Home Theater Audio Systems

Transcription

1 Home Theater Audio Systems

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3 Home Theater Audio Systems 3 HOME THEATER AUDIO SY STEMS In the early days of the movies there was just the visual image to enjoy. Motion picture technology had not yet advanced to the point where synchronized sound tracks were possible. This detail, however, did little to dampen the enthusiasm for films produced during this silent era, movies were big stuff, but it is interesting to note that in an attempt to compensate for this sound deficiency many studios released features with sheet music scores so that piano players could play along with the film. As we know now, sound is a critically important part of the motion picture experience, but in the early days, this was just becoming known. The Silent Era lasted until the late 1920s. By this time, several studios had developed synchronized motion picture sound tracks systems and began showing them theaters. By today s standards, these early talkies films were pretty crude, but the concept of sound to accompany the on-screen picture was rapidly embraced by the viewing public. Just a decade later, in a continuing quest for technical achievement, the Hollywood studios began to experiment with multi-channel cinema sound. Interestingly, one of the first multichannel films was Walt Disney s well known Fantasia, which was released in 1941 in Fantasound. This 6 discrete channel (!), high-fidelity release caused quite a stir and soon Hollywood became awash in a sea of incompatible, multichannel formats issued by many of the major studios. Standardization on a common multichannel format did not occur until the 1960s when Ray Dolby, a young engineer with Ampex corporation, developed a process for recording multichannel sound optically onto movie film. This was considered a major advance in the motion picture industry because the Dolby-encoded optical sound tracks could be played in both the old mono/stereo movie theaters, and in the new Dolby Stereo four channel surround theaters. Almost overnight it became the standard for cinema sound. Although Dolby Stereo, and it s home version, Dolby Surround, were the best and only systems for a long time, the advent of digital technology in the 1980s advanced sound technology even further. Today most major movies for theatrical viewing are released in one or more of the following digital formats: Dolby Digital, DTS, or SDDS,

4 4 Home Theater Audio Systems (More on these later). How The Original Dolby Stereo System Works In the original commercial, analog Dolby Stereo cinema sound systems, the left and right audio tracks are read optically off the film strip and fed to a Dolby Stereo Cinema Processor (see the diagram below.) This processor decodes the Dolby Stereo signal into four different channels, in particular: The Center Channel. Here the filmmakers place most of the on-screen dialogue spoken by the actors. The Left and Right Front Channels. These channels carry more traditional stereo sounds, such as music and sound effects, but they are also used for dialogue and effects when actors move off to either side of the screen. (Example: When you see a shot of a train coming onto the screen on the right and going across to the left, the director will do the same to the train sound. It will progress from right speaker to the center speaker and then on to the left speaker.) The Surround Channel. First, it is important to note that there is only one surround channel. In many theaters there is an array of speakers at the rear but with the Dolby Stereo system, only one channel from the Dolby Stereo decoder drives them. The surround channel is for two things. The first and foremost is for ambience. In other words, in order to give a feeling of being there, directors often mix the sounds that lead to spaciousness, like the sound of rain, for example, into the surround tracks. Another thing surround channels are often used for is for directional sound effects. (Example: as helicopters roar overhead, directors often mix the sound tracks so that the choppers pass from the front speakers to the rear speakers in a thundering roar. Since it s release in the late 1960s, over 6000 movies have been recorded with Dolby Stereo-encoded sound tracks. Whether the movie is on videocassette, laserdisc, from your cable system or over the air, chances are it is in Dolby Stereo. The New Theatrical Digital Formats Today most movies for commercial theater viewing are released in one or more of the following digital formats: Dolby Digital, DTS, or SDDS. These new digital sound systems are incompatible but share the following characteristics: 1) they are pure digital; 2) they use compression algorithms to fit multichannels either on the film (Dolby Digital and SDDS), or on a separate CD (DTS); and 3) they have six discrete channels, left front, center, right front, left surround, right surround, and low frequency effect channel(lfe), referred to as 5.1 with the.1 for the LFE. (Note: SDDS provides 7.1 channels, with two additional front channels. At this point, Sony, the developer of the system have any plans to develop a consumer

