April 08, 2008
| by Dennis P. Barker
Movie-going is a way of life for all of us. Except for television, it’s probably the biggest spectator sport in the world. However, even sporting events cannot draw the crowds that movies like “Star Wars,” “Indiana Jones” or the “Terminator” franchise can generate. It’s an experience to be savored and enjoyed in a darkened cinema. Home theater is an extension of the movie-going experience in that it attempts to recreate the same sense of awe and wonder within the confines of your living/family room. While a picture is worth a thousand worlds, sound is more than fifty percent of the equation. The goal of films today is to aurally engulf you and place you right in the middle of the action so that you viscerally feel it as well as see it. Multi-channel sound is now a way of life for us.
Surround sound comes in many varieties and permeations. Surround processes aural information to give you added depth and sense of realism not heard from 2-channel stereo. Surround sound makes the sound so real that it’s in front of you, behind you, and all around you. Surround for the home began in the early 1990s as an outgrowth of the sound heard at the movies. While Dolby Stereo was the first commercial use of multi-channel sound in theaters some thirty years ago (1977) with “Star Wars,” digital surround sound was not introduced to movie-goers until 1993 by Steven Spielberg (and MCA/Universal) in his colossal hit “Jurassic Park.” This was also the first commercial use of DTS (Digital Theater Systems). Today, DTS is now employed on the soundtracks of several thousand feature films, including many recent blockbusters.
How Does Surround Sound Work?
Basically, it takes two-channel stereo sound and splits it into dialogue and primary sound up front and effects sound to the rear. It’s certainly a vast improvement over plain vanilla two-channel stereo. The Pro Logic processor extracts four channels from the two stereo encoded channels and steers or directs them to the appropriate speakers, e.g. dialogue to the center channel and effects to the rear. Under this scheme, the rear surround channel signal is divided over two speakers, which gives it more coverage. It is, however, a mono signal. The rear channel information is derived by the simple formula of L[eft] minus R[right] with added reverb to give it a more natural like sound.
Digital 5.1 Surround
Multi-channel surround sound with 6 independent channels of sound - Digital 5.1 Surround – arrived in 1995. As the name implies, 5.1 is 5 full-frequency, discrete and independent audio channels (front left, center, front right, right surround, and left surround) plus a separate .1 channel, which is a dedicated Low Frequency Effects (designated LFE) channel. LFE directs bass information to your subwoofer. Since it is not a full bandwidth channel (of 20 - 20,000 Hz), the dedicated subwoofer only gets a .1 number.
One of the first things that you hear with digital 5.1 surround is that the side or rear surround speakers are now in stereo (versus mono with Pro Logic). Sound, music, dialogue and effects are now directed to their proper placement in terms of screen location. As the old saying goes, “you are there!” now takes on a whole new meaning in 5.1. Digital 5.1 surround is among the most realistic surround currently available. It will be delivered via 5 psycho-acoustically-matched speakers and a dedicated subwoofer.
There are only two schemes available that will provide you with true digital 5.1 surround for the home—Dolby Digital and DTS. Each scheme strives to provide the listener with the ultimate home theater experience. The major difference between Dolby Digital and DTS is that DTS is fully discrete. What this means is that specific aural information is directed toward a specific speaker giving the listening experience a more realistic tone. While Dolby Digital was devised as an evolutionary approach, DTS is more revolutionary one as it was specifically designed for multi-channel sound.
Not all material is encoded in Dolby Pro Logic, Dolby Digital, or DTS, however. Therefore, most equipment manufacturers of audio receivers and processors also offer other artificial surround modes such as Hall, Matrix, Simulated, etc. These modes can add realism to material not found elsewhere. As well, they can be employed in the playback of CDs too, sometimes giving them added depth and the feeling that the music is surrounding you. It may not be an unpleasant feeling.
Dolby Digital is a multichannel perceptual coding scheme. Initially, Dolby’s new surround system was called AC-3 (for audio coder 3). It was introduced in movie theaters in June 1992 as Dolby Stereo Digital (or Dolby SR). While Dolby Surround is a single-band-limited surround channel with a range of 100 Hz to 7,000 Hz, Dolby Digital, on the other hand, offers a full dynamic range on all five main channels of 3 Hz to 20,000 Hz. Dolby Digital has been available since late 1995, and there are now thousands of DVD titles presently encoded in Dolby Digital. Many films released today are also in Dolby Digital Surround. All DVD players now include both digital audio outputs – either/or coaxial or optical and 6-channel output to pipe the Dolby Digital or DTS audio stream. Upconverting DVD and Blu-ray players also include HDMI.
Dolby Digital’s perceptual coding seeks to eliminate the data we cannot hear, while maintaining all the information that we can hear. Its purpose is to get more information into the available spectrum. Perceptual coding has been designed to decode multichannel digital audio. It divides the audio spectrum of each channel into narrow frequency bands that correlate closely to the frequency selectivity of human hearing allowing coding noise to be very sharply filtered taking advantage of the psycho-acoustic phenomenon known as auditory masking. Coding noise stays close in frequency to the audio signal being coded. This effectively masks the noise. AC-3 uses a “shared bit-pool” arrangement plus human auditory masking to make use of transmitted data as efficiently as possible. Bits are distributed to meet the needs of the frequency spectrum. By using a model of the audio-masking scheme, bits are distributed among the various channels according to need. Basically AC-3 allows proportionally more of the transmitted data to represent audio, which, according to Dolby Labs, yields higher sound quality. In turn, it allows multichannel surround sound to be encoded at a lower bit rate than required by just one channel on a CD. Today, all receiver brands now include a Dolby Digital decoder built into every A/V Receiver. There are 28 versions of Dolby surround.
For HD on television, and all movies on disc now include a Dolby Digital soundtrack as do most TV shows like C.S.I.: Crime Scene Investigation, C.S.I. – Miami, LOST, ER, Desperate Housewives, Gray’s Anatomy, Battlestar Galactica, or The Tudors.
Dennis has been involved with Consumer Electronics forever it seems. His 25+-year career includes a 12-year tour of duty at Consumer Reports magazine, as well as stints as a product reviewer, market analyst, technical editor, and consultant for the electronics industry. He lives in Ossining, NY with his two children, one demanding cat and piles of A/V equipment.