I am having difficulty identifying a modulation scheme. It appears to be a combination of pulse width modulation and pulse distance modulation. I have identified four different symbol combinations but would like to know if there is some specific format I can reference to reinforce my findings. The identified combinations are as follows:

Symbol 1: 2ms high, 2ms low
Symbol 2: 2ms high, 1.6ms low
Symbol 3: 1.6ms high, 2ms low
Symbol 4: 1.6ms high, 1.6ms low

 

Trigger a 1.8ms monostable on every signal edge.
At end of 1.8ms monostable pulse.
Compare signals conditions at end and start of 1.8ms pulse using a basic latch for memory or separate discriminators for rising and falling edges in which case the start was always high or low for that edge. You use an XOR gate to decode the information based on that comparison (or XNOR to invert signals information values).

If the signal interval was 1.6 ms you get a 1 output because the value changed, and if it was a 2ms you get a 0 output because the value is still the same. That is using the XOR gate.

It is a basic non return to zero modulation scheme. You get something over 500 baud from that signal line using 250 to 312.5 Hz because you use positive and negative edges for information.

That is a pretty low frequency.

 

I'm kind of following you. Can you provide a link to a more in-depth description with example(s)?