JBL CINEMA SOUND SET UP User Manual

CINEMA SOUND SYSTEM MANUAL

January, 1998

JBL CINEMA SOUND SYSTEM MANUAL

Table of Contents

INTRODUCTION..

II.

BASIC

SYSTEM

Analog

Digital

and

A-

Evolving Dynamic Range Requirements in the Cinema..

..................................................................................................

CONCEPTS..

Film

Formats..

Film

Formats

B-chains

.........................................................................................

..............................................................................

................................................................................

...................................................................................

E. Integration of Loudspeakers into the Acoustical Environment F. G. Coverage

III.

ACOUSTICAL

Power

Noise

Response and

Requirements for Proper Stereo Reproduction

CONSIDERATIONS

Criterion

(NC)

B. Control of Reverberation and

Role

The

IV.

SPECIFYING THE CORRECT LOUDSPEAKERS AND AMPLIFIERS..

Hardware

Advantages

Cinema

New JBL

Mechanical

Subwoofers

Surround

Screen

Use of

MOUNTING

of the Acoustical

Class vs.

Biamplification

Playback

Driver Developments

Details of JBL Screen

................................................................................................

Requirements..

Losses..

Multiple

High

REQUIREMENTS..

A. General Comments

Platform C. D.Surround

VI.

ELECTRICAL

Wiring

Wiring C.

Wiring

Wire

Dividing

System

References..

Subwoofer

and Baffle

Mounting..

Mounting

INTERFACE

for Non-biamplified Diagram for Surround

Gauges and

Network Characteristics..

Setup

and

...............................................................................................................

Power-Flat Systems

......................................................................

Requirements..

..........................................................

Discrete

Consultant..

Room

Size..

................................................................

.....................................................................

Level

Calibration..

..........................................................

..................................................................

Loudspeaker Systems

...........................................................................

.........................................................................................

Frequency Elements..

............................................................................

.....................................................................................

Construction..

..............................................................

...............................................................................

...................................................................................

..................................................................................

Installations..

Biamplified Installation..

for a

Channels..

Line Loss

..................................................................

Calculations

..............................................................

Checkout..

....................................................................

................................................

Reflections

....................................

.....................................................

.................................................

...................................................

............................................

..................................................

...........................

.....................

..........................

...............

.........................

.2 .2 .2 .4 .5 .7

.12 .12 .13 .15 .15 .15

.17 .18 .18 .26

.29 .30 .31 .31

.31 .32 .33 .35 .35 .35 .37 .38 .38 .39

.41

page

INTRODUCTION

The decade of the 1980’s saw many improvements in the quality of cinema sound. Dolby Laboratories had begun the cinema sound revolution during the middle 1970’s with the introduction of noise reduction and equalization of cinema loudspeaker systems. In 1981, JBL demonstrated the first flat power response loudspeaker systems at the Academy of Motion Picture Arts and Sciences. In 1983, Lucasfilm introduced the

THX@

system, along with their program of cinema certification. As the 1980’s progressed, Dolby stereo optical sound tracks gained in favor, increasing the number of stereo houses significantly. The application of Dolby Spectral Recording (SR) to cinema release prints represented another step forward in sound quality.

By the mid

199Os,

three digital systems had been introduced into the cinema, Dolby SR-D. Digital Theater Sound (DTS), and Sony Dynamic Digital Sound (SDDS). These systems have similar digital performance characteristics, and they all provide analog stereo optical tracks for overall compatibility and operational redundancy, should the digital portion of the system fail, or momentarily go into a mute mode. DTS makes use of a synchronized CD-ROM for its digital program, while the other two include the digital information on the print itself.

As new cinema complexes are being pianned and constructed, acoustical engineers are now more than ever before being engaged to deal with problems of architectural acoustics and sound isolation between adjacent exhibition spaces. More attention is being paid to the specification of sound equipment and its careful integration into the cinema environment.

JBL has a strong commitment to the cinema sound market. We have become the acknowledged leader in the field, and our products are routinely specified for major studios and

post-

production houses throughout the world. JBL continues its rapid pace in new product development

aimed at increasing performance levels in the cinema.

