Data Sheet
2. Ring Burst Detection
CCA does not support the dual tone alert signal
(refer to Dual Tone Alert Signal Detection section).
Instead, CCA requires that the TE be able to detect a
single burst of ringing (duration 200-450ms) that
precedes CLIP FSK data. The ring burst may vary
from 30 to 75Vrms and is approximately 25Hz.
Again in a TE designed for CCA CLIP, the TRIGout
high to low transition may be used to interrupt or
wake-up the microcontroller. The controller can thus
be put into power-down mode to conserve power in a
battery operated TE.
3. Ring Detection
In Bellcore’s CND/CNAM scheme, the CID FSK data
is transmitted between the first and second ringing
cycles. The circuit in Figure 3 will generate a ring
envelope signal (active low) at TRIGout for a ring
voltage of at least 40Vrms. R5 and C3 filter the ring
signal to provide an envelope output.
The diode bridge shown in Figure 3 works for both
single ended and balanced ringing. A fraction of the
ring voltage is applied to the TRIGin input. When the
voltage at TRIGin is above the Schmitt trigger high
going threshold V
T+
, TRIGRC is pulled low as C3
discharges. TRIGout stays low as long as the C3
voltage stays below the minimum V
T+
.
In a CPE designed for CND/CNAM, the TRIGout
high to low transition may be used to interrupt or
wake-up the microcontroller. The controller can thus
be put into power-down mode to conserve power.
If precise ring duration determination is critical,
capacitor C3 in Figure 3 may be removed. The
microcontroller will now be able to time the ring
duration directly. The result will be that TRIGout will
be low only as long as the ringing signal is present.
Previously the RC time constant would cause only
one interrupt.
Dual Tone Alert Signal Detection
The BT on hook (idle state) caller ID scheme uses a
dual tone alert signal whose characteristics are
shown in Table 1.
Bellcore specifies a similar dual tone alert signal
called CPE Alerting Signal (CAS) for use in off-hook
data transmission (see Table 1). Bellcore states that
the CPE should be able to detect the CAS in the
presence of near end speech. The CAS detector
should also be immune to imitation from near and far
end speech.
MT88E43
In the MT88E43 the dual tone alert signal is
separated into a high and a low tone by two
bandpass filters. A detection algorithm examines the
two filter outputs to determine the presence of a dual
tone alert signal. The ESt pin goes high when both
tones are present. Note that ESt is only a preliminary
indication. The indication must be sustained over the
tone present guard time to be considered valid. Tone
present and tone absent guard times can be
implemented with external RC components. The
tone present guard time rejects signals of insufficient
duration. The tone absent guard time masks
momentary detection dropout once the present
guard time has been satisfied. StD is the guard time
qualified detector output.
Item
Low tone
frequency
High tone
frequency
Received
signal level
BT
2130Hz
±1.1%
2750Hz
±
1.1%
-2dBV to -40dBV
per tone on-hook
a
(0.22dBm
b
to
-37.78dBm)
-46dBV
(-43.78dBm)
up to 7dB
Bellcore
2130Hz
±0.5%
2750Hz
±
0.5%
-14dBm
b
to
- 32dBm per tone
off-hook
-45dBm
up to 6dB
Signal reject
level
Signal level
differential
(twist)
Unwanted
signals
Duration
Speech
present
<= -20dB
(300-3400Hz)
88ms to 110ms
d
No
<= -7dBm ASL
c
near end speech
75ms to 85ms
Yes
Table 1 - Dual Tone Alert Signal
Characteristics
a. In the future BT may specify the off-hook signal level as
-15dBm to -34dBm per tone for BT CIDCW.
b. The signal power is expressed in dBm referenced to 600 ohm
at the CPE A/B (tip/ring) interface.
c. ASL = active speech level expressed in dBm referenced to
600 ohm at the CPE tip/ring interface. The level is measured
according to method B of Recommendation P.56 "Objective
Measurement of Active Speech Level" published in the CCITT
Blue Book, volume V "Telephone Transmission Quality" 1989.
EPL (Equivalent Peak Level) = ASL+11.7dB
d. SIN227 suggests that the recognition time should be not less
than 20ms if both tones are detected.
5-57
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