s
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 hears things differently.
First of all, any sound that is equal in the front channels (one which
would be centered between the two front speakers) is completely rejected
by the rear speaker (as mentioned above); it will not be heard in the surround
channel. Any sound which is out of phase between the two front channels
(as the surround channel will be) will be passed through and heard at full
volume in the rear speaker. Since the surround channel is out of phase
in the front speakers, the listener cannot distinguish where, in relation
the front speakers, the sound is coming from. As a result, by the time
the surround channel reaches your brain, it seems to come completely from
the rear speaker.
The second problem Pro Logic® attacks is that sounds toward
the edge of the TV screen tend to be pulled off the screen altogether;
someone talking near the edge of the screen will seem to be heard off screen.
This is once again caused by leakage of the front channel sound to the
rear channel. Pro Logic® adds a fifth speaker between the two
front speakers. This fifth speaker is fed with an equal mix of the front
channel signals, mixed in phase; sort of the opposite of the rear channel.
This pulls sounds back to the screen. Placing your front speakers close
to our TV helps solve this problem also. (Do not place them too close as
the magnets in the speakers will cause color shifts on the TV screen.)
Two Speaker Systems
There are some two-speaker surround sound systems on the market. These
systems use sophisticated phasing and time-delay circuits to fool the auditory
system into hearing sounds from points in space that are not between the
speakers. I have yet to listen to one of these system myself. Reviews in
magazines report that sounds that should come from behind actually come
from the side. They also report that some sounds come from random directions.
A four speaker system is cheaper and works better. (Lately, I've noticed
that two-speaker surround sound is being pre-encoded into some TV commercials.
In these commercials, w