THX

What is THX? This sound system was developed by George Lucas after he found that sound carried by cinemas did not meet to his criteria and satisfaction. He develop patents.

Actually, THX system in cinema consist of approved speakers and sound processor. Strict accoustic requirement for noise, echoes and reverbation, screen size and seating position must be met. THX engineers checked them periodically.

Do not be mislead to think that THX , Dolby Surround and DTS are competing surround sound. They are not. In fact THX builds on Dolby Surround to create an even better surround sound.

There are two types of THX equipment. Ultra is more expensive and are meant for larger rooms of sizes larger than 3,000 cubic feet. Select equipment are meant for smaller rooms of about 2,000 cubic feet.

In a smaller room, speaker are placed much closer to the listener and therefore less maximum power is required by the speaker. Secondly, making Select equipment does not require as much vertical directivity as Ultra equipment since the audience is nearer to the speaker.

Ultra front speakers have special capability to control both vertical and horizontal directivity. Vertical directivity is such that reflection off both ceiling and floor are effectively minimized while horizontal directivity covers as much ground as possible so that everyone in the room hears sound well.

THX surround speaker are special too. They are placed at the side of the listener and are dipole speaker with drivers pointing both in front and at the back. Sound is therefore directed to the front and the back. The direct path from the speaker to the listener is a null zone where no sound is heard. It creates an effect of envelopment.

THX basically works on three processes.

THX Re-equalization

The sound system in the cinema has already reduced the high frequency of the movie sound. THX engineers thereby compensate by making the soundtrack much brighter

( increase in treble). However for the home listener, this will come across as too bright though. THX seeks to overcome this problem by reducing the treble.

In movie theater, the sound equipment and hardware tapers off the high frequency sound. Secondly, the long distance between the speaker and the audience seat causes the high frequency sounds to be dampened. Lastly, sound absorption properties of the seats as well as the curtains and fabrics lessen the reverberation of high frequency sounds.

The sound engineers for the film soundtrack make the treble even higher to compensate for these factors.

However, when you listen at home , the soundtrack becomes too brilliant or harsh. THX requalization reduces the treble to produce a more balance sound at home.

THX Surround Decorrelation

In a home theater environment there are only two surround speakers. This is a very real possibility of localisation. Whereas in cinemas there are huge arrays of surround speakers along the sides of the cinema walls to lessen and diffuse the sound.

This process takes the monoaural signal and changes the sound specfically the treble and mid range frequencies

This technology is even more pertinent to Dolby Surround as the signal is monaural. Even in Dolby Prologic and Digital there may be parts of the sound track in monoaural. If the signal is in stereo , the decorrelation process recognises it and is switched off.

Dolby Surround system produces a monoaural sound for the surround speakers. By by changing the time and phase of the sound signal the sound becomes slightly different for the left and right surround speaker and helps to prevent localisation.

Timbre Matching

If you ever noticed, the same sound can sound different coming from a different location.

As all the speakers are placed in different location you will hear different harmonics.

This is especially true during a scene when an aircraft flies overhead and you will hear different harmonics as the sound moves from one speaker to another.

THX seeks to minimize and reduce the differences in harmonics or timbre from different speakers.