This manual has several goals. First, it will provide a background in basic systems concepts,

and then move on to acoustical considerations in the cinema. The subject of electroacoustical

specification will be discussed, as will the problems of mounting and aiming of the components. Electrical interface and system checkout will be covered in detail. JBL believes that the more dealers

and installers know about the basics of sound in the cinema, the better will be the results of their work

in all areas.

II. BASIC SYSTEM CONCEPTS

A. Analog Film Formats

There are two film sizes for theatrical exhibition: 35 mm and 70 mm. The most common

projection image aspect ratios (horizontal vs. vertical) for 35 mm can be either 1.851 (“flat”) or

(“scope”). Seventy mm prints are normally projected at a ratio of

2.2:1.

The advantages of 70 mm

2.35:1

have, in the past, been the availability of six magnetic tracks and large image area. The cost of a 70

mm print is quite high, and these prints have normally been made in limited quantities for exhibition in premier houses in large metropolitan locations. Today, the general practice with 70 mm is to use three channels behind the screen (left, center, and right) and a single surround channel feeding multiple

page

loudspeakers. Options are to use the two remaining magnetic tracks for subwoofer signals and/or split (dual channel) surrounds.

The 35 mm format was modified during the 1950’s to handle four magnetic tracks: three screen channels and a single surround channel. At the same time, the standard monophonic variable area optical track was maintained. Figures IA and B show the channel layout for both 70 mm and 35 mm magnetic standards. At present, the 35 mm magnetic standard is no longer in general use.

A. 70

0.35

’

MAGNETiC

STRIPING

Figure 1. 70mm six-track magnetic format (A); 35mm four-track magnetic format (B)

Figure 2A. 35mm Dolby Stereo Optical format

page

rOUTPUTS

--QL

-_o

--_o R

-_o

SURROUNDS

SUBWOOFER

RTO

INPUTS ----

ADAPTIVE

MATRIX

MASTER LEVEL

CONTROL

AUDIO DELAY

i kHz

LOW-PASS

FILTER

Figure 28. Block diagram of the Dolby Stereo Optical playback matrix

B~TYPE

DECODER

Today, the Dolby Stereo Optical system is virtually a standard format on non-digital 35 mm

film. In this process, the dual bilateral variable area optical sound tracks, which were formerly

modulated with a monophonic signal, are now modulated in stereo, as shown in Figure 2A. Recording on the two sound tracks is accomplished through a matrix, which accepts inputs for the three screen channels and the single surround channel. The signals intended primarily for the left and right screen

loudspeakers are fed to the left and right channels. Program material intended for the center screen

loudspeaker, including most on-screen dialog, is fed to both stereo channels in phase. The in-phase

relationship between the stereo channels triggers the playback matrix to steer that information

primarily to the center screen loudspeaker, through a combination of gain control and altering of

separation coefficients within the matrix. In a similar manner, information intended for the surround

channels is fed to both stereo channels so that there is a 180” phase relationship between them. This

phase relationship triggers the playback matrix to steer that information primarily to the surround

loudspeaker array.

Figure 2B shows details of the playback matrix used in Dolby Stereo Optical soundtracks. The

surround channel is delayed relative to the other channels so that, by the precedence (or

Haas)

effect, the surround channel will not dominate the perceived sound field in the middle and back of the house. The reason for this is that the matrix output contains certain “leakage” signals that may be disturbing to a listener if such signals were to be heard from the surround loudspeakers. in practice, the surround channel is delayed with respect to the screen channels so that the most distant listener in the cinema will hear that channel delayed by a minimum of 20 milliseconds. Since the ear will “lock in” on earlier arrival sounds, localization will be maintained in the direction of the screen for all patrons, while effects

intended only for the surround channel will be

problem is further addressed by rolling off the response of the surround channel above about 7

clearly heard from the surround loudspeakers. This

kHz.

