Certified surround sound

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Having established certification programs for commercial cinema, dubbing stages, and home theater, Lucasfilm THX introduced its third program in 1999, with design and performance standards geared toward the increasing use of professional, small-scale (that is, smaller than a typical dubbing stage) facilities for surround sound mixing and monitoring. The name of this program is pm3, which stands for Professional Multichannel Mixing and Monitoring. It is basically a certification program that bridges between THX’s standards set for dubbing stages/cinema and home theater.

Small-scale surround sound facilities have proliferated, largely, of course, because of the growth of DVD, resulting in the necessity to specifically mix and monitor multichannel productions for this format. Dubbing stages were meant for motion picture sound projects for theatrical release, and were usually booked solid for that purpose. Also, monitoring and mixing soundtracks for DVD simply made sense in a smaller room, since the ultimate destination of a DVD production would be in a home environment that is likely to be comparably sized.

Smaller rooms for working with multichannel sound are also relevant to other applications, including telecine, mixing and monitoring for HDTV, preparation of film soundtrack pre-mixes, and 5.1 music production. With the increasing use of such facilities for multichannel audio purposes, THX saw both an opportunity and a necessity for creating a new standard, a new area for which the THX logo would identify facilities as having met a set of criteria for performance, and bearing a level of prestige that has been established from the company’s prior achievements.

As you will soon discover, pm3, in a sense, “borrows” from aspects of the THX professional dubbing stage program, and, to a lesser degree, from the home THX program. In theory, the small-scale facility that is pm3-certified has been optimally designed, equipped, and set up so that it allows for an ideal sonic transition from the THX-certified dubbing stage environment.

Similarly, this facility would ideally provide for a transition to the THX home theater. The overall objective of THX is to provide for the best possible consistency in performance, from creation to presentation.

The first steps that are required in order to receive pm3 certification is the completion of the pm3 Site Information Forms and the provision of architectural drawings. THX then performs a feasibility study, and, if favorable, then works with the facility’s architect and acoustical consultant to finalize the design of the facility, and the equipment installer as well. THX personnel act as consultants throughout the process, and, if necessary, will modify the dimensions of the room for considerations of console placement, positioning of the speakers, and room gain for low frequency performance. Following construction of the room, the final step for certification is the alignment of the sound system and then a check of the completed facility by a THX engineer to ensure that all required standards have been met. Thus far, all pm3-certified sites are new constructions; however, THX will also certify existing locations, provided that they meet all of the necessary qualifications and requirements. Like the commercial cinema and professional dubbing stage programs, there are criteria for room acoustics as well as the equipment. A pm3-certified room must meet the NC-25 noise level limit, the same requirement for dubbing stages, though it is recommended that facilities be designed and equipped to meet NC-20. Reverberation times need to be below specific limits between 125 Hz and 8 kHz, for the room’s specific size. Furthermore, discrete echoes need to be minimal at the primary, or central, console seating position. One of the important reasons why THX interfaces in the design stage is to recommend construction guidelines to minimize HVAC noise and other noises intruding from the exterior. The wall structure for the room will also be assessed, and, if necessary, the client will be advised to consult on improving acoustical isolation. THX generally recommends that the walls be constructed to meet an STC rating of 70 to 75.

Also similar to THX’s commercial and professional theater certification programs, a pm3 licensee can only use speakers and amplifiers from an approved equipment list. Thus far, there are eight speaker manufacturers on this list, including Apogee, Dynaudio, Genelec, JBL, M&K, Mackie Designs, Meyer, and Pilote Films. Selected power amplifiers are available from BGW, Crest Audio, Crown, and QSC. And like the certification programs for dubbing stages and theaters, the equipment isn’t licensed, as is the case for the home THX program, but rather has been selected to meet specific performance requirements.

In addition, every pm3 facility uses the THX CC4 5.1-channel subwoofer crossover-controller. This component provides for bass management of five satellite speakers, for which low-end content below 80 Hz is summed, along with the LFE, to the subwoofer. Along with bass management, the THX CC4 has a number of additional functions. There’s a switch for re-equalization for the main channels, the ability to select between up to four 5.1 sources, trim level setting for all channels, and the capability to monitor any channel by itself.

For equalization, a protocol is used that is similar to that for theaters and dubbing stages. The THX R2 spectrum analyzer is employed by the engineer, which is interfaced with a laptop and provides for equalization with the aid of a real-time analyzer, as well as broadband pink noise test signals. Since pm3 facilities are small rooms, the equalization would be flat across the spectrum. However, if the site is considerably larger, and also will be used for film soundtrack production, then the room would be equalized to the standard SMPTE 202M/ISO 2969 (or “X”) curve. Four calibration microphones are used for the equalization, and are positioned such that the primary microphone is at the central console seating position. The others are placed in the vicinity of this position, or at other client seating locations if they exist. The THX R2 also provides for tests to determine background noise level (NC, or Noise Criteria rating), and reverberation time (RT).

For the surround speakers in a pm3 facility, THX specifies that they create the diffuse-type of sound that is encountered when listening to an array of speakers along the side and rear walls in a theater. This could be accomplished using speakers with dipole-type radiation or an array of direct radiators (at least three per channel). (M&K is currently the only pm3-approved manufacturer offering a speaker with dipolar radiation.) Recognizing that some of their clients will devote a pm3 facility to 5.1 music production, and therefore prefer to monitor the surrounds using a pair of direct radiators, THX offers to the client the option to switch between direct and diffuse surround modes. By having the opportunity to monitor with a diffuse soundfield, the client can obtain an impression of what the mix will sound like in a home environment with dipole speakers. M&K’s Tripole speakers are particularly useful in this regard, since they allow for switching between direct and dipole radiation, and also the combination of both (Tripole).

For the front left and right speakers, the recommendation is that they be placed such that the angle formed by the speakers and the console seating position is 45 degrees (�3 degrees, according to THX), for film, video, or television soundtrack production.

However, recognizing that some clients desire the ITU-R standard for 5.1 music monitoring, which have the front left and right speakers positioned together with the console to form a 60-degree angle, THX allows for this configuration, provided that the pm3 facility is going to be devoted to 5.1 music production.

As is the case for the certification programs for theaters, the pm3 license is valid for one year before the facility has to be tested for compliance prior to a renewal of the license. There are currently 26 pm3 facilities around the world, and THX is said to be gearing up for significant growth in 2001. Some of the more notable sites include 5.1 Entertainment and the 20th Century Fox Studios postproduction facility in Los Angeles, Intersound in West Hollywood, La Curva in Mexico City, and Sony PCL in Tokyo.

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