Preliminary Information
The diode bridge shown in Figure 3 half wave
rectifies a single ended ring signal. Full wave
rectification is achieved if the ringing is balanced. 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, TRIGout high to
low transition may be used to interrupt or wake up
the microcontroller. The controller can thus be put
into sleep mode to conserve power.
Dual Tone Alert Signal Detection
According to SIN227 the Idle State Tone Alert Signal
allows more reliable detection of Caller Display
Service signals. The Idle State Tone Alert Signal
follows the line reversal and a silence period. The
characteristics of the BT’s idle state alerting tone is
shown in Table 1.
MT8843
Bellcore specifies a similar dual tone alert signal
called CPE Alerting Signal (CAS) for use in off-hook
data transmission. Bellcore states that the CPE
should be able to detect, in the presence of voice,
the CPE Alerting Signal.
The dual tone alert signal is separated into the high
and low tones with 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.
Detect and non-detect guard times can be
implemented with external RC components. The
guard times improve detection performance by
rejecting signals of insufficient duration and masking
momentary detection dropout. StD is the guard time
qualified detector output.
•
Dual Tone Detection Guard Time
When the dual tone alert signal is detected by the
CNIC2, ESt is pulled high. When the alerting signal
ceases to be detected, ESt goes low.
Figure 4 shows the relationship between the St/GT,
ESt and StD pins. It also shows the operation of a
guard time circuit. The guard time circuit improves
detection performance by rejecting detections of
insufficient duration and by allowing momentary ESt
dropouts once the duration criterion has been met.
The total recognition time is t
REC
= t
GP
+ t
DP
, where
t
GP
is the tone present guard time and t
DP
is the tone
present detect time (refer to timing between ESt, St/
GT and StD in Figures 15 and 18).
The total tone absent time is t
ABS
= t
GA
+ t
DA
, where
t
GA
is the tone absent guard time and t
DA
is the tone
absent detect time (refer to timing between ESt, St/
GT and StD in Figures 15 and 18).
Bellcore states that it is desirable for an off-hook
capable CPE to have a CAS detector on/off switch.
The switch was conceived so that a subscriber who
disconnects a service that relies on CAS detection
(e.g., CIDCW), but retains the CPE, can turn off the
detector and not be bothered by false detection.
SW1 in Figure 4 performs the above function. In the
B position, the comparator input, hence StD, is
always low. The CAS detector will not be enabled
and its output will not cause interrupts (except for the
system power up condition described in section
“Interrupt” on page 28’).
BT states that the idle state tone alert signal
recognition time should be no less than 20ms when
5-25
Item
Low tone
frequency
High tone
frequency
Received
signal level
BT
2130Hz ±1.1%
2750Hz±1.1%
Bellcore
2130Hz± 0.5%
2750Hz±0.5%
-2dBV to -40dBV -14dBm to -32dBm
per tone on-hook
a
per tone off-hook
(0.22dBm
b
to
-37.78dBm)
-46dBV
(-43.78dBm)
up to 7dB
-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
near end speech
75ms to 85ms
Yes
c
Table 1. Dual Tone Alert Signal Characteristics
a. The off-hook signal level is -15dBm to -34dBm per tone to be
specified in the BT CIDCW specification in the future.
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.
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