Advance Information
2130 Hz and 2750 Hz
CAS/DT-AS Characteristics
Frequency Tolerance
Signal Level (per tone)
Reject Level (per tone)
Maximum Twist (V
2130Hz
/V
2750Hz
)
Duration
Reject Duration
Signal to Noise Ratio
Speech
Speech
Bellcore
a
(Off-hook only)
+/-0.5%
-14 to -32 dBm
d
-45 dBm
+/-6 dB
75 to 85 ms
+/-6 dB
75 to 85 ms
ETSI
b
(Off-hook)
+/-0.5%
-9.78 to -32.78 dBm
(-12 to -35 dBV
e
)
MT88E45
BT
c
(Off-hook = ‘Loop State’)
(On-hook = ‘Idle State’)
Off-hook: +/-0.6%
On-hook: +/-1.1%
+0.22 to -37.78 dBm
(-2 to -40 dBV)
On-hook: -43.78 dBm
(-46 dBV)
+/-7 dB
Off-hook: 80 to 85 ms
On-hook: 88 to 110 ms
Off-hook: <=70 ms
On-hook: <=20 ms
Off-hook: Speech
On-hook: >= 20 dB
(300-3400 Hz)
0 dB
TBD
Hybrid Op-amp (GS2) Gain
Vdd = 5V +/- 10%
Hybrid Op-amp (GS2) Gain
Vdd = 3V +/- 10%
0 dB
-3.5 dB
0 dB
-3.5dB
a. SR-TSV-002476, Issue 1 Dec 1992
b. ETS 300 778-2 Jan 98. The DT-AS plus FSK variant of ETSI on-hook signalling described in ETS 300 778-1 is not supported
because on-hook DT-AS uses the GS1 op-amp. With the GS1 gain in Table 4, the minimum DT-AS level will be below the
MT88E45’s minimum accept level.
c. SIN227 Issue 3 Nov 97, SIN242 Issue 2 Nov 96
d. dBm - Decibels above or below a reference power of 1 mW into 600 ohms. 0 dBm = 0.7746 Vrms.
e. dBV - Decibels above or below a reference voltage of 1 Vrms. 0 dBV = 1 Vrms
Table 3 - CAS/DT-AS Characteristics
CAS Detection
In North America, CAS is used in off-hook signalling
only. In Europe (ETSI) it is used in off-hook
signalling, and by BT in both on and off-hook
signalling. ETSI calls it the Dual Tone Alerting Signal
(DT-AS). Although the ETSI on-hook standard
contains a DT-AS specification, BT is the only
administration known to employ CAS in on-hook
signalling. (BT calls it Tone Alert Signal.) The CAS/
DT-AS characteristics are summarized in Table 3.
Table 3 shows the Hybrid op-amp gain for operation
at 3V and 5V nominal Vdd, with a
±
10% Vdd
variation. For 3V operation, the Hybrid op-amp gain
should be reduced from the 5V setting to maintain
the CAS reject level and to maintain the talkoff
immunity: the CAS threshold is directly proportional
to Vdd, when Vdd is reduced the threshold becomes
lower, hence lower level CAS are accepted. If the
gain is not reduced, the MT88E45 will be more talkoff
prone.
When CAS detection is selected, the dual purpose
output pin DR/STD is STD. STD goes low when CAS
has been detected, and returns high after CAS has
ended.
CAS Guard Time
The guard time circuit shown in Figure 5 implements
a timing algorithm which determines whether the
signal is a CAS. Proper selection of the guard time(s)
is key to good speech immunity. The first indication
that there might be a CAS is when EST goes high.
EST high indicates that both tones are present. EST
low indicates that one or both tones is not present.
STD low indicates that CAS has been detected.
When STD returns high it indicates that CAS has
ended.
The timing algorithm consists of 2 components: a
tone present guard time (t
GP
) and a tone absent
guard time (t
GA
). t
GP
sets the minimum accept
duration for CAS. That is, both tones must be
detected continuously for t
GP
for STD to go low to
indicate that CAS has been detected. For STD to
return high to indicate that CAS has ended, one or
both tones must have disappeared for t
GA
. The
purpose of t
GA
is to bridge over momentary EST
7
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