5 Home Theater Audio Systems 5 SDDS in the future. So, at least at this point, we only have to deal with DD and DTS in the following discussion.) First, DD and DTS are incompatible systems. If a movie theater wants to show both DD and DTS movies, it must have two separate decoders. Similarly, if you want to use both DD and DTS encoded software in your home, you must have the capability to decode both. What comes off any digital software is a digital bit stream, a series of ones and zeros. It s exactly what comes off your stereo CDs, but there s a difference between CDs and DD or DTS. A CD has just two discrete channels and the bit stream contains all of the music (i.e. no compression). But DD and DTS has six channels or three times the information. And while CDs are typically one hour, movies are typically two hours. So now, there s twelve times the information. Imagine the size of a CD that holds 12 times what a current CD holds! To get around the massive information storage problem, engineers have developed perceptual coding algorithms which radically reduce the size of the bit stream, but in such a way, that ideally it sounds no different from a full bit stream. This is done by exploiting commonly known properties of human hearing. Here is a simple example: When you are listening to a symphony orchestra, some instruments or sections of the orchestra are playing loudly, and others softly. To your ears, the loud sections overcome the soft ones, so you do not hear them. Hard to believe, but true. So the engineers at DD and DTS use perceptual (what you perceive) coding which eliminates the played, but unheard, sounds and thereby reduces the bit stream. (The original term for DD, AC-3, is, in fact, the designation for DD s perceptual coding system) A DD or DTS decoder takes the bit stream, using the algorithm, divides it into 5.1 channels, converts it into 5.1 or 6, analog channels and it s ready to be played by your six channel audio system. But here s the problem. The perceptual coding algorithms developed by DD and DTS are entirely different. So if you want to listen to both formats, you need outboard DD and DTS processors, or a preamp/processor or receiver which decodes both. This was prohibitively expensive, but with the development of one solution, IC chips that perform both DD and DTS, prices are coming down. Whats Needed In Home Theaters So which do you need in your home theater, DD, DTS, or both? Honestly, this question is more easily asked then answered, but we will review software that is and will become available in future CDs and this may help with your decision. Compact Discs: There are a few CDs recorded in DD. There are many more DTS CDs, but still fewer than 100. Many more are planned. But, to confuse the situation, audio-only DVDs will soon be released which will, of course, require new audio DVD players, which may are

6 6 Home Theater Audio Systems How Dolby Digital works In Commercial Theaters Dolby Laboratories developed Dolby Digital for compatibility with existing optically based film-sound systems. The challenge was to fit both the optical and Dolby Digital sound tracks onto one film stock. To do this, the engineers at Dolby placed the AC-3 digital data between the film sprocket holes. On the decode end, during projection, Dolby Digital decoders take the digital bit stream and decode it into 5 discrete full-bandwidth channels and one subwoofer bandwidth-limited channel (.1). These 6 channels are then fed to amplifiers, and then to the theater s speaker system. Filmstrip showing optical stereo and Dolby Digital sound tracks may not incorporate the video DVD function. Further complicating the matter is that two separate DVD audioonly discs will be released. One, released by Sony/Philips, has two layers-one compatible with CD players, and another multichannel layer. The DVD consortium has another DVD audio disc which also be multichannel, but not compatible with CD players. The two systems will encode their bit streams in entirely different ways. Further, neither uses DD or DTS. The multichannel decoding will be executed inside the player and output as 6 channels. As you can see, the situation with CDs and DD/DTS encoding is messy and evolving as we speak. LaserDiscs: There are quite a few LDs in the DD format and an increasing number in DTS. The point of confusion here is how the digital bit streams are placed on the LDs. In the days of old, the audio track of LDs was an analog signal similar to an FM radio signal, with or without CX noise reduction. With the advent of CDs, LDs were released with the Dolby Surround (analog in theaters) track placed on the LD as a CD PCM (pulse Code Modulation) system. The old analog tracks were used for the directors commentaries and the like. With the advent of digital movie theater sound, DD and DTS proposed two different systems of encoding the digital soundtrack. DTS proposed replacing the PCM digital signal with a DTS signal which would require little compression. DD proposed replacing the left analog signal with it s bit stream modulated as an FM carrier in order to preserve the PCM/Dolby surround signal for those without DD decoders. And that s how they re released today. For DTS LDs, you just go from the digital output of the laserdisc player into the digital input of the DTS processor or receiver. For DD LS, you must first pass the signal through an RF demodulator which strips the FM carrier off the digital bit stream. The output of the demodulator is then input into the digital input of the DD processor or receiver. But this discussion may be academic. With the release of DVDs, sales of LD software have plummeted. For anyone starting out new, an LD is not recommended. Buy a DVD player. But thousands have LD players and LD collections. Unless they plan on replacing their LDs with DVDs as they come out (and many old movies may not be released on DVD for years), this discussion is still relevant. Digital Video Discs: The DVD standard requires that all DVD releases have a PCM track or a DD track. It s a practical matter, all but a handful have been released with DD. Great, you think. All movies are 5.1 channels on DVD because they re DD. Unfortunately, no. While all DD theatrical releases and nearly all DD and LDs are 5.1 channels, not all DD DVDs are. Here s why. For DVDs, DD only refers to the perceptual coding used. DD DVDs can be mono, stereo, 4 channel (dolby surround), 5.0 channel )No LFE) or 5.1 channel (!). The reason for this is simple. DD DVDs can only encode and play the original sound track. Since nearly all pre star wars movies were mono, the DD DVD will also be DD mono. Operas and concerts recorded in Stereo will be in DD stereo. Dolby Surround movies will be 4 channel. And digital 5.1 movies will be 5.1. (Here another confusing thing-dts theatrical releases are released as DD DVDs). The only exception to this is where studios have remixed the Dolby Surround movies to be 5.1 releases. DTS has announced that it will be releasing DTS encoded DVDs. These DVDs have a DTS flag on the DVD which the DVD must be able to read. Unfortunately, no first generation, and only a few second generation DVD players can read the DTS flag. However, the cost to the manufacturer of making a DVD player DTS compatible is minimal. If DTS establishes a foot hold in the DVD market,

