MD380 direct talk noise

vk3net
Posts: 8
Joined: Tue Nov 22, 2022 7:38 pm

MD380 direct talk noise

Post by vk3net » Fri Dec 16, 2022 7:37 am

I did a search but did not find the issue I have. Please let me know if I missed anything.

I get noise when two OpenGD77 talk to each other. I recorded two short videos:

Noise on the receiver side: https://youtube.com/shorts/aw_w5xMbODY?feature=share
SDR receiver shows the signal up and down: https://youtube.com/shorts/aHYOFnqkNUE?feature=share

I have 2 x MD380 and both are running on OpenGD77. My plan was to use one of them as a hotspot to extend the range. After setting them up, the noise was found on the receiver side. With more checking, it shows the point to point transmission also has the same issue. Few more points:
  • Same issue regardless which MD380 is the receiver
  • The noise does not increase with the volume turn up
  • No noise when use FM point to point, only happens on DMR
  • With MMDVM hotshot, no noise found either on transmission or receiving. The sound is clear
  • It seems the transmission does not continue (TimeSlot?) and causes the receiver noise
Can you please let me know what causes the noise and how to resolve it?

Thanks!

EU1AEQ
Posts: 141
Joined: Sat Jan 30, 2021 7:25 pm

Re: MD380 direct talk noise

Post by EU1AEQ » Fri Dec 16, 2022 8:26 am

I'm joining you.
I use 1801 as a node transmitter. I hear the same noise. There is no noise when working through city repeaters.

G4EML
Posts: 919
Joined: Sat Nov 16, 2019 10:01 am

Re: MD380 direct talk noise

Post by G4EML » Fri Dec 16, 2022 5:09 pm

The noise you are hearing is the un-squelched FM Baseband noise in the unused timeslot.
When receiving DMR the the FM baseband signal is routed to the HRC6000 chip, this then decodes and generates the the received audio which is routed to the Audio Amplifier. Unfortunately the design of the MDUV380 circuitry in this area is not perfect and a very small amount of the FM Baseband Audio leaks through to the audio amplifier. The level of this is very low and seems to vary a lot between individual radios. We don't have the full schematics of the MDUV380, so at the moment we don't know if this can be improved.

The noise will only affect DMR reception on the speaker of the MDUV380. It won't affect the its use as a hotspot and it won't appear on a transmission.

The reason you don't hear the noise when listening to a repeater is because that transmits with a 100% duty cycle, so there is no unsquelched noise on the unused slot.
The same applies to most hotspots. Even though they may be operating Simplex they normally transmit with a 100% duty cycle, filling the unused timeslot with blank carrier.

Your spectrum plot of the SDR receiver is perfectly normal and does not show any problem. The pulsed nature of DMR DMO mode will always look like that.

Colin G4EML

vk3net
Posts: 8
Joined: Tue Nov 22, 2022 7:38 pm

Re: MD380 direct talk noise

Post by vk3net » Fri Dec 16, 2022 8:36 pm

Thanks Colin for the very detailed explanation, it is well received.

Just one more small question here:
G4EML wrote:
Fri Dec 16, 2022 5:09 pm
The same applies to most hotspots. Even though they may be operating Simplex they normally transmit with a 100% duty cycle, filling the unused timeslot with blank carrier.
Is it possible to make MD380 also fill the unused timeslot with blank carrier when it works as hotspot or direct talking?

VK3NET

EU1AEQ
Posts: 141
Joined: Sat Jan 30, 2021 7:25 pm

Re: MD380 direct talk noise

Post by EU1AEQ » Fri Dec 16, 2022 9:05 pm

G4EML wrote:
Fri Dec 16, 2022 5:09 pm
Colin G4EML
Thanks. You can also clarify that this noise on the rt 3s is also on the factory firmware when working with the node on OpenGD. On RT3, there is no noise when working through a node on OpenGD.

G4EML
Posts: 919
Joined: Sat Nov 16, 2019 10:01 am

Re: MD380 direct talk noise

Post by G4EML » Fri Dec 16, 2022 9:18 pm

In theory it would be possible but that is not compatible with the DMR specification so is not something we should be doing.

It is difficult to tell from your video how loud the noise is on your radio. Certainly on both of mine it is noticeable if I listen closely but is at a very much lower level than any incoming speech. So it is not really a major issue. It may be that some radios, or batches of radios, are worse than others.

I will have to reload the official firmware and see if the problem is still present there. If it isn't then that might point to some, currently unknown, method of reducing the audio breakthrough.

As mentioned before if we had a copy of the schematics it would help. We do have a partial schematic of the MD380 but the MDUV380 is a significantly different radio.

Colin G4EML.

vk3net
Posts: 8
Joined: Tue Nov 22, 2022 7:38 pm

Re: MD380 direct talk noise

Post by vk3net » Sat Dec 17, 2022 2:24 am

Thanks Colin,
G4EML wrote:
Fri Dec 16, 2022 9:18 pm
It is difficult to tell from your video how loud the noise is on your radio. Certainly on both of mine it is noticeable if I listen closely but is at a very much lower level than any incoming speech. So it is not really a major issue. It may be that some radios, or batches of radios, are worse than others.
I do not need to listen closely to hear the noise, which is very noticeable in a quiet room. What I have actually are one MD380 and one RT3S, purchased in the last 3 months but from different sellers. They may be produced at the same time in the same factory, but out of my knowledge to identify. They are at the same noise level.
G4EML wrote:
Fri Dec 16, 2022 9:18 pm
I will have to reload the official firmware and see if the problem is still present there. If it isn't then that might point to some, currently unknown, method of reducing the audio breakthrough.
I will also try to do the same test for one of the radios and post the results here.

Cheers,
VK3NET

vk3net
Posts: 8
Joined: Tue Nov 22, 2022 7:38 pm

Re: MD380 direct talk noise

Post by vk3net » Sat Dec 17, 2022 2:33 am

EU1AEQ wrote:
Fri Dec 16, 2022 9:05 pm
You can also clarify that this noise on the rt 3s is also on the factory firmware when working with the node on OpenGD. On RT3, there is no noise when working through a node on OpenGD.
Sorry, I am a bit confuse. Do you mean when using OpenGD as hotspot, there is NO noise with factory firmware, but HAS noise with OpenGD as receiver firmware?

SP1KC
Posts: 15
Joined: Sat Dec 17, 2022 8:11 am

Re: MD380 direct talk noise

Post by SP1KC » Sat Dec 17, 2022 8:25 am

Hello,
I have the same noise. TYT MD-UV380 on the original firmware works fine. On OpenGD77 sometimes there is noise instead of voice like in the video of the first post. I also noticed that sometimes you can't hear anything at all, but on the dashboard you can see a transmission on the same talkgroup and slot. As if the radio did not see the transmission. MMDVM Dual Hat with the latest firmware, OpenGd77 as well.
Best wishes,
Kris SP1KC

vk3net
Posts: 8
Joined: Tue Nov 22, 2022 7:38 pm

Re: MD380 direct talk noise

Post by vk3net » Sat Dec 17, 2022 9:03 am

vk3net wrote:
Sat Dec 17, 2022 2:24 am
I will also try to do the same test for one of the radios and post the results here.
I have reloaded the factory firmware and can confirm SAME noise can be heard.

The difference is the noise can be fully muted when turning the volume all down for the one with factory firmware, but not the one with OpenGD. The noise with OpenGD firmware does turn up or down with volume knob change.

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