Call Progress Tone Detection
Call progress tones are audible tones sent from switch-
ing systems to calling parties to show the status of
calls. Calling parties can identify the success of a call
placed by what is heard after dialing. The type of tone
used and its timing vary from system to system, and
though intended for human ears these signals can pro-
vide valuable information for automated calling sys-
The M-980-02 is a signal detector sensitive to the fre-
quencies most often used for these progress tones.
Electronic equipment monitoring the DETECT output
of the M-980-02 can determine the nature of signals
present by measuring their duty cycle. See Figure 4 for
a diagram of a circuit that could be used to permit a
microcomputer to directly monitor tones on the tele-
phone line. Much of the character of the progress
tones is in their duty cycle or cadence (sometimes
referred to as interruption rate). This information, cou-
pled with level and frequency indication from the M-
980-02, can be used to decide what progress tones
have been encountered.
Table 5 shows some call progress tones with on/off
times; 0.25/0.25 being 250 ms on, 250 ms off on a
repeating basis. For example, dial tones as shown in
the table are usually “on” continuously and last until the
first dial digit is received by the switching system. Line
Busy, on the other hand, is turned off and on at a rate
of 1 Hz with a 50% duty cycle, or an interruption rate of
60 times per minute (60 IPM). The tones can be dis-
tinguished in this way.
It should be noted that while such techniques will usu-
ally be effective, there are some circumstances in
which the M-980-02 cannot be accurately used.
Examples include situations where ringback tone may
be short or not even encountered. Ringback may be
provided at ringing voltage frequency (20 or 30 Hz)
with some harmonics and may not fall in the detect
range, and speech or other strong noise may obscure
tones making cadence measurement difficult.
Detection of “answer” is most difficult for many rea-
sons. One way to determine if a called party has
answered is by looking for a short burst of DETECT
indications without a cadence match (produced by a
click and “hello” at the far end). Some applications will
require special methods like speech detection, but
most can be reliably handled with the M-980-02 and
simple cadence measurement.
As can be seen, the tones used for the same purpos-
es in different systems may not be the same.
Standards do exist and should be consulted for your
particular application. In North America AT&Ts “Notes
on the Network” or EIAs RS-464 PBX standard should
be reviewed. In Europe tone plans may vary with
locale, in which case the CEPT administration in each
country must be consulted. Outside these areas,
national PTT organizations can provide information on
the systems within their borders.
A valid tone will always be detected if it is at least 40 ms long.
tID 40 ms @ -38 dBm Inter-tone gaps must be detected if greater than this duration.
85 ms @ -10 dBm
Drop-outs of valid tone of up to 18 ms will be ignored.
Application or removal of high level signals outside the must detect range may cause momentary detection, which may be filtered by time guarding the output.
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