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<h1>Acoustic Echo Cancellation (AEC)</h1>

<p>On the MS (Mobile Station) side, the microphone picks up
the voice coming from the earpiece of the MS, and an acoustic echo will travel
by air and along the body of the MS. The GSM recommendation 3.50 states that
a handset and a hands-free MS should perform acoustic echo cancellation. In
other words, the mobile should have a built-in echo suppresser or canceller,
and no acoustic echo cancellation should be needed on the network side. However,
it seems that some mobiles are not capable of removing the acoustic echo sufficiently,
and the subscriber may sometimes hear the mobile originated echo. In an MS-to-PSTN
call, this means that the PSTN subscriber hears his/her own voice as an echo
with a delay of about 200 ms if the echo cancellation in the MS is not successful.
The same problem exists also in MS-to-MS calls although in these cases the
delay is naturally about 400 ms. In these cases, the echo cancellation should
be done by the built-in echo canceller of the MS as the normal echo cancellation
in the MSC does not perform any echo cancellation in the uplink direction.
Because of the reasons explained above, the Acoustic echo cancellation feature
has been included into the TRAU software.</p>


<p>The new version of the Nokia AEC operates with all three
GSM speech codecs (FR, HR, and EFR), and the uplink DTX state will no longer
be relevant to proper operation. Another improvement in the new version of
the AEC is the possibility to compensate long fixed transmission delays by
giving a command from the user interface to adjust the AEC window to the known
additional delay (0-620 ms) in 20 ms steps.</p>


<p>The AEC support for each codec can be switched on/off,
and additional delay can be controlled by the transcoder MML.</p>


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<dl class="note-notification">
<dt class="note-caption">Note</dt>
<dd>AEC for HR and EFR is not available for TCSM (TCSM2 required).</dd>
</dl>


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<dl class="note-notification">
<dt class="note-caption">Note</dt>
<dd>The additional delay improvement feature can be used to
handle delay caused by whatever transmission equipment (satellite connections
are one example) in the AEC operation.</dd>
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