Chapter 3: HT Audio

In the previous chapter I explained how to reproduce in your home both the quality (high resolution) and shape (widescreen format) of a theatrical picture. Similarly, a home theater should reproduce both the quality and shape of theatrical sound. By shape here, I mean the location of the sound—theaters are designed with an arrangement of speakers around the audience, and we want an equivalent arrangement in the HT room. This is called surround sound.

I’ll start our discussion of HT audio by explaining how surround sound is encoded in various sources. Then I’ll talk about speakers and where they’re positioned, and finally introduce a key part of an HT system, the audio/video receiver.

Principles of Surround Sound

Your Home Stereo Won’t Work

Nearly all HT program sources provide stereo analog sound. You can easily reproduce this sound, in stereo, with whatever quality your home stereo system is capable of simply by connecting the audio from the source (e.g., VCR, digital cable TV) to your stereo receiver, instead of to the TV. Then turn up your stereo and turn down the TV.

The problem is that, while the quality may be high, most modern films were created with surround sound, which has more than just the two stereo channels. Typically there are five distinct channels: one in front mostly for dialog, one each at the left and right front, and one each at the left rear and right rear (called the “surround” speakers), as shown in Fig. 3-1. Since the creators of the film planned the sound for five channels, you won’t hear the film at home as it was intended to be heard in the theater (the goal of HT) unless your home system can also play five distinct channels.

So, your existing stereo system won’t do. Instead, you’ll need:

·      Electronics to decode the sound into its distinct channels.

·      Five amplifiers, one for each channel.

·      Five speakers arranged around the listening area, each connected to one of the five amplifiers.

And, you really should also have:

·      A sixth amplifier/speaker combination to handle low frequencies, called a powered subwoofer.

The first two jobs (decoding and amplifying) are performed by an a/v (“audio/video”) receiver, which I’ll describe in more detail shortly. You could use your existing stereo speakers for two of the five speakers you’ll need for HT, but it’s best to replace even these so all the speakers will match. You can, however, keep any source equipment you have, such as CD and tape players and even a turntable if you still have one. And, if your current stereo has a suitable subwoofer, you can keep it.

Two Ways to Encode Surround-Sound Channels

There are two main ways to encode the multiple audio channels for surround sound. The names are, unfortunately, very confusing.

·      Dolby Surround/Pro Logic: Piggybacking on the analog stereo audio by mixing two additional channels with the stereo: one for the front center speaker, and one for rear speakers. This mixing is the Dolby Surround part. The corresponding decoding circuit in the a/v receiver that recovers the four channels from the mix is called Dolby Pro Logic. We’ll write it as “Surround/Pro Logic” to remind you that they go together.

·      Dolby Digital: A digital encoding of 6 or more distinct channels that can be decoded only by a special circuit. (The full name is “Dolby Digital Surround,” but I’ll call it Dolby Digital to avoid overusing “surround” and making the terminology worse than it already is.)

(“Dolby” is the name of the company that originated these surround-sound technologies.)

What’s confusing about the terminology is that you’ll see media, such as videotapes, marked as Dolby Surround, but the feature that plays it back as 4 channels is called Dolby Pro Logic. Dolby Digital goes by the same name in both contexts, but there’s a problem there too because it used to be called AC-3, and you’ll sometimes see that term used instead.

The advantages of Dolby Surround/Pro Logic are that it can be recorded on any medium (e.g., videotape) that can handle plain stereo, and even be broadcast over the air. Also, it’s compatible with stereo systems: If the receiving system can’t interpret the 4 channels, it just comes out as regular stereo.

The disadvantage of Dolby Surround/Pro Logic is that the side/rear channel is mono only and capable of a smaller range of frequencies than the other three channels, so it doesn’t reproduce a true surround-sound effect.

The advantages of Dolby Digital are that each of the 5 surround channels is completely independent and of full frequency range, and there’s even a sixth low-frequency effects (LFE) channel for things like the rumble of space ships. The commonly-used notation “5.1” refers to the 5 main channels and 1 LFE channel. Normally the LFE channel goes to the subwoofer, as shown in Fig. 3-1.

