MX800 Transceiver
Technical Manual
1.6 Five Tone Encoded / Decoder (SELCALL)
Selective Calling or SELCALL is a type of squelch protocol used in radio communications
systems, in which transmissions are preceded by a brief burst of sequential audio tones. A typical
application of SELCALL is for controlling the squelch of receivers based on the transmitted tone
sequence. Another application of sequential tone sequences is Automatic Number Identification
(ANI) where each radio in the system is assigned a unique tone sequence that is sent either on the
initiation of a call (leading edge ANI) or at the end of a call (trailing edge ANI). Sequential tone
sequences can also be used to remotely control other radios in the system.
Most SELCALL systems utilise a sequence of 5 tones, each of which can have any of ten
frequencies, plus 2 extra frequencies for 'Group' and 'Repeat'. The SELCALL system uses the
'Repeat' character when transmitting a sequence of sequentially repeated characters, e.g. when the
sequence '11122' is to be transmitted, the SELCALL encoder will translate the sequence as
'1E12E' where the character 'E' is the 'Repeat' character
.
1.6.1 SELCALL Tone Sets
The nominal tone durations for each SELCALL system is given in the table below:
Table 1-3 Nominal SELCALL Durations
Duration
EEA
System
CCIR System
CCIR-70
System
ZVEI
System
Start
interval
100 ms
(min)
100 ms (min)
70 ms
(min)
140 ms
± 15 ms
Tone
Duration
40 ms
± 4 ms
100 ms ± 10 ms
70 ms ± 10
ms
70 ms ±
15 ms
The start interval is the minimum silence period that must be satisfied before the transmission of the
SELCALL sequence. The actual silence period can vary depending on whether the radio is already
transmitting. If transmission is initiated by a SELCALL sequence then the silence preamble will be
extended to allow the radio and the system to settle prior to the transmission of the SELCALL sequence.
The extended preamble will be user programmable and is added to the standard SELCALL silence
preamble, see
Changing the SELCALL transmit preamble
and
. The tone frequencies assigned to each tone set implemented in the T19/T26
option board is shown in
Table 1-4 Standard SELCALL Frequencies
.
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© SPECTRA ENGINEERING 2010 Revision 1.0.4