sing to Add a Third Channel to a Two Channel System
Encoding surround sound is very simple. The third channel is simply
mixed into the left channel as if it were any other microphone channel.
However, before being mixed with the right channel, the phase of the third
channel is reversed. This has the same effect as if the wires on the right
speaker were reversed, but only for sounds encoded for the third channel.
Therefore, sounds intended for the left and right speakers will sound normal.
Sounds encoded for the third channel will be hard to locate and may even
be lost in the shuffle unless deliberately listened for.
Decoding the Surround Channel
In order for a speaker to work there has to be an electric current
flowing through it. As the current through the speaker changes, the speaker
cone moves back and forth. The more current flowing through the speaker,
the farther the cone moves. The current actually reverses direction so
that the speaker cone is alternately pushed out and pulled in, creating
[missing diagram of typical speaker]
To decode surround sound is also very simple. All you have to do is
add a third speaker. The secret is how the third speaker is hooked up to
the system. Normally a speaker is hooked up to the red and black (or +
and -) connectors on the stereo amplifier. The third speaker has both wires
hooked to the red (or +) connectors on the amplifier. This is the simplest
way to create a surround sound system.
Figure 5: Surround sound with sounds for front channels
Notice in Figure 5 that current flows through the front speakers, in
one side and out the other. Current doesn't flow in the third speaker because
current is trying to flow in both sides at once.
Figure 6: Surround with sounds for the rear channel
With sounds encoded for the third channel, the current flows out of
phase for the front speakers. Notice in Figure 6 that when the current
flows out of phase in the front speakers, it is presented to the rear speaker
in such a way that it is able to flow through the speaker. Therefore, sounds
encoded for the rear speaker are also heard in the front speakers, but
are out of phase so they are difficult to locate. On the other hand, these
sounds are heard solidly in the rear speaker so are only perceived to come
from there. It actually works better if you use two speakers for the surround
channel. When two speakers are used for the surround channel, the black
(or -) wires are connected together as shown in Figure 7.
Figure 7: Decoding surround sound with two rear speakers
In the above example the regular stereo sounds are considered to be
equal, thus heard halfway between the front speakers and totally unheard
in the rear speaker. In reality, sounds are often unequal in the front
speaker and thus heard off center. With these sounds there is a proportional
leakage to the rear speaker. This has two effects: first, the off center
sounds are "pulled" farther off center; second, it adds a pleasing ambiance
to the sounds in the front speakers even when no third channel is encoded.
This is exactly what Radio Shack's Quatravox four channel simulator did.
If you happen to have a Quatravox it will decode surround sound.
Dolby Surround and Dolby Surround Pro Logic
So what about Dolby Surround® and Dolby Surround Pro
Logic®? Dolby Surround® adds a 20 millisecond delay
to the rear channel. This makes your room sound larger. Since the sounds
from the rear channel arrive late, it seems as though they came from a
greater distance. Pro Logic® add a sophisticated "gain riding"
amplifier to the system. This amplifier attempts to determine where the
loudest sound is and boosts the appropriate amplifiers to emphasize that
sound. The reason for this is to help solve the problem of signal leakage
from the front channels to the rear channels, as mentioned above. Mathematically,
there is only a 3db separation between the front and rear channels. Theoretically,
this separation would be barely detectable; you would have to listen hard
to tell that there was any difference at all. However, in reality the human
auditory system hea