Make your own free website on Tripod.com

A LISTENER'S GUIDE

TO DOLBY SURROUND


Introduction


Millions of TV viewers and music lovers around the world today are enjoying the unique entertainment experience provided by "home theatre" audio/video systems. Common to all these systems, from the simplest to the most elaborate, is Dolby Surround sound thrilling, spacious multichannel sound that previously could be heard only in well-equipped movie theatres.

If you have such a system or are planning one for your home, this Guide is for you. In it we touch upon most of what you need to know to select, set up, and get the most out of a home theatre system with Dolby Surround. With a little time, effort, and knowledge, your investment will pay rich dividends for many years to come.

Where it came from
For more than forty years, multichannel stereo sound in movie theatres has been creating a more involving experience for the audience. In addition to left and right screen channels, theatres also use a center channel to sharpen the perspective of on-screen sounds, plus a surround channel played over loudspeakers along the sides and rear of the auditorium to immerse the viewer in ambiance and special effects.

At first, multichannel cinema sound could be provided only by means of costly film prints with magnetic soundtracks, limiting its availability. Then, in 1976, Dolby Stereo revolutionized the film industry by placing multichannel sound onto affordable prints with optical soundtracks. This technology was then extended to bring multichannel soundtracks to viewers at home, and was called Dolby Surround to differentiate it from the professional Dolby Stereo cinema system

What it is
Dolby Surround is a two-step, encode/decode process involving both recording and playback. When a Dolby Surround soundtrack is produced, four channels of audio information left, center, right, and surround are encoded onto two audio tracks using equipment manufactured by Dolby Laboratories. These two tracks are then carried on stereo program sources such as video tapes and TV broadcasts into your home, where they can be processed by a Dolby Surround decoder to recreate the surround sound experience.

Today thousands of theatrical films on home video, as well as many television shows, audio cassettes, and CDs, are Dolby Surround encoded. If you listen to them over a regular two-channel stereo system, they will sound much like any other conventional stereo programs. With a Dolby Surround decoder, however, you will retrieve the "missing dimension" that lies within the encoded soundtracks: sound from all around you that brings you into the action on the screen

What you need
A home theatre system with Dolby Surround can take many forms, and need be neither elaborate nor expensive (although there is no limit to what you can do). If you have a television set and a stereo music system, you can simply add a few extra speakers and a decoder/amplifier or an audio/video (A/V) receiver equipped with Dolby Surround decoding. Some speaker manufacturers offer packages featuring a specially designed center speaker plus a pair of surround speakers for just this purpose. Dolby Surround is also available in televisions, separate control preamplifiers, stereo rack systems, satellite receivers, and compact music centers. This means that there is a surround system right for you, regardless of your present equipment or budget.

Basic Dolby Surround vs. Pro Logic
There are two kinds of decoders: basic Dolby Surround and Dolby Surround Pro Logic. Both recover the surround information from encoded program material and feed it to a pair of surround speakers placed up on the side walls adjacent to the listening area.

In a basic Dolby Surround system, left and right front speakers are fed with the entire program in normal stereo without any processing, while the surround speakers are fed with a surround signal derived by a relatively simple passive matrix decoder. As with regular two-channel stereo, the left and right front speakers create a "phantom" center channel but only for those relatively few viewers seated on center.

By using the same directional enhancement system found in professional Dolby Stereo cinema processors, however, Pro Logic decoders derive a separate center channel to keep dialogue and other central sounds firmly localized on the video screen. Pro Logic also supplies higher separation among all four channels and more accurate sound positioning, which along with the center channel enable a greatly expanded listening area. Dolby Surround Pro Logic is the best way to accurately reproduce the Dolby Stereo theatre experience in your home and will ensure that you hear Dolby Surround soundtracks as the producer intended.

Basic Dolby Surround decoders feature:

Dolby Surround Pro Logic decoders feature in addition:

Some Pro Logic decoders also offer an additional decoding mode, Dolby 3 Stereo, which provides left, center, and right channels only. This mode is for use where full surround is not required, but a wide stereo soundfield without a "hole in the middle" is desirable. A TV set, for example, could be supplied with detachable speakers for a wide stereo spread, with an integrated center speaker to improve dialogue positioning as with any Pro Logic system. The Dolby 3 Stereo mode can be useful if you are unable to install surround speakers right away, and with a full surround system it may prove preferable for such program sources as synthesized "stereo" TV broadcasts or music recordings with vocal solos.


