by Richard Elen,
Originally published on AudioRevolution.com
In the beginning was mono... a single, "monophonic" channel of recorded sound. In fact, in the beginning there was one microphone, as well: a horn attached to a mechanical recording machine. Later, the advent of electrical recording brought the possibility of mixing the signals from multiple microphones together to produce a final result.
But as early as the 1930s, Bell Labs in the United States and EMI in Britain (under the guidance of audio genius Alan Blumlein) were working on stereophony, using two audio channels to create a more complete audio picture. "Stereophony" really means "solid sound", and carries the idea of creating a true three-dimensional sonic image within the listening environment. But we generally understand the term "stereo" to mean audio with two channels.
There are several ways of creating a stereo soundstage, and the most common is to use two or more microphones whose signals are positioned in different places across the soundstage from left to right - this is derived from the Bell Labs approach. Another approach, favored by Blumlein and quite widely used in Europe in general, is to use a special stereo microphone or microphone array in which a pair of microphone elements are placed very close together (a "coincident pair"), pointing at an angle to each other (normally 90 degrees). This technique produces remarkable realism and depth to the sonic image, but it is less easy for the recording engineer and producer to control.
Stereo, as we generally understand it, delivers a sound stage that extends between two speakers at about 60 degrees apart as far as the listener is concerned. While some techniques can make the soundstage appear to extend beyond the speakers, of in front or behind them (Blumlein stereo is particularly good for this), it is difficult to make the sound stage wider with only two channels. If you move the speakers further apart, you get a hole in the middle, for example.
In the 1970s, several attempts were made to extend stereo into a second dimension: depth. Most approaches relied on adding to the front pair of stereo speaker a second pair at the rear. This was referred to as "quadraphony", or "quad" for short. Ideally, in the studio, a recording would be mixed to four channels, and those four channels would be fed to the four speakers in the listening environment - this was referred to as "discrete quad".
Unfortunately, there were few four-channel distribution media available at the time other than tape, which was not widely used. Instead, several techniques were developed to attempt to convey four channels on two-channel media, particularly the vinyl disc. Most of these involved "matrixing" the four channels down to two, and then decoding the signal at the other end, to recover the original four channels - in the meantime insuring that the 2-channel medium was playable on regular stereo equipment.
Regrettably, to do this matrixing with 100% accuracy is mathematically impossible, and as a result even the leading systems, Columbia's SQ system and Sansui's QS, had major performance problems.
Other techniques that used a high-frequency subcarrier to carry the surround information in a similar way to FM stereo also suffered problems, and ultimately, with its host of incompatible, poorly-performing systems, "quad" was discontinued.
It was not only quad's transmission method that was problematical. The approach to creating surround-sound itself was not entirely successful. Although we can pinpoint an audio image very accurately across a 60-degree soundstage in front of us, our hearing is far less accurate across a rear stage, and our ability to localize sounds to the side is even worse.
Luckily, special effects for the cinema do not rely on pinpoint localization accuracy: they require instead impressiveness. Dolby Laboratories developed a surround sound matrix based on QS, and began to promote it for theatrical use. There was a problem, however. The "quad" setup had speakers either side of the screen at the front - way wider than the recommended 60 degrees for most of the audience. The result was that center-front sounds, such as dialog, tended to get lost: a major shortcoming.
The solution was to add an extra channel, center-front, to handle dialog. In addition, logic circuitry was added to improve the localization performance of the system as a whole.
The resulting system rapidly caught on. There was also a need to handle powerful, ultra-low-frequency sounds, such as dinosaur footfalls or asteroids crashing into the Earth. These couldn't be handled by the existing speaker systems because of distortion. As a result, an additional audio path was added to handle Low Frequency Effects (LFE). As the LFE only needs to handle low frequencies, it doesn't require full bandwidth: it's just a "0.1" channel.
Today, cinemas and home theater systems use a discrete six channels to carry audio to the loudspeakers: Left front, center front, right front, left surround and right surround, plus the LFE. The result is the surround-sound system we today refer as "5.1".