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(Echo/Echo Canceler)
The signal from FAX1 reaches FAX2 via stations 1 and 2, but the reflection signal at station 2 also
returns via station 1 (echo). As the distance between station 1 and station 2 is far, the echo returns to
FAX 1 at a max of 600 msec after transmission.There is a possibility that this signal is detected
erroneously as the signal from FAX2. For a normal call, there is also a possibility that the echo of
their own voice will make the call difficult to understand. For this reason, each station (station1 and
station 2) attaches echo cancelers (S1 and S2) for international lines or long distance lines. For the
echo canceler, the level of the transmission signal from FAX 1 is compared with the level of the
reception signal from FAX2. When the transmissionsignal is larger, S1 is closed while S2 is opened
when it is smaller. In other words, for transmission from FAX1, S1 is closed and S2 is open, so that
the echo does not return to FAX1.
(Causes and Countermeasures)
No. Countermeasure
Echo Communication Problem Example
Side
1
Sending side
Some time is needed to compare the level of the receiving
and sending signals for the echo canceler. The header of the
training signal lacks due to a switching delay to close S1.
2
Receiving side
The echo canceler function stops according to a CED signal
frequency of 2100Hz (S1 and S2 are both ON), a DIS signal
is returned as an echo, and a DCS signal from the sending
side overlaps theDIS echo. Then the receiving side FAX
Receiving side
cannot retrieve the DCS signal.
(Refer to Fig. a)
Sending side
3
Sending side
Communication failure occurs in a long distance
communication on the telephone line without an echo
canceler.
4
Sending side
or
Receiving side
There are some cases (e.g. Mobile comms.) which causethe
collision of TX / RX signals due to the delay / echo and noise
5
Sending side
of the network / terminal. (Refer to Fig. e)
Receiving side
file://C:\Documents%20and%20Settings\Admin\Plocha\SCHÉMATA\VIDEA\PAN...
Fig. a (Overlapping the Echo of the DIS signal and DCS signal)
Fig. b (Countermeasure by Changing the CED Frequency)
http://servis-manual.com/
Countermeasure
Add a dummy signal to the beginning of the
training signal.
Change to a 1100Hz CED signal frequency.
(Refer to Fig. b)
Change the regular rime of 75 msec between
the CED signal and DIS signal to 500 msec.
This will give at least 250 msec to recover
the echo canceler operation. (Refer to Fig. c)
The sending side FAX sends a DCS signal
not after receiving the 1st DIS signal but after
receiving the 2nd DIS signal. (Refer to Fig.
d)
Decrease the transmission level from -10
dBm to -15 dBm and the echo level will
decrease.
Decrease the receiving sensitivity from -13
dBm to about -32 dBm so an echo signal will
not received.
Set additional Pause time (Service mode:
code No. 774) in between the original and its
repeated signals, to prevent the collision of
the signals at both end.
Stránka č. 2 z 4
Service Code
Service code (521)
(International mode
select)
This countermeasure
becomes the default
value.
Service code (520)
(CED frequency
select)
Service code (593)
(Time between CED
and 300 bps)
Service code (594)
(Overseas DIS
detection select)
Service code (596)
(Transmit level set)
Service code (598)
(Receiving
sensitivity)
Service code (774)
(T4 timer)
27. 10. 2004
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