About the Channels


The left and right channels
The left and right channels in a surround system, as in a conventional stereo system, contain the full audio bandwidth. In addition, in a Pro Logic system, they carry center channel bass information if the Normal center mode is used (as described below). Thus the left and right speakers should reproduce the full frequency range, have good spectral balance, match each other closely, provide adequate loudness capability, and feature low distortion. In other words, whether conventional multiple-driver designs or satellite/subwoofer systems, they should be the best you can afford. All in all, the requirements for a good stereo music system apply equally to a multichannel surround system.

The Center Channel
The center channel in a Pro Logic system carries not only dialogue, and so keeps it firmly centered on the screen regardless of where you're seated, but also carries a significant share of other on-screen sounds, special effects, and music. It also should keep the timbre, or tonal quality, of sounds from changing as they move from one channel to another across the front. Ideally, therefore, the center speaker and amplification would be the same as used for the left and right channels.

Space and cost considerations may preclude using the same speaker model for the center. Pro Logic decoders feature a Center Mode Control, described below, that lets you use a smaller center speaker and amplifier without seriously compromising performance. The center speaker should still sound as similar as possible to the left and right speakers in the mid and high frequency regions, however. Some surround decoders also provide equalization that allows adjusting the center speaker's frequency response for a better match.

Be sure to audition center speakers carefully, particularly those that use multiple small drivers to achieve a low profile. While many are excellent, some may provide colored sound for viewers seated off to the sides, insufficient mid-bass output, or both.

Pro Logic center mode control
All Pro Logic decoders provide a choice of Normal and Phantom center channel operating modes: some units offer the third option of a Wide mode.

Normal removes the low bass (frequencies below 100 Hz) from the center channel and redistributes it to the left and right speakers to maintain the program's original bass level (this has no effect on the sound's spatiality because low bass does not provide directional cues). In most instances, this mode lets you achieve good overall system performance with a smaller center channel speaker driven by about one-half (but not less than one-third) the power provided for the left and right channels. The actual amount of power necessary is affected by such factors as speaker sensitivity and whether or not center channel equalization is used.

Phantom is for use without a center channel speaker; it redistributes all center channel information to the left and right channels, providing conventional stereo across the front. For viewers seated in the middle, center channel information will appear as a phantom image between the left and right speakers. Dialogue and other central sounds. however, will tend to "pull" away from the screen towards the nearer speaker for viewers seated off center. Therefore, be sure to install a center speaker as soon as possible, to take advantage of the wide listening area that is one of Pro Logic's most useful benefits.

Wide means "wide range": that is, the entire audio bandwidth is delivered to the center channel speaker. Use this mode only if you have a full-range center speaker that can reproduce extended bass. In this mode the center channel amplifier will be called upon to reproduce bass levels equal to those in the left and right channels, so it should have the same power output as the left and right amplifiers.

The surround channel
The surround channel in Dolby Surround programming deliberately does not contain extremely low frequencies (below 100 Hz) or high frequencies (above 7 kHz). Upon playback, the surround channel signal passes through a 7 kHz low-pass filter and a modified Dolby B-type noise reduction processor in the decoder to reduce both noise and distracting high frequency signals that might leak through from the front channels. The surround signal is also delayed slightly to increase the apparent separation between the front and surround channels.

Although there may be some leakage of low bass from the front channels to the surround channel on Dolby Surround programming, there is no need for the surround speakers to reproduce it; only the front channels' low bass needs to be heard. On the other hand, while not needed for Dolby Surround, full treble response is recommended for the surround speakers, because other processing modes and future delivery formats (such as Dolby Surround Digital) may take advantage of it.

All in all, the requirements for surround speakers are much the same as for the center channel in a Pro Logic system set to Normal. Thus you can use the same speaker models for the surrounds and the center, which will also help to assure good timbre matc hing. Home theatre systems that use five identical satellite speakers for left, center, right, and surround, plus a separate subwoofer for the non-directional low bass, are particularly effective at combining accurate channel matching with cost-effectiveness and ease of installation.

While the surround channel is monophonic, most A/V products provide two separate amplifiers for the two surround speakers. In general, their combined power output need be only about one-half that of one front channel (left or right) for satisfactory perfo rmance. With any given installation, however, the actual amount of power required for the surrounds or any other channel is affected by how loud you like to listen, speaker sensitivity (efficiency), the use of tone controls or equalization to boost sounds, and how much sound is "soaked up" by the listening room's furnishings.

