Surrounded by Sound

When the world of audio reproduction went from monaural to stereo, a new sonic vista was revealed. Individual sound sources (instruments, voices, and
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FIG. 1: In a 5.1 surround-sound system, the front left and right speakers form a 60° angle with the listener at the apex, and the center-channel speaker is directly in front of the listener. Direct-radiating surround speakers are often placed at an angle between 100° and 120° from the front-center line.

When the world of audio reproduction went from monaural to stereo, a new sonic vista was revealed. Individual sound sources (instruments, voices, and so on) could be localized within the left-to-right stereo sound field, presenting a much more interesting and involving sonic image to the listener.

In an attempt to improve the front-to-back imaging and general sense of envelopment, quadraphonic recording and playback was introduced in the 1970s. In this system, four channels of audio were directed to four speakers, which were generally placed in the corners of a room. However, quad never achieved widespread popularity for various technical and marketing reasons.

Despite quad's failure, the merits of multichannel audio (often called surround sound) are quite clear. This is demonstrated every day in commercial cinemas around the world. Modern movie soundtracks are routinely mixed to several channels, and speakers are placed around the theater. The sources of the apparent various sounds in the movie are appropriate to what's happening on the screen, heightening the audience's sense of being "in the action."

Now surround-sound playback systems are proliferating in "home theaters," and multichannel music-only recordings are becoming more common as well. So it's important for musicians, engineers, and producers to be familiar with the concepts and technology of surround sound.

The most common type of surround-sound system uses six discrete channels of audio: five full-range channels and one channel that is limited to the low frequencies (typically below 120 Hz or so). As a result, the system is called 5.1.

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The speakers that correspond to the five main channels are placed around the room (see Fig. 1). Two are placed just like a stereo pair, in front and to the left and right of the listener. The third speaker, called the center-channel speaker, is directly in front of the listener. The two remaining speakers are behind or to the sides of the listener; these are called the surround speakers.

The three front speakers are generally standard, direct-radiating designs. The surround speakers can also be direct radiators, but they are often of a different type called bipolar. In bipolar design, two sets of drivers are mounted into a cabinet, one set facing toward the front of the room and one set facing toward the rear. This means that such speakers should be placed to the sides of the listener. Most commonly, these drivers are angled into the room slightly. Bipolar speakers produce a greater sense of envelopment than direct radiators, and they make it more difficult to pinpoint the location of individual sound sources, which is desirable in most movie soundtracks.

The remaining channel is often called the low-frequency effects (LFE) channel, and it requires a specialized type of speaker called a subwoofer. Because low frequencies are relatively nondirectional, the subwoofer can be placed almost anywhere in the room; typically, it is in the front between the center and either the left or right speaker. Sometimes it is placed in a corner in order to reinforce the low frequencies.

Of course, each speaker needs a channel of amplification. Most modern receivers include five or six amplifier channels. (Many subwoofers have their own internal amp, so they don't need external amplification.) More sophisticated playback systems include separate power amplifiers. A lot of home-theater amps provide five or six channels; alternatively, you could use three stereo amps or five or six monaural amps (often called monoblocks). Of course, you can also use powered speakers all around.

The next step back along the signal path is the surround processor. This device takes a digital signal from the source (for example, a CD or DVD player), decodes it into discrete, analog channels, and sends them to the amplifiers. (Receivers have built-in surround processors in addition to amplifiers.)

This brings us to the source, which is typically a CD or DVD. (Other digital sources, such as laserdiscs and digital satellite feeds, can also carry 5.1 surround-sound signals.) How are 5.1 channels encoded on these media? And what surround-sound standards have been established to encourage the widespread acceptance of multichannel sound?

Over the past few years, two standards have been vying for dominance in the surround-sound market. By far the most successful is Dolby Digital, which has been adopted as the standard surround-sound format for DVD-Video discs and the new digital-television (DTV) system.

