Unable to transmit with P25 radio or RF Link app

K6RBJ

I just received my Openspot 4 Pro. I set it up for P25, and am using a Midland STP-404A handheld. I can receive P25 audio, and it sounds great. I cannot transmit with the radio. The hotspot light flashes green that says it is receiving, but no audio is transmitted out over the TG.

I also tried the RF Link app on my Samsung A53 phone and Samsung tablet. I cannot hear any audio from the hotspot, nor can I transmit even though the app asks for mic permission.

I have upgraded to the latest beta firmware with no luck regarding either issue. I have not tried using my DMR radio yet, but will try it tomorrow.

Any info on setting up the Midland STP-404A to work with the Openspot and/or getting the RF Link app working would be greatly appreciated.

Bryan - K6RBJ

HA2NON

You won't be able to use the app or cross mode to P25 as it uses an older, incompatible IMBE voice codec. You can only listen/transmit P25 if you have a P25 or a Yaesu C4FM transceiver (in VW mode).

K6RBJ

Well, it appears my OP may have been misinterpreted. So let me clarify.

I am not trying to cross mode anything. I am simply trying to get the Midland P25 radio mentioned in my OP to work with the Openspot 4 Pro. The Midland is programmed to the correct frequency and NAC, and I have entered the two talkgroups I am attempting to access. One is WMRA TG 7941, and the other is PAPA Bridge TG 31078. I can receive audio from both of these P25 systems, but I cannot transmit to either talkgroup. I don't know if this is an issue with the radio programming, or a setting in the Openspot 4 Pro.

Any useful things to try would be much appreciated. I purchased the Openspot specifically to use with my otherwise unused Midland STP404A P25 radio. FYI, I did verify the Openspot transmits and works fine with my DMR radio, so it does not appear to be an Openspot hardware issue. I'm hoping it is something I overlooked in radio or TG programming, or a box that needs to be checked or unchecked in the Openspot settings.

Thank you,
Bryan Johnson K6RBJ

HA2NON

Please run P25 AutoCal, or manually adjust the RX offset on the Modem page in +-50Hz steps from -500Hz to +500Hz for the lowest BER. You can verify settings using the openSPOT's built-in local echo service on P25 ID 9999.

If AutoCal does not work for you then the RX offset between the transceiver and the openSPOT is greater than 500Hz. What you can do is to switch frequencies on the openSPOT, but keep the transceiver's frequency. For example, if the transceiver is on 433.900MHz, then test by setting the openSPOT to the following frequencies:

433900500 Hz
433900600 Hz
433900700 Hz
...
433899500 Hz
433899400 Hz
433899300 Hz
...

You can use the built-in local echo service to test your own voice on P25 ID 9999.

K6RBJ

Ran AutoCal twice and it came up with a setting of -460 both times. I saved those settings and tried the 9999 echo service and no go. Also tried to transmit on the above mentioned TG7941, again with zero luck. Don't know what other settings to try.

HA2NON

What you can do is to switch frequencies on the openSPOT, but keep the transceiver's frequency. For example, if the transceiver is on 433.900MHz, then test by setting the openSPOT to the following frequencies:

433900500 Hz
433900600 Hz
433900700 Hz
...
433899500 Hz
433899400 Hz
433899300 Hz
...

K6RBJ

I'll give it a try. But I don't quite understand why my P25 radio doesn't transmit out the Openspot at all. I understand high BER leads to packet loss, but I would think it would still transmit out. Or am I missing something here? Like, is it a syncing issue between the radio and Openspot? Please explain.

K6RBJ

So after trying AutoCal again, it indicated an offset of -460. I saved that setting and tried transmitting with my Midland P25 radio several times. I did get a parrot back one time, with very low volume. I have not been able to duplicate that result again. That is, the Openspot still does not transmit my audio to any P25 talkgroups. Super frustrating!

K6RBJ

Still can't get my P25 radios to work with this thing. I have two Midland P25 radios, and neither one will reliably TX thru the 4 Pro. I did get one to Echo a couple times, and I appear to have a very low BER, so I don't get why the radios won't work.

I also don't understand why the Shark Link app won't allow me to TX in P25 mode, since no radio is involved. So freq, BER, etc. should not even come into play. But yet I can't TX on the selected P25 TG's even with the app. DMR radio works fine, and I am able to TX in DMR mode with the app.

A friend wondered if the Openspot is looking for 6.25khz or 12.5khz bandwidth? My older Midland radios only do 12.5khz on P25, so could that be the issue?

Would really like to get this working, so I am not limited to only using my DMR radio with it.

Thank you,
K6RBJ

HA2NON

P25 uses the old IMBE voice codec, which is no longer supported by the onboard AMBE hardware codec chip so the SharkRF Link app can't transmit or receive in P25. You should use 12.5KHz channel spacing in P25.

K6RBJ

But my P25 radio should be able to transmit and receive with the Openspot correct? Something I'm still not able to do. I've tried moving the receive frequency up and down as suggested. I did get the Openspot to Echo my transmission a couple times, but no repeatable results. I'm wondering if the issue is with the radio, but I have two identical Midland P25 radios and neither work with my Openspot.