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C H A P T E R
6 :
If the length of the subscriber A line is up to 9 km (5.5 miles), delay will be less
than 1 ms. Because the hybrid adds some distortion and multiple reflection, in
general the width of the echo is up to 8 ms.
The delay of the far-end echo will depend on the call type (for example, local
call, national call, international call). If the network is a full four-wire network
only the far-end hybrid will generate the echo. The delay of this echo depends
on the group delay of all devices used by the network.
If the network is still using two-wire devices to carry communications,
additional echo will be generated [NMS99].
In VoIP networks there are many additional potential sources of echoes and
because of the typically much longer network latencies, the echo delays will
also be significantly longer. Multiple echoes are a very common problem in
VoIP networks as well. With mobile networks the situation is even worse. Here
the acoustical path between the speaker and the microphone of the mobile
device is another potential source of echoes, especially with modern, very small
phones.
The algorithm to calculate
the stimulus for the measurement. Consequently, the values obtained by this
algorithm may not be compared to an Echo Return Loss ("ERL") measured
according to ITU-T G.122.
On the other hand, the results achieved when an echo is measured with a
speech signal may differ from those measured with a sine tone as a test
stimulus. For most applications the echo of a real conversation is more
interesting than the echo of a sine tone. The algorithm used to determine the
echo is based on the long term correlation between the two signals. The values
require some time after the start of the measurement until a steady state is
reached.
6.7.2 Interpretation of Echo Parameters
Echoes shorter than app. 5ms are also known as side tone and not perceived as
disturbing, they are even wanted and required to perceive a conversation as
natural.
6.7.3 Signal Acquisition
Figure 6.30 clearly shows there are two kinds of sources that signals can be
obtained from. In all OPERA™ versions files can be assessed. When working
with the OPERA system version including a POTS telephony board or the audio
interface option, OPTICOM's signal acquisition software
which is described in detail in chapter 4. Online measurements with phone
lines are currently not supported.
T E L E P H O N Y
B A N D
T E S T I N G
the echo on a telephone line uses real speech as
149
V O I C E
Q U A L I T Y
OptiCall™
can be used
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