7 Home Theater Audio Systems 7 DTS vs DD - Is There A Real Difference Sonically? Honestly, this is a fight we are reluctant to take sides on, there simply are a lot of good arguments illustrating the claims of both systems. We urge you to do some reading, and listening, and form your own conclusions! However, as you collect information, here are some of the issues you will find. The DTS system uses significantly less compression, and it s proponents claim it sounds closer to the original sound tracks. Dolby responds that it s DD system in A/B comparisons is indistinguishable from the master tapes, so who cares about compression amount. DTS also claims it has a more robust bass and distinct surround channels. DD responds that DTS artificially boosts those channels(!) so that is why some listeners form those conclusions. If you are thinking of doing your own comparison, keep this in mind. Rarely, even on the same movie released in DD and DTS consumer versions, is the same film mix used, so A/B tests of the same release will often sound different. Finally, three of the leading consumer home theater magazines performed rigorous, blind A/B comparisons using DTS/DD LDs with the exact same mix. The result? One said there was no detectable difference; the second said there were some differences and the third said DTS sounded better than DD. Warning: your results may differ! all players will be DTS compatible. Because all DVDs must contain a DD track, all DTS DVDs will contain a DTS track. But here s the trick. Remember, for DVD, DD does not equal automatic 5.1. So a DTS DVD may have a 5.1 DTS track, but only a 2 channel DD track, most likely matrixed analog Dolby Surround. But few decoders on sensing a stereo digital signal will automatically switch to Dolby Pro Logic (the home analog Dolby surround), and most cannot be forced into that mode, so you ll just have a stereo signal. Steven Spielberg is a part owner of DTS, and rumor has it that he will only allow his movies to be released in 5.1 DTs, not 5.1 DD. So, for DVD, DD vs DTS may not be merely a choice of which 5.1 format you prefer with duplicate releases. Digital Television: At least for DTV, the situation is simple. DD is the mandatory standard, with no alternatives. Thank God, this one is simple, at least for now.

8 8 Home Theater Audio Systems The THX Story... Understanding Home THX In the late 1970s, blockbusters such as STAR WARS and others drew the public s attention to the potential of high quality sound in the cinema sound. However, George Lucas suspected that all the attention that he and other filmmakers put into the sound tracks of their movies was being sabotaged by inferior sound systems in theaters around the world. So Lucas hired a young engineer by the name of Tomlinson Holman to look into the matter and dubbed the project THX, for Tom Holman s Experiment, and, not so coincidently, the name of Lucas s first commercial film: THX The results? Holman s theater survey confirmed their worst fears: The sound systems in theaters around this country and the rest of the world ranged from acceptable to down right terrible. Upon receipt of this conclusion, Lucas immediately set Holman to the task of writing a set of standards for the entire movie sound chain that would help. In specific, he wanted theaters to reproduce the audio quality achieved on the finest Hollywood dubbing stages. (Note: The dubbing stage is a highly specialized, theater-sized sound track mixing environment. The various sound elements of a film, such as dialogue, sound effects, music scores, etc., are mixed by skilled sound engineers in this special studio. The dubbing stage is the reference standard for a film s audio tracks.) Thus began the highly successful Lucasfilm Ltd. THX program. Today, the THX name has become synonymous with the highest level of theater sound system reproduction. Arguably the most important theater in the world, the 1000 seat Samuel Goldwyn Theater of the Motion Picture Arts and Sciences (it is here that Academy Award nomination screenings are held), is a THX theater. Currently, there are over 950 theaters and dubbing stages around the world that have been designed or re-engineered for THX reproduction. After the massive success of the commercial THX business, the next step, naturally, was to design a system that would reproduce the dubbing stage experience in the home. In order to achieve this, Holman identified six goals that all Home THX systems must achieve: 1) Accurate frequency response. Dubbing stages are equalized with the industry standard X curve. This standard is defined by the International Standards Organization (ISO) in their specification ISO and is designed to compensate for the acoustics of large movie theaters. Lucasfilm realized that this equalization does not apply to a home system, so a proprietary Reequalization circuit is specified for home use. The

