Theory of Operation

WOW is an aggregated adaptation of four processing techniques pioneered by SRS Labs. First, a set of Head Related Transfer Functions (HRTFs) expands and elevates the apparent size of the stereo image, so that the sound stage takes on a scale of immense proportion when compared to the actual physical speaker locations. Then, bass performance is enhanced through a proprietary psychoacoustic technique that restores the perception of low frequency fundamentals by dynamically augmenting harmonics that the speaker can more easily reproduce. For monophonic audio sources, an extremely natural stereophonic sound image can also be synthesized through a patented technique that avoids the pitfalls of traditional stereo synthesis techniques. Finally, spatial cues responsible for positioning sound outside the horizontal boundaries of the speakers or headphones are equalized using a second set of HRTFs. These HRTF correction curves account for how the brain perceives the location of sounds to the sides of a listener even when played back through speakers in front of the listener. The result is the presentation of instruments and vocalists in their proper place, including the indirect and reflected sounds all about the room. The HRTF curves utilized for headphones create a sound image that is perceived to be outside the listener's head.

This powerful audio processing suite contains numerous controls and options for tuning to each particular product and playback environment. In addition, the components of WOW can be used together or individually to address specific audio issues in different applications.

SRS 3D can be applied to retrieve and restore spatial information, directional cues, and other sonic nuances which are either missing or altered by the electronic reproduction of stereo and/or the microphone mixing process SRS FOCUS can be applied to elevate the stereo sound image providing clarity of vocals, a vertically elevated sound stage and can be used to reposition non-optimally placed speakers from a lower physical location to an appropriate hearing level (i.e. Car or projection TV). The TruBass psychoacoustic bass enhancement technology provides deep, rich bass up to an octave below the F0 frequency of a speaker driver. It can be applied to any speaker or headphone, but is especially effective with very small speaker systems to negate the need for a subwoofer or additional physical components.

WOW Block Diagram

Theory of Operation
The human hearing system uses several methods to determine the direction from which a particular sound is coming. Since our hearing is binaural, these methods include relative phase shift for low frequency sounds, relative intensity for sounds in the voice range, and relative time of arrival for sounds having fast rise times and high frequency components.

WOW consists of three components based on SRS Labs' patented technology, each of which is described below.

SRS 3D
SRS 3D processes the signal in such a manner that the spatial cues lost in the record/playback process are restored. Since the human hearing system is involved and is actually part of the loop, its transfer function is made a part of the system transfer function. At the same time, SRS 3D system processing avoids an objectionable buildup of frequencies where the human ear has increased sensitivity in the difference channel and is effective over a wide area so that the listener is not restricted to a favorable listening position between the two speaker. In a stereophonic signal, frontal sounds produce equal amplitudes in the left and right channels and are therefore present in the "sum" or L+R signal. Ambient sounds that include reflected and side sounds produce a complex sound field and do not appear correlated in the left and right channels. They are therefore present in the "difference" or L-R signal. Although these two signals are normally heard as a composite signal, it is possible to separate and process them independently and then remix them again into a new composite signal which contains the required spatial cues that the stereo recording process did not provide. The directional cues are mostly contained in the difference signals, so these can be processed to bring the missing directional cues back to their normal levels. The processed difference signals can then modified spectrally with an HRTF based front/side corrections and increased in amplitude in order to increase apparent image width.

In addition to creating 3D Stereo images from stereo program material, it is often desirable to expand monaural (mono) signals to wider image formats. This can be supported by SRS 3D technology because it also contains a mono-to-stereo synthesis feature based on SRS Labs' patented SRS 3D Mono technology. This technology makes use of constant phase filters to produce the synthetic left and right output signals resulting in tightly focused and centered bass and vocals in the output.

Creating SRS 3D Sound

FOCUS
As with SRS 3D, FOCUS technology is based on the fundamentals of the human hearing system and the psychoacoustics of sound. Specifically, FOCUS is modeled on vertical HRTF's (Head Related Transfer Functions), will SRS 3D is based on horizontal HRTS's. HRTF's are special filters that have a frequency response with specific curves which model the human hearing system. The shape of the ear allows us to determine the direction from which the sound is coming. By manipulating the contour of the sound from a pair of speakers, it is possible to use a contour that the auditory system interprets as being located at a particular angle or location.

