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

[Doug Crompton]

Travis,

   I believe your problem is not levels as much as it is twist which
relates to frequency response. Each DTMF digit has one tone from a low
group of tones and one tone from a high group of tones. These two tones
have to be not only dead on frequency but very close in level for DTMF to
decode. The difference in these levels is called twist. These requirements
are so it doesn't false on voice. I suspect that you have a frequency
response or noise problem in the interface. The only way to determine this
is to look at it on a scope.

Did you build the radio-less node? What microphone are you using?  Most
mics use 8 volts from the radio. It is possible that the USB 5V is not
enough to drive the mic. Also (and this may be the case) if the microphone
has a 5V regulator in it it may not be operating properly with 5V input.
You could try powering the microphone from an external supply of around 8V
- make sure this is just to the mic and does not feed back into the other
circuits.

There have been reports of certain microphones causing issues when running
on 5V.

I can tell you there is nothing wrong with the DTMF decoding and that the
levels are not that critical as long as the proper tones are coming in.  If
I had to guess I would say the proper tones - levels, twist, possible noise
on the input all could cause this problem.

If we do find out that this is a voltage issue a voltage boost circuit
which would boost the 5V to 8V could be used. Also the mic, if it does in
face have an internal regulator, could possibly be bypassed.


*73 Doug*

*WA3DSP*

*http://www.crompton.com/hamradio <http://www.crompton.com/hamradio>*


On Thu, Mar 1, 2018 at 4:38 PM, "Travis French via arm-allstar" <
arm-allstar at hamvoip.org> wrote:

> 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
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