“We are the art gallery curators,” announced Tom Holman at the first home cinema dealer training I attended at Skywalker Ranch in Marin County, CA in July of 1995. Many readers will know that the mastermind behind THX (whose initials formed the ‘TH’ of the three-letter logo) probably contributed more to our academic understanding of what a home cinema actually is than any other living audio engineer or acoustician. Of course, it’s fitting that Holman is now the guru at Apple’s audio labs, and his ongoing research into psychoacoustics and multi-channel sound reproduction will certainly continue to promote its long-lasting and reverberating effects.
With quadrophonic audio for the most part already consigned to the trashcan of history by the late 1970s, but with matrix Dolby Surround processors starting to find their way into living rooms by the late 1980s, it was becoming clear that a unified theory – for want of a better term – was needed to address the misunderstanding surrounding multi-channel audio in the home. By 1993, Dolby Labs conducted a non-scientific home survey into its employees’ surround speaker arrangements, only to find that the type and placement of those speakers were pretty inadequate to say the least.
THX formulated a plan for professional cinemas in 1983. George Lucas decided that the complex mix of Return of the Jedi should be presented optimally in as many theaters as possible. For a cinema operator to have a screen ‘THX-Certified’ and be able to show the legendary Broadway ‘Deep Note’ trailer before a feature – and under licence – theatrical exhibitors had to conform to a number of strict rules. These revolved around achieving nothing less than excellence in film presentation and realizing “what the director intended” (a term originally coined by Lucasfilm marketeers but frequently borrowed since). To extend the art gallery metaphor in the opening paragraph, Tom Holman would also explain that, “if the filmmakers are the painters, then THX are the picture framers.”
A THX cinema had be able to reproduce dialogue accurately and intelligibly, elicit the “quietest whisper to the loudest explosion,” be agnostic to audio or vibration leaks from adjacent auditoriums, and mitigate background noise to a measurable NC30. Even the viewing angle to the screen from every seat in the house should be no less than 32°. The THX team provided a number of design services and recommendations for each customer, which included details of how to build double-stud walls and a behind-screen baffle, plus an approved list of amplifiers and speakers which could be deployed to achieve reference level audio. A proprietary THX crossover was also mandatory in every projection booth, which in turn had to be furnished with a double glass enclosure to mask the sound of a 35mm projector from the audience. This is merely skimming over the surface, but a THX engineer had to measure all the results in a finished auditorium with his accompanying ‘R2 analyzer’ before granting certification.
The challenge, however, with developing a home theater system (which achieved the same goals as the professional version) was how to play back a film soundtrack in a small room that was originally mixed in a large room and was designed to be played back in a large room. The results of Holman’s research led him to design an architecture of hardware and electronic features to solve those problems.
A 4-channel matrix Dolby Surround (or later discrete 5.1) AV preamp or receiver must contain a proprietary circuit which included signal processing with three primary functions: Re-EQ, Decorrelation and Timbre Matching. Re-EQ was an electronic predetermined roll-off which took the edge off the shrillness of film soundtracks. High frequencies are extended in a film mix so they can carry in a large room or professional cinema. Decorrelation split the two mono rear channels to add a sense of spaciousness in the surrounds and Timbre Matching negated the effects of tonal shifts as sounds pass from in front of our ears to behind. A steep roll-off crossover of 80Hz was also the optimal point at which a subwoofer should take over from the five main channels with non-directional audio and effects, and took place after the primary signal processing. (The 80Hz recommended crossover position is now a feature of all modern-day AV receivers.)
These electronic features worked seamlessly with a number of physical attributes in the speaker designs themselves. The front LCR speakers should have focused vertical dispersion, minimizing floor and ceiling first reflections, and therefore enhancing dialogue intelligibility and lending precision of audio cue placement in the front soundstage. The surrounds were dipolar in design, had to be placed to the sides of the listener in the ‘null’, and could replicate the array of side and rear wall speakers in a professional cinema.
While the focused vertical dispersion characteristics of the LCRs were extremely effective on film soundtracks, there was a corresponding compromise in their ability to image well on two-channel music sources, leading to much controversy in the hi-fi press. Furthermore, dipole speakers arguably became less relevant with the advent of 5.1 discrete audio and its accompanying split surround information. Both speakers and power amplifiers were only passed for certification – and licensed – if they could replicate the entire accepted audible hearing range of 20Hz to 20kHz and achieve flat response across the spectrum.
Although the landscape – or soundscape – has changed dramatically in recent years, particularly with the arrival of object-based audio in both the cinema and home, the fact is that a lot of THX’s rulebook still applies in one form or other to both environments. Much of the doctrine is now in the public domain, but Tom Holman’s work during those early years rarely gets the recognition it deserves. As a dealer trainer in the mid-1990s at Lucasfilm, I can say with certainty that there was always a moment of realization as attendees started to process the scale of the project THX had undertaken. Putting on a movie show that a filmmaker will be proud of is a worthy cause, and one that those who purport to love the art of film must never overlook. There will always be a role for the art gallery curator.