Monaural and Stereo Signals

Monaural, or single source sound signals, combine all audio information into a single channel. This is characteristic of older signals, but monaural signals are also currently used in AM radio and in many large auditorium sound systems. Mono is used in AM radio because of bandwidth limitations. For auditorium sound systems, the use of ordinary stereo is problematic because those listeners close to one of the stereo speakers will perceive all of the sound to be coming from that speaker, whereas the real sound source may be center front. The resulting sound image problems are so severe in large auditoriums that a single sound cluster is typically used over the sound source to keep the sound image centered.

Stereo, or two discrete channel sound, gives a much more realistic sound in a home listening room, allowing you to localize the instruments in an orchestra for example. Discrete stereo with high channel separation is possible from any signal source which can present a left and right channel, and can be broadcast with FM radio. Further enhancements to sound fields can be made with surround sound.

Surround Sound

Surround sound is a term applied to five channel sound (regular R & L stereo, center front, plus left and right rear speakers). It typically means Dolby Surround, the home application of Dolby Stereo, which was introduced into movie theaters in the 70's. It is a matrix encoding scheme which puts four channels of information on two recorded channels to be decoded into L, R, Center, and Surround upon playback.

A popular adaptation of surround sound for home sound systems is called Dolby Pro-logic. To obtain five discrete channels of information (including separate signals to LB and RB speakers requires digital surround sound.

Dolby Pro-Logic

Dolby Pro-Logic refers to a system for decoding Dolby Surround signals and directing them to five speakers. Receivers which are designated as "surround" may passively separate the surround signal and send the center channel signal equally to R and L front channels. Dolby Pro-Logic goes further in that it extracts the center channel signal to send to the front center speaker and actively cancels it from the L & R speakers. It attempts to steer the signal to the correct channels, and in so doing it creates a more realistic sound field, but it cannot overcome the limitations of the matrix encoding scheme. The L to R channel separation is high, but between center and left or right the separation is limited to about 3 dB. The separation between surround and left or right is similarly limited. All surround speakers receive the same signal. Precise sound localization requires 15 to 20 dB of channel separation (Riggs). More precise localization is obtainable from digital surround systems. One such system is called AC-3.

AC-3 Digital Surround Sound

AC-3 is the name given to an encoding scheme for a digital surround system which is called a 5.1 channel system (five discrete channels plus a subwoofer output). The digital approach permits new ways of dealing with the limited available bandwidth. Since most of the bandwidth of video/audio media must go to the video portion, the audio signals must be squeezed into the remaining limited bandwidth. Squeezing the additional discrete channels into the digitally recorded signal is aided by what is called perceptual encoding, making use of the fact that some sounds are inaudible and others are masked by louder ones; these signals can be simply removed by the encoder to make room for more important sounds.

AC-3 also allocates bits between channels of the discrete system to shift signal-handling capability to the channel with the greatest current demand. AC-3 was originally developed for HDTV. AC stands for Audio Coding and 3 is the generation of the design. The designation "Dolby digital" is sometimes used as a name for this system.