December 2001

Update on 6.1 Surround Sound

Dolby enters the 6.1 home-theater fray

Last Month I reported that Dolby Labs did not directly license their proprietary 6.1 matrix processing for home-theater products, and that the license for this processing on consumer products is available from THX (the co-developer of this technology) as THX Surround EX, with Dolby providing the licensing for professional-theater products. Well, just after we published that column, Dolby announced that they would begin licensing Dolby Digital EX for consumer home-theater products beginning with the Outlaw 950 processor, which will become the first product available with this designation. By doing so, Dolby is making the "official" and licensed 6.1 matrixed-surround version of Dolby Digital available to a much wider range of products other than just those with THX Surround EX certification.

For consumers there are some important distinctions to be aware of. Dolby Digital EX specifications are less stringent than those of THX Surround EX. For instance, Dolby Digital EX does not require Timbre Matching, Decorrelation, or Re-EQ for the surround channels. Additionally, the bass management requirements are less complicated with simpler crossovers needed. THX Surround EX processors and receivers also have to meet other specifications to receive THX certification, which invariably puts these products in the mid- to upper-price ranges. One advantage that Dolby Digital EX may have is that it uses Dolby Pro Logic II processing to extract the matrixed-surround information, which may sound better than standard Pro Logic or other generic matrix processing methods.

One more thing, where do you place the extra speaker(s)?

THX and Dolby both recommend placing the left and right surround speakers to the sides of the listening position (not behind) and also recommend the use of two center-surround speakers mounted in the rear. This is presumably to provide a more diffuse rear soundfield rather than having the sound come from a single point directly behind the listener.

Ideally, all speakers used should be of the same design and placed the same height and distance from the listener. In reality, many of these conditions will not be met and it may be especially difficult to place the center-surround speaker(s) a sufficient distance from the listening position since many people have their sofas or seats placed right up against or just slightly away from the rear wall. However, all Dolby Digital EX and THX Surround EX processors and receivers will allow for the setting of delays for the surround channels to compensate for differing distances to the speakers. Probably the worst situation, and something to try to avoid, is having only one center-surround speaker placed very close to the listening position.

Although THX still recommends the use of certified dipole speakers for all of the surround channels, their specifications do allow for the use of two direct-radiating speakers for the center-surround channels. On the other hand, Dolby does not recommend any particular type of design for the surround speakers and simply states that they should be of the same brand and of similar design.

Oh wait, I forgot to mention THX Ultra 2

THX has recently released a set of specifications for their new Ultra 2 certification so that their certification now comes in three levels: Select, Ultra, and Ultra 2. This latest version of THX includes two types of 7.1 surround processing called THX Ultra 2 Cinema Mode and THX Ultra 2 Music Mode. These 7.1 surround modes can be used on any 5.1-channel program material to apply "proprietary processing that blends the directional and ambient surround information prior to replay through four surround speakers." Basically this is what all of the current matrix 6.1 and 7.1 surround processes do, but I presume that this is THX’s own proprietary implementation of it. THX also seems to be touting Ultra 2 as a one-stop solution to the confusion stemming from all the competing 6.1 and 7.1 formats.

So conceivably, you could now end up having a 6.1 or 7.1 surround processor or receiver with THX Ultra 2 modes as well as THX Surround EX, Dolby Digital EX, and DTS-ES (both Discrete and Matrix), as well as any other generic 6.1 or 7.1 processing that the manufacturer sees fit to include, not to mention Dolby Pro Logic II and DTS Neo:6.

Confused yet?

If you are feeling a bit overwhelmed by all of these surround formats, you are not alone. I remember speaking to a well-respected loudspeaker designer and he admitted to me that the audio industry often shoots itself in the foot with these types of competing, confusing, and sometimes incompatible formats. At the time he was only referring to Dolby Pro Logic, Dolby Digital, and DTS! Unfortunately, the situation may only get worse before it gets any better with the proliferation of audio-only formats such as multichannel SACD and DVD-Audio to add to the confusion.

...Roger Kanno
roger@hometheatersound.com

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