Captives of Two-Channel Sound
For most of us, the word "stereo" implies two
channels of sound information, and that seems intuitively right: we have two ears, so we
only need two sound sources. The word actually derives from the Greek for
"solid" and carries the sense of something with an extra dimension.
Stereographic photographs are 3D and stereophonic recordings allow you to mentally
position musical instruments or other sounds across an imaginary soundstage. In audio
terms, stereo can mean any number of channels (except one), and in many instances, two is
far from ideal.
In fact, there are times when mono is more than adequate.
For some years, I lived in a place that included a large, reverberant room that I used as
my primary listening area. From time to time I found that I had left the system switched
into the mono mode rather than stereo, but only noticed it when I got very close to the
speakers. Some stereo recordings even sounded better in mono.
The reason was that the room itself had an acoustic
environment complex enough to impart a real sense of space and depth to the sound, whether
mono or stereo. And in most large performance venues -- theaters and concert halls --
beyond a certain distance from the stage you hear nothing but reflected sound; any spatial
clues from the direct sound are thus lost, yet few would argue that listening to a concert
from the rear half of such an auditorium is an unsatisfying audio experience. That's what
used to happen in my big room as well.
Most listening rooms are small, however, and acoustically
dead. So various means have been tried over the years to include a sense of acoustic space
in the recording itself. Even in the days of mono-only recordings, engineers often added
in lots of echo to make the sound seem more spacious.
Real stereo, when it came, was a big improvement, because
it did contain genuine directional clues. But the two-channel version available for home
use had -- and still has -- some limitations.
One has to do with what's called "imaging." Our
hearing apparatus is adept at playing psychoacoustic tricks on us. If someone talks to you
from directly in front, the bits of waveform that make up his voice arrive at your ears at
precisely the same time. That's the clue that tells your brain he's right in front of you
even if your eyes are closed. With a stereo system, if you're sitting exactly the same
distance from two speakers, and they reproduce the same material, it will also reach your
ears simultaneously. The illusion is that the sound is coming from directly in front
rather than from the speakers off to the side; other sounds arrayed across the soundstage
similarly seem to come from their proper positions between the speakers. A sound coming
from slightly to the right of center will arrive at our right ear slightly before it gets
to the left, which is how we assign its position.
We're extraordinarily sensitive to these time clues; if we
move our heads only inches off the midway line between speakers, the center image
immediately snaps to the nearer speaker. Thus, stereo listening means sitting still, and
can usually only work properly for one person in the room.
Some early stereo systems provided a center-channel output
to fill in the gap but these were never very satisfactory. Only with Dolby Pro Logic
technology was this problem solved; it derived a real center-channel signal by directing
dialogue and suchlike to the center channel and removing it from the left and right
channels. This resulted in a rock-solid center image wherever you sat in the room because
the sound was actually coming from the center.
The other main drawback of two-channel stereo is that it
creates a curtain of sound across the front of the room, but very little ambience. From
time to time, speaker manufacturers have attempted to fill out the sound field by making
speakers that fire backwards or sideways as well as forward, and various electronic
schemes have been used to give a sense of wideness a normal two-channel stereo lacks, but
these have tended to remain curiosities.
Over the years, most audio experts have agreed that the
only way to simulate a truly three-dimensional audio field is to provide extra speakers to
reproduce the reverberant sounds of the location where a recording was made, rather than
relying on the listening room's own characteristics. That was the theory behind quad 30
years ago, and when a demonstration was properly mounted -- as it almost never was -- it
could be spectacularly effective. The main problem was that the technology was not up to
the demands of four-channel sound in those days. Dolby Surround fared much better, and its
digital equivalent finally put the last technological fillip on multichannel sound. Even
so, most of us are likely to do a lot of our listening from two speakers for the
foreseeable future.
Why? If two channels are inadequate, why was this system
adopted in the first place?
The answer seems to lie in the scramble to develop some
sort of home stereo in the mid-‘50s. Most of us first heard stereo in movie theaters,
where some of the big first-run films had six-channel surround tracks not unlike those
being produced today. Six channels were possible because the film companies could apply
that many magnetic strips down the edges of the film and record on them. It wasn't too big
a stretch to imagining the same sort of thing could be done with ordinary tape recorders,
too, but nobody seems really to have tried (although two-channel stereo showed up first on
tape).
The assumption was that the LP would be the prime music
medium for a long time, so a method had to be found to put multiple channels on vinyl. One
innovator tried recording one channel on the outer half of a disc and the other on the
inner half; two cartridges were used to play this record, but it never took off. For one
thing, keeping the spacing of the cartridges was a nightmare. Also, it yielded only half
the playing time of a normal record and the inner channel was notably inferior.
There was even an ambitious attempt to "bury"
extra information on a disc by means of high-frequency subcarriers, which might have
permitted more than two channels, but it was very hard to make it work -- then, or 20
years later when the technique was used again for quad.
In the end, the recording industry adopted a system that
had been patented by an Englishman named Alan Dower Blumlein as early as 1931. This was
founded on the physical fact that a record's groove wall had two sides, so you could
record one channel on each. By the time newer technology came along that might carry much
more information, the two channels were so entrenched that no one seems to have considered
more. Maybe they just remembered quad, shuddered, and settled for the familiar.
...Ian G. Masters
ian@mastersonaudio.com
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