Advance Information
MT88E45
c
BT
a
b
2130 Hz and 2750 Hz
CAS/DT-AS Characteristics
Bellcore
ETSI
(Off-hook = ‘Loop State’)
(On-hook = ‘Idle State’)
(Off-hook only)
(Off-hook)
Frequency Tolerance
Signal Level (per tone)
Reject Level (per tone)
+/-0.5%
+/-0.5%
Off-hook: +/-0.6%
On-hook: +/-1.1%
d
-14 to -32 dBm
-9.78 to -32.78 dBm
(-12 to -35 dBV )
+0.22 to -37.78 dBm
(-2 to -40 dBV)
e
-45 dBm
On-hook: -43.78 dBm
(-46 dBV)
Maximum Twist (V2130Hz/V2750Hz
)
+/-6 dB
+/-6 dB
+/-7 dB
Duration
75 to 85 ms
75 to 85 ms
Off-hook: 80 to 85 ms
On-hook: 88 to 110 ms
Reject Duration
Off-hook: <=70 ms
On-hook: <=20 ms
Signal to Noise Ratio
Speech
Speech
Off-hook: Speech
On-hook: >= 20 dB
(300-3400 Hz)
Hybrid Op-amp (GS2) Gain
Vdd = 5V +/- 10%
0 dB
0 dB
0 dB
Hybrid Op-amp (GS2) Gain
Vdd = 3V +/- 10%
-3.5 dB
-3.5dB
TBD
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
signalling. (BT calls it Tone Alert Signal.) The CAS/
DT-AS characteristics are summarized in Table 3.
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.
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.
The timing algorithm consists of 2 components: a
tone present guard time (tGP) and a tone absent
guard time (tGA). tGP sets the minimum accept
duration for CAS. That is, both tones must be
detected continuously for tGP 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 tGA. The
purpose of tGA is to bridge over momentary EST
dropouts once EST has met the minimum tone
duration so as to decrease the likelihood of a long
talkoff being broken up into several talkoffs. Usually
tGA is set very short or removed altogether because
there is another way to deal with the problem (by
ignoring further detections for 2 seconds after every
detection).
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)
7
MITEL [ MITEL NETWORKS CORPORATION ]