The use of surround is an art that varies from program to program. Some programs feature the surround channel extensively for prominent effects, while others use it only to create ambiance. Most of the time, mixers use surround to envelop you in continuous low-level "atmospherics," such as wind through the trees, and only occasionally for discrete special effects. This avoids drawing your attention away from the action on the screen.


Speaker Placement


The left and right speakers
In a Pro Logic system, the left and right speakers should form an angle of approximately 45 degrees to the listening position (see illustration). This duplicates the speaker arrangement used both for mixing surround soundtracks and for a center seat about two-thirds of the way back in a well-designed movie theatre. It also works well for conventional stereo music sources.

If you're not using a center speaker, you may need to place the left and right speakers closer to the screen for good integration of sound and picture but not so close that their magnetic fields disturb the picture. If necessary, use magnetically shielded speakers.

The Center speaker
For best directional effect, try to have the center speaker at the same height as the left and right speakers (2) and as close as possible to the screen. Place it directly atop or beneath the TV set only if it is magnetically shielded, as the magnetic field from a conventional speaker could distort the picture's color.

The front of the center speaker should also be aligned with, or slightly behind, the left and right speakers. Avoid having it forward of the left and right speakers, as that could adversely affect sound perspectives for listeners seated off center.

It may also be possible to use your TV's built-in speakers for the center channel if there is adequate power and fidelity, and if the set has appropriate audio inputs (check your owner's manual).

The surround speakers
Properly conveying both directional effects and diffuse ambiance requires an evenly distributed surround soundfield comprising both direct and reflected sound. This is achieved in movie theatres by means of many direct-radiating surround speakers (typically ten to twenty) along the sides and rear of the auditorium.

Listening tests have confirmed that it is the side speakers that contribute the spaciousness or "openness" associated with good surround sound. Theatres use speakers at the rear only because without them, the surround soundfield would appear to be in front of, rather than around, those seated in the back rows. In home environments, just two surround speakers, one to each side, are usually enough to achieve proper results.

Too much direct sound from the surround speakers can make their location too obvious, or create an earphone-like, "in-the-head" sound image. On the other hand, too much diffusion may disperse the image such that it is utterly directionless, seemingly everywhere at once. Finding the right balance of direct and reflected sound means considering speaker placement, room design, and the acoustic characteristics of both.

If you use direct or dipole radiating surround speakers, place them on the side walls alongside the seating position, two or three feet above seated listeners' heads. If the seating area is unusually deep, two surround speakers along each side instead of one may be desirable. In-wall speakers, while more difficult to install, can resolve any decor quandaries.

In most cases, direct radiating surround speakers should be aimed straight across the room towards each other, not down at the listeners, to create a proper blend of direct and reflected sound. If the room is unusually "live" (with lots of bare sound-reflecting surfaces), however, it may be advisable to tilt the surround speakers down slightly towards the listeners, to increase the ratio of direct to reflected sound for greater clarity. Conversely, in a particularly "dead" room (thick, sound-absorbing carpeting, heavy drapes, etc.), aiming the speakers toward the rear wall or ceiling can increase reflections for greater diffuseness.

If it is not possible to locate the surround speakers in a preferred way, experiment creatively with their placement, keeping in mind the goal of a diffuse and enveloping but not overly vague soundfield. Here are some alternative locations:

Special surround speaker designs
Not all surround speakers are conventional direct radiator designs. There are single-cabinet models, for example, designed for placement directly behind and above the listening position, that can be very effective when the usual pair of surround speakers is precluded by room layout or aesthetic considerations. These models use multiple speaker drivers firing upward and to the sides, creating a good surround soundfield by means of ceiling and side wall reflections.

There are also many so-called "dipole" designs that radiate most of their sound to the front and rear, placing the listeners in a "null" so that comparatively little direct sound reaches them. These units generally work best when mounted on the side walls as recommended for direct radiating surround speakers. As their soundfield consists mostly of back and front wall reflections, however, dipole designs are more dependent on the room and its acoustics than conventional models.

In any case, be sure to consult your surround speakers' owner's manual for any special installation considerations.

Subwoofers
Because they reproduce only non-directional low bass, subwoofers do not contribute to sound localization, and therefore need not be in line of sight to the viewer (in many cases they can even be tucked away under or behind furniture). The accuracy and smoothness with which they reproduce low bass, however, does depend on their placement.

