Polycom EF2241 - Manual (Page 42)

T
ROUBLESHOOTING
V ORTEX EF2241 Reference Manual 40 Technical Support: 800.932.2774
and the level of this signal that is reflected as the microphone input (after microphone
amplification).
If the electrical level of the reflected signal picked up by microphone is the same as
the level of the electrical signal sent from the AEC to the loudspeaker output, the
room gain of this microphone channel is said to be 0 dB. If the reflected signal picked
up by the microphone is higher than the level of the signal sent to the loudspeaker out-
put, that microphone channel has positive room gain. The more positive the room
gain, the harder the AEC must work to determine which signal is an echo and which
is a local speech signal.
Excessive Room
Gain
Excessive room gain can be caused through a number of mechanisms:
1. The most common is excessive amplification of the remote (reference) signal at
the local loudspeaker output. This may be explained as follows. If the reference
signal is too low coming into the EF2241, i.e. the codec audio signal is too low,
the room audio amplifier is usually used to compensate and bring the room audio
to an acceptable level. For example, if the reference signal is 12 dB too low, the
room audio will need to be amplified by approximately 12 dB to bring it to a rea-
sonable listening level. This adds 12 dB to the room gain, which will most likely
cause it to exceed the amplifier room gain limit (See “Verify Room Gain” on
page 14). This situation can be remedied by applying enough gain to the codec,
phone or program audio inputs (Inputs A-D) which will make up the Reference
input signal so that the acoustic echo canceller (AEC) sees a good reference sig-
nal rather than trying to compensate at the amplifier.
2. Another common cause of room gain failure is excessive microphone amplifica-
tion. For example, if a microphone is "hot" by 6 dB, then the reflections of the
loudspeaker output signal which are picked up by the microphone will be ampli-
fied by 6 dB more than necessary. This adds 6 dB to the room gain, which may
be sufficient to cause room gain problems. This situation could easily arise if, for
example, the conferencing equipment is set up so that participants are too far
from the microphone. In such a situation, after correct microphone setup the
local microphone audio level may be too low because of the distance from the
Figure 28. Room Gain.
AEC
A
B
Room Gain = 20 log (B/A)