Dolby Surround
Dolby Surround is the name of the system that reproduces the sound in your living room in exact the same way as a movie theatre does. A surround set consists of the 2 speakers from the conventional stereo-set, an extra center speaker for the dialogues and at least 2 surround speakers for the rear information.

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.

Dolby Surround Pro Logic, which is the full name of the home system, has been on the Dutch market a few years now. Over 50 million systems are sold world wide of which half a million in the Netherlands. Producing in Dolby Surround costs barely more than normal audio post production. In contrary to what is usually thought, you don't have to pay an additional fee for broadcasting or for video copies. Mixing in Dolby Surround, 5.1 and 7.1, for the living room or for the cinema, most modern (especially D&R equipped) Studio's can cater for it all.

More about Dolby Surround

To many people, the term surround implies that something new has been added to a stereo audio signal something requiring more than two speakers for reproduction. While this is true, it should also be realized that stereo itself is not confined to just two channels of sound; it can mean three, four, six, or any number so desired. Somewhere along the way, home stereo became synonymous with two channels, while at the movie theatre several multi-channel stereo formats appeared–and disappeared–in the 1950's and 60's.

By the latter 1970's, Dolby Stereo was established as a stereophonic reproduction system having from three to as many as six channels of sound to enhance the action and drama of theatrical presentations in ways only approached by two-channel systems. The most obvious feature of Dolby Stereo is that an additional channel of sound is reproduced along the sides and back of the theatre to "surround" the audience with sound.

Dolby Laboratories then devised a simple method to emulate the overall effect of Dolby Stereo in a home environment by recovering these extra surround sound effects. Introduced in 1982, Dolby Surround products today enjoy widespread popularity, with millions sold worldwide and the pace accelerating due to technical breakthroughs in home audio/video delivery formats, especially stereo television and satellite.

The advanced Dolby Pro Logic Surround system appeared in consumer products in the fall of 1987, quickly gaining favor with home theatre enthusiasts for its improved spatial articulation and expansive listening area. This leap in performance, however, came at a price; the decoder circuitry was significantly more complex, in effect limiting Pro Logic technology to high-end A/V products.

Just one year later came the breakthrough needed to make Pro Logic decoding as economically attractive as it is sonically: a custom integrated circuit was developed, consolidating a significant number of processing circuits into one package. Since that time several more analog and digital integrated circuits have become available, providing high performance multi-channel sound in a wide range of cost-effective home systems.

Today, A/V systems have taken on new dimensions; televisions with 27- to 35-inch screens are popular, with a move toward even larger screens and 16:9 "home theatre" aspect ratios underway. Rear projection sets from 40 to 60 inches are becoming mainstream products, as are projection systems with 6- to 12-foot screens. Larger screens coupled with increased video resolution bring the home viewer more of the movie theatre experience, and benefit greatly from expanded sonic dimensions to balance the presentation. These factors led to Dolby's introduction of Pro Logic, the second generation in Dolby Surround decoding technology.

The Pro Logic system
Pro Logic is an active process designed to enhance sound localization through the use of high-separation decoding techniques. The system is a direct descendant of the one used in Dolby Stereo cinema processors, and features a center channel speaker along with the left, right, and surround outputs. Just as the L-R differential stage is the heart of a passive decoder, the adaptive matrix is the heart of a Pro Logic decoder. Two main signals go in (L-total and R-total), and four resultant signals emerge (L, C, R, and S).

To summarize, Pro Logic operates by continuously monitoring the encoded soundtrack, evaluating the inherent sound field dominance, and applying enhancement in the same direction and in proportion to that dominance. Notice that the adaptive matrix employs two parallel paths: a relatively direct audio path (L and R inputs go straight to the combining networks) and a complex control path. Most of the decoder's electronics are used to condition and analyze the input signals rather than to actually process the audio itself.

Time delay
is often used to create an echo effect, which can help give music reproduction a feeling of greater spaciousness. However, that is not why it is used in Dolby Surround decoders. The real reason is to improve the sense of clarity and directionality of front channel sounds. This is done by taking advantage of the "Haas" or precedence effect, which enables the main frontal sound to arrive at your ears before the surround sounds. The time delay stage compensates for the travel time of sound through the air, which takes about 1 millisecond per foot distance. By knowing how far the listening position is from the front and the surround speakers, it is possible to adjust the time delay for optimal results.

There are generally two kinds of time delay available in Dolby Surround products: adjustable, or un-adjustable (fixed delay). As would be expected, the adjustable delay allows a wider range of distances to be used than the fixed delay.

