How it works
Build a passive matrix speaker network which decodes surround sound
Build a wireless surround sound system for as little as $40.00
© 1993, 1998 by Robert
The circuits and wiring diagrams below have been tested on several hi-fi
stereo systems with no damage to the systems. However, I cannot be responsible
for some hair-brained engineer who may have used a radical design that
might be damaged by an unorthodox speaker hookup. Use these circuits at
your own risk.
History of Surround Sound
After reading many articles about surround sound I have come to the
conclusion that most of the writers haven't done their homework. Some writers
trace the origin of surround sound to the 1976 movie Star Wars.
Others trace it to the abortive attempts at marketing quadraphonic (four-channel)
stereo in the early 1970's. Where did surround sound originate; what was
the first surround sound movie? So far, the earliest reference I have seen
to what is now called surround sound was a documentary on the making of
the 1940 Disney movie, Fantasia. The concept of encoding a third
channel into a stereo signal is so simple that I'm sure it was thought
of as soon as stereo sound was developed.
What about Star Wars? Star Wars was released in a six-channel
stereo format which was already at least 15 years old in 1976. I suspect
the truth is that, starting with Star Wars, surround sound became
almost universally used for movie prints destined for theaters lacking
the more expensive multichannel sound systems. Home theater surround sound
is simply a by-product of these prints. There is no need to re-record the
sound for home video. Once a movie is encoded with surround sound, it remains
with the movie for the home video release.
How Surround Sound Works
With standard stereo, the separate signals are sent to separate speakers,
one to the right of the listener and one to the left. The sounds produced
by the speakers may seem to come from one speaker or the other, or may
seem to come from empty space anywhere between the speakers.
Figure 1: speakers in unison, sounds appear to come from between
If the sound level in both the right and left speakers are equal, the
sound will seem to come from a point in space half-way between the speakers.
Figure 2: If a sound is louder in one speaker than the other
the sound will be heard closer to the louder speaker
If one speaker is louder than the other, the sound will seem to come
from a point in space closer to the louder speaker.
When a stereo signal is recorded, microphones are placed so that the
sounds in each speaker is at the correct level to make sounds seem to come
from the directions they originated form. Because of this all of the instruments
of a symphony orchestra will seem to come from their proper places between
the speakers; violins, trumpets and timpani toward the left; violas, woodwinds
and horns toward the center; cellos, basses and trombones toward the right.
Stereo sound, as described above, works only if the speakers are in
phase, meaning that the speakers are wired to work in unison. To be in
phase, the speaker cones must move together; they must move toward the
listener at the same time and must move away from the listener at the same
If you accidentally wire one (but only one) speaker backwards, the
speakers will work out-of-phase; one speaker cone will be moving toward
the listener while the other speaker cone is moving away from the listener
and vice versa. This causes a total collapse of the stereo effect. All
sounds will seem to come from everywhere between the speakers at once or
may seem to come from both speakers at once. To be sure your speakers are
in phase; tune to an AM radio station (or use another non-stereo signal);
adjust the volume to be equal in both speakers. The sounds should seem
to be coming from a single point in space between the speakers and nowhere
else. If this is not the case, the speakers are out-of-phase. To fix the
phasing, reverse the wires on one (and only one) speaker.
Figure 3: speakers out of phase, listeners can't tell where
sound comes from
Using Pha