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© 2006 ADAPTIVE DIGITAL TECHNOLOGIES, INC. all rights reserved
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DATA SHEET
ADT G.168
G.168
v
9 ECHO CANCELLER Adaptive Digital Technologies, Inc.
The Adaptive Digital Technologies patented, industry standard, carrier class G.168 echo canceller is
available on many DSP and general purpose processors. Adaptive Digital’s echo canceller qualified as
toll-quality at AT&T’s Voice Quality Assessment Labs in Middleton, NJ. AT&T’s Voice Quality
Assessment Lab evaluated Adaptive Digital’s echo canceller using its stringent series of performance
tests including AT&T’s Mean Opinion Score (MOS) subjective tests as well as the standardized set of
G.168 objective tests. The subjective and objective performance of Adaptive Digital’s echo canceller
surpassed even the performance of AT&T’s benchmark lab echo cancellers.
The importance of subjective testing should not be overlooked. Although the ITU’s G.168
recommendation is well thought out, this type of objective test is no guarantee that an echo canceller
will sound good to a human listener. By doing both subjective (MOS) and objective (G.168) testing,
Adaptive Digital ensures the optimum voice quality, which is, after all, what it’s all about!
Another important reason for solid subjective performance is that G.168 is not a bit-exact specification.
When an algorithm is specified in a bit-exact way, every compliant implementation will sound identical.
This is the case for most of the standardized vocoders. In contrast, G.168 specifies test conditions,
excitations, and required minimum output performance. As a result, the voice quality achieved by
different echo cancellers varies greatly. Beyond all the lab testing that can and should be done, there is
no substitute for lessons learned through years of real-world deployment.
The overall importance of echo cancellation should also not be overlooked. From the users
perspective, echo is arguably the worst type of impairment that can be encountered during a telephone
conversation. Typically, if a circuit has echo, the two parties agree to hang up and dial again because it
is so difficult to speak in the presence of one’s own echo.
The moral of the story is – use a top-notch echo canceller. A reputation for good voice quality is difficult
to achieve. A reputation for bad voice quality is easy to achieve and difficult to overcome.
ECHO CANCELLATION OVERVIEW
Echo in the telephone network is caused by hybrid circuits that convert between two-wire analog
interfaces and four-wire analog interfaces, as seen in figure 1
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