Surround Starts with Stereo
Many aspects of producing in Dolby Surround are the same as producing in stereo. The main difference is that the mixing console must ideally have at least four buss outputs to feed a Dolby Surround encoder, and some additional speakers and amplifiers are needed to monitor the center and surround channels via a Dolby Surround decoder. In most cases, the finished two-channel encoded soundtrack is all that will be recorded. However, it is sometimes desirable to record the four-channel "stems" (encoder input signals) onto separate tracks when further elements are to be added later, such as with music pre-mixes for movie soundtracks.

Necessary Equipment
1. Dolby Model SEU4 Surround Encoding Unit.

The SEU4 encodes four input signals (Left, Center, Right, Surround) into two output signals (Lt, Rt).

2. Dolby Model SDU4 Surround Decoding Unit.

The SDU4 decodes two input signals (Lt, Rt) into four output signals (L, C, R, S) using Dolby Pro Logic Surround decoding technology. The unit also provides switch able stereo and monaural monitoring modes for evaluating compatibility. A ganged master fader allows all output channels to be varied together. A note about Dolby’s products: The SEU4 and SDU4 are 1U high, 19" rack mount professional units. Both use XLR type connectors with balanced line levels adjustable from -10 to +8 dBr (0 dBr=0.775 Vrms). Pin 2 is "hot." For unbalanced connections, pin 1 is tied to pin 3 for proper operation.

Mixing console.
Mixing will be performed in four channels, so a four-buss output structure is needed. Conventional stereo consoles with sufficient sub grouping and auxiliary sends may be used to start. Dedicated consoles such as the Cinemix and Octagon consoles are designed in away that monitoring your surround results are as easy as stereo monitoring. This is mostly lacking on ordinary stereo consoles and a pain in the ass to work with. Automation is not required, but may be beneficial for fine-tuning special effects. Four channel pan pots are also desirable for certain effects, although even basic pair-wise left-right and center-surround panning facilities may be adequate for some productions. The SDU4 mode and level controls simplify connection directly to monitor amplifiers. Program meters (with peak and average weighting) are recommended to read the four encoder input signals and two encoder output signals.

Speakers and amplifiers.
Three speakers are needed across the front to produce the left, center and right channels. Ideally, all three will be the same model for best acoustic matching. Unfortunately, it may not be possible to use the same speaker in the center due to space restrictions. In such cases, finding one that blends well sonically is important. Using a smaller model from the same product line as the left and right speakers is recommended. Use of a magnetically shielded speaker is necessary whenever that speaker needs to be positioned near a video monitor.

The center speaker needs to be at the same height as the left/right pair for best imaging. It is also important to confirm that the polarity (phasing) is correct in all three front speakers so that panned effects create proper phantom images between the center/side speaker pairs.

At least two surround speakers will be used, but room shape and area of coverage may dictate that four will be preferred for even coverage. Refer to the room layouts in Figures 2 and 3 for typical speaker arrangements. The requisite characteristics for the surround speakers are smooth response from 100 Hz to 7 kHz and wide dispersion. The same type of small speaker used for the center will often be used in the surrounds to assure sonic matching. Generally, good quality two-way bookshelf speakers are sufficient for surround channel use.

Surround Speakers
Surround speakers are typically located two or three feet above ear level, and should not be aimed directly at the mixing position. The goal here is for an evenly dispersed sound field, with no prominent hot spots or "in the head" images. While the surround speakers should be "in-phase" with each other, their absolute polarity has no relevance to the front channels since the decoder introduces time delay into the surround channel.

The amplifiers used for center and surround channels should be similar to the ones in the left/right channels. Note that only one amplifier is needed to drive all the surround speakers, since the total acoustic output requirement is no greater than in any other channel. Series-parallel wiring, as appropriate, should be used.

Ancillary Equipment.
Most studios have an array of signal processors available, and some are indispensable for proper Dolby Surround production. Limiters top the list, and some may need to be dual- or triple-ganged in special cases. Time delays, phasers/flangers, echo/reverb and equalization are also possibilities. A Dolby Surround mixing consultant will be instrumental in sorting out specific processor needs for a given production.

Dolby Surround Mixing Consultants
Dolby Surround mixing consultants are available to assist with the initial installation of the equipment and to perform the alignment of the encoder, decoder and monitoring system. Instruction on the use of equipment may also be available at this time. Dolby consultants are also available for staff training. All services are on a time plus travel service contract basis and are billed at half day or full day rates.

Dolby Surround Trademarks
The Dolby Surround program is administered by Dolby Laboratories Licensing Corporation under various agreements. Certain conditions apply to the use of Dolby logos for trademark protection purposes. A license to use any Dolby Logo is required of the owner of the work (not the studio in most cases). Please contact the Dolby Office nearest you for additional information:

Needs at home
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.

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 sound field 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 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 matching. 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 performance. 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.

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.

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.

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We like to thank Paul Weijenberg and Dolby Laboratories

for the use of this article

for further understanding of the Surround Phenomena.

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