There are 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?

A list of factors affect DTMF performance"

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.


Dual-tone multi-frequency signaling



DTMD and AllStarLink

Doug Crompton doug at crompton.com
Mon Jun 15 13:38:36 EST 2015
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  If you go into the Asterisk client - asterisk -rvvv  - you should see each numeral/character displayed on the screen when you send it from your transceiver into the node radio.

Failure to see anything or proper tones is almost always a level or equalization issue.  DTMF is two tone and if the tones are of unequal level (twist) it will not decode properly.

Another clue to not decoding is the tones will not be muted - the other end will hear your tones distinctly.  

Go into simple-usb-tune-menu and make sure the rx level - item 2 - is kiciking up on peaks to just above 5khz with voice.

If it is then perhaps you are using a node radio that needs deemphasis - you can set that to yes and uncomment it in /etc/asterisk/simpleusb.conf  - then restart Asterisk -  astres.sh  -  and recheck the levels.

If noether of those things work then perhpas you have a bad DTMF source. Did you try more than one radio? Also the Chinese radois are notorious for lousy DTMF.  
73 Doug

Setting your levels and deemphasis correctly
almost always solves the problem of not decoding DTMF. 


MT8870 DTMF Decoder





Glenn Lyons VK4PK
Ver:gnl20200116 - pre published v0.9