Power - Polycom HDX 9006 Integrator's Manual

Integrator's Reference Manual for Polycom HDX Systems Room Design and Layout

Power

Power is a function of loudspeaker efficiency and total available system power. Most speakers operate in a
power range that is broader than the range in which they operate without distortion. For the purpose of
conference communication, we are interested in sound that has little or no distortion. Sound that is
reproduced accurately (with no distortion) will most accurately represent the voice of the people from the
far-end (our primary goal). Accurate reproduction also will aid the echo-cancellation circuitry in the system,
minimizing the amount of echo that the system sends back to the people at the far-end, and thereby
increasing perceived ease of intelligibility and understanding. Remember that any distortions present in the
playback audio system—whether harmonic, amplitude (gain compression) or temporal (time delays)—will
be recognized by the echo canceller as "new audio information," and it will send those distortions to the
far-end, perhaps wreaking havoc on the system audio quality. In short, speaker power should be matched
to overall audio subsystem power. The speakers should provide adequate coverage and be able to present
approximately 80 to 85dBA-SPL (continuous) at the local site with the system operating at nominal power
utilization, and have a peak reserve of 15 to 20dB before distortion.
Range/Frequency Response
The human ear is able to hear sounds in a very wide range of frequencies (as low as 70Hz and as high as
12,000Hz). The human voice is able to produce sounds in a narrower range (100Hz to 8,000Hz). Most
spoken communication occurs, however, in a range that is only 150Hz to about 6,000Hz. This means that
we need to select speakers that operate with ideal performance in a fairly narrow range for human voice (as
opposed to speakers used for music, that may have ranges of 20Hz to 20,000Hz). We must also be alert to
the crossover characteristics of the speakers we select. Many coaxial and paraxial speakers have their
crossover within the middle audio frequencies, thereby inducing potential distortion within the spoken
frequency range and creating anomalies within the system that hinder voice communication.
Video Elements
As a general rule, any display used in a videoconferencing environment should be sized for the number of
attendees, the physical distances involved and the type of material presented onscreen. The screen size
should allow for clear and easy viewing at the various distances experienced within the room. A measure
of required screen size that often is applied to projection technology is: no closer than 1.5 times the diagonal
measure and no farther than 7 times that measure. Nobody should have to sit closer than 2 times the screen
diagonal measure, nor farther than 8 times that measure.
Direct viewed tube-type displays (monitors) almost always are sharpest and brightest in a
videoconferencing environment. "Retro-projector cabinet" displays (which look like largescreen TVs) are
next in sharpness and brightness, and "front-screen" projectors come in last. Glare and uncontrolled
ambient room lighting adversely affect the quality of the image most with front-screen projectors and least
with direct view tubes. A very limited number of frontscreen projection systems have sufficient brightness
and contrast to be useful in a properly lit videoconference room.
Video Projection for Use in Videoconference
Many installations make use of video projection devices. The most important thing to remember in the
planning of video projection for a videoconference space is that front projection is vastly inferior to rear
projection. Front projection systems are less expensive and easier to implement, but the conflicting interest
between the camera and the projection display makes this form of display a very poor choice. Front
projection setups operate best when the lighting in the room is dimmed or doused. When this is done, the
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