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If it does not choose properly, speech may be distorted by the AEC or the canceller may go out of convergence. Since modes change frequently during conversations (especially when there are more than two people participating), state machine performance is extremely important. The basic factors of the state machine’s performance are 1. The accuracy of determining the correct state 2. Impact on the signal if the wrong state is selected 3. How gracefully it switches between states. 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 only way to evaluate state machine performance. 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. 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 acoustics have such a large impact. If this cannot be arranged, at least consider the operating environment differences in each case in the final decision. If possible, listen to the room acoustics with the echo canceller disabled so the effects of the different rooms can be compared. Seven Things to Listen For • Residual Echo. If there is excessive residual echo, the sound may have a hollow, distant quality or there may even be distinct audible echoes. This is especially noticeable during the receive mode, when there is no near speech to mask the echo. If this is due to a short tail length, the residual echo may sound delayed. • Loss of Convergence. When the echo canceller loses convergence, the result is an audible residual echo that could be louder than an echo with no echo canceller at all. This is generally caused when the state machine mistakes a double-talk situation for a receive state. If this happens, the echo canceller begins to adapt to the near talker’s speech as well as the echo, and goes out of convergence. • Howling. Pitched squealing noises may occur when both parties have hands-free systems with open speakers and mics. This is caused by either a lack of howling rejection, or howling rejection that is not working properly. • Attenuated Speech During Double-talk. Noticeable changes in volume levels may occur...

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