Uncompressed audio at 44.1 kHz with 16-bit resolution requires a bandwidth of about 700 kilobits per second (kbps) for each channel, so six channels need a total of 4.2 Mbps. In order to conserve bandwidth, Dolby Digital uses a lossy data-compression and encoding algorithm called AC-3 to reduce the required bit rate to roughly 400 kbps. Because it uses this algorithm, Dolby Digital is sometimes mistakenly called AC-3.

The other contender is called DTS, from Digital Theater Systems. Derived from the company's commercial-cinema surround-sound system of the same name, this format has been slow to reach the market. DTS also uses lossy data compression to bring its bandwidth requirements down to about 1.4 Mbps, which is less severe compression than in Dolby Digital, but still substantial. Whether this decreased compression improves the sound quality is hotly debated within the home-theater industry.

Both formats require a player that can output the appropriate data stream from the disc and a processor that can decode it. Virtually all DVD players can output Dolby Digital, and most can now output DTS as well. Similarly, all modern receivers and surround processors can decode Dolby Digital, and most can handle DTS.

In addition to Dolby Digital and DTS, there are a couple of other formats you should be familiar with. A new DVD format called DVD-Audio will soon be available (see "Tech Page: CD? No, DVD!" in the July 1998 issue of EM). Unlike DVD-Video, the new format is designed primarily to store audio-only data on standard DVD discs (although it can include a limited amount of graphics, such as album covers, lyrics, and artist bio information).

Because a DVD disc has such a large storage capacity (4.7 GB and up), uncompressed, multichannel audio is easily accommodated. As a result, DVD-Audio allows multiple channels of audio at several bit resolutions and sampling rates from 16/44.1 to 24/192 (although 24/192 is limited to two channels).

In addition, different rates and resolutions can be used on different channels within a single mix. Even so, it is likely that lossless compression will be used to reduce bandwidth requirements by a factor of two. In fact, a scheme called Meridian Lossless Packing (MLP) has recently been adopted as the standard compression for DVD-Audio. DVD-Audio players should become available by the end of the year.

Another new type of audio disc is the Super Audio CD (SACD) from Sony and Philips (see "Tech Page: CD-The Next Generation" in the March 1998 issue of EM). This type of CD stores multichannel audio using a very high sampling rate (2.8 MHz) and 1-bit resolution, as well as a proprietary encoding algorithm called Direct Stream Digital (DSD). In addition, an SACD can include two layers: one with multichannel DSD data and one with standard stereo CD data, which means that such hybrid discs are fully compatible with current CD players. However, Sony's first commercial SACD player is stereo only.

WHY 5.1?
Most engineers, producers, composers, and musicians have been working in stereo for their entire careers, but they are becoming 5.1 converts in droves. Why? Because of the effect it has on the listener. The sense of realism, the apparent acoustic space, the impression of "being there" can be overwhelming to ears accustomed to stereo. Two well-known mixing engineers actually claim they've been driven to tears of joy while mixing in 5.1 because of the format's impact.

Others who are a little more restrained simply point out that humans are able to localize sounds that originate from any direction, and 5.1 is much more capable of stimulating this ability than stereo. Musicians and composers are particularly attracted to 5.1 because of the artistic possibilities it offers.

However, there is some disagreement about how 5.1 should be used. Most of these arguments revolve around aesthetic decisions that the producer, engineer, or composer must make. Some want to use the extra speakers to experiment with radical spatial placement of sounds. Others abhor this approach, and prefer to use 5.1 to heighten the listener's sense of being in an acoustic space. Debates of a more technical nature, such as which elements of the mix should be routed to the LFE channel, also abound.

It should be noted that 5.1 has its own technical limitations. This is not the ultimate be-all and end-all of channel formats. For example, experts agree that the surround-sound effect can be greatly enhanced by adding even more channels and speakers. Dolby is experimenting with a 6.1 format for commercial cinemas called Dolby Digital EX, in which a center speaker is placed in the rear. In addition, several surround processors synthesize 7.1 channels of sound from 5.1 material. Systems with 12 or more channels are being tested for use in theaters and other specialty locations. However, the advantages of adding more channels must be weighed against the immense difficulty of transmitting, storing, and reproducing an increased amount of audio data.

