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THX basics

Enjoy authentic cinema sound at home

thx_smallAs televisions grow ever larger with features like high definition and 3D available at very affordable prices, more and more people are able to appreciate cinema-grade picture quality. Unfortunately, many are so enchanted with their big screen HDTVs that they don’t stop to reflec that their sound is as flat as their screen.

A quick glance at the history of ciema reveals that sound has long played second fiddle to picture. The first moving pictures, after all, were just that – there was no sound at all until 1927’s “The Jazz Singer.” Once movie sound was finally established, picture quality continued to pull ahead of cinema sound with a range of amazing innovations that took grainy black and white and transformed it into dazzling Technicolor followed by more realistic color print film. Screen-size also became ever larger with formats like Cinemascope and VistaVision.

By the 1960s, audiences could enjoy highly realistic shot-on-location films like Bonnie and Clyde, but heard a soundtrack reproduced using technology that had not been improved since the 1940s. In the 1960s, Dolby Laboratories developed a system for reducing the noise – mainly, background hissing – in film soundtracks. Movie soundtrack remained mono, however, until the mid-seventies when Star Wards popularized Dolby Stereo – a new, practical, and less expensive method for reproducing stereo effects in cinemas.

In the early 1980s, the THX standard was introduced. This was not a proprietory audio standard but rather a certification system for cinemas that ensured they employed audio systems capable of reproducing the increasingly advanced surround sound standards that were being introduced at the time. Championed by George Lucas and the audio engineer Tomlinson Holman, THX grew out of the need to ensure that the best soundtracks could actually be played back as they were intended by the sound engineers.

The art of cinema sound

When thinking of movies, the soundtrack should not be seen as an add-on to the picture, but as an integral part of the experience. Just as a cinematographer balances perspective, movement, and background in any shot, a sound engineer needs to ensure that the sound effects do not overwhelm the dialogue and that a realistic sound image is produced. To this end, sound engineers work with sound channels. In order to localize a sound on the screen, the tone will be mixed in such a way that it switches from the left to the middle to the right channels depending on how the source of the sound moves on the screen.

The three front channels reinforce the visual impression of the picture as the camera pans over a scene. Together with rear effect speakers, the impression of being in the middle of the action can be realistically conveyed. Sound technicians need to have a good feel for the relationship of the sound to the events on the screen when they mix the sound, all the while referring to the picture. They adjust levels, mix sound effects into the action and remove others until the sound seems a perfect fit for what’s on the screen. Only then will the picture and the sound work together as one.

Each sound channel is independently mixed, enhanced, and reproduced. Voices are usually mixed so that they radiate from the middle of the screen. In a home cinema system, this means that the centre speaker is very important in reproducing these channels. Anything to the left or right of the screen is reproduced by the left and right front speakers. The rear speakers, also known as the “effect” loudspeakers produce all of the sounds that are not directly represented by something onscreen such as birdsong, traffic, or the wind howling through the trees. A subwoofer takes care of the deep tones such as the rumbling of thunder or a car engine. By nature, very low bass frequencies cannot be easily localized so that these tones seem to envelope the listener from all sides regardless of where the subwoofer is placed.

Cinema Front In film studios, soundtracks are played back under ideal acoustic conditions. Whoever wants to experience a movie as it was truly intended needs to hear it in the studio where it was made. The next best things is a cinema or home cinema system able to simulate these conditions. The THX licensing system for cinemas and home theater systems developed by Lucasfilm Ltd. was developed in order to allow viewers to experience the movie as its creators intended it to be enjoyed.

THX Technology for the home

THX home cinema systems are comprised of control units, amplifiers and loudspeakers capable of meeting the performance and quality standards originally set by Tomlinson Holman together with George Lucas. Home THX components that fulfill these standards are capable of creating a sound quality comparable to that experienced in a production studio.  

The THX home cinema control units and A/V receivers decode audio signals from major surround sound processing technologies such as Dolby Pro Logic, Dolby Digital or DTS. Unlike other system that simply amplify and reproduce four to six channels, a THX home cinema control unit or A/V receiver processes the signals so that they are completely optimized. One way they do this is by a process called re-equalization. Re-equalization or re-EQ, adjusts the treble levels intended for movie theater sound for the conditions of a smaller rom. Without this process, higher frequencies would be otherwise be much too loud, experienced as a preponderance of sibilant and bright tones. That's because, unlike cinemas, small rooms are not able to dampen treble tones very well. The re-equalization process from THX home cinemas therefore simple reinstates the original linear frequency range before the signal is amplified and sent to the loudspeakers. 

Another process that THX A/V receivers and control units undertake is Timbre Matching. This ensures that the tonal qualities of a signal do not change as it moves from one speaker to the next. This is important with effects like the passing of an airplane overhead. Without Timbre Matching, the viewer would notice a difference in the tonal qualities. This difference is the result of our ears reacting differently to sound that radiates from a source in front of us compared to those that radiate diffusely from the sides.

THX home cinema specifications oblige license holders in not only how they implement the procedures but also regarding the compliance of the most stringent standards regarding amplification, loudspeakers and additional components. Compliance with these specifications guarantees that THX home cinema systems consistently produce intelligible dialogue, exceptional localization of moving objects, and an integrated sound image.

Dipole The dispersion effects of the front loudspeakers have an extremely significant effect on the quality of sound reproduction in a THX system. The vertical speakers purposely radiate the sound in a very narrow range into order to better target the sound and to reduce reflections from the floor and ceiling. The horizontally radiating loudspeakers, on the other hand, disperse sound very broadly so that even viewers seated to the sides can enjoy the entire sound panorama. This characteristic was, however, relaxed in the new THX Ultra 2 norm in order to better realize high-end music playback.

An especially diffuse surround sound is as important in home cinema systems as in theaters. In large theaters, this effect can be achieved by many loudspeakers. At home, it is most effectively produced by dipole speakers. The sound from these speakers is dispersed from two directions, neither of which radiates directly towards the listener thereby creating an effect of being situated within a sound image instead of before or behind one.

The front loudspeakers in a THX system reproduce the frequency range from 80 Hz to 20 kHz. The frequency range of both rear speakers extends from 125 Hz to 8 kHz with Dolby Pro Logic and 80 Hz to 20 kHz with Dolby Digital and DTS. The subwoofer is responsible for all frequencies between 20 and 80 Hz. All loudspeakers need to be capable of sound pressure levels up to 105 dB.

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