Home theater audio begins with understanding surround sound.
Although movies, TV, and other video are thought of as visual media, creative professionals know that the soundtrack is half the experience. No one wants to watch a movie on the best possible screen with no sound, or with low-quality sound. A truly immersive experience requires sound reproduction that tricks your brain into thinking that you are actually in the middle of the action, as if you were “in” the movie. Today’s sound recording, encoding, and rendering systems are able to achieve an unbelievable level of realism. A big part of that realism comes from surround sound—the ability to play back sound in a way that places the apparent source of any object almost anywhere in your room.
Stereophonic (stereo) sound recording and reproduction provides a somewhat two-dimensional sound stage (left to right based on the volume of a sound source in the left vs. right speaker, and near to far based on the volume of that source). Recording engineers and producers place instruments or sounds anywhere along those two dimensions. For recorded music, this was a big improvement over monophonic sound. Because humans have two ears, positioned on opposite sides of our head (binaural hearing), we can often determine the position of an object even without seeing it. This is because sound coming from an object on our left will arrive in our left ear slightly earlier and with slightly higher volume than in our right ear. Sound engineers can simulate this effect by panning an individual track in a multi-track recording left or right to position individual sound sources (instruments, people, or sound-producing objects or events).
Although the near-far axis can be simulated by adjusting the volume of a particular source, the apparent location of a source in a stereo recording will always come from the pair of speakers (such as the front of the room), as opposed to the other side of the room (such as the back of the room, or overhead).
Home video distribution started with VHS and Betamax tape, and these formats included stereo audio, also known as two-channel audio.
Early surround sound consisted of 4 channels, with speakers positioned in the 4 corners of a room (left front, right front, left rear, and right rear). Commercial quadraphonic LP record and tape formats were introduced in the early 1970s. A small number of quadraphonic recordings were released, but since these formats were a commercial failure, they disappeared by the late 1970s.
5.1 Surround Sound
The first commercially successful surround sound recordings use 5 channels (left front, center front, right front, left rear, and right rear), along with a low frequency effects channel. While 5.1 channel surround sound playback can provide the illusion of sound sources toward the rear or sides of a room, localization of the sound source by the listeners is not consistent, and the results vary when the position of the listener is not in the center of the room. The center channel is often used for voices in the soundtrack, which helps keep the apparent position of the people speaking on the video screen, regardless of the position of viewers/listeners in the room.
Dolby Digital Surround Sound
Dolby developed the Dolby Digital (AC-3) 5.1 channel surround sound format in 1986, but it wasn’t until the introduction of DVDs in 1996 that consumers could get an inexpensive, accurate surround-sound format in their homes. Dolby Digital led to a revolution in amplifiers, receivers and speakers, as consumers recognized that their home video experience was much better with surround sound. Dolby Digital supports up to 6 discrete channels, and these are normally organized as 5.1 channels, with 5 full-range audio channels (left, right, center, left rear, and right rear) and a subwoofer channel (denoted as .1).
7.1 Surround Sound
By adding speakers to the left and right side walls, midway between the front and back of the room, a 7.1 channel audio mix can provide much better spatial localization versus 5.1 channel audio. Still, there is one dimension that is missing... the height dimension. For that you need Spatial Audio.