Over the last few weeks I’ve been experimenting with the duplex MMDVM board, and it continues to have some problems which I thought I should share the details of…
- Not all of my radios connect reliably when the board is in duplex mode.The GD-77 seem to have the least issues connecting.
The MD-380 will only connect if I change the DMRTxLevel to a higher value than is normal (somewhere between 53 and 57 seems to work).
AFIK this setting changes the FM deviation of the 4FSH being produced by the ADF7021 transceiver chips on the board.
I’ve had reports where MD-380’s would not connect at all, but I’ve also spoken to people who have a MD-380 and didn’t need to change the Tx Deviation.
Also… when I change this value, the GD-77 has problem connecting.
Even when my MD-380 manages to connect, the board seems to occasionally confuse which TimeSlot the MD-380 is transmitting on, with the result that no audio is received by anyone who is listening.
I discussed this with Andy CA6JAU who writes the firmware, and he said there are general problems with the MD-380 connecting even to the simplex versions (e.g. Zumspot and JumboSpot), which are normally resolved my adjusting the DRMTxLevel.
The other possible cause of the problem is time delay though the board, between when the MD-380 sending the connect (wake-up) message and when the MD-380 receives the response. However at the moment its unclear quite what is causing this issue, and unfortunately I don’t think Andy has time to investigate.
- When leaving a pause (break) before pressing the PTT in response to the last incoming over, the GD-77 seems to occasionally not connect – but does not give any notification that its not connected.It appears that if I press the PTT within one or perhaps two seconds, that the GD-77 seems to connect on most occasions, but anecdotally leaving a pause / break, seems to increase the chances that a connection fails.
Whats strange is that if the GD-77 can’t connect, it normally beeps and stops transmitting after around 1 second. But in this case it continues to transmit, and the Pi-Star dashboard indicates that the hotspot is not receiving the transmission.
This could be an issue with the way Pi-Star displays the status, however I think that the duplex hotspot board is not signalling to MMDVM Host that it has an incoming signal, but is still sending data back to the GD-77 hence it thinks that everything is working fine.
I need to do more tests to attempt to confirm whether this problem is exacerbated by leaving a pause, or whether it happens at random, and to also determine where in the hierarchy the problem lies. i.e MMDVM_HS firmware or perhaps MMDVMHost or even DMR Gateway.
My guess would be an issue on the board / firmware.
- Operating in duplex mode, the board is unable to support DMR and other modes at the same time (i.e where board scans for each mode for a short time.)
I’ve posted about this before, but basically the ADF7021 chip on the MMDVM hotspot, has to be setup to receive different types of modulation when listening for DMR to when listening for D-Star or YSF etc. Hence board listens for each type of modulation for a short time, in its scan loop.
The problem is that for duplex DMR operation, where the transmit and receive frequencies are different, the transceiver e.g. GD-77 sends a connection (wakeup) data packet to the “repeater” and expects a fairly instant response.
However if the duplex hotspot happens to be listening for D-Star (or YSF or P25 etc) when the transceiver sends this connection data, the duplex hotspot will not send back a response to the transceiver, and hence it won’t connect.
The transceiver should make multiple (probably 3) connect attempts, with a small gap between each ( I think around 250 mS between attempt), but its possible that the duplex hotspot would not be listening for a DMR signal during any of the connection requests.
The firmware already listens for DMR signals for something like 10 times longer than it listens for the other modes, but this is not long enough to guarantee it listening at the correct time.
There may be a partial workaround for this, to change the duplex firmware, to listen for DMR signals for at least the as long as the time required to hear two connection requests, and to not listen for other modes, for longer than the gap between connection requests.
This would probably mean that the scan loop would need to be changed from (for example)
However I don’t know if this would impact on the detection of D-STAR or YSF or the other modes
It could also potentially result in the loss of the first second or two of audio for other modes.
- Even with the fix that Andy wrote; my board still has a timeout of around 4 minutes.
Its possible that some boards have a worse timeout, as the value seems related to the difference in clock frequency in the transmitter e.g. GD-77 and the hotspot.
My understanding of this problem is that there is no way to synchronize the data packets arriving from the transceiver (e.g. GD-77) and the data being sent back to from the hotspot. This is because the ADF7021 does not have a “recovered” clock output.
I think there may be some hardware and firmware changes which may be able to fix this in the long term, but there is no simple fix for this without modifying the hardware.
I suspect there may be other problems that I am not aware of, so please feel free to comment if you have another problem with these boards.
My overall view of these boards is still that they are generally good value, as they cost less than $10 more than the Jumbospot. But it is very new technology and you may potentially have to run it in Simplex mode, either permanently or until specific problems are fixed.