Ever since Thomas Edision invented the phonograph in 1877, the quest has been on to make the reproduction of sound as real as sound heard in its original environment. Today's surround sound technologies are just a continuation of this quest.
Dolby Pro Logic IIz: Surround Sound Goes Vertical
Dolby Pro Logic IIz processing is an enhancement being implemented in some home theater receivers that extends surround sound vertically and fills the space above and in-front of the listener. Dolby Prologic IIz offers the option of adding two more front speakers that are placed above the left and right main speakers. This feature adds a "vertical" or overhead component to the surround sound field (great for rain, helicopter, plane flyover effects). Dolby Prologic IIz can be added to either a 5.1/5.2 channel or 7.1/7.2 channel setup. It is also compatible with two-channel and surround sound sources, including, if applied correctly, Dolby TrueHD and DTS-HD Master Audio.
When added to a 7.1 or 7.2 channel setup, you end up with both surround back and front height speakers - However, you would then need amplification for all 9 channels. Since some home theater receiver only provide amplification options for 7.1/7.2 channels, you must forgo the surround back channel option when using the Pro Logic IIz feature when using a 7.1/7.2 channel home theater receiver. This means that you are actually using a 5.1/5.2 channel setup and adding the Dolby Pro Logic IIz height channels to obtain a 7.1/7.2 channel setup.
In order to use Dolby Pro Logic IIz to is maximum effect, the front height speakers should be mounted approximately 3ft directly above the front left and right main speakers. In addition, to retain the character of the original surround sound mix, the speaker level settings for the height channels should be set slightly lower than that of the main left and right front speakers. However, you tailor the speaker levels to your preference.
The Motivation Behind Dolby Pro Logic IIz
The motivation that guided the development of Dolby Pro Logic IIz is observation that humans hear more from the front and sides than from the rear.
In other words, in the effort to create an optimum surround sound listening experience, it is more advantageous to emphasize sound coming from the front, sides, and above the listener than it is to add more emphasis from sounds originating from the back of the listener.
In the case of current surround sound technology, the observation is that the traditional 5.1 channel surround schemes now commonly employed provide enough rear audio information for the listener, and adding one or two more surround back channels, as is promoted with current 7.1 channel home theater receivers, really doesn't give the listener that much more of a surround sound experience. In addition, in smaller room environments, adding one or two surround back channels is physically impractical.
For more details about the implementation of Dolby Pro Logic IIz, check out the official Dolby Prologic IIz page.
Related Technologies to Dolby Pro Logic IIz
Although the familiar Dolby brand name garners attention to Dolby Pro Logic IIz amongst consumers, there are similar technologies from other companies that are also available.
Yamaha offers a similar technology on some of its home theater receivers called Presence. This surround sound scheme also employs the addition of two front height speakers to add a full surround experience from the front and above the listening position.
Also, Audyssey, a company well-known for its speaker setup, room correction, and sound processing software, offers DSX (Dynamic Surround Expansion). like Dolby Pro Logic IIz and Yamaha Presence, DSX adds front vertical-height speakers, but, in a twist, it also provides for left/right wide speakers positioned between the front left and right and surround left and right speakers. For a more detailed explanation and speaker setup illustrations, check out the Official Audyssey DSX Page
SRS Circle Surround II
In addition to Dolby, Yamaha, and Audyssey, some branded home theater receivers include SRS Circle Surround II, which fills in the sound space between the front left and right speakers and the listener without the need for separate height or wide speakers. Circle Surround II is able to generate its sound field within the physical constraints of a traditional 5.1 or 6.1 channel speaker setup. SRS uses a digital sound processing technology to produce the desired effect.
You are probably asking yourself, "Is my current home theater receiver obsolete if it doesn't offer any of these technologies?". The short answer is "NO". If you have a 5.1 channel system, good speakers and good speaker placement go a long way to providing a good surround sound experience.
I wouldn't replace a home theater receiver just to get the ability to add two more front or side speakers. Other things, such the ability to perform Dolby TrueHD/DTS-HD Master Audio decoding and HDMI connectivity would be a more logical reason to upgrade. However, if the receiver you are considering also has Dolby Pro Logic IIz or any of the other technologies mentioned above, then that might be an added, but not required, bonus.
On the other hand, just because you have and setup that incorporates Dolby Pro Logic IIz, Yamaha Presence, DSX, or Circle Surround II, doesn't mean you will magically transform your surround sound listening experience. A lot has to do with the acoustical properties of the room, the speakers you use, the speaker placement, speaker setup levels, the type of material you listen to, and finally, your own personal surround sound preferences.
For an in-depth look at surround sound technologies, read my article: The History and Basics of Surround Sound