The all-encompassing pleasure of surround sound is one of the stimulations of modern film experiences. It is incredible that AR/VR device manufacturers are seeking ways to bring this experience out of the cinema and into the realm of wearable headsets (also known as near-field surround sound). In this article, we will first introduce the limitations of common audio amplifier solutions, and then present an easy-to-use (plug-and-play) micro Class D amplifier, and demonstrate how to use it to achieve this ambitious goal.

Introduction
    The screen is a window through which people can see a virtual world. The challenge lies in making that world look real, act real, sound real and feel real. Ivan Sutherland, an American computer scientist and Internet pioneer widely regarded as the "father of computer graphics", concisely expounded on the challenges of creating truly immersive augmented reality or virtual reality (AR/VR) experiences. Although many efforts have been focused on creating stunning visual panoramas, attention is now beginning to shift to further enhancing the audio and haptic components of AR/VR adventures. In fact, Jeffrey Yau, a researcher at Johns Hopkins University in Baltimore, summarized the results of recent experiments on the relationship between sound and touch. We listen with our ears and feel with our skin, but our brain may combine this information in a specific way. The frequency information from the two senses always seems to be combined. For developers, this might mean that future improvements in AR/VR audio and haptic aspects will take place simultaneously rather than independently and in parallel.
    In this design solution, we have considered the types of audio experiences that high-end AR/VR device manufacturers hope to offer, as well as the challenges of achieving this goal within the limited space constraints of wearable headsets. Before proposing an efficient, plug-and-play digital Class D amplifier, we discussed the significance of haptic feedback for AR/VR audio experiences, which can be easily expanded to achieve the desired audio and haptic experiences in a space-saving, compact solution.

近场环绕声
    Surround sound is defined as a stereo system that involves three or more speakers surrounding the audience to provide a more realistic modern cinema effect. It uses a 7.1 configuration (3 left, 3 right, one center channel and one subwoofer) to create a more realistic surround sound experience. The task of placing eight speakers (plus an amplifier) in different positions is no small matter, even in a spacious room, to provide the best sound quality. It is incredible that AR/VR device manufacturers are now seeking to reproduce the same near-field surround experience in wearable headsets.
    Although this may sound unlikely, they are also looking for ways to use haptic joysticks to create audio synchronization reverb, thereby providing the wearer with a more satisfying sensory experience. Anyone who goes to the cinema to watch a movie or to the auditorium to listen to a classical orchestra knows that sound is only part of the experience. The overlapping vibration felt by the audience greatly enhanced the overall experience.

Head-mounted 3D audio
    Facing the arduous task of creating an immersive 3D audio design in a head-mounted installation, the key to achieving this goal lies in choosing the appropriate audio amplifier to meet the design specifications. Interconnecting up to eight speakers within the framework of a wearable head-mounted device requires the use of an audio IC, which not only adopts a micro-package but also allows for direct circuit board wiring. The amplifier should be as efficient as possible (90% or higher) and be capable of operating at the lowest possible supply voltage. This helps to minimize power consumption, thereby extending the battery life of the headphones. Like all modern audio applications, high-fidelity sound quality (high PSRR, low noise and wide dynamic range) is almost beyond description.
    Another key specification of the amplifier is its EMI performance. Because multiple traces traverse different lengths, the audio amplifier must suppress unwanted signal frequencies, which may become perceptible if passed to the speaker. It seems that meeting the audio requirements is not strict enough; the amplifier must also meet the fast conduction time needed for synchronous connection with the haptic joystick. If the conduction time exceeds a few milliseconds, the tactile circuit must always remain in an awakened state, even when not in use, which will waste power. By using an amplifier with a fast conduction time, the tactile circuit is activated only when needed.

A typical but not insignificant solution
    To meet the requirements of high efficiency, a Class D audio amplifier needs to be used. These devices are provided in different forms, operate under different power supply voltages, and have different efficiency levels. Figure 3 shows that <> I2S are connected to the S input of the audio processing unit. Class D speaker amplifiers typically require four independent I2S ports.

Figure 7. Connect GAIN_SLOT to VDD or GND to obtain the desired gain setting.

    For applications that require variable gain Settings, GAIN_SLOT pins can be easily routed using a tiny trace of 2mil and a 2mil gap, as shown in Figure 8.