Mixing in Surround

A BIT OF HISTORY

Surround sound is almost universally acclaimed to be a more realistic and pleasing experience to the listener than stereo. This applies to just about any type of program, from music to motion pictures to television. People that can't tell the difference between mono and stereo can immediately hear and appreciate the difference between surround and stereo. It is a development so dramatic that it will change the way we listen, record, mix and enjoy music forever.

Surround sound in one form or another has actually been with us for more than 50 years. Film has always used the three channel "curtain of sound" (developed by Bell Labs in the early 30's) since it was discovered that a center channel provided the significant benefits of anchoring the center by eliminating "phantom" images (in stereo the center images shift as you move around the room) and better frequency response matching across the sound field. The addition of a rear effects channel to the front three channels dates back as far as 1941 with the "Fantasound" four channel system utilized by Disney for the film Fantasia and in the 1950's with Fox's Cinemascope, but it didn't come into widespread use until the 60's when Dolby Stereo became the surround de facto standard. This popular film format uses four channels (left, center, right and a mono surround, sometimes called LCRS) and is encoded onto two tracks. Almost all major shows and films currently produced for theatrical release and broadcast television are presented in Dolby Stereo since it has the added advantage of playing back properly in stereo or mono if no decoder is present.

With the advent in the 80's of digital delivery formats capable of supplying more channels, the number of surround channels was increased to two and the Low Frequency Effects channel was added to make up the six-channel 5.1 system, which soon became the modern standard for most films (the Sony SDDS 7.1 system being the exception), music and DTV.

And of course, there's Quad from the 70's, the music industry's attempt at multi-channel music that killed itself as a result of two non-compatible competing systems (a preview of the Beta vs. VHS war) and a poor psychoacoustic rendering that suffered from an extremely small sweet spot.

TYPES OF SURROUND SOUND

5.1 is the mostly widely used surround format today, being used in motion picture, music and digital television. The format consists of six discrete speaker sources: three across the front (left, center and right), two in the rear (left surround, right surround) plus a sub-woofer (known as the Low Frequency Effects channel or LFE), which is the ".1" of the 5.1 (see Figure 13). This is the same configuration that you hear in most movie theaters, since 5.1 is the speaker spec used not only by THX but also by popular motion picture release formats such as Dolby Digital and DTS.

Figure 13 A 5.1 Surround System

Surround System

Graphic courtesy of Dolby Labs

The LFE Channel

LFE stands for Low Frequency Effects and is sometimes referred to in film production circles as the "boom" channel because that's what it's there for: to enhance the low frequencies of a film so you get the extra boom out of an earthquake, plane crash, explosion or other such dramatic scene requiring lots of low frequencies.

The LFE channel, which has a frequency response from about 25Hz to 120Hz, is unique in that it has an additional 10dB of headroom built into it. This is needed to accommodate the extra power required to reproduce the low frequency content without distortion.

BASS MANAGEMENT

The Bass Manager (sometimes called Bass Redirection) is a circuit that takes all the frequencies below 80Hz from the main channels and the signal from the LFE channel and mixes them together into the subwoofer. This is done to make use of the subwoofer for more than the occasional low frequency effect, since it's in the system already. This enables the effective response of the system to be lowered to about 25Hz.

Since the overwhelming majority of consumer surround systems (especially the average low end ones) contain a bass management circuit, if you don't mix with one then you're not hearing things the way the people at home are. And, since the Bass Manager gives a low frequency extension below that of the vast majority of studio monitors, the people at home may actually be hearing things (like unwanted rumbles) that you can't hear while mixing.

OTHER TYPES OF SURROUND

There are many other widely used surround formats. Three-channel (stereo front speakers with a mono surround), four-channel (three front speakers with a mono surround) such as Dolby Prologic, five-channel (three front speakers with a stereo surround but no LFE channel), and eight-channel (the Sony SDDS format with five front speakers and an LFE channel) all abound.

There are other non-standard formats that use as many as ten channels for height and extra rear and side channels as well. The Star Wars prequel Episode 1The Phantom Menace introduces a Dolby Digital Surround EX 6.1 format in which a center rear channel is used. Many amusement rides such as Universal's Back to the Future ride have used as many as 14 channels.