Home Theater
Surround Sound Journey: From Soundtrack­ to Speakers
A look at the technology behind movie soundtracks and how the sound is delivered on surround systems.
soundtrack mixing
July 03, 2008 by Phil Lozen

I remember the first time I experienced stereo sound while watching a movie at home. My parents had just purchased a stereo from Sears. I wasn’t older than 13 but I distinctly remember hooking up our Hi-Fi VCR to the new stereo and marveling at the sound of the jets moving from one side of the room to the other in “Top Gun.”

We’ve come a long way since then.

Today’s home entertainment rooms aren’t complete without at least five speakers, and the move to seven speakers (plus a subwoofer) is in full force. But unlike that day in the late eighties, setup isn’t as simple as hooking a red and white composite cable from your VCR to your stereo, and soundtracks feature a lot more than just stereo or mono sound.

Today, the options are endless when it comes to home entertainment. Technologies from Dolby and DTS offer producers more choices than ever before, and the options for home theater setup are always expanding. This article will look at three of those options as they relate to speaker setup, and how the technologies behind the soundtracks ultimately determine what you hear when you settle in to watch a movie.

The Basics
In a 5.1 setup, five speakers are used; a center channel, two front surrounds and two rear surrounds, along with a subwoofer or low-frequency effect (LFE) channel. A 6.1 setup adds a rear center channel, while 7.1 eliminates the rear center channel but adds two side surround speakers.

When it comes to content, virtually all DVDs created today are mixed in at least 5.1. HD DVD and Blu-ray opened the door for 7.1 content, and to a much lesser extent there are 6.1 mixes floating around.

Every soundtrack is created using one of two main technologies: Dolby and DTS. Both Dolby and DTS feature multiple profiles, such as Dolby Digital Plus and DTS-HD Master Audio. There’s also THX, which is not a format but a certification process. Soundtracks and equipment can each be THX Certified, meaning they have passed a rigid set of tests required to earn the THX badge.

When a recording is being produced, a decision has to be made about what type of soundtrack to include. As the soundtracks get more full-featured, the amount of data on the audio portion of a disc grows. Directors are often forced to walk a delicate line to get the highest quality audio and video they can on a disc, at times sacrificing one for the other.

Bit-rate & Compression
Bit-rate and compression determine the final size of a soundtrack. Bit-rate is the amount of data passed per second during playback, the higher the bit-rate, the more data there is. The highest quality next generation audio formats, Dolby TrueHD and DTS-HD Master Audio, support up to 18Mbps and 24.5 Mbps, respectively. In the profiles the next level down, Dolby Digital Plus and DTS-HD High Resolution Audio, the bit-rate drops to 6Mbps. By comparison, the bit-rate for the standard 5.1 surround sound formats for both Dolby and DTS is no more than 1.5Mbs.

Compression also factors in to the size and quality of a soundtrack. A final mix that replicates the studio master bit for bit is called a lossless recording. Currently only Dolby TrueHD and DTS-HD Master Audio offer this. Most soundtracks have some amount of compression, resulting in a lossy recording. Techniques vary, but some methods compress the audio stream to as little as five percent its original size.

When compressing a soundtrack, the author is actually removing bits of data from the audio track. Soundtrack authors determine the pieces that either cannot be heard or clearly perceived by the human ear or sounds that occur at the same time as others and therefore are not distinguishable. The first pass of compression usually involves removing these pieces of data. As the sound is more compressed and the bit-depth more reduced, more noise is introduced into the recording. If you’ve ever heard a bad MP3 file, that’s the result of too much compression introducing too much noise. Techniques do exist to help hide the noise that’s created when reducing bit-depth, commonly known as noise shaping.

Some people will argue that any compression results in a loss of fidelity, while others say controlled amounts, done the right way, can be indistinguishable to the human ear. Compression is a necessary evil, however, when it comes to creating home entertainment soundtracks.

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