Polycom Vortex EF2280, Справочное руководство

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T
ROUBLESHOOTING
V
ORTEX
EF2280 Reference Manual 44 Technical Support: 800.765.9266
this signal that is reflected as the microphone input (after microphone amplifi-
cation).
If the level of the reflected signal picked up by microphone is the same as the
level of the 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 loud-
speaker output, that microphone channel has positive room gain. The more
positive the room gain, the harder the AEC must work to determine which sig-
nal 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 EF2280, i.e. the codec
audio signal is too low, the room audio amplifier is usually used to com-
pensate 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 reasonable listening level
for the local participants. 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 16). 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 signal rather than trying to com-
pensate at the amplifier.
2. Another common cause of room gain failure is excessive microphone
amplification. 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 amplified by 6 dB more than necessary. This adds 6
dB to the room gain, which may be sufficient to cause room gain prob-
lems. This situation could easily arise if, for example, the conferencing
Figure 28.
Room Gain.
AEC
A
B
Room Gain = 20 log (B/A)