Audio for Home Theater and Multichannel Music [part 1]

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The explosive growth of home theater and high-definition television has expanded the possibilities for a home-audio system. Today’s technology allows us to enjoy 2-channel music, multichannel music, and film soundtracks in surround sound in our homes—all through the same audio system.

In this section we’ll look at the components needed for multichannel-music and home-theater reproduction, as well as ways of enjoying home theater without compromising the system’s musical performance. (For a more in-depth treatment of home theater, including video components, check out our other guides.)

Overview—Surround Sound

Home theater is made possible by multichannel surround sound. Instead of two speakers in front of you, as in a stereo system, home theater surrounds you with five, six, or even seven. Three speakers are placed in the front of the room, with two, three, or four surround speakers located to the rear or sides of the listening/viewing position. Surround sound is what provides the sense of envelopment, of being in the action, that makes the home-theater experience so engrossing.

Multichannel surround sound is encoded on DVDs, some satellite and cable TV transmissions, and over-the-air high-definition television (HDTV) broadcasts in the Dolby Digital format. This format, introduced in 1997 to replace Dolby Surround (and its decoding variant, Dolby Pro Logic), encodes six discrete audio channels in a single digital bitstream. The six channels are called left, center, right, left surround, right surround, and low-frequency effects (LFE), and correspond to loudspeakers arrayed in the pattern shown in Fig. 1. The LFE channel carries only high-impact, low-bass information (below 100Hz), and can be reproduced by any of the loudspeakers, or by a separate subwoofer. Typically, the LFE channel drives a subwoofer. With five full-bandwidth channels plus the 100Hz LFE channel, we call Dolby Digital a 5.1-channel format. The “.1” in 5.1-channel sound is the LFE channel. The competing DTS surround-sound format, found on some DVDs as an option, also delivers 5.1 discrete audio channels to your home-theater system.

Home-theater audio systems are increasingly moving from 5.1 channels to 7.1 channels. Many audio/video receivers and controllers can accommodate 7.1 channels. The two additional speakers in a 7.1-channel system, called surround-back speakers, are mounted on the rear wall directly behind the listener. This loudspeaker array takes advantage of the newer surround-sound formats that deliver an additional signal to drive the surround-back speakers. In addition, A/V receivers and controllers with 7.1- channel capability can synthesize signals to drive the surround-back speakers from 5.1- channel sources such as Dolby Digital and DTS.

(Note that some receivers have six amplification channels; in that setup only one surround-back speaker is employed directly behind the listener, this configuration is, however, uncommon; if you’re going to add surround-back speakers, it’s not much additional effort or cost to add two of them.)

A home-theater audio system typically employs three speakers across the front of the room, two surround speakers to the room sides or rear, and a subwoofer.

Should You Choose a 5.1-Channel or a 7.1-Channel System?

A fundamental question facing the home-theater shopper is whether to buy a 5.1- or 7.1-channel audio system. The vast majority of today’s film soundtracks are encoded in

5.1 channels, but newer films may be mixed in 7.1 channels for theatrical exhibition as well as for home theater. The two new high-definition disc formats vying to replace DVD (Blu-ray Disc and HD DVD) can carry 7.1-channel soundtracks. The future might be 7.1, but the present is rooted in 5.1 channels.

There are several arguments for choosing a 5.1-channel system. First, most films are mixed in 5.1 channels. Second, mounting and running cables to the additional surround-back speakers isn’t worth the small improvement in Performance (except in very large rooms). Third, many extremely satisfying home-theater systems are based on 5.1-channel playback. Fourth, a 7.1-channel loudspeaker package distributes your speaker expenditure over seven speakers plus a subwoofer ‘at her than over five speakers and the sub, which could compromise the c good surround speakers are better than four mediocre ones. Finally, if your living room dictates that your couch be positioned against the wall opposite the video display, here’s little point in adding surround-back speakers on that wall; it’s not likely that they will perform as intended.

