A few weeks ago I posted about a few problems I encountered using the Duplex MMDVM hotspot, one of which being, what appeared to be a 60 second timeout.
Following this discovery I contacted Andy CA6JAU who writes the MMDVM_HS firmware, and he was able to reproduce similar problems on his duplex hotspots, but with a timeout which was consistently over 60 secs.
Andy fairly quickly tracked down the problem, and let me know that …
The root of the problem is clock bit rate drift between radio and HS. Because this depends on TCXO small differences (radio and HS), I modified a little the code to minimize this effect. At least here works for all my duplex boards.
Andy released version 1.4.7 to resolve this issue, but he has indicated that this is not a perfect solution.
It just increases the time before the transmitting rig (e.g. GD-77) will disconnect, because ultimately the reason for the timeout is that the TX chip (ADF7021) on the duplex hotspot, does not have its clock signal locked to the incoming signal (e.g. from my GD-77) and over a period of time, either the data buffer in the hotspot under-runs’ (empties) , or over-runs (overflows).
I know some people including Steve K2GOG are investigating using a clock clock rate generator chip, as the clock source for both the Rx and Tx ADF7021, as the clock frequency could be adjusted by the firmware. However I’m not sure if the cheap clock rate generators, like the Silicon Labs Si5351, have too much jitter to be be usable.
If I hear from Steve that he’s got this working, I have a couple of Si5351 breakout boards, so I could attempt to modify one of my duplex hotspots, and I will write a post about it.
In the mean time,… I tested 1.4.7 and my timeout is now 4 minutes.
I’m not sure if this timeout is being caused by the Brandmeister network, server or whether my 60 secs timeout has been multiplied by 4 to now be 4 minutes.
But I think this is an acceptable timeout as most repeaters enforce a 3 minute timeout. So to all intends and purses this bug has been fixed.