First, temporarily put the subwoofer near your favorite listening position. Then, as you play program material with significant low bass content, go stand and listen at likely subwoofer locations in the room. That location which delivers the best bass sound clean and full, but not "boomy" or "thuddy" will be the best for final subwoofer placement.


Surround Decoders


When it comes to choosing the component that implements the surround decoding, bear in mind both your current equipment and any likely future expansion. Among your considerations should be the following:

1. Basic Dolby Surround or Pro Logic? If cost is a critical factor, basic Dolby Surround decoding may fill the bill. Pro Logic, however, provides the most accurate surround decoding and the most involving listening experience and it need not cost much more.

2. Configuration. Surround decoding is available built into television sets, powered surround speaker systems, A/V receivers, control preamplifiers and amplifiers, add-on decoder/amplifiers, stereo rack systems, satellite receivers, and compact music centers. Your choice will depend primarily on what you have now that will be suitable for your surround installation.

3. Input and program source flexibility. The new equipment should have inputs for all your current audio/video units and accommodate future expansion. For example, if you have an S-video VCR, be sure that any new component that has video switching provides S-video inputs so you can take advantage of the higher performance format.

4. Amplifiers. Some decoder units have no amplification, and are meant to function as a control center for an elaborate system of separate components. Others have two or three amplifiers built in, on the assumption that you will use your current equipment to power the left and right channels. A/V receivers incorporate all the electronics you'll need, from surround decoding to amplifiers for all channels.

The louder you like to listen, the more amplifier power you'll need overall. But quality is just as important as quantity, and power requirements can vary with the various channels (see earlier discussion of the center and surround channels).

5. Subwoofer output. If you expect to add a subwoofer to your system, it will be made easier by choosing a decoder unit which has a separate subwoofer output. It is possible to add a subwoofer to almost any system, except (usually) those with surround decoding built into the television set.

6. Automatic input balance. Standard on most Pro Logic decoders, this feature prevents program material imbalances from degrading the surround decoding. You won't have to worry about manually adjusting the input balance to compensate for problems with program material.

7. Adjustable surround channel delay. If your room is of average size and proportions, then the fixed 20 ms delay time standard on all Dolby Surround decoders will be fine. But, if you sit unusually close to the surround or front speakers, choose a decoder with an adjustable delay that can be optimized for your circumstances

8. Tone controls/equalization. Some decoders have tone controls only for the left and right channels, and some units defeat the tone controls when a Dolby Surround mode is selected. Better models will have tone or equalization controls active in all modes, including Dolby Surround. Some Pro Logic units also have separate EQ for the center channel that can help you match the sound of your center speaker to the left and right speakers. Although rare, EQ for the surround channel can be of similar benefit.

9. Other signal processing. Some units incorporate audio processing modes other than Dolby Surround, usually to simulate various acoustic environments for music listening). If such extra modes interest you, look for a unit that implements them, and possibly Pro Logic decoding as well, with DSP (digital signal processing) circuitry. If your interest is exclusively in Dolby Surround, products with analog-based decoders may prove the better value.

10. Analog vs. digital processing. As described above, Pro Logic decoding may be implemented by analog or digital means. Because all implementations must meet the same decoding performance standards set by Dolby Laboratories, all give consistent results; there is no "best" version when it comes to decoding surround programming in your home. Each decoder product, therefore, must be evaluated on its own sonic qualities and economic merits, and on whether or not you want such features as additional processing modes or direct digital interface with high-end CD and LaserDisc players.

11. Ergonomics. Because the unit with surround decoding interconnects with virtually all your other components and becomes a focal point of the system, be sure that it connects easily, has all the features you want (such as remote control), and can be operated easily by everyone likely to use the system.

12. Professional installation. With an elaborate system in particular, a home theatre specialist can help you choose the right equipment, and install it in a way that both sounds right and complements your decor.


Other System Considerations


Loudness and dynamic range
Dolby Stereo film soundtracks are mixed and then heard in theatres at predefined, standard loudness levels. At home, you decide how loud to play these films as well as all other programming. How loud you like it will affect your choice of speakers and amplifier power.

The maximum loudness level favored by most listeners (and their neighbors!) is well below that achieved in the well-equipped cinema. Thus home equipment enabling full theatrical loudness, while available for those who want and can afford it, usually is not necessary.

Your system's ability to reproduce quiet, subtle sounds more a function of your listening room's isolation from extraneous sounds than of the playback equipment itself should also not be overlooked.