The argument for purchasing a 7.1-channel system goes like this: Even though most films are mixed in 5.1 channels, today’s A/V receivers and controllers have processing circuits to create lie surround-hack channels from 5.1-channel sources. Although this trick doesn’t provide true 7.1-channel playback (the surround-back signals are simply derived from the left- and right-surround channels), it nonetheless increases the feeling of immersion in the soundfield. When playing true 7.1-channel sources, the film-sound mixers are able to position sounds directly behind you, an impossible feat with just 5.1 channels. Moreover, we are moving into an increasingly 7.1-channel world with the new audio formats on HD DVD and Blu-ray Disc; it’s better to get a 7.1-channel system now rather than have to upgrade later. (See the Addendum at the end of this section for more detail on surround-sound formats.)

Here’s my view. If you have a moderately sized room and you’re adding surround speakers for the first time, go with a 5.1-channel system. If your room is large, or if you’re building a theater room as part of new construction, select a 7.1-channel package. Larger rooms benefit from the surround-back speakers because they help to fill the “hole” between widely spaced left- and right-surround speakers. If you’re building a home with surround-speaker wiring in the walls, there’s very little cost and effort required to run wires for surround-back speakers.

Overview—Audio/Video Receivers, Controllers, and Amplifiers

The job of decoding the Dolby Digital or DTS bit-stream into separate audio channels, and then converting those digital data to analog signals, falls to the audio/video receiver (AVR) or A/V controller (the latter is also known as a surround-sound processor). The difference between them is that the AVR contains five, six, or seven amplifier channels; the controller has no internal amplification and must be used with a separate multi-channel amplifier. The AVR also includes an AM/FM tuner and possibly XM or Sirius

satellite radio. Other than those differences, the two components are identical. Let’s first look at the A/V controller. In addition to performing surround sound decoding (Dolby Digital, DTS, Pro Logic, etc.), the controller receives audio and video signals from the source components (satellite, DVD, tuner, CD, SACD, DVD-A, digital-video recorder) and selects which one is decoded and amplified by your home- theater audio system and sent to your video monitor for display. The controller also performs digital signal processing and bass management, adjusts the overall volume, and fine-tunes the levels for individual channels (these processes are explained later in this section). The controller’s output is six line-level signals: the left, center, right, left surround, right-surround, and subwoofer channels. Controllers that support the newer 7.1-channel surround-sound formats (Dolby Digital EX, DTS-ES, Dolby Digital Plus, DTS-HD) have eight line-level outputs rather than six. The eight outputs correspond to seven audio channels plus a subwoofer.

These six or eight separate outputs are fed to a 5- or 7-channel power amplifier (plus an optional subwoofer), where they are amplified to a level sufficient to drive the home theater’s loudspeaker system. A home-theater amp can have five, six, or seven amplifier channels in a single chassis. Alternately, some power amplifiers have three channels, others have two, and some arc available as monoblocks: one amplifier channel per chassis. A 3-channel power amplifier allows you to use your existing 2-channel stereo power amplifier on the surround channels and the 3-channel amplifier on the front three loudspeakers, for example. Most subwoofers have built-in power amplifiers to drive their woofer cones.

An audio/video receiver (AVR) incorporates, in one chassis, an A/v controller and multichannel power amplifier. AVRs are less expensive than separate controllers and power amplifiers. Separate controllers and power amplifiers usually deliver better performance than an AVR, although today’s upper-end AVRs are quite sophisticated, and many offer superb sound quality.

Overview: Home-Theater Loudspeakers

A conventional stereo system provides two audio channels, left and right, which are reproduced by the left and right loudspeakers. When correctly set up, two channels of information driving two loudspeakers produce a soundfield in front of the listener that seems to exist between and around the two loudspeakers.