B. Digital Film Formats

The Dolby SR-D format, introduced in 1992, is shown in Figure 3A. It has exactly the same optical sound tracks as shown in Figure 2A with the addition of digital information located in the otherwise unused space between sprocket holes. This new digital format provides the usual three screen channels plus a split surround pair and a single low frequency (subwoofer) channel limited to

100 Hz. This is commonly referred to as a “5.1’ channel system and uses an elaborate perceptual encoding process known as AC-3.

timecode

btween

Figure 3A. Dolby SR-D Figure 38. DTS Figure 3C. SDDS

data

optical

tracks and picture

Figure 3B shows the format used in DTS. Here we see only the stereo optical tracks and a

sync channel for maintaining control of the associated CD ROM player.

Figure 3C shows the format used with SDDS. In addition to the stereo optical tracks, there are

two digital tracks, one at each edge of the film.

Like Dolby SR-D, DTS and SDDS make use of perceptual encoding methods for reducing the amount of digital data required for system operation. DTS and SDDS support the 5.1 channel format used in most cinemas, but SDDS also supports as many as 5 screen channels for special

applications.

All digital formats discussed here have a fall-back (failsafe) mode in which the analog tracks will be used in case of failure of the digital portion of the systems.

C. A- and B-chains

For convenience in defining responsibilities for system specification and alignment, the playback chain is customarily broken down into the A-chain and the B-chain, as shown in Figure 4. The A-chain is comprised of the preamplifiers (optical or magnetic), light source (optical), magnetic heads, solar cells (optical), associated equalization (signal de-emphasis), and noise reduction and directional decoding required for flat electrical output at the end of that chain. For digital reproduction, a digital optical reader is used and the digital signal is fed to a digital-to-analog conversion system. The analog A-chain is shown in Figure

4A,

and the digital A-chain is shown at B. The B-chain,

including split surround channels, is shown at C.

FILM

SIGNAL OUT

--$-I_

___)

‘I’

LAMP

SCREEN CHANNEL

of31

SOLAR

CELL

Figure 4A.

--)

PREAMP

--)

Block diagram of analog A-chain

NOISE

REDUCTION

SIGNAL OUT

Figure 48. Block diagram of digital A-chain

MASTER

FADER

SURROUND CHANNEL

il Of

SUB CHANNEL

,:,pg

1 3 OCTAVE

_,,

Figure 4C. Block diagram of B-chain with split surrounds

The B-chain is comprised of one-third octave equalization, dividing networks (low- or high-level), power amplification, and loudspeakers. JBL Professional products are used extensively used in the chain of the system.

D. Evolving Dynamic Range Requirements in the Cinema

Figure 4D shows details of the headroom requirements of current and future cinema formats. According to Dolby Laboratories, the level of dialog in the cinema will remain as it currently is, while the added headroom will be used primarily for more realistic peak levels for sound effects and music.

Depending on specific signal content, the peak level capability of Dolby SR analog tracks can be 3

greater in the mid-band than with Dolby A, rising to about 9 dB at the frequency extremes. The digital formats can provide 12 dB headroom relative to Dolby A, with overall characteristics that are flat over the frequency band. This peak capability translates into acoustical levels, on a per-channel basis, of

103 dB-SPL in the house. All of the loudspeaker systems discussed in this manual will meet these

new specifications, consistent with the size of the cinema for which the systems will be specified.

110

100

B-

-1

16K

tiz

Figure 40.

:r(

375

125

253 500

Peak power levels

(Al

A-type.

(8) SF?. (CI SR.D

Dynamic range requirements for Dolby-A, Dolby SR and Dolby SR-D formats

E. Integration of Loudspeakers into the Acoustical Environment

In order to present a clear picture of the interaction of loudspeakers and the acoustical environment, we will begin with the previous era in cinema loudspeaker design. Through the end of the

1970’s,

way designs composed of multicellular or radial high-frequency horns and hybrid horn/reflex

the loudspeaker systems which were current in the cinema were the tried and true

two-

low-

frequency systems. These systems had been developed by Bell Laboratories as far back as the

1930’s,

and the versions used until just a few years ago were essentially the same as has been developed and refined by Lansing and Hilliard (1). These systems were well engineered in terms of efficiency, ruggedness, and low distortion, given the acoustical performance demands of the day. Their designers had also successfully coped with the problems of frequency division and arrival time

page

differences between high and low frequency sections. The chief weakness of these systems was their

lack of uniform coverage. System design stressed output conversion efficiency, because of the small

power amplifiers available at the time.

