Polycom OMNISMART OMNIVS500U - Manual (Page 12)

A Short Glossary of AEC Related Terms
Near end The end of the connection where the echo canceller is located. This is where the echoes take
place. Also, see far end.
Far end The end of the connection opposite the echo canceller. The user on this end could be using a
handset or another hands-free system. This is where the performance of the echo canceller are most
noticeable. Also, see near end.
Double-talk The state of the echo canceller when there is speech on both ends of the connection. This is
the most difficult state to detect accurately, and most problems with audio quality occur during double-talk.
Bandwidth The frequency range the canceller passes without attenuation. For narrow-band (telephone)
applications, this is defined as 300-3000 Hz. For wide-band (videoconferencing, etc) applications this is
defined as 50-7000 Hz.
Initial Convergence Time The time it takes for the echo canceller to converge to a specified amount of
cancellation. In G.167, the canceller must have an Initial Convergence Time of 1 second, and must achieve
20 dB of cancellation within this time. Manufacturers may alternatively refer to the Convergence Rate of
the echo canceller on their data sheet.
Convergence Rate The rate at which the echo canceller converges when it begins to converge. This is
measured in dB per second. In order to meet the Initial Convergence Time of G.167, an echo canceller must
have a convergence rate of at least 20 dB/sec.
Howling Pitched, squealing tones that occur when hands-free systems at both ends of a connection have
open speakers and microphones. This is similar to microphone feedback in an auditorium. Howling can
cause damage to audio components if it is not attenuated.
Half-duplex Behavior of most speakerphones, which prevents howling and acoustic echo by only
allowing one party to talk at a time.
Tail length The length of the filter which cancels echoes (measured in ms). The more reverberation a
room has, the longer the tail length will need to be.
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