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Panning is the spread of a sound signal (either monaural or stereophonic pairs) into a new stereo or multi-channel sound field. A typical physical recording console pan control is a knob with a pointer which can be placed from the 8 o'clock dial position fully left to the 4 o'clock position fully right. Audio mixing software replaces the knob with an on-screen "virtual knob" or slider for each audio source track which functions identically to its counterpart on a physical mix console.
The control knob is designed to distribute its source sounds with constant power, so that at the full left extreme, with the knob pointing to the 8 o'clock position, the sound appears in only the left channel, with all the energy going exclusively to the left channel (speaker). Conversely, when placed in the far right extreme, that is the 4 o'clock position, the sound only appears in the right. In the middle, at the 12 noon position, the sound in each channel is evenly distributed, but decreased in each channel by the law amount, thereby distributing the left and right energy equally across the two output channels, so that the overall sound power level is always constant regardless of the knob position. Because of the phantom center phenomenon, any sound placed at the 12 noon position is perceived as coming from precisely between the left and right speakers, because the brain processes sounds equally from left and right, and concludes that the sound must be coming from a source located between the two speakers.
The pan control in audio gets its name from panorama or panning action in moving image technology. The audio pan control can be used in a mix to create the impression that a source is moving from one side of the soundstage to the other, although ideally there would be timing and reverberation differences present for a more complete picture of movement within a defined space. Simple analog pan controls only change relative level; they don't add reverb to replace direct signal or change delay timing.
Panning can also be used in an audio mixer to reduce or reverse the stereo width of a stereo signal. For instance, the left and right channels of a stereo source can be panned 'straight up', that is sent equally to both the left output and the right output of the mixer, creating a dual mono signal.
The pan control or pan pot (panoramic potentiometer) has an internal architecture which determines how much of each source signal is sent to the two buses that are fed by the pan control. "Pan pots split audio signals into left and right channels, each equipped with its own discrete gain (volume) control." The power curve is called taper or law. Pan control law might be designed to send -4.5 dB to each bus when centered or 'straight up' or it might have -6 dB or -3 dB in the center. If the two output buses are further combined to make a mono signal, then a pan control law of -6 dB is optimum. If the two output buses are to remain stereo then a pan control law of -3 dB is optimum. Pan control law of -4.5 dB is a compromise between the two.
"Signal passes through both channels at an equal volume while the pan pot points directly north." A pan control fully rotated to one side results in the source being sent at full strength to one bus (either the left or right channel) and zero strength to the other. "Tracks thus seem to move in the direction that [one] point[s] the pan pots on a mixer, even though [one] actually attenuate[s] those tracks on the opposite side of the horizontal plane."
The pan pot is not the same as a balance control on a consumer stereo receiver. The balance control takes a stereo source and varies the relative level of the two channels. The left channel will never come out of the right speaker by the action of a balance control. A pan control can send the left channel to either the left or the right speakers or anywhere in between. Note that mixers which have stereo input channels controlled by a single pan pot are in fact using the balance control architecture in those channels, not pan control.
Before pan pots were available, "a three-way switch was used to assign the track to the left output, right output, or both (the center)". Ubiquitous in the Billboard charts throughout the middle and late 1960s, clear examples include the Beatles's "Strawberry Fields Forever" and Jimi Hendrix's "Purple Haze". In the Beatles's "A Day In The Life" Lennon's vocals are switched to the extreme right on the first two strophes, on the third strophe they are switched center then extreme left, and switched left on the final strophe while during the bridge McCartney's vocals are switched extreme right.
See also 
Further reading 
- Rumsey, Francis and McCormick, Tim (2002). Sound and Recording: An Introduction. Focal Press. ISBN 978-0-240-51680-6