Tweak3D.net - 3D Audio: How It Works

Posted: April 10, 2000
Written by: Tuan "Solace" Nguyen

Expanded Stereo

Stereo sound was invented in the 1950's using two speakers, each one being monophonic. Each speaker carries a separate channel. And each channel carries its own discrete signal. Stereo, while being greatly superior to mono, has always been limited by its sound stage, or the size of its audio image perceived by the listener. The major problem was that stereo had a relatively minute optimum listening point which is where the listener must position themselves in order to get the best acoustic image.

Numerous techniques have been used to widen the stereo sound stage and even the optimum listening point by using delays or filters. Most of these however, were poorly designed and implemented and would create muffled sounds, out of phase signals with a great deal of frequency response loss. The bass would just die out and the high trebles sounded "oily".

The most well known companies who specialized in expanded stereo (and usually found on many PC speaker systems) include SRS, Spatializer, and QSound. They use sophisticated algorithms to widen the sound stage more effectively without destroying the original signal. Expanded Stereo is the most convenient method when you're tight on desktop space and low on cash. Some expanded stereo methods entail that you connect a small device between your soundcard and your speakers, adjusting the settings to your liking. Some are done via software such as Power Technology's Widener. A lot of low-to-midrange priced speakers have the technology built in. And even though its sound stage is still limited and it will never do positional effects, it is still a step up from plain stereo.

Surround Sound

Dolby Pro Logic Surround

The first of three types of surround sound is Dolby Pro Logic Surround. It is the oldest surround technology and consists of four channels of audio information: Left, Right, Center and Surround. It is actually composed of two stereo channels. The center channel consists of equal signals from the left and right channel while the surround channel is the left and right channel thrown out of phase with one another. If you try to play a Pro Logic encoded sound through two speakers you may hear a slight echo or sound shift in the audio output. Some home receivers and sound cards have signal processing capabilities that virtualize the center channel with just the front left and right speakers.

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