Power amplifiers
Basic Dolby Surround requires a minimum of three channels of amplification, for left, right, and surround channels. Many decoding units, however, provide separate amplifiers for two surround speakers, and Pro Logic requires an additional amplifier channel for the center speaker. Many audio/video receiver models therefore have as many as five amplifiers built in, while for more elaborate systems component power amplifiers with up to six channels are available.

As with any sound system, home theatre amplifiers should have low distortion, provide flat frequency response, and deliver enough power to let you play the system as loud as you prefer. While having too much power for one or more channels will be of no benefit, too little power on even one channel can be a weak link that limits the entire system's clarity and overall loudness range. The left, center, and right channels, and the surround channel as a whole, should be capable of equal loudness. The amplifier power that will make this possible will depend on speaker sensitivity and variations in bass content among the channels as explained in the earlier discussion of center and surround channel requirements.

As with any stereo system, be sure that the amplifiers will operate properly if you use speakers of less than 8 ohms impedance. This also applies to units with a single surround amplifier to which you are expected to connect two speakers in parallel.

Subwoofers
Many film soundtracks feature powerful low bass effects (explosions, the rumble of spaceships, etc.) that help heighten your emotional involvement with the action on the screen, and many cinemas today are equipped with subwoofers special speakers dedicated to reproducing just the low bass. Because loudspeakers that are adequate for home music reproduction often cannot reproduce these powerful bass effects convincingly, home theatres, too, benefit from adding subwoofers. Should you decide to do so, be sure to follow the instructions supplied with the subwoofer and your surround decoding unit.

If your surround decoder has a proper subwoofer output that carries the low bass from all three front channels, simply connect it directly to the subwoofer amplifier input. If there is no subwoofer output, your subwoofer may be designed to accept (and internally combine) separate left and right input signals; check its instructions. If so, be sure to operate your system with the center channel mode set to Normal, which will redirect the center channel's low bass to the left and right channels, and thus on to the subwoofer.

Program sources
Dolby Surround programming can arrive in your home via broadcast TV, cable TV, VCR, laser disc, or satellite receiver. Laser discs and many satellite transmission formats utilize digital audio, which, in addition to their high picture quality, makes them particularly excellent sources.

Each source must be capable of proper two-channel stereo to deliver the surround encoding. For example, while the VCR does not have to be a Hi-Fi model, it must at least have stereo linear tracks, and it must also have line level audio outputs.

While the RF output that lets you play a VCR over channel 3 or 4 on your TV can deliver stereo from the VCR's internal tuner, it will provide only a mono signal from tapes, even those that are recorded in stereo. What's more, regardless of the soundtrack format, both picture and sound quality are improved by using your video components' audio and video line outputs.

The audio outputs from all your video sources should be connected to the surround decoding unit. Because there are an increasing number of music recordings being produced in Dolby Surround, you may also want to connect your CD player and/or cassette deck.

Video screens
A satisfying home theatre installation does not require a large, expensive screen. Many viewers happily continue to use their existing television set when adding a surround system, finding that the expanded soundfield enhances their viewing sufficiently. But others, wishing to get closer to the movie theatre experience, upgrade to a larger direct view set (up to 35 inches), a rear projection model (45 to 60 inches), or a front projection system delivering a 100-inch or larger picture. With front projection, you can even get a perforated screen and place your front three speakers out of sight behind it, just like in a movie theatre.

The size of the screen is not the only factor in the perceived picture size; distance from the screen is equally important. A 32" picture, for example, can seem just as big as a larger picture viewed from a greater distance. Viewing distance is usually constant for a given room, however, so that the larger the screen, the larger the perceived picture.

Due to resolution limits, such as those imposed by scanning lines, a viewing distance of about four times the picture's height usually provides the biggest perceived picture without undue degradation. This distance will vary with the set's quality and that of the source material, whether you use any image enhancing accessories such as a "line doubler," and your personal tolerance level for picture flaws. As shown at the right, a 10 to 15 viewing angle is appropriate for normal TV; about 20 is possible with normal TV combined with image enhancement; and 30 is likely for the high-definition television (HDTV) system now under development. The 45 or greater angle from a good seat in a theatre, however, is unlikely to be practical in the home in the foreseeable future.

In any case, video quality will be greatly enhanced by properly adjusting contrast, brightness, color saturation, and hue. Also keep room lighting from reaching the screen in a way that compromises deep black picture elements.