There are several pieces of gear that are unique to 5.1 mixing, but owners of modern project studios probably already have some of the items that are required to get started in 5.1.

First of all, you need to have enough speakers and amplification to reproduce five full-bandwidth channels and one low-frequency channel. Several companies, including Event, Genelec, Tannoy, Dynaudio, JBL, and Bag End, offer speaker packages specifically for 5.1 studio monitoring. Many of these packages include five matched speakers for the main channels and a subwoofer for the LFE channel. However, some engineers prefer to use smaller speakers for the surround channels, imitating the configuration common to many households.

Next, you need a way to pan sounds among 5.1 channels. Many newer mixing boards targeted toward the project-studio market are capable of mixing in 5.1. The Yamaha 02R and 03D, Mackie D8B, Tascam TMD-8000, Panasonic DA7, and the Roland VM series of digital mixers all provide this capability. It is possible to create 5.1 mixes with boards that do not include surround-panning features, but it is quite difficult, and the impressive cost/performance ratio of the new mixers makes them very attractive.

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FIG. 2: Minnetonka Software''s Mx51 is a PC-based DAW with the ability to do 5.1 panning.

Several digital audio workstations now include 5.1 panning in their core software or allow the use of plug-ins that perform this function. Minnetonka Software recently introduced Mx51, a PC-based DAW with the ability to do 5.1 panning with the Digital Audio Labs V8 card or the Yamaha DSP Factory card as its audio hardware (see Fig. 2). Sonic Solutions' new Sonic Studio HDSP workstation includes surround panning as a standard feature.

Pro Tools owners can use the SmartPan Pro plug-in by Kind of Loud Technologies to perform 5.1 or even 7.1 panning on Pro Tools 24/Mix or MixPlus systems. SonicEngineering has introduced the Panhandler plug-in for DirectX hosts, Innovative Quality Software's SAW, Adobe Premiere, and Syntrillium's Cool Edit Pro. The 5.3 version of SEK'D's Samplitude 2496 includes extensive surround mixing, and Steinberg's forthcoming Nuendo audio production software will also offer surround support. Rest assured that many more 5.1-capable DAWs, plug-ins, and mixers will appear in the near future.

What sort of device should you use to record your 5.1 mix? Any machine that lets you record six full-bandwidth audio channels will do. At present, the most commonly chosen option is multitrack tape. The nearly ubiquitous DA-88 and ADAT formats are excellent choices because of their popularity and their ability to record a 5.1 and stereo version of the same mix on the same tape. Mixing to a DAW is a great choice, especially if it lets you achieve better-than-CD audio quality, such as 24/96.

Depending on the delivery format you are mixing for, you might need to encode your 5.1 mix using a data-compression method such as Dolby Digital. This requires an appropriate encoder. Hardware encoders are expensive and sometimes available only by leasing, so the best option is usually a software encoder. Sonic Foundry offers SoftEncode, a PC-based application that allows six audio files to be encoded into 5.1 Dolby Digital. Astarte has announced the imminent release of A.Pack, which performs the same feat on the Mac and PC. Sonic Solutions offers a Dolby Digital encoder for its Mac-based workstations that provides real-time encoding.

The new 5.1-channel format provides audiences with a greatly enhanced listening experience, and it provides artists and engineers with an expanded palette of creative possibilities. In addition, it produces these impressive results while remaining within the limits of the existing and emerging technologies that are (or soon will be) in the hands of the public. Enthusiastic acceptance by audiences and leading audio professionals indicates that 5.1's future is fairly well assured.

Of course, the 5.1 format does have its limitations, and debates about its "proper" use will simmer for quite some time. It is important to remember that this is typical for a new format. Nevertheless, 5.1 is an amazing advance beyond stereo, and the equipment to produce it and play it back is commonly available and relatively affordable. All of these ingredients add up to one conclusion: It is high time to take a closer look at 5.1.

Vance Galloway is a San Francisco-based experimental musician and new-media consultant. Scott Wilkinson is a contributing editor for EM and a journalist in the home-theater industry.