There used to be two disadvantages of Dolby Digital, but they’re less important today. The first is that a/v receivers that could decode Dolby Digital were very expensive, but that’s no longer the case. The second is that the source has to have a separate track to record the Dolby Digital audio, since the stereo channels are already in use. So, Dolby Digital is supported only by newer media such as laserdisc, DVD, digital satellite, and HDTV, but never by videotape or analog television. The video quality of these older sources isn’t very high anyway, so you may not care that much about the limited surround sound.

One more cause of confusion is that just because a source is Dolby Digital, that doesn’t mean that it’s surround sound, since Dolby Digital encoding can also be used to encode mono, stereo, and 4-channel surround sound, as well as 5.1. But as far as setting up your HT goes, you don’t need to worry about this. If the sound isn’t true 5.1, your a/v receiver will figure out how to handle it.

A Simple Approach

Here’s how to make planning and installing your HT easier: Forget about Dolby Surround/Pro Logic and just set it up for Dolby Digital. This is practical because almost all a/v receivers now contain the circuitry to decode Dolby Digital, and you need the same five speakers anyway.

If you have a source such as a videotape that has only Dolby Surround/Pro Logic, it will still play just fine, as all receivers that handle Dolby Digital also handle Dolby Surround/Pro Logic. What’s more, usually your a/v receiver will detect which surround technology is in use and switch itself automatically.

When you’re buying or renting a source (videotape or DVD), you don’t have to decide between Dolby Surround/Pro Logic and Dolby Digital, because:

·      Videotapes don’t come with Dolby Digital, and

·      DVDs come with either Dolby Surround/Pro Logic only or with both that and Dolby Digital, in which case your a/v receiver will automatically lock onto the Dolby Digital track. (You might have to choose that track from the DVD’s menu, however.)

So, it’s really not so bad. Put together a Dolby Digital system, play whatever source you want, and everything will work itself out.

More Surround-Sound Terminology

So far I’ve introduced four terms for surround-sound terminology: Dolby Surround, its playback counterpart Dolby Pro Logic, Dolby Digital, and its former name AC-3. Here are some additional terms that you might encounter:

·      Dolby Pro Logic II, which is a newer version of Dolby Pro Logic that decodes Dolby Surround audio into full 5.1 surround sound (that is, 6 channels). It helps with sources that can’t support Dolby Digital but can support Dolby Surround, such as videotape, but it’s not needed or used if the source has Dolby Digital encoding, as is usually true for DVDs and HDTV broadcasts. Generally, any a/v receiver that supports Dolby Pro Logic II will also support Dolby Digital. (Note that the encoding, Dolby Surround, is identical for both Pro Logic and Pro Logic II—the improvement is all in the decoding circuit.) Dolby Pro Logic II also enhances ordinary stereo sources (e.g., music CDs) to give a very pleasing surround effect that uses all the speakers, not just the two main front ones.

·      DTS (“digital theater system”), which is a competitor to Dolby Digital in theaters (and slightly better technically). DTS has made little inroad into the home, chiefly because HDTV uses Dolby Digital, so that’s what mainstream equipment and nearly all DVDs support. Many new a/v receivers support DTS along with Dolby Digital, so you’ll probably have DTS support in case you ever need it. Occasionally a DVD title (e.g., Saving Private Ryan) is available in two versions, and you can choose the DTS version if your receiver can handle it.

·      THX, which is a specification for movie and home theaters that can support an especially high level of audio quality. It’s not a surround-sound encoding scheme. You might see the THX logo on equipment that’s built to the THX standards, but it’s not important unless you’re going for the absolute top end.

·      Surround EX, which adds an additional “surround back” channel to even out the sound coming from the left and right surrounds. It’s also known as 6.1, which is one more than 5.1. For most home systems it’s going way beyond what you need, although it does more accurately mimic movie theaters, which have speakers along the back as well as the sides.

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Surround Sound Speakers

Each of the 6 surround channels in a 5.1 system gets its own speaker. Each is wired to its own connector in back of the a/v receiver, so there are at least 12 wires (six cables of two wires each) going from the speakers to the a/v receiver. This is similar to how an ordinary stereo is wired, but with more speakers and more wires.

The surround-sound effects will be best if the speakers are carefully matched. But, if your budget is tight and you have 4 speakers from existing stereo systems, you can certainly start with just a new center speaker (why is explained below) and then upgrade the speakers later.

The next several sections explain what’s important about each of the various speakers. I’m going to talk only about the typical setup, not exotic arrangements with extra rear center speakers, multiple side speakers, and so on, as that would be much too confusing and wouldn’t add much in the way of improvements anyway.