Equalizers
An audio equalizer can help to improve the overall sound of your system by smoothing its frequency response. Do not, however, connect an equalizer so that it affects the stereo signal prior to surround decoding. Altering the original level and phase relationships of an encoded signal could result in inaccurate decoding. Any equalization should occur only after surround decoding, at the left and right line outputs of the surround decoder (many A/V receivers provide external processor connections for just this purpose). If you use a two-channel stereo equalizer, use the Normal Pro Logic center channel mode so that center channel bass is routed to the left and right, where it can be affected by any low-frequency equalization you might apply.

The room: the forgotten component
The listening room contributes as much to what you hear from your system as any of its components. Fortunately, the typical home listening room is usually friendlier to good sound reproduction than a large hall or theatre. A family-size audience means low background noise. Typical home furnishings help prevent echoes and reverberation that could muddy the dialogue. And it is both easier and less expensive to achieve wide frequency range and ample loudness with low distortion in a living room than in a large public place.

Room characteristics that contribute to good hi-fi music reproduction are just as appropriate for good home theatre sound. Carpeting on the floor and drapes on large picture windows will cut down on mid- and high-range sound reflections that can add harshness to music or muddy the dialogue. If you have the luxury of choosing among several potential sites for your system, avoid those rooms that have any two dimensions the same (such as a square room), or that have any one dimension exactly twice another (such as a room just twice as long as it is wide). Such dimensions aggravate "standing waves," low-frequency sound resonances that in some cases may color the sound.

Arrange the seating area so that it is centered between the side walls on which you mount the surround speakers. Speaker systems mounted in walls or furniture may require extra sealing, baffling, and/or damping to minimize reflections and refractions that could color the sound. Any cloth used to hide speakers must have a weave that is "open" enough to let high frequencies through. And of course, you should prevent other furnishings from rattling or vibrating on strong bass notes or sound effects.

Surround modes for music
Many surround decoders provide processing modes other than Dolby Surround, variously called "Hall," "Stadium," "Jazz Club," etc. These attempt to recreate the effect of hearing music performed in their namesake environments. A "Matrix" mode may also be provided to give either mono or conventional stereo programs added spatial dimensionality.

Depending on how the signal processing is accomplished, the effectiveness of these modes can vary widely from product to product. They are often preset to very high levels to make impressive demonstrations in the store, so check to make sure that they can be turned down once you get the unit home; too much processing quickly becomes very tiresome.

Enhancements for movie viewing
The goal of cinema is to transport the audience to a different world by means of a huge, involving picture and all-encompassing multichannel sound. When the lights go down in the theatre, we are to "suspend disbelief" and forget where we really are. The goals of home theatre are much the same, particularly now that Pro Logic decoding makes it possible to hear a film's soundfield as originally crafted by its producers.

Nevertheless, there is still something uniquely attractive about experiencing movies in a well-designed, full-size Dolby Stereo cinema. This sensation, particularly the impact of effects, derives in part from the acoustics of the theatre itself. If your goal is to duplicate in your home these unique, more ephemeral aspects of the theatrical experience, Pro Logic serves as the starting point. Highly sophisticated surround decoders often offer additional processing designed to work in conjunction with Pro Logic to achieve a more "theatrical" experience. Some, for example, offer "Cinema" modes that use digital signal processing (DSP) to simulate the acoustics of real theatres. These can sound quite convincing, but only at considerable cost, so evaluate such units carefully before making a choice.

Other units provide "THX Cinema" processing that strives to emulate the effect of a large theatre without actually simulating its acoustic reflections. THX-licensed processors use special equalization in all channels, and "decorrelation" of the surround channel to increase its diffusion. THX-licensed speaker systems must meet directivity criteria and other performance requirements. THX-licensed amplifiers are designed so that, in conjunction with licensed speakers, your home theatre can achieve virtually the same loudness levels reached in movie theatres.

These various additional modes are not new sound delivery formats, but additional forms of sound processing that are combined with Dolby Surround Pro Logic decoding. For more information on the processes and their intended function, consult the manufacturers of the products that incorporate them.


System Calibration


Systems with basic Dolby Surround
Speaker levels. Adjust the left/right balance as you would for any stereo system. Be sure in particular that dialogue is centered, apparently emanating from the screen for those viewers seated on center.

You can, if you wish, simply adjust the surround level so that it "sounds right." It should be set to deliver ambiance so subtle that you are hardly aware of it, but would immediately miss it were you to shut off the surround channel. Prominent surround effects are rare, but should be clearly audible when they do occur.

