Stereo and Surround Sound
From DSPWiki
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Stereo Reproduction
The proper placement of stereo speakers is at 30° angles from the center of the listener. This can also be imagined as an equilateral triangle between the two speakers and the listener as an optimal listening situation.
With off-center listening, the position of the center image is problematic. Everything that should be heard in the middle will sound more like it is coming from the closest speaker. This is an important concept to keep in mind while mixing, because a mix should be durable, and work in a variety of settings.
Movie Sound
Strictly stereo sound in movies is unintelligible from seats at extreme angles because the speakers are so far apart, and the difference in the delay time between the speakers causes an echo-like disturbance. Because of this, dialog is always mixed to the center channel, regardless of where it takes place on screen. Music and effects are mixed to the other channels.
Dolby Pro-Logic Surround Sound
Dolby Pro-Logic is was an analog attempt to interpolate 4 channels from the 2 tracks available on standard film at the time. The channels were left, center, right, and rear. The rear channels had a narrower frequency range, and since there was only one rear channel, it didn't really "surround" anybody.
Panning
Stereo panning results not only in amplitude differences between the speakers, but also a phase difference. Since there is no acoustic isolation of the left speaker from the right speaker, the signal leaks from side to side, and this is called "crosstalk".
Two different strategies for panning are available: amplitude (linear) panning, and intensity (constant power) panning. Constant power maintains equal power in the center, not equal amplitude. This tends to make the center image more pronounced, which is beneficial in certain situations, such as with high frequencies or in large spaces.
Panning Inaccuracies:
Sometimes high or low frequencies can pan too fast compared to the other frequencies. This explains some of the reason people can claim one pan pot is better than another.