A home-theater system provides multiple channels, each channel feeding a loudspeaker located in front of, alongside, and/or behind the listening/viewing position. Specifically, a home-theater system has three front loudspeakers located across the front of the room, and two, three, or four surround loudspeakers behind or to the side of the listening position. Two of the front three loudspeakers flank the video monitor; the third is positioned above or below the video monitor. The front loudspeakers are called left, center, and right.

The left and right loudspeakers reproduce mostly music and sound effects. The center loudspeaker’s main job is reproducing dialogue, and anchoring onscreen sound effects on the television screen. Having three loudspeakers across the front of the room helps the sound’s location more closely match the location of the sound source in the picture. For example, in a properly set-up home-theater system, if a car crosses from the left side of the picture to the right, you hear the sound of the car move from the left loudspeaker, through the center loudspeaker, and then to the right loudspeaker. The sound source appears to follow the image on the screen.

The surround loudspeakers have a different job. They’re generally smaller than the front loudspeakers, and handle much less energy. Consequently, they can be mounted unobtrusively on or inside a wall. Surround speakers mostly reproduce “atmospheric” or ambient sounds, creating a diffuse aural atmosphere around the listener. In a jungle scene, for example, the surround loudspeakers would re-create sounds such as chirping birds, falling raindrops, and blowing wind; in a city scene, the viewer would be surrounded by traffic sounds. The surround loudspeakers’ contribution is subtle, but vitally important to the overall experience. Correctly set-up surround speakers should not be able to be heard directly but should, instead, envelop the listener/viewer in a diffuse sound-field.

Let’s look at each of these components of a home-theater system in more detail…

A/V Receivers

The audio/video receiver is a marvel of technological sophistication, packing advanced features and technologies into a chassis about the size of yesterday’s stereo receiver. I ‘he following discussion of AVRs applies equally well to A/V controllers, with the exception that controllers have no built-in amplification or radio reception.

A modern A/V receiver, shown in Fig. 2, performs five vital functions in a home-theater system: the source-switching functions of a preamplifier (selecting which source to listen to or watch), surround-sound decoding, six (or eight) channels of digital-to-analog conversion, an electronic crossover to split up the frequency spectrum, and five, six, or seven channels of amplification.

A modern AVR is packed with sophisticated digital signal processing. (from Rotel)

Specifically, an AVR performs these functions:

1) receives video and audio signals from various source components (digital-video recorder, satellite, cable, DVD, HD DVD, Blu-ray Disc, etc.) and selects which are sent to the video monitor and home-theater audio system (this function is called “source switching”);

2) performs surround decoding, whether Dolby Pro Logic, Pro Logic II, Pro Logic IIx, Dolby Digital, DTS, Dolby Digital EX, DTS-ES, or one of the newer high-resolution surround-sound formats (the Addendum to this section explains the many surround-sound formats and their variations);

3) controls playback volume;

4) makes adjustments in system setup, such as the individual channel levels;

5) directs bass to appropriate loudspeakers or subwoofers (called bass management);

6) performs THX processing (if so equipped);

7) amplifies the five, six, or seven channels to a level sufficient to drive a loudspeaker array.

The AVR is the master control center of your home-theater system. When you first set up an AVR, its onscreen display will guide you through selecting such things as whether or not you’re using a subwoofer, if your surround loudspeakers arc large or small, and in calibrating individual channel levels and delay times. You will also control the system on a day-to-day basis through the AVR’s front panel and remote control. This includes source selection, volume adjustment, and the setting of individual channel levels (described later in this section).

above: Many of an AVR’s set-up parameters are accessed via on on-screen display. (from Rotel)

How to Choose a Home-Theater Receiver

Receivers range from entry-level units costing about $200 to state-of-the-art models that will set you back as much as $8000. This wide price range reflects the significant differences between receivers in their features, connectivity; power outputs, and sonic performances.

When choosing a receiver, first make sure the receiver has enough inputs for your system, provides your preferred surround-format decoding (Dolby Digital EX and DTS-ES, for example), offers a friendly user interface, has THX processing and certification (if desired), is well-built, and has good sound quality.

