Installation
C-21
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, the result of falsing is the loss of repeat audio
for a syllable or two. In addition, an accidental digit is stored into the controller’s
command buffer. This last item is not usually a problem, however, because the
digit will be discarded after a few seconds when the controller discovers that
the digit was no part of a valid command. In any case, falsing is annoying and
can usually be cured to an acceptable extent.
Several solutions to falsing are available:
• Turn OFF the DTMF muting feature. Use the Select DTMF Decoder Mute
Delay command on page 11-8. If you do this, all DTMF digits will be
repeated. This is a simple fix, but it may not be acceptable for repeaters
that need security for DTMF commands.
• 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 (2V p-p),
turn the audio level down. Overdriving the decoder will not help it decode
any better.
• 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 40mS 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,
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 will solve 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.
Real-Time Clock
The Controller features a year 2000-compliant hardware real-time clock and
calendar circuit with a lithium power source. Accurate time keeping is
maintained during loss of main power.
The clock and calendar circuit is accurate to ±1 minute per month. For
improved accuracy, commands are provided to manually or automatically
adjust the clock to compensate for drift. (See page 8-1.)
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