Figure 5A shows the on- and off-axis curves of a typical horn/reflex system, while polar

response of a typical multicellular horn is shown at B. Note that the off-axis response of the

low-

frequency system falls off considerably at higher frequencies. The typical reverberant room response of a system composed of these elements is shown at C. Note here the double hump, which indicates that the total power output of the system is far from uniform. At the same time, however, the on-axis

response of the system may be fairly flat, when measured under non-reflective conditions.

A. Off-axis response of ported horn system

C. Reverberant (power) response of a cinema system composed of elements similar to those shown in Figures 5A and 58

Polar characteristics of a 2 x 5 multicellular horn

( -Ii:

'25

.Hi

+1c

-5

I-

3'5 63

Mulhcellular horn (2 x 5) 1000 Hz vertical Multicellular horn (2 x 5) 2000 Hz vertical

(soliu);

(so/id); horizontal (dashed)

Figure 5.

horizontal (dashed)

Theatre equalization of old-style cinema system

Multicellular horn (2 x 5) 10 (solid); horizontal (dashed)

If any attempt is made to equalize the response of this system in the cinema, then the response along the major axis of the system will be anything but flat. This is precisely the problem which Dolby Laboratories encountered when they introduced equalization into cinemas during the 1970’s.

Page 8

kHz

vertical

F. Power Response and Power-Flat Systems

The discrepancy between on-axis and reverberant room response in the older systems was solved with the introduction of a new family of systems based on uniform coverage high-frequency horns and simple ported low-frequency enclosures. Figure 6A shows the horizontal off-axis response of the JBL 4648A low-frequency system. Note that the response is uniform below 500 Hz over a wide angle. At 6B we show the vertical off-axis response of the 4648A system. Note that the response begins to narrow just below 200 Hz. The net result of this pattern narrowing in the horizontal and vertical planes is that they make a good match for the pattern control of the JBL 2360A horn at the normal crossover frequency of 500 Hz.

Figure 6C shows the off-axis response curves for the 2360A Bi-Radial horn, coupled to a JBL 2446J high-frequency driver which has been equalized for flat power response. Note that the off-axis curves are essentially parallel, indicating that the horn produces a solid radiation angle which is

uniform with respect to frequency. The need for equalization of the compression driver comes as a result of the natural high frequency roll-off which occurs in high frequency drivers above about 3.5

kHz.

This frequency is known as the “mass break point” and is a function of diaphragm mass and

various electrical and magnetic constants in the design of the driver.

When the 4648A or 4638 low-frequency system and the 2360/2446 combination are integrated

into a full range system for cinema use, the -6 dB beamwidth above 500 Hz is smoothly maintained at 90” in the horizontal plane and 40” in the vertical plane out to 12.5

kHz.

At lower frequencies, the

system’s coverage broadens, eventually becoming omnidirectional in the range below 100 Hz.

Figure 6.

(A) Horizontal response; (B) Vertical response; (C) Off-axis response of a JBL 236OA horn

equalized for

flar

power response

When the system described above is equalized in a typical cinema environment, both direct

sound and reverberant sound can be maintained quite smoothly, as shown in Figure 7A. The system’s

reverberant response is proportional to its power output, or to its power response, and the matching of

the system’s on-axis and power response indicate that the reflected sound field in the cinema will

have the same spectral characteristics as the direct sound from the loudspeaker. When this condition exists, sound reproduction, especially dialog, will sound extremely natural. (The frequency response

contour shown in Figure 78 is the so-called “X-curve” recommended for cinema equalization, as

specified in

IS0

Document 2969.)

*xis

---

POWER

RESPONSE

RESWNSE

UNEOUALIZED

Figure 7.

