6 Hz Tone signal
Bin 64 600.0 Hz Tone Signal
Bin 65 609.4 Hz Tone Signal
Bin 66 618.7 Hz Guard
Bin 67 628.1 Hz Noise
Bin 68 637.5 Hz Noise
Bins 69 to 75 are not used and the 750 Hz related bins start at 76.
Higher numbers of "Bins per tone" use more of the in between bins until they are almost all used.
"Peak" and "Ave" refer to the way that the Tone signal bins are treated. Either the average of all of them is compared with the noise, or the largest power is. For wide band signals, like rain scatter, average would be best. For drifting signals, peak might be---these are areas that need further exploration.
OPERATION WITH LHL-7 CODE - This operation is quite similar to CW. Symbols to be sent are typed in and show on the very top line. Transmission is initiated by the Home key.
Reception is monitored in the Alt-A box and can
sometimes be seen
on the waterfall. Only those tone combinations that create a valid
symbol will produce an output on the display.