Understanding the Mechanics of THX

Frequent moviegoers are likely familiar with THX. As the lights fade and the screen illuminates, the THX emblem emerges, accompanied by a powerful, resonant audio. The message "The audience is listening" flashes, signaling an immersive sensory journey.

Many believe THX is merely another audio format for films, but in reality, THX is something entirely different. This article explores what THX truly is, its functionality, and how cinemas implement it.

Short History

THX, inspired by THX 1138 (George Lucas' debut film), was created at Lucasfilm in the early 1980s. George Lucas aimed to guarantee that films would maintain consistent visual and audio quality across all venues. Tomlinson Holman, the former technical director at Lucasfilm, along with a team of THX engineers, developed a standardized set of criteria to address this goal.

They identified several issues in theaters that could detract from the audience's film experience:

THX is fundamentally a collection of standards designed to address these issues. To showcase the THX logo, a theater must implement these standards and receive certification from Lucasfilm's THX division. The first film screened in a THX-certified theater was Return of the Jedi in 1983. Today, over 2,000 theaters worldwide have achieved this certification.

Several steps must be completed before a theater can achieve certification:

The following sections will delve into each of these steps in detail.

Licensing & Evaluation

The initial phase involves the theater owner and THX finalizing a licensing agreement. The owner agrees to lease THX crossover equipment and pays for the privilege of displaying the THX logo. After signing the agreement, THX evaluates either the architectural plans for new auditoriums or conducts a survey of existing ones to assess current conditions.

The theater owner collaborates with a THX team to create a design that meets THX standards. Once the design changes are implemented, THX helps the owner select appropriate equipment from their approved list. The finalized design and equipment list are then sent to THX for approval. After approval, construction or renovation can commence. CES: TXH Tech Pages provides insights into the architectural specifics of building a THX-certified theater.

Certification

Here’s an overview of the THX certification process. THX evaluates several theater attributes, categorized into four main areas:

Many theater auditoriums lack sufficient soundproofing to block external noise. Have you ever been distracted by the sounds of gunfire or explosions from a neighboring movie or the persistent hum of HVAC systems? THX-certified auditoriums must maintain a noise criteria level no higher than NC-30 per octave. This measures background noise, ensuring minimal intrusion from external sources. NC-30 is akin to the quiet ambiance of a church.

The THX team evaluates reverberation and echoing by timing how long a 60-decibel (dB) tone takes to fade completely. For certification, this duration must align with specific limits based on the auditorium's size. A THX-certified theater ensures no audible echoes.

The projection system and screen's brightness must comply with standards set by the Society of Motion Picture and Television Engineers (SMPTE). The projector must align with the screen's center within five percent (ideally three percent) of its width and height.

For movies with a 2.35:1 aspect ratio, viewers in the back row's end seats must have a viewing angle of at least 26 degrees, with 36 degrees being optimal.

Every seat must provide an unobstructed view, and the audio must remain clear and precise from any location in the auditorium.

Sound

High-quality audio is central to the THX experience. THX engineers focused on two key components of the sound system when establishing their standards:

The A-chain includes the components responsible for decoding audio from its format and processing the signal to eliminate noise. This involves the audio pickup on the projector and the cinema sound processor. Formats like DTS, Dolby Digital, and SDDS, along with noise-reduction systems such as Dolby A and Dolby SR, enhance the A-chain's performance.

The B-chain encompasses the remaining parts of the sound system, including the THX crossover, power amplifiers, and speakers. THX engineers recognized that even with excellent audio from the cinema processor, a subpar B-chain could degrade the experience. To address this, THX patented a sound system design featuring a specialized crossover and components from a list of THX-approved equipment. Each piece of equipment undergoes rigorous testing to meet THX's stringent standards.

The THX crossover is a unique sound processor that divides low and high frequencies to deliver clearer audio through the theater's sound system. Theater owners lease this crossover as part of the THX licensing agreement. It employs 24-dB-per-octave high-pass and low-pass filters, ensuring high frequencies don't reach the subwoofer (low-pass) and low frequencies don't go to the main speakers (high-pass). This page illustrates a high-pass filter, while this page demonstrates a low-pass filter.

In the 1990s, THX expanded into home theater. VHS and DVD movies featuring the iconic logo undergo the THX Digital Mastering Program, ensuring top-tier standards for sound, video, compression, and replication. Visit THX.com: Certified Products for a list of THX-approved home theater components. However, unless your home theater room meets the same rigorous specifications as commercial theaters, the THX experience may not match that of a cinema.