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TONE SIGNALING FORMATS
Signaling Format Compatibility
In tone remote systems, the industry standard 2175 guard tone is filtered out by the tone remote adapter at the
base station so that it is not heard over the air. This filter can affect signaling tones close to 2175 Hz. Tones within
+/-70 Hz could be attenuated to a level such that they cannot be decoded by receiving equipment. There are
several ways to deal with this issue:
1. Choose a format that will not be affected.
2. Do not use the affected tone(s).
3. Change the guard tone.
The last solution involves re-tuning the notch filters on the TRC. This can be done at the factory upon request
(recommended). It also requires that the guard tone and notch filters on the tone remote adapter be changed as
well. While Midian’s products allow for this, not all do. Please note that this issue often applies to decoding as
well. Many tone remote adapters notch 2175 out of the receive audio as well. This possibility should be
investigated if decode problems are experienced. The following sections address the formats and tones that are
likely to be affected.
ZVEI:
All ZVEI formats use 2200 Hz for either the ‘0’ or the ‘9’ tone. This is only 25 Hz away from the standard
guard tone frequency of 2175. The best solution to this problem is to simply not use the ‘0’ tone when encoding
DZVEI and not use the ‘9’ tone when encoding ZVEI or DDZVEI formats. Alternatively, the guard tone could be
changed to another frequency. If this is done, it should be at least 150 Hz away from the nearest frequency used
by the format. 2970 Hz is a common choice.
CCIR and EEA:
CCIR and EEA both use the tone 2110 Hz as the ‘repeat’ tone. This tone indicates that the
current digit is the same number as the last digit. The best solution to this problem is to simply not use codes that
have repeating digits. That is to say, do not use a code where a digit appears next to itself in the code such as
‘12334’ which has two 3’s in a row. Another solution is to change the guard tone. If this is done, it should be at
least 150 Hz away from the nearest frequency used by the format. 2970 Hz is a common choice.
Other Affected Formats:
REACH two-tone, tone group A, uses 2274, 2196, and 2121 Hz to represent the digits
6, 7, and 8. Do not use tone group A if using this format. PLECTRON tones 2260 and 2164 should not be used.
Formats Not Affected:
The following formats do not use tones between 2105 and 2245 and therefore should be
unaffected by the guard tone filter: AVCALL, DTMF, EIA, EUROSIGNAL, GE, MODAT, NATEL, QUICK CALL I,
and QUICK CALL II.
