Digital Signal Processing Reference
In-Depth Information
Figure 15-6.
Packet discarded at the receiver because of their late arrival (delay larger than 20
ms) as a function of the bit error rate. Adaptive vs. not-adaptive technique.
shows the percentage of packets discarded at the receiver due to late arrival (de-
lay > 20 ms). If the speech frame dimension is adapted to the channel condition,
packets tend to arrive on time for successful playback, leading to higher percep-
tual quality.
Even more positive results were obtained for wideband speech transmission,
for which a larger range of source rates is available. The GSM AMR wide-band
codec [2] at rates of 23.85 kb/s and 6.6 kb/s was used in the same wireless sce-
nario. The behavior of a single VoIP communication without interfering traffic
is plotted in Figure 15-7 where the even larger advantage of the adaptive solu-
tion over the constant bit-rate transmission is clearly noticeable with respect to
Figure 15-4: with BER equal to
the packet loss rate is more than
halved (from 4.3% to 2%).
