GOODSOUND!GoodSound! "How To" Archives

Published February 15, 2003

 

Playing Music in Surround: Part One

In the 1970s, quadraphonic recordings made their public debut in a variety of formats. These included Columbia’s SQ, Sansui’s QS, and CD-4 Quadradisc. A lack of interoperability among these different surround standards was one of the reasons early quad never managed to catch on.

Nowadays, there’s a similarly diverse set of standards for surround-encoded music on the market: Dolby Digital, DTS (Digital Theater Systems), SACD (Super Audio CD), and DVD-A (DVD-Audio). But unlike in the earlier surround format wars, the computing horsepower available in modern equipment means that many pieces of playback equipment are capable of playing back two or more of these formats, with some units handling all four.

Analog versus digital audio

There are three main ways audio equipment connects to a surround system. Presumably you’re familiar with using speaker cable to connect your speakers. Usually components like a CD player or tape deck are plugged in with an interconnecting cable terminated with what are called RCA jacks. Both the speaker signal and the line-level interconnects carry an analog signal that varies continuously up and down, based on the recording.

In contrast, the content on a CD or DVD is actually encoded digitally using a series of 0s and 1s that represent the audio signal sampled at a high rate of speed (at least 44,100 times per second). Many CD players and all DVD players include a digital output that lets this information transmit directly to a receiver, a surround processor, or a similar device with a digital input. One advantage of this: The sound quality doesn’t degrade easily when a digital signal is moved from one component to another. The other advantage: These digital signals are capable of holding surround sound instead of just stereo music.

In order to decode most surround formats, a digital cable runs between the CD or DVD player and the receiver/surround decoder. Eventually these signals switch back to analog again before they reach the speakers. (Only a handful of speakers have facilities for receiving a digital input.) Some DVD players include the capability to decode the surround sound and convert it back into analog from inside their chassis. In this case, they deliver the signal via a series of six analog outputs. One thing to watch for is that most receivers only have a single six-channel analog input, so you could run into problems trying to connect multiple DVD players in this fashion. This generally comes up only if you’re trying to use, say, a dedicated player for SACD and another for DVD-A.

At their core, almost all of the digital sound-encoding schemes on the market, from the original one used on the CD to the latest DVD-A titles, are fundamentally based on technology called Pulse Code Modulation (PCM). PCM schemes are defined by two main parameters. The sampling rate is how many times per second the audio is digitized. Typical sampling rates are 44,100 times per second (or 44.1kHz as it’s abbreviated), 48kHz, 96kHz, or 192kHz. This determines the maximum frequency the system is capable of handling, which is one half the sampling rate, so the CD-quality 44.1kHz gives a maximum frequency of 22.05kHz. The bit depth of a PCM recording determines how many distinct volume levels are capable of being distinguished, and that determines the dynamic range of the medium. CD quality consists of 16 bits, which translates into 65,536 levels capable of handling 96dB of dynamics.

Dolby Digital

Although Dolby Digital first appeared in theaters in 1992 with the release of Batman Returns, most consumers initially encountered it with the introduction of DVD players to the market in late 1997. Most DVD surround-sound content is available in Dolby Digital. Note that Dolby Digital doesn’t necessarily imply surround: The format is flexible enough that it can even handle a mono soundtrack.

Using an encoding scheme that Dolby refers to as AC-3, Dolby Digital normally includes what’s referred to as 5.1-channel surround: five main speakers (left, center, right, left surround, right surround) and the ".1" channel, which has content specifically designed for a subwoofer. In most cases, a digital cable transmits this 5.1-channel signal from the DVD player to a surround receiver. Many DVD player models include a Dolby Digital decoder inside of them, usually with six separate analog outputs for use in this mode. Normally, the Dolby Digital decoder inside a receiver has better capabilities for surround decoding than the ones built into a player. Common extra features are improved subwoofer support (or "bass management" as it’s often called) and more programmable delay times for rear speakers.

More recent Dolby Digital titles are actually encoded with 6.1 channels, adding a back surround speaker(s) directly behind the listening position. If you’re trying to transmit that over analog cables, it takes seven of them to handle this newer format.

The main advantage of Dolby Digital is its ubiquity: All DVD players are required to support it, every modern surround receiver supports it, and you rarely encounter any issues playing the titles back. The only real disadvantage of the format is that Dolby Digital usually has lower sound quality than the others listed here because it compresses the audio into a smaller amount of space.

DTS

In 1993, theaters across the country first shook to the sound of the Tyrannosaurus rex in Jurassic Park, brought to you via DTS surround. DTS first appeared in the home by a somewhat odd path. The DTS Entertainment Company decided to release DTS-encoded music in surround on standard compact discs. Your CD player would read this data, treat it as standard music, and then output it over its digital output. The surround decoder at the other end would recognize that it was actually a compressed DTS signal rather than a regular stereo one, and then switch to a surround mode to play it back.

DTS CDs are still on the market today, normally packaged in regular CD cases but with warning notes that they will only play in a DTS system. Recently some of these (like the wonderful Alan Parsons surround version of On Air [HDS 710215-4414-2-9]) have been re-released in the larger case size normally used for DVD-Audio. Make sure to check the labels to confirm you’re getting what you expect. Note that the original DTS CDs were usually somewhat forced surround remixes, putting lots of effects in the rear channels. Try to listen before you buy so that you know you’ll like the mix.

When the first DVD players shipped, they couldn’t handle a DVD with DTS sound on it. All current DVD players from any of the major vendors include support for DTS DVDs, but to be sure you can check for the DTS logo on the player or in its manual.

DTS normally dedicates more space on the DVD to sound than Dolby Digital recordings do, and many believe DTS sounds better partly because of this. Some DVD titles, both movies and music, ship with both types of surround sound on the disc. A menu function on the DVD normally lets you swap between the two soundtracks. A good example is the DVD Peter Frampton: Live In Detroit [Image D88161GDVD], which sounds great both ways. The video is pretty good too.

Next month we’ll look at the two newest emerging formats for surround sound: DVD-Audio and SACD.


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