[arm-allstar] Digits 2, 3 Not Decoding

Doug Crompton doug at crompton.com
Fri Aug 5 23:44:51 EST 2016

DTMF requires good equalization. It uses two tones, one from a low band group and one from a high band group. The levels of the two tones must be within a certain margin. This is often referred to as twist. Here is the specs on levels -

At the transmitter, the maximum signal strength of a pair of tones must not exceed +1 dBm, and the minimum strength is -10.5 dBm for the low-group frequencies and -8.5 dBm for the high-group frequencies. When not intentionally sending DTMF tones (including the inter-digit interval), any leakage of these tones must not exceed -55 dBm. The frequencies generated by the transmitter must be nominally within 1.5% of the stated values and the receiver must not accept signals that deviate more than 3.5% from the stated values. 

Absolute levels do not usually matter a great deal. That is there is not some very narrow level setting that is needed to be set for it to work. More important is that you don't have some level disparity across the spectrum. The lowest tone is 697hz. and the highest is 1633hz.

Most Allstar systems work fine but if you have pre or deemphasis set wrong it will most certainly cause problems. Some transceivers notably Chinese models have difficulty. Make sure if you are using a Baofeng radio like the UV5R that you turn OFF DTMF feedback to the speaker because they do no mute the mic. If the speaker feedback is turned on the decode will hear both the direct tones and those through the mic and get confused.

73 Doug

Date: Fri, 5 Aug 2016 18:09:09 -0500
To: arm-allstar at hamvoip.org
Subject: [arm-allstar] Digits 2, 3 Not Decoding
From: arm-allstar at hamvoip.org
CC: wb5nfc at gmail.com

I'm helping a friend get a new node on the air. We're using a Raspberry Pi3 with the latest image and simpleusb. Digits 2 & 3 do not decode, but all others do. The same problem persists with two different HTs into the node. Audio levels sound good in and out, but we don't have access to a monitor / signal generator at this time. Suggestions?


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