Cinema equalization of power flat systems

JBL pioneered the concept of flat power response in the cinema

EQUALIZED

(2,3).

It has become the

guiding principle in much of JBL’s product design, and it has been adopted by the industry at large.

G. Coverage Requirements for Proper Stereo Reproduction

In the cinema, it is expected that all patrons will be able to appreciate convincing stereo reproduction. By contrast, standard two-channel stereo in the home environment often imposes strict limitations on where the listener must sit in order to perceive correct stereo imaging. The factor that makes the big difference in the cinema is the presence of the center channel. Not only does the center loudspeaker lock dialog into the center of the screen, it further reduces the amount of common mode information the left and right channels must carry, thus making it possible for listeners far from the axis of symmetry to hear the three channels with no ambiguity or tendency for the signal to “collapse” toward the nearer loudspeaker. In the Dolby stereo matrix, the same convincing effect is largely maintained through gain coefficient manipulation during playback.

Ideally, each patron in the house should be within the nominal horizontal and vertical coverage

angles of

all

the high-frequency horns. This requirement can usually be met by using horns with a

nominal 90” horizontal dispersion and by toeing in the left and right screen loudspeakers. In very wide

houses, the spreading of high frequencies above approximately 5

kHz,

as they pass through the

screen at high off-axis angles, actually helps in providing the desired coverage.

Another desirable condition is maintaining levels as uniformly as possible throughout the house. We have found that aiming the screen systems, both high- and low-frequency, toward the back wall helps in this regard, by offsetting normal inverse square losses with the on-axis “gain” of the

screen systems. Measurements made at the Goldwyn Theater of the Academy of Motion Picture Arts and Sciences in Beverly Hills, California, show that, over most of the frequency range, front-to-back levels in the house are maintained within a range of 5 elements toward the audience would produce front-to-back level variations of up to 10 to 12

dB.

By contrast, aiming the high-frequency

dB.

An important requirement here is that the back wall of the cinema be as absorptive as possible. If the rear wall is not highly absorptive, then tilt the high frequency loudspeakers down, with the horn’s axis pointing at the seating area two-thirds of the way back in the house.

This performance is seen in Figure 8. At A, we show in plan view the direct field coverage given by a JBL 2360 horn aimed at the absorptive back wall of a large theater with sloped floor. Coverage at 2

kHz

is within a range of

+/- 3dB,

front to back. If the horn is aimed downward to a point two-thirds the distance from front to back, the coverage is as shown at B, and coverage at the rear of the house is compromised. The coverage given by one of the outside horns, aimed at the rear wall, is shown at C. It is customary to toe in the left and right systems toward the center, whether or not the screen itself is curved, and the aim is to provide adequate coverage for all patrons, with response maintained within a total range of 6

dB.

Figure 8.(A) Direct field coverage at

2kHz,

aimed at rear wall; (B) Same, horn aimed 2/3 distance front to back;

page

The surround ensemble of loudspeakers, if properly specified, can easily produce a sound field that is uniform throughout the back two-thirds of the house, and level variations can often be held within a range of 2 to 3

dB.

Details of surround system specification will be covered in a later section.

When all of the above points are properly addressed, the sound in a cinema can approach that which we take for granted in a post-production screening facility director intended it to sound. It is only when such details as these have been carefully worked out that the effects intended by the sound mixer can be appreciated by the viewing audience.

which is, after all, how the picture

.-.

III. ACOUSTlCAL

CONSIDERATIONS

A. Noise Criterion (NC) Requirements

The usual sources of noise in a cinema, outside of the patrons themselves, are air handling and transmission of noise from the outside. In the case of multiplex installations, there can be leakage from adjacent cinemas as well. Not much can be done about a noisy audience, but it is true that at the

post-production.stage,

encountered in the

mixing engineers take into account certain masking noise levels which may be

field

and even do the final mix under simulated noisy conditions (4).

125

FREOUEW

Figure 9. Noise Criterion (NC)

octave

band

(Hz)

data

2ll

wrves,

.(*

FREOUENCV

(Hz)

Figure 10. Sound Transmission Curves

+ 30 hidden pages