Inputs, Outputs, and Source Switching

Let’s start with the receiver’s most basic function: selecting the source you listen to or watch. The receiver accepts audio or A/V (audio and video) signals from all your source components and lets you select which source signal is sent to the power amps and video monitors. A basic receiver will offer two analog-audio inputs and perhaps four audio/ video (A/V) inputs. Most receivers today offer one USB input for connecting your computer to play music files from your hard disk. In addition to the main outputs that drive your TV and power amplifiers, two record outputs are often pro vided, to drive two video recorders.

When choosing a receiver, make sure its array of inputs matches or exceeds lie number of source components in your system. Your system is likely to expand in he future, so look for a receiver with at least one more input than you need right now.

All receivers have inputs for digital audio signals as well as for analog ones. These inputs receive the digital-audio output of a DVD player, satellite receiver, digital- video recorder, or other digital source. This digital connection can transmit a variety of digital signals, including Dolby Digital, DTS, Dolby Surround, and 2-channel PCM (Pulse Code Modulation) signals. The latter signal appears on a CD player’s digital-out put jack, or a DVD player’s digital-output jack when playing CDs. As just mentioned, a SB input is handy for playing files from your computer through your main audio system. Some AVRs also have a port for connecting an iPod. A few offer a signal-processing mode that reportedly improves the sound of compressed MP3 files to “CD-quality” (a doubtful claim). Also consider the type of video jacks on the A/V inputs. Most receivers offer S-video jacks on all inputs, with perhaps two inputs with component- video connections. Component video, carried on three RCA jacks, delivers better performance than S-video. If you plan to connect multiple sources via component video, he sure the receiver can accommodate all your component-video sources.

Today’s AVRs and controllers provide HDMI switching, seen here as the top right row of jacks. (from Anthem)

As more source components and video monitors incorporate High-Definition Multimedia Interface (HDMI) inputs, receivers are now offering HDMI switching. That is, the receiver may have several HDMI inputs and one HDMI output, allowing you to select which HDMI-equipped source is displayed on your video monitor. If your receiver lacks this feature, you’ll need to run the HDMI output from your high- definition source components directly to the HDMI inputs of your video monitor. Some video displays have only one or two HDMI inputs; if you have more HDMI equipped source components than HDMI inputs on your video display, you’ll need HDMI source switching. Fig above shows the rear panel of a controller equipped with HDMI switching, and an HDMI connector is shown elsewhere in this guide.

All AVRs have a video output that connects to your video display, with the receiver selecting which video input feeds the display. Some AVRs have an extremely useful feature called upconversion or trans-coding (the terms describe the same feature) that converts all video sources (composite video, S-video, component video, or HDMI) to an HDMI output. This allows you to run just a single HDMI cable between your receiver and video display. Without this feature, you would need to connect as many types of video cables as you had different outputs on your video sources. For example, if you had a DVD player with component-video output and a satellite receiver with 5- video, you would need to run both component-video and S-video cables between the receiver and television. HDMI upconversion solves this problem.

Digital Signal Processing (DSP) and Surround-Decoding Formats

The availability of powerful Digital Signal Processing (DSP) chips has revolutionized receivers in the past few years. A DSP chip is a number cruncher that operates on specific instructions (the software) controlling it. DSP chips are the heart and brain of the receiver, performing surround-sound decoding, signal processing (equalization, crossovers), and THX post-processing (if the receiver is THX-certified). Today’s advanced receivers boast the computing power of a late-1980s mainframe computer.

The first job of the DSP chip is decoding, that is, converting a stream of digital data into separate digital signals that can be converted to analog audio. All receivers today decode the three major surround-sound formats: Dolby Digital, DTS, and Dolby Surround. Dolby Digital is the mandatory surround-sound format for DVD (DTS is optional), and has been chosen as the surround-sound format for high-definition television (HDTV). Dolby Digital Plus and DTS-HD decoding are required in HD DVD and Blu-ray disc players, with studios free to choose either format. Dolby TrueHD and DTS-HD Master Audio are optional on HD DVD and Blue-ray discs.

