Five speakers

5.0 Audio

Soon after Dolby Surround came out, it was replaced by Dolby Pro-Logic surround. This added probably the most important channel, the center channel. Again, the source for Dolby Pro-Logic was still a two channel stereo source, but engineers knowing how the decoders would interpret various phase patterns in the stereo signal were able to record with this in mind. This created a very successful surround sound system, with predictable distribution of sound, while using a two channel source. Limitations to this system included surround channels that were limited in frequency response in addition to the technical limitations of this narrowband format. Subwoofers were added to many Dolby Pro-Logic systems, but it was the decoder that distributed the sound to the subwoofer.

Around this time, THX home was first introduced which added a few new tweaks to Pro-Logic audio. THX added an adaptive correlation algorithm that created discrete rear channels. They use information from the extrapolated surround channel and the front channels to create a stereo rear surround channel. Cinema re-equalization was also added to roll off the high frequencies in the front channels. The reason for this was that in the movie theaters, the speakers need to fire through the perforated movie screen, tracks are therefore mixed with additional treble to compensate for the loss going through the screen. When these movies were brought directly to laserdisc, they used the original sound mixes intended for the theaters. In your home, since the speakers are for the most part unobstructed, THX removes this added equalization. With the advent of DVD, it is not clear how the tracks were mixed, especially with non THX discs. With THX discs, it is assumed that they benefit from going through a THX decoder. Recently, the specifics of what THX does has become less clear, THX simply says that they make the experience better. With many DVDs this is true, having the option to use THX or not is a great feature to have.

In addition, THX advocated the use of multiple subwoofers to best replicate the theater experience. Their standards called for subwoofers to play all frequencies below 80hz. THX also promoted controlled dispersion of the speakers. Front and center channel speakers were designed to have a limited vertical dispersion with wide horizontal dispersion to limit interactions from sound bouncing of the floors and ceilings. Rear speakers were dipole to create a wider, more diffuse rear sound stage. THX amplifiers were sending 5.1 signals to the speakers, discrete channels for all speakers including the subwoofer all from an analog stereo signal.