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<h1>Adaptive Gain Control/Downlink</h1>

<p>In a transcoder it is also possible to choose an adaptive
gain control in the downlink direction. In order to have an overall solution
to the problems caused by too low speech volume in GSM phones and variable
volume level in the PSTN, an adaptive gain has been implemented in the downlink
direction to the TRAU to guarantee sufficient volume level in the MS.</p>


<p>The gain is attenuated immediately in steps of 1 dB if
any clipping is present in the signal. Clipping is detected by examining the
PCM samples in the transcoder in blocks of 160 samples (20 ms) before conversion
from A-law to linear. Full attenuation is reached in 120 ms. Clipping cannot
be heard as the attenuation is done very fast.</p>


<p> After the attenuation, there is a short waiting period
after which the amplification is started again slowly in 1 dB steps if no
clipping is present. The value of the downlink (DL) gain can vary between
0 dB and 6 dB in TCSM and between 0 dB and 9 dB in TCSM2, and the minimum
and maximum limits are adjustable by MMI in steps of 1 dB within this range
in the TCSM2. The short waiting period is five seconds, and the interval in
1 dB steps is one second.</p>


<p>If there is no speech present in the downlink direction,
the gain is not increased. Also the short waiting period is not updated when
there is no speech present. This is done to avoid increasing the amplification
in cases the other party of the telephone conversation is silent. This may
cause amplification of acoustic echo in mobile-to-mobile calls.</p>
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