[arm-allstar] DTMF Decoding / Zeroing in on RX Levels

[Travis French]

Good Afternoon Everyone,

I too am experiencing a DTMF decoding issue myself similar to a recent discussion but mine it not related to a walkie-talkie. I have built a radio-less node copying the schematic that Doug had posted. With an Alinco DR-135 in use for my RF node, I had a lonely DTMF mic that was just begging to be used.

That being said, I am unable to get consistent DTMF decoding from it while at the same time having good voice audio. While adjusting levels, I can get various tones to decode. #1 seems to work the best through a broader range of levels while the higher numbers I just can't seem to get to decode reliably. It's hit or miss. When it's decoding the best, the echoed (voice) audio is the worst. 

Two questions:

1. Without a service monitor, if there a way to determine if I need to adjust up or down to get the best decode reliability? Maybe I am sweeping through in too large of increments leading me to miss the sweet spot. I suppose it is possible.

2. I see in the mic (EMS-57) a small variable resistor labelled audio level.  Looking at schematics online from previous Alinco mics, this VR adjusted the DTMF levels. I can't find a schematic for the newest mic and was not sure based on the board label if this was mic or DTMF level. The layout is very different. Does anyone have any experience with these? Maybe this is where I need to adjust.

Lastly, when the echoed voice audio is sounding good the #1 key is the only thing that gets decoded if I am lucky. Again hit or miss. That and the DTMF tone echoed back has one heck of a wallop followed by what seems to be a rise in amplitude and it then settles to a steady tone.

Any thoughts on the matter would be greatly appreciated.

Thank you in advance,

Travis E. French - K2PCB