Further tests have been conducted and observations made of normal on-air traffic.
The question that arises is why everything else I have tried except
OT-USB and OT3 devices reliably decodes the Foxtrak (DK7IN, aprstracker 0.11, TT1), and the OT-USB and OT3 devices reliably
decode everthing else I have tried (including TT3+) except the Foxtrak (DK7IN, aprstracker 0.11, TT1).
In all tests, transmitters were adjusted for 3kHz devn at 2200Hz (well below limiter threshold), and pre-emphasis checked to be within 2dB, my own receive tests were always with pre-emphasis (error < 2dB) except for the T-135 which I measured no pre-emphasis in tx and I suspect no de-emphasis on rx. I have no knowledge of how network OTx devices were configured.
Notably in the mix of everything I have tried is a TT3+ which is the
same hardware as the Foxtrak, same parts in the waveform generation, but different firmware (which generates the waveform quite differently to the former).
So there is clearly an interoperability problem, but I would not point a finger at the OT devices or Foxtrak, as the Foxtrak may or may not be out of Bell202 spec, and there is observable jitter on the 2200Hz tone generated, but a bunch of modem chips in TNCs I have (VK6 Flash TNC (AM7910), TNC-X (MX-615), Tiny2Mk2 (TCM3105), MFJ-1270B (XR2206)) decode FoxTrak reliably.
I think that what emerges from measurements and observations so far is that if you want to install infrastructure such as a digi or even an iGate and you want to decode Foxtrak and TT1, some OT devices have a problem.
That the TT3+ works better is an indication that more can be done with the hardware than the three types of firmware I tried in the FoxTrak manage. If you were biased, you might cite that as proof that problems are entirely in the FoxTrak, but on balance, all the other modem chip style TNCs tolerate FoxTrak/TT1 and that is to their advantage.
So, if you want a tracker that works with more receivers, avoid Foxtrak with DK7IN, aprstracker 0.11, or TT1 firmware, or TT1 itself.