PD98x - PHONE PATCH - Microphone feedback and DTMF dialing during Full-Duplex calls

[iu0lty]

Good morning friends.
I'm working in this little project, and I need your help to solve a seemingly unsolvable problem.

This is the scheme:
A PD985 (radio A) is wired to a Phone Patch interface, that is connected to an analog PBX.
With another PD985 (Radio B) I set up full duplex call to Radio A.
Then I dial the DTMF code for the extension number I want to reach:
doing this enables me to make "phone calls" to my place via DMR radio.

The idea seem to work on paper, but I'm facing two problems.

DTMF DIALING
It seems that as soon as I set up the full duplex call, the 985 can't send out DTMF codes. At the moment I worked around this by sending the dtmf code in half duplex, and only after this I can set up the FD call.
The main problem with this scheme is that sometime, on the phone side, the called party picks up the call before I can open the FD communication, hears nothing and hangs up.
There is a way to send out DTMF during a full-duplex call? It's possible to access the radio phonebook during full-duplex calls?


MICROPHONE FEEDBACK
This is the most annoying issue.
On the phone side, no problems: the called party can hear me loud and clear, and so I can hear him.
On the radio side, big problem: when I try to speak, my audio comes back. It seems like the audio from Radio B goes inside the phone patch interface and then the Radio A microphone picks it up again and sends it out to the air.
Indeed, this happens: I speak, the phone-side party hears me, and then my audio comes back to me.
It's really annoying, since I hear back everything I say in the radio.

I'm thinking that this is some kind of wiring problem: as you can say from the images attached, this is the only wiring diagram that works for me:


PD9 PHONE PATCH

SPK + ------------- SPK

SPK -_______________ GND (SPK- and MIC- are connected together)
MIC -

MIC + --------------MIC

PTT ---------------- PTT


As you can see, the Phone Patch interface has a commong ground for speaker and microphone, but the PD9 have three separate pins for SPK-/MIC-/GND. Furthermore, if I connect the PD9 GND pin to the phone patch GND, both Mic and Spk audio disappear.

Do you think there is a workaround for this?


Thank you for your time friends.

 

[dazey77]

Most basic solution, make a FD call and play the DTMF into the 985 microphone from something like a phone. Sure you can find an app for playing DTMF noises. Appreciate its not as good as having the 985 do the DTMF itself.

On the audio feedback, could be an issue with the phone patch (giving sidetone) as opposed to a radio issue.

 

[iu0lty]

Most basic solution, make a FD call and play the DTMF into the 985 microphone from something like a phone. Sure you can find an app for playing DTMF noises. Appreciate its not as good as having the 985 do the DTMF itself.

On the audio feedback, could be an issue with the phone patch (giving sidetone) as opposed to a radio issue.

About the DTMF:
At the moment I send the DTMF string on half-duplex, then set-up the FD call. And that is a bit more pratical than sending the DTMF with an external generator. The thing that I'm finding strange is that Hytera completely locks-up the keyboard during FD calls: I'm finding this really strange, and I'm feeling that I'm doing something wrong.

About the audio feedback:
Problem solved. I just had to tweak around with the radio volume and the trimmers inside the phone patch.

Thank you.

 

[buddrousa]

If I remember right your radio making the call has to be a full duplex radio also not just a plain over the counter radio.
We had a few full duplex phone patch setups in the early 80's and they were special radios.

 

[iu0lty]

If I remember right your radio making the call has to be a full duplex radio also not just a plain over the counter radio.
We had a few full duplex phone patch setups in the early 80's and they were special radios.

As I said, the radios are two Pd985 with SFR licence. In DMR you have 2 timeslots, and you can use one TS for RX and the other TS for TX. Back in the '80s, my father used the same phone patch interconnection interface, just under an homemade analog repeater.

 

[buddrousa]

Back in the early 80's I setup and maintained over 15 Zetron 48b's 2 in each county 1 VHF and 1 UHF
Also not up to date on new full duplex radio models was only throwing out a thought.