6-ChannelAnalog Input

To play multichannel music, the receiver must have a six-channel (or eight-channel) analog input. This input is fed from the six analog outputs from a multichannel SACD or DVD-Audio player. (See Digital for a complete explanation.)

Analog-Bypass Mode

For the audiophile shopping for a receiver that will also serve as a 2-channel preamplifier for his system, one of the most significant considerations is its performance with 2-channel analog sources and signals from an SACD or DVD-Audio player. If uncompromised music performance is important to you, you’ll want a receiver that has an analog-bypass mode. Without an analog-bypass mode, all analog signals will be converted to digital and back to analog as they pass through the receiver. Digital conversion is far from sonically transparent, so the sound will suffer.

Automatic Calibration

Some receivers offer automatic calibration of individual channel levels and delay times. This calibration is covered in detail later in this section. Briefly, home-theater systems require that you adjust the level of each channel so that the listener hears the same volume from each speaker. Similarly, you must tell the AVR the distance between each loudspeaker and the listening positions; the receiver delays the signal from some channels so that the sound from each speaker arrives at the listening position at the same time. Automatic calibration in AVRs makes this process simple and foolproof you position a measurement microphone at your listening seat, press a few buttons, and the system calibrates itself. This feature, once reserved for very expensive AVRs and controllers, is now found even in budget models.

7.1-Channel Playback from 5.1-Channel Sources

Some home-theater controllers and receivers provide 7.1 -channel playback from 5.1 -channel sources such as Dolby Digital and DTS. These products have eight channels (seven channels plus a subwoofer) rather than the six channels of most Dolby Digital- and DTS equipped products. The two additional channels drive two extra speakers placed behind the listener, augmenting the two surround speakers at the sides of the listening location.

Making effective use of the additional surround speakers requires sophisticated signal processing in the receiver. Receivers with THX Ultra2 processing incorporate a circuit that creates a 7.1-channel signal from any source, allowing you to use your 7.1 -channel loudspeaker system on all source material, not just on those movies that have been encoded with 7.1 channels of information in the Dolby Digital Plus, Dolby TrueHD, DTS-HD, or DTS-HD Master Audio formats (see the end of this section). Dolby’s Pro Logic IIx will also generate from 5.1-channel sources a 7.1-channel signal that can drive the eight loudspeakers in some systems.

Multi-Room and Multi-Zone

Some upper-end AVRs offer a feature called multi-room or multi-zone. Both terms describe the receiver’s ability to send a line-level signal to an amplifier and speakers in another room. This allows one person in the family to watch a movie in the living room while another listens to XM or Sirius satellite radio in the bedroom, for example. If you see a button on the front panel marked ZONE 2, the receiver has multi-room capability.

Power Output in AVRs

AVRs vary in their power outputs from a low of about 6OWpc to a whopping 200Wpc. When choosing an AVR, match the AVR’s output power to the sensitivity of the loud speakers it will be asked to drive (or vice versa). The criteria used in matching an amplifier’s output power to a loudspeaker’s sensitivity described in detail in Sections 8 and 9, apply equally to an AVR and a multichannel speaker system.

Note, however, that AVRs generally don’t perform as well as separate amplifiers in their ability to drive loudspeakers. The designers must squeeze into a chassis of manageable size the power supplies, output transistors, and heat sinks—all large and heavy items—for five, six, or seven channels. The power supply is what delivers electrical current to the receiver’s output transistors—current that ultimately pushes and pulls the cones in your loudspeakers back and forth. Consequently, the first performance attribute sacrificed in inexpensive products is the ability to deliver current when the loudspeaker demands it. In fact, some receivers will overheat and shut down if asked to drive loudspeakers with an impedance of less than 6 ohms. The receiver’s output transistors simply aren’t up to the job. (A more thorough explanation of this phenomenon is included in Section 8.) This rarely occurs in separate power amplifiers because of their greater capacity to deliver current to the loudspeakers.