9 Home Theater Audio Systems 9 diagram approximates the Home THX re-equalization curves. 2) Intelligible dialogue. With rare exception, every word spoken in a movie is intended to be heard clearly by the audience. THX systems assure that this is the case by specifying a center channel speaker with a narrow vertical dispersion pattern and driven by an electronic processor utilizing a Lucasfilm THX proprietary dialogue equalization circuit. 3) Full dynamic range. Filmmakers spend a great deal of time adjusting the components of their sound tracks to play at specific levels (see the diagram at the right). THX specifications require that a Home THX system be calibrated so that the standard 70 db THX reference tone is played back at exactly 70 db Sound Pressure Level (SPL.) 4) Accurate on-screen localization. The location of the sound field coming from an on-screen image, such as a glass breaking, should appear in the same place as the visual image. In order to localize on-screen sources, Lucasfilm specifies that front speakers also be designed with narrow vertical dispersion patterns, and be precisely located in reference to the screen. 5) Smooth panning. This requires a set of well matched speakers, properly placed for minimum room reflections. Speakers that are mismatched and misplaced, can cause a panned or moving sound source (like a actor s dialogue) to jump about or change in tone. This can ruin the effect desired by the director. 6) An enveloping sound field. The sound coming from the surround speakers is for diffuse ambiance. It shouldn't be localized (the source of the sound located), and THX systems use a special decorrelation circuit and dipole speaker radiation pattern to assure that this is the case. How To Get THX Sound In a Home Theater For you to achieve what the Lucasfilm creative forces have designed, the cinema dubbing stage experience in your own home, several items are required: A Home THX controller. This proprietary circuitry is THX specifications require that dipole speakers be used in THX home theaters. Dipole speakers create acoustic nulls in the audience seating area. This prevents localization of the speaker and a mor e realistic feeling to the surround field.

10 10 Home Theater Audio Systems licensed from Lucasfilm and is built into many A/V receivers and stand alone processors. The diagram above illustrates the signal processing that a THX licensed unit performs. THX licensed A/V receivers and processors are available from a number of manufacturers. With the advent of DD and DTS, there are now two THX standards for a controller, THX 4.0 and THX 5.1. THX 4.0 is used for analog Dolby Surround and THX 5.1 is used for DD and DTS. Most THX standards (subwoofer crossover, reequalization) are the same. But in 5.1 the decorrelation circuits for the surrounds shuts off when the surround channels are carrying discrete information, and kicks back in when the surround channel information is the same for the l and R surround, maintaining a diffuse sound field. A Home THX speaker system. The loudspeaker requirements of the home THX system are tightly specified and include requirements for loudspeaker distortion, directional properties, frequency response, sensitivity, impedance and power handling. For example: Front Speakers: The three front speakers (left, center and right) must be identically matched, with a flat response over 80hz to 20kHz. They also should have identically focused dispersion patterns and be magnetically shielded to prevent distortion in nearby televisions. Surround Speakers: Home THX systems are required to utilize two dipole design surround speakers to prevent localization of the surround information. See the diagram above. Subwoofers: THX systems are required to have at least one subwoofer to reproduce the bass range (20hz to 80Hz) of the three front channels. The crossover of the subwoofer and the front speakers must be electronically matched with 24 db/octave slopes. THX subwoofers must be able of sustaining a 105 db SPL, in a 3000 cubic foot room, in the farthest seating position. Note: as the diagram below illustrates, for best sub-woofer performance, place the subwoofers in the corner of the room. Home THX amplifiers. Home THX amplifiers must pass minimum electrical specifications and be capable of producing a constant sound pressure level without overload. For example, THX amplifiers must provide at least 100 watts in order to drive the speakers to the upper limits of a film s sound tracks (105 db). Furthermore, at all levels of amplification, the THX amplifiers must not produce audible distortion. Further, they must be able to deliver their rated power into a 3.2 Ohm load, all channels driven, on a continuos basis. Home THX Receivers. THX also certifies receivers for home use, both with Dolby Surround and Dolby Digital. No doubt, DTS/DD/THX receivers are in the offing. Receivers must have THX controller circuitry (crossover, reequalization, timbre matching, decorrelation). THX receivers also have stringent receiver power amplifier requirements which far exceed the typical receiver. They are, however, as stringent as the requirements for separate power amplifiers (eg no 3.2 Ohm power requirement).