SRS Labs employs a set of curves within the FOCUS technology to move the image vertically along the axis between a set of speakers (for stereo) or vertically above a single speaker (mono). Although the FOCUS elevation process may be used alone, SRS 3D processing is considered an integral part of most applications and the bass enhancement technology added as part of the WOW technology suite completes the audio enhancement ensemble. When used with FOCUS, SRS 3D serves three functions. SRS 3D expands the sound field making the stereo image much wider, especially important for TV applications using the internal speakers. SRS 3D also removes the sweet spot so that it is not necessary to sit directly in the center between the speakers to hear good stereo. It is possible to listen at extreme angles, beyond the speaker placements, and still hear a distinct stereo image. This is beneficial in TV environments, where it is common that people watch a television from the sides of the room. A wide sweet spot is also critical in cars, where none of the passengers are likely to be centered between the speakers.

FOCUS elevates the sound field to the optimal speaker position, which is ear level.

TruBass
SRS Labs' TruBass technology is based on the fundamentals of the human hearing system and the psychoacoustics of sound. In order to understand how TruBass is able to create the perception of deep bass from smaller speakers, it is helpful to understand the psychoacoustic principle upon which TruBass is based.

The human hearing system exhibits non-linearity, which produces intermodulation distortion in the form of additional overtones and harmonics that do not exist in the actual audio received at the ear canal. These non-linear effects are most pronounced at low frequencies. TruBass takes advantage of this phenomenon in its operation. When the human ear is presented with specific harmonic frequencies relative to a missing fundamental tone it will fill in the fundamental frequency based on the higher harmonics that are present. By accentuating the second and higher frequency harmonics of a fundamental tone present in the audio signal, yet outside the range of reproduction by a certain speaker size, TruBass gives the perception of greatly improved bass response. For example, if two tones are generated from a speaker at 100Hz and 150Hz, the hearing system will produce an intermodulation component at 50Hz, which is the difference of the two actual frequencies.

TruBass generates the perception of greatly improved bass by taking the difference between the two actual frequencies.

The process of sound reproduction does not simply stop at the acoustic energy produced by the loudspeaker, but includes the shape of the outer ear, the auditory nerves, the brain, and the recognition process of the listener. All of these elements are used to translate acoustical vibrations in the ear into nerve impulses, and finally into a "sensation", or perception of sound. TruBass creates the perception that low-frequency sounds are emitted from a loudspeaker by selectively processing higher frequency bands, depending on the frequency and amplitude of the fundamental input signal. The brain extrapolates this set of boosted harmonics to restore a bass signal in the audio source that is significantly attenuated or lost by the physical limitations of the speaker. None of the original audio is removed or altered. The bass enhancement is added in such a way that the audio quality is not compromised. The TruBass enhancement is dynamically processed, with little or no harmonic boost when there is no low frequency content, and increased as strong bass is detected in the audio source.

The frequency range of the boosted harmonic frequencies can be tailored to the speaker's capability. Any frequency range can be selected by the product engineer to match the characteristics of the speaker drivers used in the application. In today's latest version of TruBass, there are eight frequency settings from 40 Hz to 400 Hz to choose from. Each product designer can determine which setting is the most appropriate to use by looking at the frequency drop off of the product's speaker or headphones.

After harmonic processing a proprietary compression algorithm is applied to control driver excursion and create additional bass impact.

By nature, low frequency audio lacks discernible stereo separation. As a result, it is not necessary to process both stereo inputs in parallel. The recommended implementation calls for both stereo inputs to be mixed together, processed for TruBass enhancement, then equally mixed back into each output. Besides reducing the processing requirements, the bass energy is evenly distributed between both channels.

In summary, TruBass selectively enhances frequencies that the speaker can easily reproduce to stimulate the ear/brain system to perceive intermodulation components. The brain extrapolates these tones back to the much lower bass frequency that the speaker is unable to generate.