MMDVM and AT-868UV issue

[KD2EOY]

I have an old DV MEGA from when they came out 2016ish running the latest pi-star and everything works perfect.
I recently bought the 868 so I bought the new MMDVM dual band hat from eBay. The problem is the 868 does not always pass audio through.
I can see the receive light light up but don't hear anything. this does not happen all the time, sometimes it's fine, other times I may hear half of an over. My MD 380 works fine and I can hear everything all the time. The new hats are 0 offset according to the sticker on them, but the BER was high.
I adjusted the RX offset and found that -225 gives me a 0.2% on Fusion and 0.3% to 0.5% on DMR but that didn't make any difference with the audio problem. Turning on Digi monitor didn't help either. Anyone have any ideas?

[KD2EOY]

BTW, thats not a dual BAND hat, its a DUPLEX hat. Dual band would be two different bands.

I was referenceing the add:
Features and Technical Specifications:
"- High performance 32-bit ARM processor
- Fully tested
- Supports DMR, P-25,D-Star and System Fusion
- Onboard LEDs to show status (Tx,Rx,PTT,Mode)
- Up to 10mW RF power
- SMA antenna connector,Antenna included
- Mounts cleanly on all current Raspberry Pi's including the Pi Zero
- The firmware is pre-loaded and is easily upgraded via software.
- Open source 3D printable case available
- UHF 430-450 MHz,433mhz and 900MHz
- VHF 144 220 Opera FREQ
- Without shell
- Not include the raspberry pi
- The firmware is pre-loaded. if you want update pls Click Updated site
The upper part is TX,the lower is RX antenna.
UHF+VHF
SMA,antenna
Modem select:STM32_HS_GPIO
Set TX frequency,RX frequency
TX,RX frequency had better 10M or more apart.
SLOT1,SLOT2 support."

Don't believe that sticker, take the time and figure out the proper settings for both RX offset and TX offset.

Yes that's why I adjusted the RX Offset to get the BER down.

I have a hand held frequency counter coming to try and see if that will help, I can't get to a spectrum analyzer due to my ankle being just fused and this monstrosity of rings and pins in it
Thanks for your reply and help!!

[KD2EOY]

Also any idea why it works fine when using the DV MEGA but not the new MMDVM hotspots?

[KD2EOY]

Yes that's why I adjusted the RX Offset to get the BER down.

Adjusting the RX offset is tuning the hotspots RECEIVE to get closer to the frequency of the transmitter.

Your problem is the transmitter on the hotspot and the receive on the HT. Big difference.

Also any idea why it works fine when using the DV MEGA but not the new MMDVM hotspots?

If thats the case then the DV MEGA is transmitting closer to the frequency expected by the HT in question.

Every radio is going to be off frequency one way or another, they are never exact from the factory.


Example:

You have three different radios and they all display that they are set on 444025000Hz.

if radio 1 is actually transmitting at 444025150Hz (notice it is not perfectly on the frequency it says it is, that is important)

and HT1 is actually receiving at 444025000Hz That is a difference of -150hz

But HT2 is actually receiving at 444024000Hz That is a difference of -1150hz

If the digital mode can only withstand a separation of +/- 1000hz, then HT2 isn't going to work very well.

This is just a crude example with some simple math to try to get you to understand the concept and I'm sure someone out there can tell you exactly what that separation number is for every digital mode, but I don't have the numbers off of the top of my head. I just know its much tighter than plain old FM voice.

Gotcha Thanks for the info!

[KD2EOY]

Yes that's why I adjusted the RX Offset to get the BER down.

Adjusting the RX offset is tuning the hotspots RECEIVE to get closer to the frequency of the transmitter.

Your problem is the transmitter on the hotspot and the receive on the HT. Big difference.

Also any idea why it works fine when using the DV MEGA but not the new MMDVM hotspots?

If thats the case then the DV MEGA is transmitting closer to the frequency expected by the HT in question.

Every radio is going to be off frequency one way or another, they are never exact from the factory.


Example:

You have three different radios and they all display that they are set on 444025000Hz.

if radio 1 is actually transmitting at 444025150Hz (notice it is not perfectly on the frequency it says it is, that is important)

and HT1 is actually receiving at 444025000Hz That is a difference of -150hz

But HT2 is actually receiving at 444024000Hz That is a difference of -1150hz

If the digital mode can only withstand a separation of +/- 1000hz, then HT2 isn't going to work very well.

This is just a crude example with some simple math to try to get you to understand the concept and I'm sure someone out there can tell you exactly what that separation number is for every digital mode, but I don't have the numbers off of the top of my head. I just know its much tighter than plain old FM voice.

So after adjusting the offsets 3000 both ways and pulling my hair out I decided to try the 2M side, changed 3 channels frequencies to 2m in the code plug and uploaded it to the radio, but it wouldn't key the hot spot at all. Went back into the code plug, and low and behold it's set to TS1. Fixed that and now it's playing nicely. Now the MD380 it also set to TS1 and it hears everything. Just shows how open they really are.
Just wanted to say thank you for your help Jason!

[KD8DVR]

Took me a long time to get my Anytones to play nice with my duplex spots. After a lot of rx offset finagling, I got them to work.

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