Theory of Operation

Circle Surround II™ decoding is capable of delivering up to 6.1 channels of audio from mono, stereo, matrix-encoded or Circle Surround-encoded source material.

Circle Surround II features SRS Labs' TruBass® and Dialog Clarity™ technologies, both patented post-processing techniques that enhance the multichannel audio experience and address problems that occur when playing back material mixed for cinema. Even in the finest home theater environments, dialog can often become difficult to understand, but Dialog Clarity solves this problem by significantly improving dialog intelligibility from all source material. TruBass utilizes proprietary psychoacoustic techniques to restore the perception of low frequency fundamentals and enhance bass performance. TruBass processes information present in the original source material to create the perception of bass that is lower than the normal capabilities of a given speaker driver. As a result, when a person listens to audio processed with TruBass, they perceive a heightened sensation of low bass tones - bass that is amazingly full and alive.

Theory:
The basic Circle Surround decoder is capable of deriving up to 5.1 channels of audio from a CS-encoded Lt/Rt source. CS II provides enhanced decoder performance, 6.1 channel operation, and the addition of pre and post-processing for mono mode, dialog enhancement and bass enhancement.

A block diagram illustrating the overall signal processing structure of the core CS II decoder is shown in Figure 1. Key technical features of the CS II decoder include multi-band steering, variable time constant steering and advanced techniques for isolating dialog and vocals to the center channel. The CS II decoder also provides full bandwidth stereo surround channels with separation of up to 31db between surrounds for encoded sources.

In "music" mode, the consumer version of the decoder provides a highly stable front sound stage with the presentation or ambient information in the surround channels, creating a highly musical and effective surround synthesis from stereo material. For more detailed information on the CS II decoder, please refer to the SRS Labs, inc. publication "Circle Surround Principles of Operation".

Figure 1

PRE AND POST PROCESSING

The Circle Surround II decoder incorporates both pre and post-processing functions that can be selected at the user's discretion to enhance the surround experience. These functions were included to address market requests for desirable features as well as to improve upon areas that are sources of common complaints in surround sound reproduction. None of these features in any way degrade the operation of the CS system and, since all are incorporated in the CS II decoder, no special encoding considerations or changes from the original CS encoding system are required.

Monaural Input to Surround
Due to the continued prevalence of mono material and broadcasts throughout the world, it became apparent that the ability to create a creditable surround experience from a single channel input would be desirable. However, conventional techniques such as the use of reverb to simulate ambience in the rear channel were deemed unacceptable as they created very artificial and undesirable side effects. The most obvious of which is a "voice in a tunnel" effect on dialog. It was also recognized that it was highly desirable to retain front/center dialog localization, even though dialog cannot actually be separated from a monaural signal.

To address these issues and create a very natural surround experience from a single input, an SRS Labs patented technique was implemented. This technique uses constant phase filters to synthesize a stereo signal that is then applied to the CS decoder. In addition, center dialog localization is maintained for a wide range of listening positions.

Dialog Enhancement
A universal complaint among users of surround sound systems of any type is that dialog cannot always be clearly understood. This is frequently due to the high level of music, effects, and surround information in modern soundtracks. As exciting as these effects can be, nothing is more annoying than the inability to follow the story due to a lack of clearly understandable dialog.

To address this problem, SRS adapted one of its patented techniques originally developed to elevate the stereo image vertically in automotive applications. Manufacturers of television receivers quickly adopted this frequency contouring method, which is based on vertical HRTF's (Head Related Transfer Functions), when it was found that, when used in moderation, dialog clarity was significantly improved.

Based on this experience, a refined version of this method is applied as a user selectable post process to the center channel output of the CS II decoder. This process is available in both true center and phantom center modes. The result is significantly improved dialog intelligibility, providing the listener the opportunity to relax and enjoy the film without straining to understand the dialog.

Virtual Bass Enhancement
Much of the audio excitement in cinema is produced by the impact of low bass from explosions and the ominous tones of the cellos and double basses in a suspenseful scene. Frequently, however, the drivers available in less expensive home surround systems, even with the use of subwoofer, are not capable of fully reproducing the low frequency energy contained in film soundtracks mixed for high-powered commercial cinema systems. Larger, more expensive home theater systems can also benefit significantly from additional low bass impact.

To address this and provide the most exciting listening experience possible, SRS Labs' patented TruBass technology has been integrated into the CS II decoder. Based on the "missing fundamental" principle, TruBass selectively boosts a series of low bass harmonics within the range that a given driver is capable of reproducing. Various speaker size settings are available to permit the TruBass process to function effectively with a wide range of drivers. Other filters within the TruBass process are designed to carefully avoid "muddiness" or a lack of clarity in any part of the frequency spectrum.

As implemented in the CS II decoder, the TruBass processor receives its input from the original Lt/Rt source prior to decoding. TruBass processing is then applied and the output is made available for mixing with either the left and right main signals, the subwoofer signal, or both, based on user selection.