Chat-Mode Path Simulator Tests



I've been testing a variety of popular chat modes to see which performed best under various HF path conditions. The modes selected were those with reasonably fast typing speeds of 40 to 60 words-per-minute and bandwidths of 500 Hz or less.

That criteria did exclude modes like Olivia 16-500 and others that are much slower than 40 wmp. In order to include Olivia, I had to use the less robust 4-500 variant to achieve 40 wmp. Both PSK31 and RTTY-45 were included in the tests since they are certainly the most familiar chat modes and arguably the most popular.
The charts below show the results of the simulated HF path tests. Readability is based on the percentage of error-free characters copied. Good readability is achieved when the error-free copy is greater than 90% while marginal readability begins at 80 to 85%. Anything below 70% is pretty much unreadable. 

The first chart shows the results of the weak signal test. The path uses a low s/n ratio with an HF path model that simulates a quiet mid-latitude path. The result is a multi-path channel that produces sporadic fading that momentarily drops the signal below the modems sensitivity threshold.  

The next chart shows the results of a static crash test which includes the same quiet path model. Periodic noise bursts simulate the heavy static often found on the lower bands. The results should be of interest to those who use 40 and 80 meters. 

The last chart shows the performance of each mode while subjected to a highly disturbed trans-polar path. MFSK modes tend to perform well on such paths while PSK modes fail miserably. Disturbed polar paths impart random Doppler shifts to the signal as if being frequency-modulated. The phase distortion that occurs causes havoc with PSK mode reception. 

Hope the results of these path tests are useful. It would be interesting to hear from those who have experienced similar results on-air. I suspect low-band static-crash performance is of special interest to those who use NVIS for emergency communications.  

Tony -K2MO

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