Amplifiers; Loudspeakers - Polycom SoundStructure C16 Design Manual

that the echo canceller will detect both the local talkers speech and the side-tone (interpreted as the remote
talkers' speech). This will prevent the acoustic echo canceller from converging properly, degrading the
conferencing experience.
The line echo canceller shown in the previous figure above is an adaptive filter that uses the transmit signal
to adapt and remove the line echo that appears on the receive side of the line echo canceller. Similar to the
acoustic echo canceller, there may also be some non-linear suppression to remove residual echoes when
the line echo canceller is not fully converged. The line echo canceller typically is adapting when there is
transmit audio present and no receive audio, in other words, just the local talkers are speaking. This is in
contrast to the acoustic echo canceller which typically is only adapting when there is remote audio present
and the local talkers are not talking.
Due to the variation in performance of telephone lines across the world, telephone hybrids will have a
country code setting that will be required to properly interface the hybrid to the local phone line
characteristics. If the phone hybrid settings don't match the communications network, the performance of
the phone hybrid will be degraded, and possibly in violation of local telecommunications regulatory
requirements.

Amplifiers

There are two broad classes of amplifiers - low impedance and constant voltage. The low impedance
amplifiers are the type of amplifier used in consumer applications and the constant voltage amplifiers are
used in larger, professionally installed systems.
Low impedance amplifiers are designed to drive audio into low impedance loudspeakers typically with
impedances between 4 and 16 ohms. These amplifiers are often used for smaller systems with one, two,
three, or four loudspeakers and are suitable for use in medium to small conference rooms. In larger systems
connecting all the loudspeakers will significantly reduce the impedance that the amplifier will see -
potentially causing the amplifier to generate more current than it has been designed to provide. As more
and more loudspeakers are connected to a low impedance amplifier, the impedance that the amplifier sees
gets smaller and smaller which requires more and more current from the amplifier until the amplifier can not
produce any more current and shuts down. In addition, if loudspeakers are removed or added to the system,
they will affect the impedance of the remaining collection of loudspeakers, perhaps requiring changes to the
volume levels to ensure that the playback signal is loud enough and the amplifier is still operating within its
designed current range.
Constant voltage amplifiers, such as 70 V systems, make it easy to have large numbers of loudspeakers
connected to the system as each loudspeaker can tap some power from the amplifier (using a transformer
that is built into the loudspeaker) without being concerned as to the overall impedance that the amplifier
sees. Large distributed loudspeaker systems are nearly always driven by constant voltage amplifiers.

Loudspeakers

Loudspeakers and the amplifier driving the loudspeakers in the local room allow the local conferencing
participants to hear the remote audio. The loudspeakers can be wall-mounted, ceiling-mounted, or even
table-mounted, although they are most often installed in the ceiling of the local rooms. As mentioned in the
Amplifiers section, loudspeakers are either low impedance or 'transformer tapped' depending on the style
of amplifier they will be connected to.
Just as microphones are characterized by their pickup pattern, frequency response, and sensitivity to
sound, loudspeakers are characterized by their frequency response (80 Hz to 20 kHz typical), power
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