CINEMA SOUND SYSTEM MANUAL January, 1998 JBL CINEMA SOUND SYSTEM MANUAL Table of Contents .2 I. INTRODUCTION.. .................................................................................................. .2 II. BASIC SYSTEM CONCEPTS.. .............................................................................. .2 A. Analog Film Formats.. ................................................................................ .4 B. Digital Film Formats ................................................................................... .5 C. A- and B-chains ......................................................................................... D. Evolving Dynamic Range Requirements in the Cinema.. ........................... .7 E. Integration of Loudspeakers into the Acoustical Environment ..................... 7 F. Power Response and Power-Flat Systems ................................................ .9 G. Coverage Requirements for Proper Stereo Reproduction .......................... 10 .12 III. ACOUSTICAL CONSIDERATIONS ...................................................................... A. Noise Criterion (NC) Requirements.. .......................................................... .12 B. Control of Reverberation and Discrete Reflections .................................... .13 C. The Role of the Acoustical Consultant.. ..................................................... .15 IV. SPECIFYING THE CORRECT LOUDSPEAKERS AND AMPLIFIERS.. ............... .15 A. Hardware Class vs. Room Size.. ................................................................ .15 17 B. Advantages of Biamplification ..................................................................... .17 C. Cinema Playback Level Calibration.. .......................................................... .18 D. New JBL Driver Developments .................................................................. E. Mechanical Details of JBL Screen Loudspeaker Systems ......................... .18 .26 F. Subwoofers ................................................................................................ .29 G. Surround Requirements.. ........................................................................... .30 H. Screen Losses.. ......................................................................................... I. Use of Multiple High Frequency Elements.. ................................................. .31 .31 V. MOUNTING REQUIREMENTS.. ............................................................................ 31 A. General Comments ..................................................................................... B. Platform and Baffle Construction.. .............................................................. .31 .32 C. Subwoofer Mounting.. ............................................................................... .33 Mounting ................................................................................... D . Surround .35 VI. ELECTRICAL INTERFACE .................................................................................. A. Wiring for Non-biamplified Installations.. ................................................... .35 B. Wiring Diagram for a Biamplified Installation.. ............................................ .35 .37 C. Wiring for Surround Channels.. .................................................................. D. Wire Gauges and Line Loss Calculations .................................................. .38 E. Dividing Network Characteristics.. .............................................................. .38 .39 F. System Setup and Checkout.. .................................................................... .41 References.. ............................................................................................................... page 1 I. INTRODUCTION The decade of the 1980' saw many improvements in the quality of cinema sound. Dolby s Laboratories had begun the cinema sound revolution during the middle 1970' with the introduction of s 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' progressed, Dolby stereo optical sound tracks gained in favor, increasing the number of stereo s 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 postproduction 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 (" ) or 2.35:1 flat" (" scope" Seventy mm prints are normally projected at a ratio of 2.2:1. The advantages of 70 mm ). 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 2 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' to handle four magnetic tracks: three screen s 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 m m 0.35 m m A I ' MAGNETiC STRIPING Figure 1. 70mm six-track magnetic format (A); 35mm four-track magnetic format (B) Figure 2A. 35mm Dolby Stereo Optical format page 3 I LT 0 INPUTS ---ADAPTIVE MATRIX rOUTPUTS --QL RTO * + M A S T E R -_o C c LEVEL --_o R CONTROL -_o SURROUNDS I SUBWOOFER 0 I AUDIO DELAY i kHz LOW-PASS FILTER B~TYPE NR DECODER Figure 28. Block diagram of the Dolby Stereo Optical playback matrix 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 clearly heard from the surround loudspeakers. This problem is further addressed by rolling off the response of the surround channel above about 7 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 data btween optical tracks and picture Figure 3A. Dolby SR-D Figure 38. DTS Figure 3C. SDDS 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. SIGNAL OUT FILM --$-I_ ___) `I LAMP ' I i h SOLAR CELL --) P R E A M P --) NOISE REDUCTION - Figure 4A. Block diagram of analog A-chain SIGNAL OUT Figure 48. Block diagram of digital A-chain SCREEN CHANNEL I1 of31 ,:,pg MASTER FADER SURROUND CHANNEL il Of 2) 1 3 OCTAVE _,, EO A SUB CHANNEL 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 Bchain of the system. D. Evolv ...