How to SETUP a Home Theater System
Click on the item to get informations on !
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THE SURROUND SPEAKERS
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Should I use two or more rear speakers ?
First of all note that the picture shows only two surround speakers (rear right and rear left)
which is the common setup while a high number of them are typically spaced around a movie theatre. They runs at a lower
volume and diffuse a more uniform surround field. This is generaly not the case in Home
Theater however more than two speakers may be used. It actually depends on your room and your
amplifiers output impedance and power. Nevertheless you need two speakers if you are using
THX or YAMAHA Cinema DSP since rear channels are stereo.3-ways dipole speakers are a very
good solution providing high sound quality and sound diffusion.
Technical Requirements
Technical requirements are quite confusing. Indeed the
Dolby Stereo encoding process provides a bandwidth limited signal
to 100Hz/7KHz on the rear channel. Theorically you should use corresponding
frequency reponse speakers. Now if you listen to a program which is not Dolby Surround encoded but may
present very interesting out of phase signals (CDs for example), any deep bass or fine treble naturally
out of phase in the original left and right sources will appear in the surround channel and may damage
your especially designed "surround speakers". There have also been press reports that some film
producers are pre-emphasizing the bass in the surround mix, so that it will still be present after the
encoding roll-off. Moreover most Digital Signal Processors such as YAMAHA send full range material
to the rear channel in every surround modes to reach HIFI sounding requirements. What I suggest
you is to buy full range speakers which are very closed to your main in terms of cones and bandwidth
although it may be more expensive. The power should be about 60 watts.
Positioning Requirements
Dolby recommends that the surround speakers be located 1.5 meter closer to
the average listener than the front speakers, and that the "surround" signal be
undirectly diffused to the audience. The best positionning is as shown on the picture by
facing them straight up. However many people prefer to slightly rotate them to the front.
Actually it depends on your room so try different positionning until youget the best one. But avoid
direct diffusion, it would give poor surround soundfield. Please make sure as well that you chose
the required delay time, given by the following obvious equation:
Where:
T is the delay setting
Nd is the net delay time in milliseconds (15)
Df is the distance in feet from the listener to the nearest front speaker
Ds is the distance in feet from the listener to the nearest surround speaker.
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THE MAIN SPEAKERS
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Technical Requirements
The main (front) channels need to have 3-ways speakers with
fairly flat on- and off-axis response, with no resonances,
no breakup or distortion at moderately loud listening
levels, and supplied by an amplifier of adequate power that
adds no problems of its own. The treble response needs to
be flat to 15KHz or more, and the bass response needs to
reach down to at least 35Hz - or 50Hz if you are using a
good subwoofer-, since
film/video programming has much more deep bass than music.
Be aware that LD soundtrack bandwidth is 20Hz/20KHz and may
damage your speakers during some very low bass scenes. The choice
of the main speakers is very important since they may be used to listen
other stereo programs such as CDs. Trebles should be very clear
listening and midrange fine in details. Most speakers do not provide enough
quality for music listening although they sounds good with movies. Please
LISTEN before buy !
Positioning requirements
Tv or screen should be located at the center of the
mains, at least one meter from each to avoid magnetic disturbances
on your TV screen and increase channels separation. You should respect
a symetric positioning of the speakers toward the screen to enhance movie
listening. Center speaker and mains should be located on the same horizontal
axis. Remember that small speakers require stands to give the best stereo spectral
image. Finally here is a little problem due to dual stereo/dolby stereo listening...
Stereo is best when left is distant of 2.5 meters from Right while Dolby stereo
is best with at least 3.5 meters! Actually if you are not a perfect audiophile you
will not get into trouble with your Home Theater System. As far as I am concerned,
I chose 3 meters which is close to both requirements.
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THE CENTER SPEAKER
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Why a center speaker?
The center output channel of a Home Theater decoder
attemps to isolate in-phase, balanced sound, usually dialog, and send it to a single
speaker and attempts to cancel it from the other outputs.. You may think
that buy another speaker for thoses in-phase signals is not important but you are
wrong. Indeed if you have no center speaker, and rely on the "center" sound
to consist of an equal-and- in-phase signal emitted from both left/right
front speakers, the sound waves will not arrive in-phase at all
listening positions and you may loose part of the original informations.
This is the so called "comb filter effect". For example, a 6-inch difference
in distances to L/R speakers results in a 180-degree group delay (and total
cancellation of the direct wavefront) at 1000Hz (and many other frequencies).
Many people will tell you to use two center speakers if you buy a front projector.
As far as I am concerned I disagree because you will add a new comb filter effect!
Technical Requirements
First of all you need a magnetically shielded speaker to avoid
to skew the geometry and color of inadequately shielded TVs (Most of them ! However this
is of course not necessary if you luckily use a front projector). As a result in most cases
you cannot use the same speaker as your mains. Please ensure to buy your center
speaker with closed timbre to your main to prevent a very distracting image. Because
of magnetics effects, you will not find low bass center speakers and I highly recommend
you to use the "normal" center mode on your decoder which roll off the center
bass below 100Hz and delivers it in the L and R main channels
or to the subwoofer channel.
Positioning Requirements
The ideal positionning of the center speaker is above or better below you TV screen
provided that L, Center and R are horizontaly aligned. It should be located
at the middle of your main speakers and be supplied with the same power
from your amplifier than the main.
If you don't plan a center speaker, the recommended positioning of the
L/R mains is "close" to the screen. If you must place the mains far
from the screen, consider using the TV's own speaker(s) as the "center".
Unfortunately, most TV speakers are junk, so you'll have to judge the
efficacy of this by trial. Be careful with "close" - the magnets in
many speakers are powerful enough to skew the geometry and color of
inadequately shielded TVs.
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THE SUBWOOFER
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Why a Subwoofer?
Films sound effects make even greater demand on bass reproduction than ordinary
stereo to produce deep and dramatic bass tones which must be felt physically rather
than just heard. However few ordinary speakers are designed to reproduces the bass tones
in film sound at realistics levels. The way to get impressive bass is to invest in a subwoofer.
A subwoofer comprises one or many large loudspeaker units and cabinets with a large volume
and which provides only bass sounds, generally between 20 to 200Hz.
Active or passive?
An active subwoofer employs its own specially designed pbuilt-in ower amplifier and low pass crossover thus
requires only a line-level signal from your amplifiers. As a result the cost is specially designed as well !
Pay attention that if you use the subwoofer filtered output of your amplifier, you now have two low
pass crossover in series which may affect and reduce the original incoming signal power and dynamic quality
. Actually the best in terms of sound quality is to use a passive subwoofer (without any filter or amplifier) supplied
by a dedicated amplifier plugged on the subwoofer output of your surround amplifier.
Positioning requirements
It is almost indifferent where the subwoofer is placed in the room since the human
ear cannot determine the direction of deep tones nearly as well as high tones. Therefore the foundation
it provides for the music is perceived uniformly throughout the room. However this is rather theory than
reality...Indeed I recommend you to locate your Subwoofer either in front or behind your listening position.
Left or right may engender amazing and distracting effects. Moreover if you are listening to stereo programs
or classical music it may be useful to unplug your subwoofer since bass sounds become too much present.
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THE FRONT PROJECTOR
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I have not enough informations at the moment about front projection to help you.
However I recommend you to visit this Web page.
Laurent TESSIER
Last updated june 1996
Double-D symbol and DOLBY are trademarks of
Dolby laboratories licensing
corporation
THX is a trademark of Lucasfilm ltd