MT88E46 Advance Information
In a telephone adjunct box application where there is
no hybrid or speech IC, if the GS2 gain with respect
to Tip/Ring is to be set to the same as that of GS1,
the GS2 op-amp can be connected as a voltage
follower to the GS1 op-amp output as shown in
Figure 5.
hybrid or speech IC. When near end speech is not
attenuated, such as from a parallel off hook CPE or
when the application is a telephone adjunct box,
talkoff is close to compliant while talkdown is close to
condition 1 (the average case). This mode is
intended for the off hook CPE situation and is
available at the GS2 (4-wire side) input op-amp only.
GS1
GS1 op-amp configured
as in Figure 3 or 4
On Hook Mode CAS Detection
IN2+
IN2-
The ‘on hook mode’ is optimized for talkdown only. It
typically meets talkdown condition 1 without near
end speech attenuation. It is intended for the MEI on
hook CPE situation and must not be used when the
CPE itself is off hook. The input is the GS1(Tip/Ring)
op-amp because in most CPE designs, to detect
CAS when the CPE itself is on hook, the signal must
come from the Tip/Ring connection since either the
telephone hybrid / speech IC is not operational or the
4-wire side signal level is severely attenuated.
GS2
Figure 5 - GS2 Op-Amp Connected as GS1
Voltage Follower
CAS Detection
In the MEI protocol, the ACK-Sender and Backup
ACK-Sender can be on hook or off hook. Therefore, if
the on hook state CPE is the ACK-Sender or the
Backup ACK-Sender, it must be able to detect CAS
when the line is in use. Additionally, an on hook state
MEI CPE must be able to detect CAS during a call so
that it can listen in on the FSK to keep its call log
consistent with the off hook CPEs, as required in
TIA/EIA-777.
When CAS detection is selected, the dual purpose
DR/DET pin is the DET output signal. DET goes low
momentarily (416 µsec nominal) after the end of
CAS to indicate that CAS has been detected, as
shown in Figure 13. The CAS detector operates in
‘off hook mode’ or ‘on hook mode’ as selected by the
CB1 and CB2 pins (see Table 1). ‘On hook mode’
and ‘off hook mode’ use different algorithms
optimized for the CPE states.
The MT88E46 on hook mode algorithm has been
optimized for talkdown because of the way the MEI
protocol works. In MEI, the following events
(described in TIA/EIA-777) occur when CAS is
detected:
Normally DET goes low after the end of CAS.
However, because of interference from speech or
music, DET may go low before the end of CAS. The
on hook and off hook mode algorithms ensure that
DET will occur no earlier than tDET1 after the
beginning of CAS (see Figure 13).
•
Each off hook CPE shall proceed to the on hook
state not earlier than 25ms and no later than
60ms after the end of CAS as measured on Tip/
Ring. (The 25ms delay is necessary to prevent
the on hook transition from corrupting the CAS
for other CPEs that may not have completely
qualified the signal. The additional 35ms that
defines the 60ms upper limit allows for variation
in CAS detection delay.)
Similarly, speech interference can cause DET to go
low later than tDET2 after the end of CAS (see Figure
13). In off hook mode, the detection algorithm
ensures that DET will occur no later than 35ms after
the end of CAS even with speech interference, as
required in TIA/EIA-777. In on hook mode, although
the detection algorithm does not limit the detection
delay from the end of CAS, talkdown typically meets
the SR-TSV-002476 talkdown condition 1 (the
average case) when only the detections which occur
from the beginning of CAS to 35ms after the end of
CAS are counted.
•
•
After detecting a line HIGH state (the line
voltage when the line is not terminated by any
CPE), the ACK-Sender shall go off hook. The
ACK-Sender shall allow the line to remain in the
HIGH state for at least 5ms but not more than
8ms. If no line HIGH state is detected within
100ms after going on hook, all previously off
hook CPE shall return to the off hook state.
Following a CAS the Backup ACK-Sender shall
monitor the line for a line HIGH state lasting a
minimum of 15ms. Once this condition has
been detected, the Backup ACK-Sender shall
Off Hook Mode CAS Detection
The ‘off hook mode’ is Bellcore talkdown and talkoff
compliant when the near end speech is attenuated
8dB or better, such as provided by the telephone
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