Specifications

4-5
4.3 Aligning the B-Chain
The B-chain is defined as those system components from the fader through the
loudspeakers. In the CP500, available adjustments include equalization, level
control, and digital soundhead, analog, and digital surround delay settings.
Adjustable mute fade-out time is also provided.
It is not practical for the entire cinema industry to standardize on a single make and
model of loudspeaker. In any event, the different acoustical characteristics of
individual theaters would, to some extent, negate any such standardized speakers.
Electronic equalization of each loudspeaker system achieves consistent results in a
broad spectrum of environments, and with a broad range of speakers. Accurate
equalization requires the use of standardized acoustic measurement procedures.
A pink noise generator provides a continuous random noise signal that covers the
total bandwidth and is used to measure and adjust the response of the loudspeakers.
The use of random noise eliminates the problems inherent with tones (standing
wave patterns in the theaters) and enables the frequency response of the entire
system to be observed. Each channel can be measured and adjusted independently
of the other channels.
A calibrated microphone or a multi-microphone setup with multiplexer is placed in
the auditorium to receive the pink noise reproduced by the loudspeaker. The output
of the selected microphone is fed to a real time analyzer (RTA) circuit built into the
CP500 cinema processor, and equalization can be performed using the CP500 front
panel display to show the audio spectrum received by the microphone(s). Pure pink
noise would yield a “flat” horizontal line on the RTA. Thus, the effect of
adjustments to the equalizers is quickly and easily seen.
One of the problems inherent in equalization is the nature of the environment. In an
open space, a perfect loudspeaker, radiating a perfectly flat response in all
directions, placed in front of a perfectly flat microphone, producing perfectly flat
response to sounds arriving from all directions, will display a perfectly flat response
on the RTA from pink noise. In an enclosed space such as a theater, the results are
different. When the pink noise generator is first turned on, all of the sound that
initially reaches the microphone comes directly from the loudspeaker; the response
is flat—for a few milliseconds. Then reflected sound from the walls, ceiling, floor,
seats, etc. starts to arrive at the microphone together with the direct sound from the
loudspeaker. This indirect or reflected sound reinforces the direct sound. The
system soon settles into an equilibrium condition. As much energy is being absorbed
at the walls, ceiling, etc. as is fed into the room. Since high and mid frequency
energy is absorbed when sound is reflected, the displayed response appears to have
a rising bass and a falling treble characteristic. At first glance, rolling off the bass
and boosting the high frequencies may appear to be the logical approach for a flat
steady-state response, but such an arrangement works only on sustained sounds.
Dialogue contains short, impulsive sounds and will yield a much-too-bright result