As you may recall, I've been reviewing the function of the various channel groups in surround sound, including a consideration of how they came to be, how they can be approached in various types of production, and how they end up being used by our beloved viewers in their homes. This month, I'll discuss the most challenging channel: the center channel. You can't live with it and you can't live without it! What're you gonna do?
Stereophony was "invented" twice, at approximately the same time (I'm leaving out a LOT of history here--full apologies to Clement Ader). In England, in the early 1930s, Alan Blumlein invented a series of two-channel devices (including microphone arrays, cutter head mechanisms and phono cartridges) that captured, transmitted and reproduced two correlated "stereophonic" signals that yielded, when played back by a matched pair of loudspeakers, a remarkable set of sensory illusions that fall into the family of audio we now call stereo. Central to these is an illusion called the "phantom image."
At roughly the same time (ca. 1933), in the United States, Harvey Fletcher of Bell Labs created a three-channel stereophonic system that utilized three microphones, three transmission channels and three loudspeakers.
The difference between these two systems is both simple and profound. In Fletcher's three-channel version, the center of the stereophonic illusion is "solid" in that it is represented by an actual sound source. In Blumlein's version, the center of the stereo illusion is "phantom," which is to say it is "inferred" by the human auditory system. How the human auditory system does this is fascinating and remarkable to me. I have spent the past 20 years observing and studying it.
Reduced to its essence, the phantom image is a brain-generated perceptual construct based on similar signals having (very roughly) similar amplitudes and (quite precisely) similar times of arrival at our ears. To perceive a phantom image midway between two loudspeakers, the identical signals must arrive at the ears within .2 milliseconds of each other. Otherwise, the phantom begins to migrate toward the earlier speaker. What this means is that for two-channel stereo, if the listener is more than six inches off the median plane (i.e. equidistant from the two speakers), the phantom will migrate to the nearer speaker. See Fig. 1 and Fig. 2.
This is a major limiting factor for two-channel stereo. It is central to our discussion of the center channel.
The first instance of released commercial stereophony has to be Walt Disney's "Fantasia," produced in the late '30s. Interestingly, the conductor was Leopold Stokowski, who participated in experimental three-channel stereophonic efforts with Harvey Fletcher as early as 1933.
Fantasia actually was a multichannel surround production (trade-named "Fantasound"). As part of that effort, there were three channels across the front, behind the screen. I speculate that this configuration set the stage, literally, for the cinema stereophony that was to follow in the 1950s.
In that cinematic version of stereophony (the version intended for commercial movie theater release with Cinemascope, ToddAO, and Vistavision, among others), all dialogue was assigned to the center channel, to keep it "on-screen."
A two-channel version, which depends on phantom images, would have been defective for theater-goers other than those lucky few on the median plane. The left/right stereo was reserved for music and some reverb, ambience and FX. This convention has been carried forward, and is in broad use today.
IN VIDEO
When we consider video production and transmission for viewing via home television in its various guises, we have to make provision for both phantom and solid images, because that transmission may be either multichannel or two-channel stereo Further, our beloved end-user's system may or may not have a center channel.
In the Dolby Pro Logic and related analog systems, where surround is derived from a two-channel signal, a steering mechanism is employed to derive the center channel signal, while suppressing all signals that exist only in left and/or right channels.
In discrete multichannel production, we tend to finesse the center channel by deriving both a solid and a phantom center, typically by sending 50 percent of the left/right mono summation to the center channel, so that both it and the left/right phantom are present, each 3 dB down. This works reasonably well.
IN MUSIC
The center channel is almost a non-starter for multichannel music production. We have fairly strict and well-established, if informal, conventions for treatment of the phantom center image, and we have come to view the solid center channel as an aberration, except for SFX.
Some music mixers skip the center channel entirely, while others have learned to use it as one side of a three-channel stereo array that employs either left surround/left/center or center/right/right surround for a more spacious and compelling "side" signal.
In addition, music mixers will often use the same 50-percent trick that I mentioned above. At the other end of the range, some mixers send nothing but a little reverb to the center channel (to reassure listeners that the channel is working), for reasons that will become clear below.
IN THE HOME
Unfortunately, the center channel is the most wildly variable element in the 5.1 surround sound playback system. It almost never is an identical speaker and sometimes there is no speaker at all. Further, because it exists in approximately the same point in space as the television, it necessarily has to have different behavioral characteristics (make that deficiencies) when compared to more conventional full-range left and right speakers.
This is a major part of the reason it is approached with such caution by music mixers. We depend on the center image (phantom or solid) to carry the kick drum, bass and lead vocal, which can be thought of as THE most important elements in a pop music mix. To assign these to a presumably deficient speaker of unknown character and quality requires a large and unjustified leap of faith.
For home theater viewing, the situation is less dire. Dialogue usually requires less bandwidth and dynamic range than in music and FX, and even a somewhat marginal center channel can comparatively easily cope with these demands.
There is a final observation to be made here: the end-user cannot reasonably be expected to understand these concerns, or to optimize his/her system to fulfill these needs.
WHAT DOES IT MEAN?
The center channel for home video, multichannel music and home theater is a hand-me-down from commercial cinema practice that is still necessary but poorly implemented and often at odds with musical issues. It needs to be dealt with conservatively and carefully. Whatever you do, don't ask for too much from it!
Thanks for listening.