A great solution is to match the AVR with high-sensitivity speakers rated at 8 ohms. Remember that choosing a speaker system of just 3dB more sensitivity than another is equivalent to doubling the amplifier power. Put another way, a speaker rated at 90dB sensitivity puts ha as much demand on the receiver as a speaker rated at 87dB sensitivity

Satellite Radio: XM and Sirius

Many of today’s AVRs are equipped with XM Satellite Radio or Sirius Satellite Radio capability XM is more prevalent in AVRs). Both services offer more than 100 commercial-free channels of music, news, sports, talk, and other programming . XM offers 70 music channels, 30 non-music channels; Sirius delivers 125 channels some of which are non-music channels such as sports, weather, and talk radio.) Unlike over-the-air radio which is free to everyone, you must purchase a subscription to XM or Sirius. XM currently charges $9.95 per month, Sirius $12.95. You must also purchase an XM or Sirius antenna to connect to the AVR.

The big attraction of XM and Sirius, aside from a wide selection of commercial-free music, is the ability to receive this programming anywhere in the country with no static, fading, or the other problems associated with analog broadcasting. If you’ve become hooked on Xlvi or Sirius in your car, look for an AVR with XM or Sirius capability.

FM Tuner Performance

If you plan to use your AVR to receive FM broadcasts, consider the receiver’s tuner specifications before purchasing. Although we can’t tell much about how an AVR will sound by looking at the specification sheet, FM tuner performance can be characterized by a few key specs.

Good tuners are characterized by their sensitivity, or ability to pull in weak stations. The greater a tuner’s sensitivity, the better it can pick up weak or distant stations. This aspect of a tuner’s performance is more important in suburban or rural areas that are far from radio transmitters.

A tuner characteristic of greater importance to the city dweller is adjacent-channel selectivity—the ability to pick up one Station without interference from the station next to it on the dial. The alternate-channel selectivity specification defines a tuner’s ability to reject a strong station two channels away from the desired channel. When stations are packed closely together, as they are in cities, adjacent-channel and alternate-channel selectivity are more important than sensitivity.

Equally important to all listeners is the tuner’s signal-to-noise ratio, a measure of the difference in dB between background noise and the maximum signal strength. A tuner with a poor signal-to-noise ratio will overlay the music with an annoying back ground hiss.

In short, a poor tuner will have trouble receiving weak stations, may lack the ability to select one station when that station is adjacent to another station, have high background noise, and be overloaded by nearby FM transmitters or other radio signal sources (taxi dispatchers, for example).

THX Certification of AVRs and Controllers

Some AVRs are THX-certified, meaning that they incorporate certain types of signal processing to better translate film sound into a home environment as well as meet a set of specific technical criteria established by Lucasfilm. THX processing in an AVR includes re-equalization, surround de-correlation, and timbre matching. re-equalization removes excessive brightness from film soundtracks for home-theater playback. Surround de-correlation improves the sense of spaciousness from the surround channels by introducing slight differences between the two surround signals. Timbre matching compensates for the fact that the ear doesn’t hear the same tonal balance of sounds from all directions. The THX timbre-matching circuit ensures that sounds from the front and rear have the same timbre (tonal balance). THX-certified AVRs must also meet certain power-output requirements. All THX-certified AVRs also have a standardized sub- woofer crossover circuit.

