Below is a list of changes I have made in the HDSSTV signal format. I don't yet have a demodulator-decoder program that takes these changes into account. The version that incorporates these changes is what I intend for people to use. Once the demodulator-decoder program that incorporates these changes is done, I will rename this mode, which uses 4 of the 8 subcarriers for redundancy, as listed in the table below.
Name Outer Code # of Redundancy Symbols per Block
----------- ------------ ---------------------------------
Wyman 11 (306,274) 32
Wyman 12 (306,242) 64
Wyman 13 (306,178) 128
Wyman 14 (306,92) 214
Some of these modes may not be used very much, if at all.
Modulation of the middle 4 subcarriers during the leader and trailer portions of the signal is intended to allow the unambiguous determination of a much larger tuning offset than can be done with the evenly spaced unmodulated subcarriers in the leader and trailer.
Shortening the symbol period and keeping the same spacing between subcarriers reduces the size of the "gap" between subcarriers. This "gap" is expected to be used to remove Doppler shift in the future.
The plot below shows the transmission time, in seconds, vs. file size, in bytes for the 4 modes listed above. The colors code used is:
The 40% redundancy mode used for the Australia to United States tests is slightly slower that the Wyman 13 mode, which transfers (after error correction) about 4,000 user bytes per minute.
For the range of file sizes shown below, the Wyman 11 mode is not very much faster than the Wyman 12 mode, thus, I would not expect the Wyman 11 mode to be used very much, since the Wyman 12 mode provides twice as many redundancy symbols in the outer code, and doesn't take very much longer.
Below is a plot of transmission times, in seconds, vs. file length, in bytes for the various cases using an (8,6) inner code. This code uses 2 of the 8 subcarriers for redundancy. The 20% redundancy case of this mode has been used to successfully transfer files from Colorado to Indiana, using 20 meter SSB transmissions.
The color code used in the plot below is as follows:
The 20% redundancy (in the outer code) case of this mode transfers (after error correction) about 11,000 bytes per minute.
The stair step nature of these plots is due to always transmitting an integer number of codeblocks. The minimum file size for a given transmission time is one that has only one information byte in the last codeblock. The maximum file size for a given transmission time is one that exactly fills all the information symbol locations in the last codeblock.
Below is a plot showing the user data rate, in bits per second, for various digital communication modes. From left to right, the various modes are:
SSTV vertical interval signal, this conveys the SSTV mode of the following transmission
MFSK16 using a 0.5 rate code
MFSK16 using a 0.75 rate code
PSK31 with no error correction
HDSSTV (306,178) outer code and (8,4) inner code. This is the code used for the Australia to United States tests.
this will be renamed Wyman 13
HDSSTV (306,242) outer code and (8,4) inner code. This was used for the transmissions through FO29 between W9NTP and W8ZCF.
this will be renamed Wyman 12
HDSSTV (358,214) outer code and (8,6) inner code. This was used between W0LMD in Colorado, and KB4YZ in Indiana over 20 meters.
HDSSTV (358,286) outer code and (8,6) inner code.
A single level (270,214) Reed-Solomon code. This mode uses 5 subcarriers, with 11 different phase change possibilities for each subcarrier. A 2000 Hz bandwidth version of this mode did successfully transfer one test image from W0LMD to KB4YZ over 20 meters during a test on 29 Nov. 2000. This mode is not suitable for use over HF channels. I plan to modify it to be useful over terrestrial VHF, and above, channels.
Below is a plot showing the bandwidth_normalized user data rates for the same digital modes listed above. The numbers for this plot were generated by dividing the data rate, in bits per second, from the above plot, by the bandwidth, in Hz, used by the various modes.
The 114 Hz bandwidth for PSK31, was arrived at by measuring the -30db bandwidth of a sample PSK31 signal from the internet.
Note that two of these, the SSTV VIS, and PSK31, modes do not have any error correction coding.
As shown below, it takes a Pentium running at 166 Mhz longer to generate the HDSSTV .wav file than it takes to make the actual HDSSTV transmission. It takes even longer to process a HDSSTV .wav file than it does to generate one. The receive side processing time will vary with the number of errors that have to be corrected. The greater the number of errors to be corrected, the longer it takes to produce a completely corrected file.
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