Theory of Operation

Introduction

SRS Labs' robust multichannel virtualization technology, TruSurround XT® (TS XT), provides an enhanced listening experience unlike any technology that has been offered for a two speaker playback environment. TruSurround XT was developed with the goal of recreating a realistic listening environment, such as one would experience in a natural setting, through the use of only two front-facing speakers or headphones. TruSurround XT is not intended to replicate the placement of speakers around a listener; rather, the objective of TruSurround XT is to restore a realistic sensation of being surrounded by sound.

What it does

TruSurround XT is the second generation of SRS TruSurround®. Building upon its patented predecessor and namesake, TruSurround XT not only solves the problem of playing 5.1 or 6.1 multichannel content over two speakers or headphones, but also utilizes proprietary algorithms from SRS Labs - Dialog Clarity™ and TruBass® - to provide listeners with a truly immersive audio experience.

TruSurround XT is a combination of three distinct and patented processing techniques from SRS Labs:

1. Dialog Clarity: Playback of dialog in home theater environments often suffers due to competing signals from other speakers. In addition, feature film content is mixed specifically for cinema playback. When translated through home systems, dialog may become unintelligible. This SRS algorithm enhances signal clarity to address these problems, thus improving the intelligibility of dialog derived from all such source material.
2. TruSurround: This is the core technology upon which TruSurround XT was created. TruSurround delivers a compelling, virtual surround sound experience through any two speaker playback system, including internal television speakers and headphones. It is fully compatible with all multichannel formats up to 6.1 channels. It carries Dolby certification.
3. TruBass: TruBass enhances bass performance utilizing proprietary psychoacoustic techniques. These techniques restore the perception of fundamental low frequency tones by dynamically augmenting harmonics, which are more easily reproduced by contemporary loudspeakers.

TruSurround XT accepts input from up to a 7 channel (6.1) audio source and processing a 2 channel output. When receiving multichannel content, TruSurround XT virtualizes the channels, creating "phantom" speakers that appear to extend all around the listener, producing an amazing surround sound effect. When receiving mono or stereo content, TruSurround XT includes the functionality of SRS WOW® to present an expansive three dimensional sound field.

TruSurround XT Processing Diagram

As signals are processed with TruSurround XT, Dialog Clarity is applied first. TruSurround then renders multichannel signals in such a way as to take advantage of the human hearing system and create an encompassing sound field. Once the two-channel output is generated, those channels are processed with TruBass.

The system can have a mono, stereo, or multichannel input signal and a stereo output signal. TruSurround processes the input signal and produces a stereo output signal, which is passed to the bass enhancement system. For this portion of the processing, the two channels of a stereo signal are mixed together, processed as a mono signal, then mixed equally into both channels to form the enhanced stereo signal. This is done because there is generally no significant stereo separation in a bass frequency signal, so there is no need to duplicate the processing for the two channels.

The implementation provides user controls for TruBass level, speaker size, Dialog Clarity level, and input type. A switch is provided to allow the signal processing to be bypassed for comparison of the processed and unprocessed signals. Depending on the application, these individual controls may be either dynamically changeable or fixed to a specific configuration.

Compatibility
TruSurround XT is compatible with signals that been decoded by a Dolby Pro Logic or Pro Logic II decoder and signals that have been encoded to stereo with a matrix encoder (e.g. SRS Circle Surround or Dolby Pro Logic). It can also process normal two channel (stereo) input by processing the signal with the SRS WOW algorithm. Additionally, TruSurround XT can process a mono signal that is input to the center channel. This channel is processed using SRS' mono to stereo algorithm and is then treated as a stereo signal. When processing a mono signal, all other inputs other than the center are assumed to be muted.

General Process
TruSurround XT processes the front and rear channels separately, and mixes the results together for the final output.

TSXT first applies an input gain to all of the input channels. By default, this gain is 0dB, but it can be adjusted to meet various system requirements, such as attenuation in systems that are prone to clipping, or gain to match bypass levels. When processing the front channels, TruSurround XT first determines if the system is in Mono or BS Digital mode. If so, it applies the appropriate algorithm to the center channel and places the result into the left and right channels. TSXT then processes the L-R signal with the front space perspective curve.

If a center channel is present, TSXT processes it with Dialog Clarity. If not, TSXT processes the L+R signal with Dialog Clarity. The enhanced L+R signal is then multiplied by the center control and mixed into the center channel.

If there is only a mono surround channel, then it is processed through the rear center perspective filter and multiplied by the rear-center control; in the passive matrix case, the same thing happens, only rear-space is used instead of rear-center. Or if the input was provided by a Circle Surround decoder, the surround information in the Cs channel is passed through the rear space perspective filter and multiplied by the rear-space control, then multiplied by the steering data in Ls and Rs channels. For all other surround input modes, the Ls+Rs signal (or the Cs signal, if present) is passed through the rear center perspective filter and multiplied by the rear-center control, and the Ls-Rs is passed through the rear space perspective filter and multiplied by the rear-space control.

Mixing these results together with the original signals with a constant gain applied and with the LFE channel generates the final TruSurround output. The left and right outputs are then processed with TruBass to generate the final TSXT output.