• Featured Article: Going to 7.1-Channel Surround Sound
• AV Tip of the Month: How Much Audible Noise/Hum Is Too Much?
• Question of the Month: What is HD Radio?

The Archives

The winner of our Algonquin Outdoor Speaker draw has been picked! Click here to find out who it is.

Going to 7.1-Channel Surround Sound
by Alan Lofft

Almost everyone loves surround sound, (except for an eccentric actor whom I know who actually prefers mono!) and what's not to love? Our ears are on each side of our head, canted forward, but we hear sounds from every direction with almost equal precision. For that reason multi-channel sound reproduction is intrinsically more realistic and believable. But how many surround speakers are required to present a plausible illusion of the real thing?

While Dolby Digital 5.1 (six channels total, including the subwoofer and two discrete surround channels) is the current standard for the digital audio component of digital TV and HDTV and has been the reigning surround format for more than ten years, there are increasing numbers of movies available that utilize the "extra" two back surround channels that define 7.1-channel surround sound.

For newcomers, "Dolby Digital 5.1" consists of three main front channels (Left, Center and Right), the center used to anchor movie dialog and/or vocalists; two surround channels (Left Side, Right Side) and one dedicated Low Frequency Effects (LFE) channel, more commonly known as the subwoofer channel, intended to handle deep bass effects below 100 Hz, hence the ".1" designation of 5.1.

So how is it there are now two additional channels (Left Rear, Right Rear) offered on virtually every current surround-sound receiver?

The short answer is realism: dating back to early experiments in the 1930s by Bell Lab engineers in New Jersey, the more channels of discrete separate channels of sound used to record and reproduce music and sound effects, the greater the realism of the auditory experience. All Bell Labs could manage back then was three channels at the front, but even so engineers and listeners considered three channels "essential" to convey the realism of a live broadcast of the Philadelphia Orchestra piped via three telephone lines to an auditorium a few miles away.

Leap forward 80 years, and digital recording technology makes it comparatively easy to add extra surround channels to the existing Dolby Digital 5.1-channel broadcast standard. The introduction of Blu-ray High-Definition disc technology with its huge storage capacity enables discrete channels of sound moving to 7.1 and even greater numbers. Tomlinson Holman, one of the early pioneers with George Lucas of THX sound systems in cinemas demonstrated a 10.2-channel system that I heard a number of years ago at the Consumer Electronics Show in Las Vegas, and he continues efforts to popularize that system. Even Dolby Lab's latest "lossy" algorithm, Dolby Digital Plus, enables up to 13 discrete channels of sound. The more popular "lossless" Dolby TruHD and dts HD Master Audio systems contained on many Blu-ray discs permit 8 channels of sound with no sonic compromises.

All About Immersion

What are the benefits of adding the two extra rear surround channels to achieve 7.1, and what circumstances would justify the upgrade? The benefits are greater surround envelopment, depth, and "immersion" in the surround effects field, both for movie effects and for musical ambience, plus much improved surround coverage for various listeners seated throughout the room.

One requirement for 7.1-channel surround is sufficient space behind your couch and listening area -- at least 5 feet or more. If your couch is jammed up against the rear wall or you have a comparatively small room of less than about 2,100 cu. ft. (length x width x height), the addition of two rear surround speakers will not likely add significant improvements in envelopment, especially if you are already using Axiom's QS8 or QS4 "multi-polar" surround speakers. However, if you have the space behind the listening area to play with, and/or your room is medium to large sized (especially so-called great rooms and family rooms), four surround speakers will deliver noticeably improved surround envelopment and coverage for all viewers in different seating locations.

While the great bulk of movie soundtracks (including all SACD and DVD-Audio music discs) are mixed in 5.1 channels, there are increasing numbers of movies (the Harry Potter movies and other movie "spectaculars") mixed in 6.1 and 7.1 channels, including a surprising number of games. But the real justification for upgrading to 7.1 is the sophistication of smart decoding algorithms from Dolby Labs, dts, and Logic7 (the latter exclusive to Harman-owned companies).

Among its many virtues, Dolby Pro Logic IIx (DPLIIx) as well as earlier versions has the ability to extract or interpolate extra surround data from a standard 5.1-channel mix or even from stereo sources, and re-direct that information to both side and rear surround channels, all of which will increase the sense of depth and precision of the surround experience. (Don't confuse this process with the "seven-channel stereo" option offered on many AV surround receivers. While pleasant, it only parcels out two-channel stereo among all the speakers in a system.)

Reasons to Believe

No matter how you look at it, there are persuasive reasons to consider upgrading to 7.1 channels with its four surround speakers. Incidentally, there is nothing fake or phony about electronically extracting surround data from existing 5.1 or even 2-channel stereo soundtracks. Just two microphones in a conventional stereo purist recording will pick up all kinds of out-of-phase information, which commonly characterizes ambient surround data, so re-directing those sounds to the sides and rear is really just placing them where they would occur in a dedicated elaborate multi-microphone recording array.

On the road to 7.1 channels, both Dolby Labs and dts previously introduced extended surround formats that use 6.1 channels (a single rear center speaker is utilized besides the left side and right side surrounds), namely Dolby Surround EX 6.1 and DTS-ES 6.1, respectively. The presence of these formats on some DVDs needn't confuse you (you may even have a 6.1-channel setup already) because both Dolby and DTS processors (and Logic7) will redistribute the surround information to a 7.1-channel setup so long as your AV receiver contains the necessary seven amplifier channels.

