DMR Audio bug?

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VK3KYY
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Re: DMR Audio bug?

Post by VK3KYY » Sat Mar 20, 2021 10:29 pm

Do you have any other DMR radios?

Echo back of the end of your own transmission is common on some networks, and the amount of audio replay is generally dependent on network latency, and can vary from nothing to longer than 1 second.

The reason some DMR radios don't hear this echo, is that they block the reception of signals on their own DMR ID.

However, with this firmware it was decided that blocking reception of your own ID is not a good idea, as it would prevent the radio hearing anyone else who was using your own ID without your authorisation.

The firmware also hears all private calls, and does not block reception of private calls to any station, unlike commercial radios which block reception of traffic which is not intended to be received by the radio's own ID.

These two decisions where taken as this firmware is for Amateur Radio use, and all traffic is deemed to be public.

VK3KYY
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Re: DMR Audio bug?

Post by VK3KYY » Sat Mar 20, 2021 10:30 pm

BTW.

What is your callsign ?

VK3KYY
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Re: DMR Audio bug?

Post by VK3KYY » Sat Mar 20, 2021 11:30 pm

Forts107 wrote:
Sat Mar 20, 2021 10:55 pm
Sorry I should have clarified.. this is strictly RX only. I'm not a ham, I'm a fire fighter and this radio is being used for RX only purposes for a local department. After every transmission the radio receives it's like the audio doesn't mute on time. I've been doing work & programming on DMR gear (mostly Motorola, some Hytera) for better than 10 years now, so I'm very familiar with their setup and operation. This OpenGD77 project is quite incredible in what it's achieved and I can't wait to see what's still in store for it!
Ah.

There is probably something being transmitted at the end of that DMR traffic which is holding the DMR audio open.

Potentially its sending trunking data (aka Tier 3) before and after the transmission, but the firmware does not support trunking / Tier 3 and I"m not sure whether the hardware in the radio actually supports trunking. AFIK the official firmware doesn't support trunking, but it may ignore the trunking data but the open firmware currently doesn't handle trunking data etc at all.
But as Ham radio networks don't transmit trunking data, its not possible for anyone to work on support for this.

VK3KYY
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Re: DMR Audio bug?

Post by VK3KYY » Sun Mar 21, 2021 6:36 am

Forts107 wrote:
Sun Mar 21, 2021 4:57 am
This isn't Tier III or trunked in any way, it's just a standalone conventional repeater.
If you are hearing audio after the audio has ended, the repeater is transmitting something that Amateur radio repeaters don't transmit, which is holding the audio system open in the radio.
We don't have this problem with Amateur radio repeaters, even Motorola and Hytera ones.

Juan Francisco
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Joined: Sat May 09, 2020 4:00 pm

Re: DMR Audio bug?

Post by Juan Francisco » Sun Mar 21, 2021 4:33 pm

I think he refers to the fact that the audio converter or amplifier after finishing any transmission is a few seconds with a soft noise and then it is total silence.

ok1pt
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Re: DMR Audio bug?

Post by ok1pt » Mon Mar 22, 2021 4:25 am

Forts107 wrote:
Sun Mar 21, 2021 6:58 pm
Yes after the TX is done there is a few seconds of a repeating noise before it mutes. It's very odd, there is nothing special about this setup. XPR8400 repeater, nothing fancy. I'll listen in to a few other stations tomorrow when I'm at work and see how they compare.
Hi there!
I know this phenomenon very well It looks like the DMR signal is not correctly ended, but continues with a lot of errors - you can hear whistling, humming, noises etc. But in my case, not only GD-77 with this firmware is doing this. Sometimes I'm checking the reception by listening to 2 - 3 different radios (all of them of the Chinese origin) simultaneously from the same source, and there are cases that a sound is produced by, say, 2 of 3, and it sounds very similar. And it differes which ones make the sound case by case. I have a possible explanation - I'm listening in a room with A LOT of electronics running around - computers, digital telephone exchanges, proprietary digital telephones (I found one model making extreme interferences and I had to replace it with another model) etc. And possibly the interference may simulate continuation of the DMR stream and inject some insane data to the codecs. RSSI without useful signal is about -105 dBm here.
As I didn't learn the low-level DMR specs yet, I don't know whethter there is a CRC or other method to check validity of the DMR frame, but if it's there, is it possible that the checks are not ideal and sometimes accept a totally wrong frames, maybe until the RF circuitry deactivates the CD signal ?
With regards & 73
Pavel OK1PT

VK3KYY
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Re: DMR Audio bug?

Post by VK3KYY » Mon Mar 22, 2021 4:57 am

There is a CRC with the DMR data and we check this, however I don't think its 100% reliable.

I think its more likely the repeater you are listening to is sending some additional information after the audio transmission.

I know some people have problems with the OpenGD77 firmware because the squelch is opened when repeaters send Roaming information, but as none of the local ham radio repeaters send roaming data its not possible for me to attempt to filter it out.

Its probably not difficult to filter the Roaming data, but as is proprietary and undocumented in many cases, its not possible to handle this data unless a source of test data is available.

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