Crossfeed is the process of blending the left and right channels of a stereo audio recording. It is generally used to reduce the extreme channel separation often featured in early stereo recordings (e.g., where instruments are panned entirely on one side or the other), or to make audio played through headphones sound more natural, as when listening to a pair of external speakers.
Crossfeed is claimed to provide relief for a small fraction of listeners who otherwise complain of "fatigue" and headaches when they listen to stereo recordings.
Crossfeed is most commonly found in headphone amplifiers and often can be toggled with a switch. Many audio player programs for computers can perform crossfeed via plug-ins.
Principle of operation 
Directional sound perception is based on the delay between the same sound reaching a person's left and right ears. In stereo speakers the sound from one speaker reaches both ears, although at different levels, and with a delay between one ear and another, since the speaker is placed away from the center. In headphones, such crossfeed does not occur, and the created stereo image is different from the same one heard from speakers. The crossfeed attempts to recreate the stereo image heard from speakers by mixing some signal from the left channel into the right channel and vice versa. It should be noted that the loudspeaker stereo image differs from a natural source in that there are two sources of sound reaching each ear(one from each loudspeaker). In that respect a headphone image could be more accurate, however most recordings are produced specifically to be played through loudspeakers, and may not sound as intended when listened in headphones.
Types of crossfeeds 
- Passive (mono):
A passive crossfeed is usually implemented by mixing the left and right channels together to some extent. Since such crossfeeds do not correct the delay between the added information and the original one, they simply reduce the amount of stereo information available in the signal, centering the image, since most of the information is now mono.
- DSP (3D audio effect):
A DSP type crossfeed implements the sound stage as one would hear it through speakers, by mixing an amount of signal from one channel to the other, delaying to mimic speaker distance and applying a HRTF to mimic the changes between the left and right ears. Examples of DSP crossfeeds are Dolby Headphone and Bauer stereophonic-to-binaural DSP.
See also 
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