Delay And Hands-Free Talking - Siemens HG 3500 V4 Service Manual

gateways_02_terms.fm
Terms
Runtime and Noticeable Effects
24
If an echo is generated on the B side (e.g. because of the 2-wire/4-wire hybrid of
analog terminal devices, also known as hybrid coil), this is noticed by the speaker,
who finds himself speaking over what he said a half a second before, for example.
With high transmission runtimes, it is therefore generally necessary to have echo-
free signals on the transmission path. With IPDA, this is achieved through active
echo cancellation in the HiPath HG 3500 and HG 3575 modules, which filters out
the signal section received from A in the send signal from B (the echo) before
transmission of the send signal from B.

2.6.2 Delay and Hands-Free Talking

A common problem is the use of hands-free equipment where there are high
transmission delays. Transmission delays are not exclusively generated by IP
transmissions. The problem can also be observed with satellite links, for example.
To suppress feedback and echo effects, the hands-free equipment lowers the
playback signal in the loudspeaker when a signal is received by the user's own
microphone (automatic gain control). Voice activity prevents the communication
partner from being heard. This does not lead to problems in situations where
communication is disciplined and the transmission delay is low. Problems arise
when the transmission delay is high. Figure 12 Delay and hands-free talking
shows how this works. Speaker A is impatient and does not wait long enough for
an answer from speaker B before speaking again. Speaker A's impatience
prevents him from hearing the answer from B which is retarded by the
transmission delay. This leads to misunderstandings.
The type of problems caused by transmission delays in hands-free talking are
generally linked to the type of hands-free equipment used. Half duplex hands-free
equipment (as described) is the most widely used and most sensitive equipment
available. Optiset telephones have half duplex hands-free talking. The hands-free
setting "noisy room" is generally the best setting for connection to an IP-based
access point (in normal rooms as well).
Optipoint telephones with full duplex hands-free talking are less likely to be
affected by the phenomena described above.
HiPath 4000 V5, IP Solutions - HG 3500 & HG 3575, Service Documentation
A31003-H3150-S104-2-7620, 03/2009