Ambisonic Surround Sound FAQ, version 2.8: Section 12

12. What is the pair-wise mixing style?

To produce a sound from the direction of a speaker requires only channel separation. To produce phantom sound images between speakers requires a mixing style. In stereo, the most popular mixing style is "pair-wise" mixing.

Pair-wise mixing is also called "pan-potting", "amplitude mixing" and "intensity stereophony". It mixes signals into the feeds for a pair of speakers to create the illusion that a sound is coming from a point somewhere between the speakers. During mixing, the apparent location of each sound is determined only by the relative amplitude of that sound in the two speakers. Almost all stereo recordings are mixed using the pair-wise mixing style.

The ear/brain localises sounds using phase differences between the ears as well as amplitude differences. (Phase is used to localise sounds with frequencies between 150 Hz and 1.5 kHz, amplitude for frequencies between 300 Hz and 5 kHz, and other cues for frequencies above 2.5 kHz. Note that the three frequency ranges overlap.) Fortunately, when a pair of speakers are in front of the listener and separated by 60 degrees or less, because each ear hears both speakers, low-frequency amplitude differences between the speakers are converted to phase differences between the ears. For most people the pair-wise mixing style works well in stereo.

Unfortunately, pair-wise mixing works poorly when the speakers are to the rear of the listener and not-at-all when they are to one side. (See the Gerzon 1985 or the Fellgett 1981 references. Better still, try it yourself!) This means that any surround sound system that relies on pair-wise mixing between adjacent speakers must fail. This is as true for the 5.1 discrete channel systems of today as it was true for the quadraphonic systems of yesterday. Such absolute statements can be made because the way that the ear/brain localises sound has not changed.

Ambisonics is completely unconnected with pair-wise mixing and does not suffer from its surround sound limitations. With the Ambisonic mixing style, sounds can originate from any direction, either 360-degree horizontal or periphonic (full-sphere).