RF to repeater has delayed Tx

[w7efs]

When a user is talking RF to the repeater, the last 1 second or so of the D-Star Tx is heard on the user's radio when the user un-keys. It sounds as if the repeater's Tx is delayed from the Rx.

We changed the RF and Net hang times to "5" from the default "20", but the result is the same. Is this a Pi-Star issue or stm32-dvm v3 hardware, please.

[w7efs]

Exploring a possible hardware issue, I received this reply from Scott, the manufacturer of the STM32-DVM:

I am doubtful it is a hardware issue since the STM32 processor processes the digital signals in near real time. (no buffering)

I'm real sure the issue would NOT be in the radios selected as they have NO buffering either.

I'm assuming you are running a Raspberry Pi3 B or B+. Any other Raspberry Pi *may* not have enough processing for "real time." In all of my testing I have never seen any signal not be real-time though the system.

Yes, we are using a new RPi3B, so it appears that the buffered audio is a Pi-Star issue, and I sincerely hope that someone can suggest a solution, please.

We'll try using dstarrepeater instead of MMDVMHost, but it would be nice to have the multi-mode capabilities.

[N6MIK]

I'm running an MMDVM_PI (from the ZumSpot folks) on a Pi3 B+. I don't use Dstar, but have the same thing happening on P25... annoying, but also serves as a real-time signal report! DMR does not have this problem.

[w7efs]

Here's some feedback from a very knowledgeable radio operator, which may help shed some light on the problem:

After reading the last post on the yahoo mmdvm groups about hearing the last second of the user transmission when un-keyed, I will have to agree it is the way the dstar signal is processed. When I use the HT at home and listen on the IC-880, I noticed there is a significant delay from when I speak into the HT and when I hear it coming out of the 880. Almost a second or more which causes me to stutter because my brain has trouble when I hear my own voice delayed. So, what this means is, the digital stream is processed through the MMDVM and Pi board causing it to be delayed by so many milliseconds (hundreds maybe). Because the PTT of the repeater TX is controlled by the stream being transmitted and not by the receiver COR, PTT will be delayed. If the users radio has a fast turn-around from transmit to receive, some of the delayed audio stream may be heard. Not sure if this can be fixed without some speed up of the digital stream processing.

... and here are the mmdvm group posts to which he's referring:

Hi, I'm running an MMDVM board with a full duplex repeater connected to it. When I transmit on DMR I hear around 4-5 seconds of my transmission when I unkey. The software is PiStar and it's running on a Raspberry Pi 2 b. Is there a setting for this or am I stuck with it.

I'm running an MMDVM duplex board on a RPi 3B+ and have zero delay on DMR, and about a 1.5 seconds on P25... Haven't been able to figure out how to fix it, but also hoping someone knows!

You won't "fix it" cuz it ain't broke.. Its inherent in the MMDVM.. It has to do with the audio processing in the Pi.. I have 5 of them and all do it..

[N7GWA]

I'm having this same issue on our repeater. Has anyone found a fix for this? The delay we're experiencing is about .5-1 second. It doesn't make it un-usuable, but is sure an annoyance. Running the Pi 3 B+ through the STM32-DVM interfaced to a Kenwood TKR-850 repeater. I do have a Pi 4 I may end up trying, but it takes some logistical planning to get it done and want to see if theres an easier fix first.