Phantom Center Phantom center channel (center speaker set to None)

What phantom center is

Phantom center mode means the receiver's Center setting is "None." Denon's manual describes the option as the one to select when a center speaker is not connected, and the receiver then plays the center channel through the left and right speakers. The reference downmix is written down in ITU-R BS.775-4: for a 2/0 stereo output, the center signal is added to each front channel at a coefficient of 0.7071, which is -3 dB. Two speakers playing the same signal produce an image between them, the same illusion that makes ordinary stereo work.

The illusion is older than surround sound. Earl Vickers' AES paper "Fixing the Phantom Center" credits Blumlein's 1931 finding that level differences between two speakers produce phase differences at the ears, which the brain reads as direction. That mechanism is strong at low frequencies and weakens above roughly 1 kHz, where phase cues become ambiguous.

Why a phantom image sounds different

The same paper lists the structural problems. Acoustic crosstalk, the left speaker reaching the right ear and the reverse, creates a comb-filter pattern of notches across the spectrum. For speakers at the standard +/-30 degrees, the lowest and deepest notch sits near 2 kHz, and it moves higher for narrower speaker angles. That band matters for consonants. Vickers cites Shirley and colleagues, who measured a small but statistically significant drop in speech intelligibility with a phantom center compared with a real center speaker.

The arithmetic of that notch is simple. At 343 m/s, the speed of sound in air at 20 degrees C, a 2 kHz wavelength is 17.2 cm, and a notch forms when the relative delay between the two arrivals is half a period, 0.25 ms, or about 8.6 cm of extra path. That delay is consistent with the roughly 2 kHz notch the paper reports, which lands in the speech band. Head shadowing reduces the combing above 2 kHz, and room reflections fill some of it in. Vickers quotes work finding that notches are almost inaudible when filled by reflections arriving within 10 ms, and that very dead rooms leave audible comb filtering.

Level also shifts. Two equal signals add to 6 dB electrically but only 3 dB acoustically in a reverberant field, so a phantom center changes balance when a mix is folded to mono, with a 3 dB center build-up.

Phantom center vs a real center speaker

Vickers notes the oldest fix is a real center speaker. Harvey Fletcher of Bell Labs used one around 1933, and Disney assigned dialogue to a center channel behind the screen for Fantasia in the late 1930s. A center speaker removes the problem of off-center listeners localizing dialogue to the nearest side speaker, because the precedence effect pulls a phantom image toward the closest source.

ITU-R BS.775-4 builds the reference layout around that fix: three front loudspeakers, with left and right at the ends of an arc subtending 60 degrees at the listening point, and the center's acoustic center at about ear height. It adds that this implies an acoustically transparent screen, and that with an opaque screen the center should sit immediately above or below the picture. Dolby's 7.1.4 speaker guide draws the front pair at 22 to 30 degrees from the center line. See acoustically transparent screen for the projector case, and TV as center channel for the middle path where the TV's own speaker serves that role.

When phantom center works

It works best when one listener sits on the center axis, because the notch and the image are both stable there. Narrow seating, speakers at the 22 to 30 degree angles Dolby shows, and a room with some reflective surfaces all help.

It struggles with a wide couch. Each seat off the center line hears the image shift toward the nearer speaker and hears a different comb pattern at each ear. No source I fetched gives a seating width at which phantom center becomes unacceptable; that threshold depends on speaker spacing, directivity and the room, so treat any single number you see online as a guess.

Worked example

Speakers 12 ft apart (6 ft either side of center), listener 10 ft back and sitting dead center: both speakers are 11.66 ft away, the arrivals are simultaneous, and the image sits in the middle. Move the listener 1 ft toward the right speaker. The right speaker is now 11.18 ft away and the left is 12.21 ft away, a 1.03 ft difference. At 0.889 ms per foot that is a 0.92 ms lead for the right speaker, and the first comb notch for a 0.92 ms delay falls at 1/(2 x 0.92 ms), about 540 Hz, well inside the speech band. This geometry ignores head shadow and room reflections, so treat it as an illustration, not a prediction.

In an AV receiver, setting Center to None on a Denon model such as the AVR-X2700H is the only change needed; the manual does not state the routing, and the ITU downmix is the reference behavior. With no center speaker, the Front speakers also carry the dialogue, so their Large or Small setting decides how much low bass the dialogue channel sends to the sub. For setup context, see speaker level calibration and the center channel buying guide.