There are various levels of THX certification. AVRs with THX-Select certification contain all the signal processing described above, but their power-output requirements are relaxed so that THX processing can be made available in less expensive AVRs that are suitable for smaller rooms. What was the original THX specification is now called THX Ultra. A third THX certification, called THX Ultra2, applies some new criteria to the certification requirements. First, the receiver (or controller) must have seven channels (rather than five or six); signal processing to convert 5.1-channel material for playback over a 7.1-channel loudspeaker array; and advanced video-switching circuits to ensure that the video quality isn’t compromised by the AVR or controller. Ultra2-certified products also incorporate features such as Boundary Gain Compensation, a bass cut invoked when the loudspeakers must be placed near a wall.

Multichannel Power Amplifiers

The power amplifier is the workhorse of your home-theater system. It takes in line-level signals from your controller’s output and amplifies them to powerful signals that will drive your home-theater loudspeaker system. A power amplifier has line-level inputs to receive signals from the controller, and terminals for connecting loudspeaker cables. The power amplifier is the last component in the signal path before the loudspeakers.

A separate multichannel power amplifier can offer higher output power than the power amplifiers found in A/V receivers. The most powerful receivers can have outputs of perhaps 200 watts per channel; separate power amplifiers offer as much as 350Wpc.

A power amplifier is described by the number of amplifier channels in its chassis. The most common home-theater power amplifiers have five channels for powering the left, center, right, surround-left, and surround-right loudspeakers. The next most common number of channels is three. A 3-channel amplifier can power the front three loudspeakers, leaving a 2-channel amplifier to handle the two surround loud speakers. If you already have a stereo (2-channel) amplifier, you can update your system for home theater by adding a 3-channel home-theater amplifier. Alternatively, a 3- channel amplifier can power the center and two surround channels, leaving your stereo amplifier to drive the left and right loudspeakers. As the Dolby Digital Plus and DTS HD formats become more popular, power amplifiers with seven channels are becoming increasingly available. Note that if you upgrade your controller to a 7.1-channel unit, you can augment your 5-channel power amplifier with a stereo amplifier to power the surround-back speakers.

How to Choose a Home-Theater Power Amplifier

High-quality multichannel power amplifiers are big and heavy, and can be expensive. The more output power provided by the amplifier, the bigger, heavier, and more expensive it will be.

Most home-theater enthusiasts listen at moderate levels to loudspeakers of average sensitivity (85 - 88dB) in rooms of average size (3000 - 4500 cubic feet) with average furnishings (carpet, some drapes), and so will need a minimum of about 70W for each of the five, six, or seven channels.

Note that driving five, six, or even seven loudspeakers places extraordinary current demands on the amplifier. If the amplifier isn’t up to the job of driving the loudspeakers adequately, the sound will be constrained on loud peaks and even distort. Just when the film soundtrack gets exciting, you hear the amplifier run out of power, distracting you from the movie. There’s a way to reduce the strain placed on multichannel amplifiers: Use high-sensitivity loudspeakers. Remember that every 3dB increase in sensitivity is equivalent to doubling amplifier power.

More specific information on power amplifiers may be found in Section 8. The discussion of sound quality, build-quality, and matching to loudspeaker sensitivity applies equally well to multichannel amplifiers as it does to 2-channel amplifiers.

Loudspeakers for Home Theater

A home-theater loudspeaker system will perform double-duty, reproducing 2-channel music sources through the front left and right speakers, and film soundtracks through all five, six, or seven loudspeakers (plus an optional subwoofer). Home-theater loud speakers can be evaluated using the criteria described in Sections 4 and 9, with one difference: Film soundtracks present somewhat different challenges to a loudspeaker system than music. Specifically, film soundtracks are generally more dynamic than most music, and have much more low-frequency energy. Consequently, great home-theater sound requires loudspeakers that not only meet our criteria for good music reproduction, but also have a wide dynamic range and the ability to handle loud and complex signals without sounding smeared or confused.

The Center-Channel Speaker

Though the center-channel loudspeaker carries a large part of the film soundtrack— nearly all the dialogue, many effects, and some of the music—only recently have center-channel speakers been elevated from afterthoughts to recognition that they are the anchor of the entire home-theater loudspeaker system.