Center Speaker

This is the most important of the speakers because it handles the dialog and sounds that come from people or objects that are on-screen, which in most movies is nearly everything. On many soundtracks it also handles much of the music. For older monophonic pictures it does all the work.

In a movie theater the center speaker (or speakers) are often behind the screen, but at home, unless you have a front-projection TV, you’ll usually put the single center speaker on top of the TV set with its front even with the front of the TV.

Unless you have front-projection, the center speaker needs to be magnetically shielded so its internal magnet doesn’t distort the TV picture. This, along with the fact that the center speaker handles most of the sound, is why it’s best to buy a speaker specifically made for center-speaker use.

Left and Right Front Speakers

These are often bigger and better than the others because they’re all that’s active when you’re playing a stereo audio-only source, such as a music CD, and many HT owners also want their system to double as a stereo system.

Align the front speakers with the center speaker. Unless they’re very tall already, it’s also a good idea to raise them so they’re more or less at the same height as the center speaker. If there will be only one or two viewers, angle the front speakers slightly so they point towards the middle of the seating area. For a larger audience, position them so their front is parallel to the TV screen.

Left and Right Surround Speakers

Ordinary speakers spread the sound directly from the front, but surround speakers should instead spread the sound out so that viewers aren’t distracted by excessive directionality. As with the center speaker, it’s a good idea to get specially-designed surround speakers, but you can use whatever you have if you want to postpone the purchase.

The best place to put these is at the sides of the viewing position, facing inward (not towards the front wall or angled toward the TV). They should be above ear level, again to minimize directionality. You can mount them on the walls, if the room has side walls. Otherwise, use speaker stands.

Often you can’t place the surround speakers exactly where you want them because the room is oddly shaped or there’s furniture in the way. You can experiment with various alternatives, including mounting them on the rear wall (facing front), on a table pointing up, on the ceiling, and so on. They key is that the sound should seem to envelope you, rather than coming from a particular direction such as from the rear.

Note that if you have a 6.1 (“Surround EX”) system instead of 5.1, you’ll have one or better, two, additional surround speakers along the back wall. But this is only for very high-end surround systems.

Subwoofer

In a stereo music system, a subwoofer handles frequencies below what the main speakers can handle, typically under 80 Hz. In a 5.1 Dolby Digital system, however, the subwoofer has an additional purpose: to handle the low-frequency effects (LFE) channel, which is the “.1” part. This is a distinct channel, not the low frequencies of the other channels.

The process of redirecting the below-80 Hz. frequencies to the subwoofer is called bass management, and it’s discussed in more detail later in this chapter.

Because the human ear can’t locate low frequencies well, you can put the subwoofer almost anywhere it fits, although you should still experiment with some different locations to see which sounds best.

Unlike the other speakers which are connected to amplifiers within the a/v receiver, a powered subwoofer has its own internal amplifier and is therefore wired with a phono-type cable to a special jack on the a/v receiver, rather than with ordinary speaker wire. It has an electrical cord that has to be plugged in, too.

Get a powered subwoofer if at all possible. That’s what most Dolby Digital receivers expect, and so they don’t even have subwoofer speaker jacks to connect an unpowered subwoofer. If you have an unpowered subwoofer, what you do instead is run speaker wires from the left- and right-front speaker jacks on the receiver to the subwoofer speaker inputs, and then another set of wires from the subwoofer speaker outputs to the left- and right-front speakers. The subwoofer takes the low frequencies it wants and passes what’s left to the speakers. (This is called a crossover.) This will work, but it’s not nearly as good as what you can get with a powered subwoofer.

If you don’t have a subwoofer at all (maybe you’re postponing the purchase), you’ll indicate that to the a/v receiver when you set it up. That tells it to send the LFE channel sounds to left- and right-front speakers, and they’ll do the best they can with them.

Phantom Surround Speakers

Speaker Impedance

Bi-Wiring

Speaker Summary

Headphones

Audio/Video Receivers

What They Do

A/V Receiver Jacks

Bass Management

Important A/V Receiver Features

Connecting Components to the A/V Receiver

A/V Receiver Summary

What About Separates?

HT-in-a-Box

©2001, 2002 Marc J. Rochkind. All rights reserved. Send comments about this site to site@htexplained.com.