DTMF

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"

• 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?

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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Russell,

  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
WA3DSP
http://www.crompton.com/hamradio

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

MT8870 DTMF Decoder

References