The center speaker is usually mounted horizontally on top of the video monitor It can also be placed beneath the video monitor or mounted inside a wall above the video display, or, if you’re using a front-projection system, behind an “acoustically transparent” screen.

The center-channel speaker should be positioned flush with the front of the television set.

Because a stereo system uses only two loudspeakers—left and right—across the front, you may be wondering why you need a third loudspeaker between them for home theater. The center-channel loudspeaker provides many advantages in a home- theater system. First, it anchors dialogue and other sounds directly associated with action on the screen in the center of the sonic presentation. When we see characters speaking, we want the sound to appear to come from their visual images. Similarly, when sounds are panned (moved from one location to another) across the front, we want the sound to move seamlessly from one side to another. For example, if the image of a car travels from the left side of the screen to the right, the sounds of the car’s engine and tires should travel with it, precisely tracking the car’s movement. Without a center speaker, we may hear a gap in the middle as the car sounds jump from the left loudspeaker to the right. The center speaker makes sure on-screen sounds come from the screen.

A 2-channel stereo system is, however, capable of producing a sonic image directly between the two speakers. This so-called “phantom” center image is created by the brain when the same signal is present in both ears. A sound source directly in front of us in real life produces sound waves that strike both ears simultaneously. The brain interprets these cues to determine that the sound source is directly in front of us. Similarly, two speakers reproducing the same sound send the same signal to both ears, fooling the brain into thinking the sound is directly in front of us.

For two loudspeakers to create this phantom center image, however, they must be precisely set up, and the listener must sit exactly the same distance from each of them—if you sit off to the right, the center image will pull to the right.

This problem is overcome by putting a center-channel speaker between the left and right speakers. Dialogue and onscreen sounds are firmly anchored on the screen for all listeners, not just those sitting in the middle. With three speakers across the front, someone sitting at the left end of the couch can still hear dialogue coming from the area of the screen—not just from the left loudspeaker. The center speaker also prevents the entire front soundfield from collapsing into the speaker closest to where you’re sitting. Moreover, the center speaker provides a tangible sound source directly in front of you; your brain doesn’t have to work to create a phantom image between the left and right speakers. Finally, the center speaker reduces the burden on the left and right loudspeakers. With three speakers reproducing sound, each can be driven at a lower level for cleaner sound.

Adding a Center Speaker to Your System

Although you can add a different brand of center speaker to your existing left and right speakers, you’re better off with three matched speakers across the front of your home theater. Speakers all sound different from one another. No matter how good the quality of a speaker, it will have some coloration, or variations from accuracy, in its sound. Remember, you want to hear a seamless movement of sounds across the front soundfield. If the center speaker has a different sound from the left and right speakers, you’ll never achieve a smooth and continuous soundfield across the front—when sound sources move from one side to another through the center speaker, the sound’s character will abruptly change.

The solution is to buy three matched front-channel speakers. Their identical tonal characteristics will not only provide smooth panning of sounds, but also produce a more stable and coherent soundfield across the front of the room. If you already have high-quality left and right loudspeakers that you want to keep, buy a center-channel speaker made by the same manufacturer. They probably won’t be as well—matched as a three-piece system, but the added center speaker is much more likely to sound similar to your existing left and right speakers. Separate center speakers sometimes use the same drivers (the raw speaker cones themselves) and other parts as the stereo speakers from the same manufacturer.

For state-of-the-art home theater, the three front loudspeakers should be identical. Although most home-theater enthusiasts will use a smaller center speaker that will fit on top of a television set and doesn’t reproduce much bass, more ambitious systems use large, full-range center speakers. These speakers are hard to position, and are generally used only with front-projector systems; the center speaker can be placed behind a perforated projection screen, just as in a movie theater.

Next: part 2

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Updated: Wednesday, 2019-07-10 17:09 PST