Matrix decoder melds multimedia

As more people have come to appreciate the benefits of surround-sound playback, they also want nonsurround program material-particularly music-to take full advantage of their multichannel playback systems. This desire is not only for home theaters, but also for car audio systems, computers and even headphone listening. The Dolby Pro Logic II 5.1-channel matrix decoder was designed with these expanded uses in mind.

Matrix surround decoding, which is the process of deriving several output channels from a two-channel delivery medium, is nothing new. When Dolby Surround Pro Logic decoding was introduced in 1987, it followed various other surround-sound technologies that had already appeared in the preceding two decades.

The popularity of Pro Logic increased dramatically through the 1990s, and helped establish the basis for surround-sound capability in future consumer digital delivery formats.

The advent of digital formats meant that source content-no longer restricted by the limitations of grooves, multiplexed radio frequency carriers or tape heads-could expand to 5.1-channel discrete audio as the new benchmark. When introduced, 5.1-channel Dolby Digital established a new level of sound field capability and articulation, but it did so using the same speaker configuration (of three in front, two in back) already established by Pro Logic.

The widespread adoption of Dolby Digital 5.1 audio on digital video disks helped elevate the standard-and expectations-of surround-sound playback in home theaters from that point forward.

Movie producers were the first to embrace discrete 5.1-channel audio for films, and it became the de facto standard for current production and release on DVDs. Many earlier movies originally released in Dolby Surround have also been reissued on DVD in 5.1 audio, allowing audiences to experience them anew with improvements in sound field spatiality, directionality and articulation. The transition to 5.1-channel audio is also well under way for digital TV over broadcast, satellite and cable systems, while the multichannel DVD audio format gives a similar boost to the enjoyment of music recordings.

Why a new surround decoder technology in 2000? Significant improvements in the way audio/video media is created and delivered to consumers have occurred in the last several years. VHS hi-fi is now the standard audio for stereo VCRs and prerecorded tapes. DVD is rapidly becoming the format of choice for movies in packaged media, and digital TV will eventually supersede analog broadcast formats around the world. The ability to deliver high-quality stereo audio has never been better. This also benefits the delivery of surround-encoded programs, which are more prevalent than ever on TV, video and DVD.

These improvements in content delivery, coupled with advancements in surround-decoding technology, allowed Pro Logic II to make advances in matrix surround decoding performance.

In previous logic decoders, including Pro Logic, the control circuit looks at the relative level and phase between the input signals. This information is sent to the variable-output matrix stage to adjust the voltage-controlled amplifiers making the level of out-of-phase signals. The out-of-phase signals cancel the unwanted crosstalk signals, resulting in improved channel separation. This is called a feed-forward design.

Matched signals

Pro Logic II looks at the same input signals and loops them to match their levels. These matched audio signals are then sent directly to the matrix stages to derive the various output channels. Because the same audio signals that feed the output matrix are themselves used to control the servo loop, the design is called a feedback logic design.

The concept of feedback control, which improves accuracy and optimizes dynamic characteristics, has long been a mainstay of such analog control systems as Dolby noise reduction. The incorporation of global feedback around the logic-steering process brings similar benefits in steering accuracy and dynamic behavior.

The Pro Logic II decoder can process any kind of source material without changing the core logic steering parameters. Since movies often contain music to a significant degree, a decoder needs to handle dialogue, music and effects equally well without creating side effects.

Pro Logic II can be implemented as a "one mode for all programs" decoder, which may be ideal for simpler products. The Movie mode works very well in that application. The Movie mode is very similar to the Pro Logic mode, with this difference: Pro Logic has a 7-kHz surround filter and a mono surround output, while the Movie mode has no surround filter and stereo surround outputs. The Pro Logic emulation mode included in the technology package is as robust as the original Pro Logic decoding mode, but does not have to provide separate decoding circuitry in the product.

The Movie and Pro Logic modes use sufficient delay in the surround channels to ensure that the sounds from the front speakers arrive at least 10 milliseconds before the sounds from the surround speakers. This creates the Haas precedence effect, which helps ensure that dialogue and other frontal sounds intended to be associated with the on-screen action are actually perceived as originating there.

The characteristics of movie sound tracks and music recordings differ in well-known ways. For example, movies (and Dolby Surround TV shows) aremixed and monitored in a calibrated multichannel environment, so the desired end result when listening at home can be obtained from a similarly calibrated home theater system. Stereo music, on the other hand, is not monitored through a surround system when being mixed, so it is not really known at that time how it will sound when played on a surround system. For these reasons, the Movie mode of Pro Logic II has preset characteristics to ensure consistent results.

In addition to decoder architecture that's identical to Pro Logic's, Pro Logic II has stereo surround capability. Since Pro Logic was introduced, the concept of bass management and the use of subwoofers has become commonplace. Pro Logic II includes a bass-management feature to derive a proper subwoofer feed or to allow bass to be reproduced from the main speakers, as appropriate for the application.

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