[arm-allstar] DTMF Decoding

[Doug Crompton]

Glenn,

 The DTMF decoding in Allstar is done in software. It is very reliable
assuming that the source and levels are correct. Some handhelds have poor
DTMF quality. So you may find that one handheld works fine and another may
have a problem. This is not a widespread issue however as we have thousands
of nodes and few issues, One problem that was known awhile back was on the
Baofeng radios they did not mute the mic when DTMF was generated thus
causing DTMF audio to have a direct path and also a path from the speaker
to the mic. In this case turning the volume too high caused erratic DTMF
decoding. In one case it was just a matter of changing a menu option.

https://kk6jyt.com/sending-accurate-baofeng-dtmf-tones-from-the-uv-5r/

You can check tone decoding in the Asterisk client. Select client from the
main menu and there you will see your DTMF keys decoded on the screen. Go
through all the keys and watch then decode on the screen. Trying this for
all your radios will give you an idea what works and what doesn't.  Check
that your levels are correct in the simpleusb menu item 2. Voice should
peak up to 5-6 khz on the bar graph display.  DTMF tones must be accurate
in both frequency and individual tone level.


*73 Doug*

*WA3DSP*

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

On Thu, Jan 16, 2020 at 5:22 PM "Glenn Lyons via ARM-allstar" <
arm-allstar at hamvoip.org> wrote:

> I am trying to get a better understand the many factors that affect DTMF
> decoding.
> Accurate DTMF decoding is essential to AllStarLink/Hamvoip functionality.
>
> Firstly, where does the decoding actually occur? Is it a DSP routine on the
> Rpi? Are there any diagnostic tools at that level?
>
> My list of factors to investigate includes....
>
> - Quality of the source DTMF encoding. Some radios (handhelds) work better
> than others. It is usually the same character that fails to be decoded when
> a HH does not work.
> - Quality of the RF transmission. Good antennas and transmission lines to
> product a clean strong signal.
> -  Audio levels in the receiver and or it's interface board. Each radio
> model has it's own specification that have to be understood and adjusted.
> -  De-emphasis setting in the receiver. I have read that audio levels and
> de-emphasis are key. I have heard that  "twisting"  occurs if set
> incorrect. What is twisting and how do you detect it? Is a
> oscilloscope helpful?
> -  Audio levels in the RIM. RIM usually have pots or jumpers that have to
> be set in conjunction with the radio 's output to control the signal levels
> entering the RIM.
> -  Audio levels set in the application software. These can be set with the
> very helpful admin utility in HamVoip.
> -  Processing capabilities of the CPU (Rpi). Is this a factor?
>
> Getting a clean audio signal has usually proved easy for me, however,
> getting DTMF
> to decode reliable has proved more difficult and caused a great deal of
> frustration.
>
> Any insights or pointers would be greatly appreciated.
>
> Glenn
> VK4PK QTH:QG62QI
> DMR-MARC ID: 5054273
> AllStar ID:48253
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