If you would like to leave less to chance and don't mind some experimentation, you can adjust your surround channel as follows to more closely approximate what the program's sound mixers intended:

1. Turn the decoder's input balance control all the way to one side to "fool" the decoder into sending equal-level signals to one front speaker and to both surround speakers (if this doesn't turn the opposite channel all the way down, return it to the center position, and disconnect one channel of the program source).

2. With the program playing, adjust the surround channel level so that both surround speakers together sound as loud as the one active front speaker.

3. Reset the input balance to center (or reconnect the disconnected input). Your surround speakers will now reproduce surround information on all Dolby Surround encoded program material at close to the originally intended level.

Input balance. The most obvious symptom of improper input balance is dialogue leakage into the surround channel. If you come across this symptom on a particular program, turn off the front speakers if you can, and adjust the input balance control until the dialogue is least audible in the surrounds (remember to turn the front speakers back on once you've completed the adjustment). If the leakage is still prominent, and/or it consists mostly of high frequency dialogue sibilance, then there may be phase errors in the source program which cannot be improved by any adjustment.

Systems with Pro Logic
Speaker levels. Pro Logic units provide a built-in test signal generator called a noise sequencer that makes it particularly easy to balance all four channels. When you activate the sequencer (the switch may be marked "Test" or "Test Tone"), it sends a brief, specially filtered noise signal to each channel in turn. As the test signal "travels" from channel to channel, simply adjust the balance controls until each of the four channels individually plays at the same apparent loudness at your favorite listening position. While sufficiently accurate balance can usually be achieved by ear, you can use an inexpensive sound level meter if greater precision is desired.

Studio sound systems used to monitor the production of Dolby Surround programming and Dolby Stereo films are all balanced in precisely this way. Therefore, once you've balanced your Pro Logic system, it will reproduce Dolby Surround program material as originally intended, with no further adjustments necessary unless you change components, move the speakers, or reinstall the system in another location.

Input balance. If, like most Pro Logic decoders, yours has an automatic balance control, you needn't worry about adjusting input balance. Indeed, even without the automatic circuit, input balance is rarely a problem because Pro Logic will continue to provide good separation. If your decoder does not have an automatic control, however, you can adjust input balance precisely by first switching off the center channel or disconnecting the center speaker (do not use the Phantom mode). Then adjust the input balance control back and forth until minimum dialogue is heard from the left and right speakers. Be sure to turn the center channel back on or reconnect the center speaker when you've finished.

Speaker polarity. Just as with the two speakers in a stereo music system, it is important that all three front speakers in a Pro Logic system operate "in phase," with their cones moving back and forth together. The terminals on virtually all speakers and amplifiers are color coded to indicate polarity, enabling you connect all your speakers consistently: the "+" and "-," or red and black, terminals on each amplifier channel should connect to the corresponding terminals on each speaker. Speaker wire is also coded to facilitate this process.

Checking the polarity of the left and right front speakers is easily done by ear. With the surround decoding off, listen to a mono program source, such as a radio announcer's voice, over the two front speakers. When the speakers are in phase, the voice will appear to emanate from a point between the speakers. If not, the voice's location will be vague, even seemingly split between the two speakers. A more foolproof method is to place the left and right speakers facing each other literally an inch or two a part. If the speakers are out of phase, bass sounds will virtually disappear. If this happens, reverse the polarity of just one speaker, and leave it connected that way.

Checking the center speaker's polarity is not quite so simple. Listen to some familiar program material, including music-only sources, for sounds that you know should emanate from between the center and the left or right speaker, or for an effect that is slowly panned across the front. Go over to a point midway between the center and left or right speakers, and listen for such sounds to appear as focused, rather than diffused and vague, phantom images. If necessary, reverse the center speaker's connections and leave them where the imaging is clearest.

The polarity of the surround speakers, too, should be consistent with the others. However, if the surround field is too "monophonic," sounding almost inside your head like earphones on a mono signal, try reversing the polarity of one surround speaker to achieve a more diffuse effect.

Time delay
In all Dolby Surround decoders, the surround channel is reproduced a split second after the front channels. This is done to enhance the localization of front sounds, and not, as is sometimes mistakenly thought, to add some kind of echo or reverb effect. According to a principle called the Haas effect, when identical sounds are reproduced one immediately after the other, the ear cannot distinguish the later sound. Therefore, a slight delay of the surround channel reduces the chances of your hearing leakage of sounds which are intended to come from only the front (such as dialogue).

