RTCM: unwanted extra de-emphasis

AllStarLink
1

So, this is an odd one...

I am helping manage a repeater that currently has three voting receivers, which are all identically programmed Motorola CM-200 VHF radios, which are all sending flat audio to the RTCM's.
With the default, I.E. de-emphasis enabled, everything sounds great for a day, maybe a couple of days. Then, it's like an extra layer of de-emphasis shows up on top of the already de-emphasized audio. The resulting audio is extremely muddy.
If I restart ASL3, everything is fine.

Likewise, if nodeemp is added to each RTCM, you get non-de-emphasized audio for a while, until suddenly, all the RTCM's are now de-emphasized after maybe a day or two, and it's that extra muddy state until Asterisk is restarted again.

What would cause this? Has anyone else experienced this behavior?

2

Just experienced this extra de-emphasis showing up between transmissions during an active net. It always clears up when Asterisk is restarted, but doesn't seem to last very long. I don't know what to look for here.

3

Here's how RTCM do filtering.

  • Pre-emphasis: RTCMs send unfiltered audio to the Mic. input of the radio. If you need pre-emphasis Tx CTCSS tone must be set. Leave it at a low volume if your radio is encoding CTCSS. The logic here is that if you are sending CTCSS you are going into the modulator input (not the Mic.) and therefore you need pre-emphasis. By keeping the CTCSS level really low you can handle the unusual case where pre-emphasis is needed but not CTCSS. These settings are in voter.conf.

  • De-emphasis: RTCMs send unfiltered audio to app_rpt when using the discriminator, which is menu item 13 COR Type set to 0=normal. By changing the COR Type to 1=IGNORE COR the de-emphasis is enabled. The logic here is that COR Type 0 is for voting and discriminator which is not filtered is needed. The de-emphasis filter is a RC network in the RTCM. There is a de-emphasis in voter.conf.

  • There is one weird exception to the above. When running duplex mode 3 there is a (hidden) setting to change emphasis. I forget the name of that setting but let me know if you are using duplex mode 3 and I'll dig into it.

Here is the voter documentation in case you don't have it.

I've heard people make the complaint you are but I've never run into it and I run half dozen or so RTCMs. I hope the above helps you sort out what might be happening. Make sure you have current RTCM firmware and ASL 3 updated.

I suspect that the RC filter in the RTCM is being toggled on and off. Sounds like you have the setting voter.conf nodeemp. What do you have the RTCM COR Type set at?