Introduction

Theoretical Basis
Design constraints often dictate placement of speaker drivers in positions that are far from ideal. The most obvious example is car audio, where speakers have been displaced from the dashboard by an increasing number of electronics. Not only are speakers now often located very low in the doors, they are pointed towards each other instead of towards the listener. The result is considerably muffled sound lacking most of the upper frequencies, even with high quality speakers.

Another consumer audio application that may have speaker location challenges are LCD, plasma, rear projection and CRT televisions. Space is very limited along the sides, between the case and picture tube. The TV controls occupy most of the area below the screen. Narrow, oval shaped speakers are squeezed into the remaining unused space. Often the speaker drivers are set back in the rear of the TV cabinet with the sound forced through tubes open at the front. Already compromised by lack of adequate acoustic damping materials, sealed enclosures, and other standard speaker design techniques, the unusual shape of the speaker mounting creates a muffled response that emanates from below the image area.

In big-screen rear projection TVs, speakers are often located under the screen and close to the floor. The high frequencies are either lost, absorbed by plush carpeting, or the image is diffused reflecting off a hard surface floor. In either case the level of the audio image is placed far below the center of the screen, resulting in a subpar audio experience.

Benefits
FOCUSTM corrects the challenges mentioned above associated with speaker location. Primarily, FOCUS elevates the stereo image of speakers high above the actual speaker placement. In a car, it appears as if the sound is coming from the windshield above the dashboard. In a TV with speakers mounted under the screen, the image is elevated to the center of the screen where the action is. The sound is much more natural and realistic when it is in the same horizontal plane as the picture. The car stereo sounds more like a home stereo and the audio in the TV becomes integrated with the visual experience.

Theory of Operation
SRS Labs’ audio technologies, including FOCUS, are based on the science of HRTFs (Head Related Transfer Functions). HRTFs 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 of sound. By manipulating the contour of the sound from a pair of speakers, it is possible to tailor the response so that the auditory system interprets the sound source as being located at a particular angle or location. The set of curves applied to FOCUS move the image vertically along the axis between a set of speakers.

FOCUS’ audio elevation technique may be split into two controls, one for low and midrange speakers, and another for the tweeter. First the main image position is set for the lower frequencies. Then, the tweeter trim is adjusted to raise or lower the high frequencies relative to the main image elevation. With zero tweeter trim, the two frequency ranges are located together in the same plane. With positive trim, a virtual tweeter can be raised above the rest of the stereo soundstage. Negative trim lowers the tweeter image below the relocated main speaker image. In a car, it is possible to have most of the sound at windshield level, while an apparent tweeter sounds like it is mounted in the headliner above the windshield. For cars that employ both midrange speakers in the door, and tweeters mounted in the corner of the side windows, the tweeter image can be adjusted independently above or below the actual positions. In this case, separate level controls can reduce the amount of FOCUS processing to the tweeters, which are already sufficiently elevated and pointed in the correct direction toward the listener.

The FOCUS algorithm also includes “bass compensation”. Just as when speakers are actually raised vertically of the floor, there will be a perceived reduction in bass when FOCUS is used to virtually raise the soundfield. The bass compensation control is designed to restore the bass level that was originally present prior to applying FOCUS to raise the vertical positions of the sound image.

Although the FOCUS elevation process may be used alone, SRS 3D processing is considered an integral part of most applications. When used with FOCUS, SRS 3D serves three functions. SRS 3D expands the soundfield making the stereo image much wider, especially important for TV applications using the internal speakers. SRS 3D also expands 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 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. SRS 3D is described in detail in a Technical Description, available from SRS.

When used with FOCUS, SRS 3D is also used to rotate the direction of the speakers if they are not pointed toward the listener, as in a car. The normal SRS 3D perspective curve is modified to have the effect of the speaker being turned, with any correction angle between 0° and 90°. Front car speakers in the door pointing inward will sound like they are focused directly toward the listener.

Block Diagram