We received the CS750 and we were able to reproduce the issue with radio firmware S3.00.14 (no newer is available on the Connect Systems website which is marked stable and not experimental,
http://www.connectsystems.com/software/software%20CS750.htm
).
We found the following issues:
- The radio completely ignores the TX preamble duration setting in the CPS.
- Radio always sends 2 Preamble CSBK PDUs followed by 1 Voice LC header.
DMR specs say in section 7.1.2.5: "The Pre_CSBK PDU is shown in table 7.7. This PDU may be used to increase robustness of non-voice (data, CSBK, etc.) delivery for scanning radios." (
https://www.etsi.org/deliver/etsi_ts/102300_102399/10236102/02.03.01_60/ts_10236102v020301p.pdf
page 74)
That's why the openSPOT2 decoded the call as a data call coming from the CS800 and CS750, because it expected a data call which should start with CSBK PDUs.
Voice calls should start with one or more Voice LC headers, and not CSBKs. Also if there is only 1 Voice LC header sent by the radio and there's a decode issue then the call can only start with a late entry. That's why other radios use TX preamble duration setting which specifies how long the radio should send the same repeated Voice LC header frame so the receiver end can always decode at least 1 incoming Voice LC header.
We've added a workaround which fixes the issue, this workaround will be available in the next beta openSPOT2 firmware which will be available next week.
We notified Connect Systems about these issues with their radios, hopefully they will ask the radio developers to release a fixed firmware.