Type of Surround Speaker

Finally, the still-debated (except by owners of Axiom multi-polar QS8 surrounds) subject of what type of surround speaker is required for the extra Left and Right rear surround channels continues to surface, in part because of some control-room monitoring setups for SACD and DVD-Audio recordings, where conventional forward-firing surround speakers were used in the back channels. While there are still some who recommend forward-firing speakers for the back surrounds, I'd point out that the original development of dipole, bipole and multi-polar surrounds was to increase diffusion and envelopment in the surround field to simulate the presence of 8 or 10 surround speakers in a large cinema. As such, the Axiom QS8 (or QS4), with its top and bottom-firing woofers and angled dual tweeters comes as close to the ideal for both ambient sound field generation without giving up the specific directional cues sometimes found in movie soundtracks and multi-channel music recordings.

How Much Audible Noise/Hum Is Too Much?

Audible noise in our music reproduction systems is always annoying. While modern solid-state design has evolved to reduce noise to theoretically inaudible levels, issues about noise and hum continue to plague most audiophile enthusiasts at one time or another.

Much has been written about ground-loop hum in this space and on the Axiom forums, and it’s this low-frequency noise that is by far the most common, almost always caused by cable-TV feeds into an A/V system. Usually a combination of trouble-shooting and/or installation of an in-line ground-isolation filter like this (http://www.axiomaudio.com/groundisolator.html) on the incoming cable-TV line will eliminate the problem.

One regular Axiom forum member, Mark J, recently raised the subject, and stated:  “I always thought it wasn't at all unusual to have some residual noise when listening up close to a speaker and having the volume up to listening levels?”

It’s true that it isn’t unusual but a more detailed explanation is in order. It’s important to distinguish between “hiss”—the higher-frequency rushing sound—and hum, which is low frequency in nature, typically around 60 Hz or 120 Hz.

Hiss is often a product of  “thermal circuit noise.” What is it? It’s residual electronic noise generated in any electrical component or conductor (resistors, transistors, etc) caused by the random thermal motion of the electrons. While we describe it as “hiss”, it’s technically “white” noise, which is equal energy per frequency across the spectrum. Since there are more discrete frequencies in the treble region (e.g. 10,000 Hz) compared to bass frequencies (50 Hz), white noise generates more hissy treble energy as the frequency increases.

The goal of careful electronic design is to keep thermal circuit noise at residual levels, below audibility at loud playback levels, but depending on parts and device quality and selection, as well as cost and design limitations, some audible hiss may intrude, especially in the gain stages of a preamplifier. Because the phono input has an extra stage of gain for moving-magnet cartridges (and an additional gain stage for moving-coil cartridges), you may hear some audible hiss on the phono input in inexpensive AV receivers, although at loud listening levels you really shouldn’t. On very high-quality and more expensive preamps, the phono stages may be as dead quiet as the low-level (CD, etc.) input stages.

Early solid-state gear from the 1970s era also tends to be inherently noisier and it’s not uncommon to hear some background hiss even from your listening seat. But well-designed modern AV receivers and amplifiers should not have any hiss or hum audible from your listening location. With your ear close to the speakers and the volume control well advanced you might hear a tiny bit of thermal hiss with some units but you shouldn't have any hum if all the grounding is correct.

Question of the Month: What is HD Radio?

Q. I’m familiar with HDTV and satellite radio, but I keep hearing about HD Radio. What is it? – G.S.

A. HD Radio has nothing to do with “High Definition” or HDTV or satellite radio from XM or Sirius. HD Radio is a proprietary hybrid analog-digital radio system developed by iBiquity Corporation in which a low-level digital signal with the same content as the analog FM and AM signal is piggybacked and simulcast with a broadcaster’s analog radio signal, a system known as IBOC (In-Band On-Channel). This enables the millions of analog radio owners who have not purchased a new HD-ready radio to continue to receive the conventional FM or AM signals. However, owners of a new HD-ready radio will receive the much higher sound quality digital radio signal when the radio detects the low-level HD signal that is carried in the sidebands of the analog signal. The HD radio will switch instantly to the digital signal for FM or AM. If the digital signal fades, the HD radio then automatically (and seamlessly, according to iBiquity) reverts to the station’s analog FM or AM signals.

Unlike subscription-based XM/Sirius satellite radio, HD radio is free and is a terrestrial radio service selected and approved by the FCC as the terrestrial digital radio system for the USA. Launched in 2002, it’s now available on nearly 2,000 US stations (Canada has approved testing and installation of HD Radio but only a few stations in big urban centers have converted.) Public acceptance in the US has been very slow with many consumers confused by the “HD” name and existing subscription satellite radio services.

A “lossy” compression algorithm similar to but not compatible with MPEG-4 developed by iBiquity is used, and sound quality on FM is claimed to approach that of CD. AM radio quality is much improved, but reports of adjacent channel interference (other radio stations close in frequency to the HD radio broadcaster) have surfaced as well as complaints of poor HD radio sensitivity (the digital signal is broadcast at only 1% of the power of the broadcaster’s analog signal). Nevertheless, prices of HD radios have come down and various car companies (Ford and others) are also offering HD-ready radios as an option or standard accessory. In addition to much-improved sound quality and resistance to interference, HD radio has the capability to carry a text data-stream with the artist or album names, or subsidiary radio services, even including 5.1-channel surround sound. The FCC in the US has not indicated that it intends to end analog radio services as it has with TV broadcasting, as there is no potential benefit for the government in selling recovered spectrum space. Rest assured, then, that your existing home, car or portable radios will continue to receive good ol’ analog FM and AM radio for many years to come. – A.L.