rs 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, with no surround sound decoding equipment at all,
sounds seem to come from places well outside the speakers.)
Now, what about controlling the balance between the front and rear
channels? There are two basic approaches to this:
One approach would be to buy a second stereo amplifier for the rear
channel. Simply wire the speakers to both + or red terminals (leaving no
connection to the - or black terminals); A second method would be to wire
in a stereo fader (available from Radio Shack). These faders are designed
for automobile stereo systems and can handle only 25 Watts per channel.
However, all you have to do is wire in the fader according to Figure 8.
Figure 8 includes a switch to switch between surround mode and regular
Figure 8: Wiring a stereo fader into the matrix
Balancing the System
In order for the above surround sound systems to work, the stereo balance
must be adjusted to null out the front channels in the rear speakers. To
do this: tune in an AM radio station (as mentioned above for checking the
phasing); disconnect the front speakers and adjust the speaker balance
control until the lowest possible sound level is heard in the rear speakers;
hook up the front speakers and you are ready to go. For the surround sound
to work properly this balance must be maintained.
Building an Active Decoder
Commercially made surround sound systems use an electronic circuit
called a differential amplifier to decode the surround sound. Since a differential
amplifier uses active electronic components, such as transistors, it is
The differential amplifier simply does, electronically, what the rear
speaker does in the passive decoder described above. The differential amplifier
rejects any signals that are identical in the front channels and pass any
differences to the rear channel. The output of the differential amplifier
is sent to the regular audio amplifier and then the rear speaker(s). One
advantage of this type of decoder is that you don't have to worry about
the null balance adjustment mentioned above. This adjustment is