1. Dual-tone multi-frequency signaling
  2. DTMF and AllStarLink
  3. MT8870 DTMF Decoder
  4. References


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



DTMF 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