Installation manual

Dolby
®
Model CP650 Installation Manual Front-Panel and Alignment Overview
3-9
RTA if you prefer.) Equalization can be performed by using an oscilloscope
connected to the RTA output test points or by using a PC running Dolby setup
software. Either display will represent the spectrum received by the microphones. The
effect of adjustments to the equalizers is quickly and easily seen. The Auditorium
Assist
TM
feature, available in system software version 2.0 and above, measures and
saves the auditorium characteristics for future checking.
One of the problems inherent in equalization is the nature of the environment. In an
open space, a perfect loudspeaker, radiating a perfectly flat pink-noise 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 output. In an enclosed space such as a theatre, 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, and the response is flat—for a
few milliseconds. Then, reflected sound from the walls, ceiling, floor, seats, and so on
arrives 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 by the walls, ceiling, and
seats 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 falling treble
characteristic. At first glance, boosting the high frequencies may appear to be the
logical solution to achieve a flat steady-state response, but such an arrangement
works only on sustained sounds. Dialogue contains short, impulsive sounds that will
yield a much-too-bright result because there is no time for reverberation to build and
add to the original sound. A standard response curve is required that favors such
impulsive “first arrival” sound and implies the same gently falling response that is
observed when the output of an ideal loudspeaker is measured with a perfect
microphone in the theatre.
The amount of reverberation varies with frequency. As frequency rises, more audio is
absorbed rather than being reflected. A typical reverberation curve in a theatre rolls
off at about 3 dB per octave above 2 kHz. This characteristic is used to define the
standard steady-state response curve for all dubbing theatres in which films with
Dolby soundtracks are mixed and for all Dolby processor-equipped cinemas.
The size of the theatre affects the reverberation time, and therefore the measurement
of frequency response. After alignment to this standard curve, some slight adjustment
of high-frequency slope may be necessary for extremely large or small theatres. The
Bulk EQ Treble Adj. menu selection can be used to reduce the output by
approximately 1 dB at 8 kHz for very a large theatre; or to increase the output 1 dB at
8 kHz for a very small theatre. Any such adjustment should be based on an
evaluation, by ear, of actual known films rather than as a rule of thumb.
Some loudspeakers used in theatres are far from ideal and require boosting of the
low- and high-frequency extremes in order to produce an approximation of the
standard reference response curve. Bass and treble controls centered on the turnover
points of typical loudspeakers lift the ends of the spectrum without the need for large
amounts of narrow-band boost from the third-octave EQ circuitry.