Preliminary Information

MT8843

5-25

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

pulled low as C3 discharges. TRIGout stays low as

long as the C3 voltage stays below the minimum

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.

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.

Item

BT

Bellcore

Low tone

frequency

2130Hz ±1.1%

2130Hz± 0.5%

High tone

frequency

2750Hz±1.1%

2750Hz±0.5%

Received

signal level

-2dBV to -40dBV

-37.78dBm)

-14dBm to -32dBm

per tone off-hook

Signal reject

level

-46dBV

(-43.78dBm)

-45dBm

Signal level

differential

(twist)

up to 7dB

up to 6dB

Unwanted

signals

<= -20dB

(300-3400Hz)

near end speech

Duration

75ms to 85ms

Speech

present

No

Yes

Table 1. Dual Tone Alert Signal Characteristics

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.

present detect time (refer to timing between ESt, St/

GT and StD in Figures 15 and 18).

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