Installing an Overhead "Envelop" Channel
Customers who bought our CS-3X professional decoder, or CS-3X Jr. Home Theatre surround decoder three years ago may have noticed we mentioned the capability of an overhead channel that is already built into these products. We suggested to the movie producers, in our publications and ads, that this easy and "free" channel would expand the listening environment to add more viewer involvement to the movie. Well, it's here!
The first movie to include extensive use of a "height" channel was "WE WERE SOLDIERS" (now out on DVD) starring Mel Gibson. This additional channel enhancement adds a new dimension to the experience and provides some exciting and dynamic realism to an already superb soundtrack.
While modern theatre sound provides three channels before the audience (plus sub-bass) and split surrounds to the sides of the audience. A Surround EX™ channel fills in the hole in the middle of the split surrounds making 6 real channels to provide a 360 degree sound field that the mixer/producer can use to create a sense of direction. This circle of sound around you is flat! It does not define sounds above or below you. A simple matrix is used to remove the EX channel from the two surround channels and place the extra sound into another amplifier and set of speakers. It does not matter if the matrix is digital or analog. The scheme is to find any "phantom" or in phase material and redirect it to another channel, while removing the same sound from the original "carrier" channels. If you do not have an EX decoder you will still hear the acoustic phantom when sitting equally between the surround speakers. However, the same matrix can also carry another separate channel that will provide distinct sound to an overhead sound channel.
Unlike "surround" channels, the overhead channel is an "Envelop" channel that completes the illusion of reality. An infinite number of sound reference points can be established by the movie producer by adding only a single additional Envelop channel. It is like sitting in a bubble of sound, not just front, sides, and back. A virtual real life experience!
Three years ago we proposed that the overhead speakers be in a multiple speaker cluster comprised of many small, inexpensive speakers. The objective is to create a diffused sound pattern and not a point source. Commercial "in-ceiling" may be used. This is ideal for auditoriums with drop ceilings. Hard ceilings will require surface mount speakers. We will discuss the speakers requirements later.
In real life, if a sound originates above you the character of the sound is relatively dull due to the distance from you (air attenuation of high frequencies) . Sounds close to you are bright and clear. In a standard multi-plex auditorium the sound from the overhead speakers, arranged in a homogenous sound pattern may give the illusion the auditorium ceiling was elevated to two or more times the actual height. This new channel should not be equalized, and will gain the advantage of a slightly reverberant acoustic environment. Overhead sounds in real life are rarely directional, but diffused.
The only requirements in selecting ceiling speakers are 1) Relatively wide range reproduction, and 2) power handling capability. Multiple 20-30 watt speakers should be used in a series-parallel or 70 volt line wiring scheme. (high quality 70 v audio transformers are very expensive.) In a small auditorium, if (12) 8" speakers are used you would be able to apply 360 Watts of power without distortion. This would be in line with the power handling capability of the other speakers in the auditorium. You must be able to cleanly reproduce the digital soundtrack headroom dynamics. Larger rooms need more speakers and higher power amplifiers.
A real advantage of the CS-3X is the ability to deliver "stereo" overhead sound, because of the steering characteristics of the patented Circle Surround™ matrix. This requires two sets of ceiling speakers, one on the left side and one set on the right. If you do not want a stereo overhead system, a switch in the unit allows "monophonizing" the channels. You may split the overhead "Envelop" speakers into two groups and power them separately. This means smaller amplifiers and easier wiring.
In a Home Theatre application it is generally not acceptable to cut holes in the ceiling to install flush mount speakers. Several manufacturers offer "pancake" surface mount systems that are low profile and compact. We suggest 4 speakers be placed over the prime listening area about 2 feet apart, in a square pattern (2 and 2.) in the center of the listening area. Since most residential ceilings are 8-12 feet in height this would be a workable compromise, producing a solid effect. Choose speakers with the widest possible dispersion pattern for good blending without hot spots. Smaller amplifier(s) are required in a home system.
None of the present Home Theatre package systems have the capability of adding an "Envelop" channel. However, the CS-3X Jr. home decoder can be connected to the power amplifier outputs of the digital surround channels and new audio information can directed to both an "EX" and "Envelop" channels using two additional dual channel amplifiers.
Since the EX channel and Envelop channel are encoded into the digital surrounds during the mixing of the movie, these channels are "embedded" in the 5.1 master that is sent to the DVD authoring house. If you heard them in the cinema, you will hear them in your home system. The effect is the same for DTS or Dolby Digital soundtracks.
The CS-3X in professional cinemas and the CS-3X Jr. in home systems provides superior decoding of these special channels.
Now that movies are being produced with the enhancement of an overhead channel we expect to see many blockbuster features include this exciting format, both in the cinema and on DVD.
Circle Surround is a trademark of SRS Labs, Inc. Santa Ana. CA. Surround EX is a trademark of Dolby Labs, Inc. San Francisco, CA.