In some units, the time delay is fixed at 20 ms, which is appropriate for the majority of home listening environments. Others provide a delay which is adjustable from 15 to 30 ms, allowing you to compensate for being seated unusually close to or far from the surround speakers. While the standard 20 ms setting will still be appropriate in most cases, you can adjust the delay. There is a wide range of acceptable results, so there is no need to be all that precise.


Dolby Surround Program Material


Every Dolby Stereo movie on stereo video cassette or laser disc, or broadcast over stereo television, cable, or satellite, is automatically Dolby Surround encoded whether or not a Dolby Surround logo is actually displayed. There are also television shows and music videos produced in Dolby Surround, and music-only Dolby Surround CDs and audio cassettes. Lists of Dolby Stereo films and Dolby Surround programming are available as described in For Further Information on the inside of the back cover.

Remember that Dolby Surround programming must reach you in stereo to be properly decoded. Most cable companies rebroadcast local stereo stations in stereo, but not necessarily satellite feeds such as premium movie channels and other cable-only services. If you have any questions, call your cable company.

You might also try switching on your Dolby Surround decoder with regular stereo programs. Dolby Surround encoding was designed for good compatibility with stereo and mono playback, so conventional stereo and surround encoded programs behave similarly when decoded. The surround information on encoded material may be more prominent, but most regular stereo program material contains naturally out-of-phase information, such as a concert hall's ambiance or audience applause, that will be decoded as surround sound. Leaving your Dolby Surround decoder on will give pleasant results most of the time, and you won't have to worry about when to turn it on and off.


Identifying Dolby Surround Programming


Although there are two types of Dolby Surround decoders, basic and Pro Logic, there is only one form of Dolby Surround encoding. Therefore encoded program material generally displays the basic "Dolby Surround" logo.

Some stereo videos of Dolby Stereo movies inadvertently reproduce only the "Dolby Stereo" logo used to identify theatrical presentations; these are also Dolby Surround encoded.

Not all Dolby Surround TV programming is easy to identify. Some encoded shows are identified by the Dolby Surround logo up front, but others my show only a generic "Surround/Stereo where available" at the beginning, a Dolby Surround logo only in the end credits or no identification at all.

The linear audio tracks on video tapes may be recorded with Dolby B-type noise reduction for higher fidelity. Such tapes may carry a "Dolby System" or "Dolby B NR ON LINEAR TRACKS" logo, which does not mean that the program is necessarily in Dolby Surround.


The Future: Dolby Digital


In 1992, a new format providing multichannel digital sound on 35 mm films, Dolby Stereo Digital, was introduced. It required the development of a new form of multichannel digital audio coding, Dolby AC-3, which is also being incorporated as Dolby Surround AC-3 in tomorrow's home theatre systems. Delivering five full range channels left, center, right, and independent right and left surrounds plus a separate bass effects channel, the new technology will be used for the US high-definition (HDTV) system slated for introduction in 1996, and a new laser disc standard expected to debut in 1995. It is also being used in two-channel form for digital cable and satellite TV and music services.

When they become available, Dolby Surround AC-3 decoders will incorporate Pro Logic decoding for compatibility with existing Dolby Surround programs. These systems will all co-exist and complement each other for years to come, so there is no reason to put off investing in a home theatre, or to assume that today's Dolby Surround will suddenly become obsolete.

Dolby Surround AC-3 has much the same speaker and amplifier requirements as Pro Logic. The separate left and right surround channels, however, will be capable of the full frequency range, and may be used by mixers for new and more elaborate creative purposes. Thus it will be more important for your surround speakers to match the fronts in the mid and high frequencies. It still won't be necessary for them to reproduce deep bass, which can be redirected to the front speakers or to a subwoofer. Therefore, the home theatre speaker systems discussed earlier that combine five identical satellites with a subwoofer will be particularly ready for the new format.

Because the new format is based on sophisticated DSP techniques, when the first Dolby Surround AC-3 products appear, possibly by the end of 1995, they necessarily will be top-of-the-line models. Some manufacturers are also planning to introduce Pro Logic units that can be upgraded later to handle Dolby Surround AC-3. If you anticipate adding it to your system relatively early on, you might look for one of these models.


Go Back Comments or Questions

1997 Dolby Laboratories, Inc.