6K V1.02 03-16-91
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4.
Check to see if a command was entered to disable the DTMF decoder from the repeater
receiver.
e.
Falsing
Some user's voices can contain frequency components that appear as DTMF digits to the decoder.
If such a digit is detected, "falsing" is said to occur. If the DTMF Muting feature is enabled, then the
result of falsing is the loss of repeat audio for a syllable or two.
Also, an accidental digit is stored into the controller's command buffer. However, this last item is not
usually a problem, as the digit will be discarded after a few seconds when the controller discovers
that the digit was not part of a valid command.
In any case, falsing is annoying and can usually be cured to an acceptable extent. Several
solutions are available:
1.
TURN OFF THE DTMF MUTING FEATURE. If you do this, then all DTMF digits will be
repeated. This is a simple fix, but it may not be acceptable for repeaters that need security
on DTMF commands.
2.
DON'T OVERDRIVE THE DTMF DECODER. The higher the audio level to the decoder,
the more likely it is to false. Check the audio level at the output of the receiver's op amp; if
over the recommended 700 mV rms (2 V p-p), turn it down. Overdriving the decoder does
not help it decode any better.
3.
INCREASE THE DTMF DECODER'S DETECT TIME. The decoder's tone detect time is
controlled by resistor R7, which is 300k as shipped from the factory. This provides a rather
fast 40 ms detect time (a telephone company standard). Increasing R7 will require a longer
detect time, thus reducing the possibility of falsing.
The disadvantage of slowing the detection time is that some DTMF autodialers generate
digits at a rapid rate; a long detection time could result in missed digits.
If autodialers are not a problem, then increasing the detection time will make a noticeable
improvement in falsing. Field experience has shown that changing R7 to 500k will solve
minor cases, and changing it to 1M solves nearly all cases. The slower response to DTMF
digits has not been a problem for users, since the increase in time is small by human
operator standards.