11 Home Theater Audio Systems 11 HOME THEATER SPEAKER SYSTEMS In a home theater, speakers need to be placed in specific locations. Frequency response and audio localization are greatly affected by where you place them. There is a great deal of academic research documenting this, so consider it a truth. Location, location, location is not just an old saw for real estate professionals. Let s consider the front speakers first. Ideally, it is best to have left, center, and right speakers of the same type and aligned in a horizontal line through the middle of the video image. Unfortunately, this is not possible in most home theaters because the front speakers need to go right smack through the middle of the picture. The next best position is to flank the television, or video screen, with the left and right speakers, and install the center channel either above or below the picture. (Note: for those that are building high-end home theaters with front projectors, several screen manufacturers offer perforated acoustically transparent screens so that speakers can actually be placed directly behind the screen. However the center speaker must match the voice characteristics of the l&r speakers, typically by using the same tweeter and midrange.) There are several schools of thought about the type and placement of surround speakers, but the most popular is to place the speakers so they are flanking the audience seating area, and somewhat above the listener's ear level. In the days of analog Dolby Surround, there was a general consensus that di-polar (multi-driver, wired out-ofphase) were best for creating a diffuse sound field. In fact, that was, and is, a THX requirement. With the advent of DD and DTS, with discrete L&R surround channels, many began to advocate front radiating point source speakers and surrounds. The problem with this approach is that when mounting as shown in the diagram, the speakers, though excellent at directional special effects, did not do well at creating a diffuse surround sound field, and were identifiable as point sources. Several A/B tests by home theater experts confirmed this. They found that di-polar speakers did a more than adequate job at localized, directional sound effects while otherwise maintaining a diffuse sound field, even with stereo music in the surround channels. THX still recommends dipolar speakers in THX 5.1. The only possible exception would be in a large room where direct radiating fronts could be placed substantially

12 12 Home Theater Audio Systems The Dolby Digital speaker system layout is similar to pro-logic layout. The main difference between the formats is that the surround channels in Dolby Digital are full bandwidth behind the listening position (at least 1/2 the front seating distance.) Finally, one or two subwoofers are a must with todays home theater systems. The LFE (low frequency effects).1 channel of DD and DTS are simply beyond the capabilities of most woofers in full range front speakers. Even Dolby Surround has bass-laden sound tracks which demand a separate subwoofer. Further, the use of subwoofer(s) can [permit you to buy smaller L&R speakers saving money and enhancing ease of placement. To finish our thoughts about placement, let s take a look at some common home theater speaker configurations. In Home Theater Room Number One. In this case we have mounted the di-polar surround channels up on a beam that traverses the room and added a second subwoofer. Alternatively, two speakers matching the fronts can be placed on stands behind the couch. LEFT but it conceals the subwoofer enclosure and makes the system less intrusive. In this particular room, placing dipole speakers on the walls was not an option because of the window locations, so circular surround speakers were placed in the ceiling. The last illustration, Home Theater Room Number Three, we show high-end home theater with a large, frontprojected video image. Here we have used in-wall speakers around the screen to give the room a clean, theater-like look. There are many manufacturers that make in-wall speakers specifically for home theater applications. We recommend you look at some of the enthusiasts magazines for specific brands. The rear surround channels are also in-wall speakers. CENTER Home Theater Room Number Two shows a picture tube type television mounted in an A/V furniture cabinet. The center channel speaker is placed on top of the television and the left and right front speakers flank it on shelves. A subwoofer is placed below the television. This may be less optimum for bass reinforcement, LEFT SURROUND Home Theater Room Number One RIGHT SURROUND SUB WOOFER RIGHT SUB WOOFER

13 Home Theater Audio Systems 13 Because this is a highend enthusiast s home theater room, we put two subwoofers in the front corners for full low frequency capability. Another option is to built the subwoofer s into the walls or into the floor, if you want to hide them. LEFT CENTER SUB WOOFER RIGHT RIGHT SURROUND Home Theater Room Number Two LEFT SURROUND RIGHT SUB WOOFER LEFT CENTER RIGHT SURROUND SUB WOOFER Home Theater Room Number Three LEFT SURROUND

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