Cell Phones; Two Or More Echo Cancellers In Series - Mitel MiVoice Business Voice Quality And Troubleshooting Manual

Analog, TDM, or PSTN Network Issues
loud noise, or ringing. Essentially, this is another instance of double talk. Background noise can
appear to the echo canceller as if someone were talking, thus causing the double-talk effect.
®
This issue is more likely to occur on Bluetooth headsets.

CELL PHONES

We have identified instances when customers were hearing acoustic echo, usually during
double-talk, while talking to people on a cell phone. This echo is due to a failure of the cell
phone's acoustic echo canceller to cancel the echo. Because of the long delays and non-linear
coding in the cellular network, only the echo canceller in the cell phone can cancel this echo.
Cell phone calls can also exhibit symptoms similar to echo when a change in cell phone towers
occurs. A change in cell phone towers can cause a brief disturbance in the signal that sounds
like echo which stops after a few seconds or less.

TWO OR MORE ECHO CANCELLERS IN SERIES

To cancel echo effectively, there should never be more than one echo canceller in each direction.
However, you cannot always avoid two echo cancellers in series, because the PSTN will
sometimes use echo cancellation, and there is no way to determine when this occurs.
In Figure 7, if echo canceller #2 is functioning correctly, echo canceller #1 will never have a
chance to adapt and there will be little impact on audio quality. However, if echo canceller #2
sometimes allows echo to pass, the echo canceller#1 will try to adapt. This scenario could
cause voice quality issues.
Figure 7: Double Echo Cancellation
19