Performance Of Echo Cancellation; Limits Of Echo Cancellation - Vlsi VS1000+1003 User Manual

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6.2

Performance of Echo Cancellation

Echo cancellation of the VoIP Speaker Phone is effective upto 128 ms and the cancellation
ratio is over 25 dB over the range of approximately 100. . . 3500 Hz. The phone requires
approximately 2. . . 10 s of clear Far-End Sound to adjust after startup. In practical
applications this is usually taken care of by the VoIP ringing tone that gives the echo
cancellation algorithm a good room model.
Echo Cancellation works best when the room is relatively quiet. Don't put the unit in
immediate proximity of e.g. computer fans.
6.3

Limits of Echo Cancellation

Echo cancellation is not an all-in-one solution. It is only one component in solid acoustic
design. It is important to take the utmost care to get linear speakers that have as little
freqeuncy response peaks as possible.
Echo cancellation cannot correct for non-linear systems. If a speaker clips or distorts
sound, echo cancellation will not work and audible pops will be heard by the Far-End
Speaker (unless a VoIP program like Skype itself happens to remove them by its fuzzy
logic echo limiter). Thus, speakers should not be overdriven.
While 25 dB echo cancellation is good, you still have to design the layout and acoustical
structures of the product in such a way that the microphone and speaker interfere as little
with each other as possible. It is particularly important to avoid resonance frequencies.
Acoustic design should not be underestimated. In the same directory as this document,
there is a separate document giving some hints on acoustic design, called VoIP Acoustic
Design (VoipAcoustic.pdf). This document, which includes a case study, may help you
to avoid some typical pitfalls.
Rev. 2.20
VS1000+1003 VoIP Speaker Phone
2009-12-07
VSMPG
6. ECHO CANCELLATION
Page 22(30)