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6.1.2
Using a Speaker and a Microphone without Echo Cancellation
Near−End
Speaker
Figure 6.2: VoIP Conversation Using Microphone and Speaker, No Echo Cancellation
Figure 6.2 shows a situation where the Near-End Speaker has forsaken his/her headset
and uses either the PC's own microphone and speakers or a speaker phone that doesn't
have echo cancellation. In this case the Far-End Signal (FES) coming from the Far-
End Speaker gets mixed with the Near-End Signal at the microphone of the Near-End
Speaker. So, instead of sending just NES, the Near-End Speaker sends a signal that
contains both NES and a delayed and distorted version of FES, called FES'.
The end result is that the Far-End Speaker will hear both his/her own speech as well as
what the Near-End Speaker says. Network delays can be up to several seconds long, and
hearing one's own voice echoed loudly with such a delay badly confuses a conversation,
leading to an unpleasant experience for the Far-End Speaker.
As an example, Skype takes some measures to limit echo from setups like this by dynam-
ically adjusting audio gains, but its power is limited. Even when gain adjustment does
work, it may sound unpleasant. The Near-End Speaker can diminish echo by turning
volume down on his speaker, but that often makes the other end hard to understand.
6.1.3
Echo Cancellation Helps Reducing Speaker Echo
Near−End
Speaker
Figure 6.3: VoIP Conversation Using VoIP Speaker Phone with Echo Cancellation
Figure 6.3 shows a conversation where the Near-End Speaker is using the "VS1000+1003
VoIP Speaker Phone" connected to the PC through USB. As in the previous image, the
microphone is recording a signal that has both the NES and unwanted FES' signals.
However, echo cancellation monitors the FES and NES+FES' signals and based on the
data builds a reverse room model. This model is used to cancel the FES' signal out so
that ideally only the NES signal passes to the Far-End Speaker.
It is important to note that echo cancellation is not meant to help you! It is meant to
help the guy on the other end. If you are hearing excessive echo on your earphones or
speaker, it is because the other end doesn't have proper echo cancelling!
Rev. 2.20
VS1000+1003 VoIP Speaker Phone
PC
FES
FES
NES+
NES+
FES'
FES'
Skype
PC
phone
FES
FES
FES
Echo
NES+
NES
cancel
FES'
2009-12-07
6. ECHO CANCELLATION
Internet
NES+
FES'
FES
Internet
FES
NES
NES
FES
VSMPG
Far−End
PC
Speaker
FES'+
NES
FES
Far−End
PC
Speaker
NES
FES
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