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The two most critical states of the echo canceller are receive and double-talk. The receive state is the only
opportunity for the echo canceller to converge correctly. It is also the time when the echoes are most
noticeable because they are not masked by speech from the other side. During the receive state, the echo
canceller must converge rapidly and apply nonlinear processing to further reduce the echo. If the state
machine does not detect a receive state correctly, echoes remain audible.
Double-talk is most frequently mistaken for other states and has the most drastic effects on the sound
quality when it is incorrectly detected. If the state machine confuses double-talk for a receive state, it may
decide to start converging. If it does, it will try to converge to the near talker’s speech as well as the room
response. This causes the canceller to go out of convergence. It may also apply nonlinear processing. This
results in excessive attenuation, noisy or scratchy speech, or half-duplex behavior, depending on what
methods of nonlinear processing are implemented.
When the state machine switches between states, there should be no audible transition. On a poorly
implemented state machine, there could be noticeable changes in volume level, changes in background
noise level, or even audible clicks as the state machine changes states. These would be especially
noticeable during the beginning and end of pauses in conversation, or even between words. The state
machine may even transition several times between modes, making an annoying series of clicks.
The Listening Test
The listening test is the most important part of the evaluation of the acoustic echo canceller. It is the only
time to evaluate the performance of the state machine, which is the most important factor for audio quality.
The listening test environment should be taken into consideration if different echo cancellers are not all
tested in the same location.
The listening test is the only
way to evaluate state
machine performance.
A panel of several people should be chosen to evaluate the echo
canceller. If possible, the same people should evaluate all of the echo
cancellers under consideration during a short period of time. These
people should listen for the common problems listed in Table 3, as well
as for the overall audio quality.
The most important part of the evaluation is on the opposite end of the echo canceller (the far or remote
end). This is where the echo would be heard in the first place and most of the echo canceller’s problems
become evident. If the echo canceller is sold as part of a complete system (including microphones and
speakers), some evaluation also should be done on the near end to ensure all of the audio components are
of good quality.
On the opposite end of the echo canceller, either a handset or another echo canceller of the same type
should be used. A listening test should not be performed with a half-duplex speakerphone or a different
echo canceller on the other end. Otherwise, it would not be clear which end had problems.
Ideally, the operating environment for testing should be similar for all of the echo cancellers, since room
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