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Tone Generation
The SH POTS system allows the injection
of user programmable tones, inde-
pendently per channel, for signalling or
user test purposes. Per channel, a tone
comprising two programmable (sine-
wave) frequencies and programmable
amplitudes can be generated (in this
The amplitude of each frequency within
the tone can be independently set from
0 to the maximum level in 256 linear
amplitude steps (8 bit value), with
n=63 corresponding to 0dBm on the
line. From this, the line signal level Vtl
for a given gain factor n is given by:
Vtl = 20 log(n/63) in dBm.
or
n = int(63*10^(Vtl/20) + 0.5)
Table 11 lists values for n for a range
of tone signal levels.
The tone frequency is given by:
Fout = 250 * N / 256 Hz, where N is a 16 bit value
(thus, N=1024 yields a tone of 1 kHz)
or
N = int(Fout*256/250 + 0.5)
The tone generated has continuous
phase if the programmed frequency is
changed during the course of a tone
(this is not so if the generator is stopped
and restarted). Tables 2 and 3 list the
values of N required to generate com-
monly occurring frequencies, and the
resulting error.
way, the most common call-progress and
information tones, melody notes or DTMF
tones, can be synthesised). The tone sig-
nal is added to the speech signal (the
user must be aware of possible clipping
which may occur if high signal levels are
programmed), or the speech
Table 12: Tone signal levels (common values)
dBm
n
3
89
1.5
75
0
63
-1.5
53
-3
45
-6
32
-8
25
-10
20
-15
11
-20
6
-30
2
-36
1
Note that it is possible to generate tones
of very high amplitude. The user must
ensure that the amplitude parameter is
23
MTK-40131
signal can also be muted during a tone
burst. The tone burst duration is under
user control only (the control bits for mute
and tone insertion occupy the same reg-
ister, which simplifies the generation of
tone bursts).
Actual
err (dB)
3.00
0.00
1.51
0.01
0.00
0.00
-1.50
0.00
-2.92
0.08
-5.88
0.12
-8.03
-0.03
-9.97
0.03
-15.16
-0.16
-20.42
-0.42
-29.97
0.03
-35.99
0.01
programmed before the tone is
enabled.
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