OpenAL Soft's renderer has advanced quite a bit since its start with panned stereo output. Among these advancements is support for surround sound output, using psychoacoustic modeling and more accurate plane wave reconstruction. The concepts in use may not be immediately obvious to people just getting into 3D audio, or people who only have more indirect experience through the use of 3D audio APIs, so this document aims to introduce the ideas and purpose of Ambisonics as used by OpenAL Soft. What Is It? =========== Originally developed in the 1970s by Michael Gerzon and a team others, Ambisonics was created as a means of recording and playing back 3D sound. Taking advantage of the way sound waves propogate, it is possible to record a fully 3D soundfield using as few as 4 channels (or even just 3, if you don't mind dropping down to 2 dimensions like many surround sound systems are). This representation is called B-Format. It was designed to handle audio independent of any specific speaker layout, so with a proper decoder the same recording can be played back on a variety of speaker setups, from quadrophonic and hexagonal to cubic and other periphonic (with height) layouts. Although it was developed decades ago, various factors held ambisonics back from really taking hold in the consumer market. However, given the solid theories backing it, as well as the potential and practical benefits on offer, it continued to be a topic of research over the years, with improvements being made over the original design. One of the improvements made is the use of Spherical Harmonics to increase the number of channels for greater spatial definition. Where the original 4-channel design is termed as "First-Order Ambisonics", or FOA, the increased channel count through the use of Spherical Harmonics is termed as "Higher-Order Ambisonics", or HOA. The details of higher order ambisonics are out of the scope of this document, but know that the added channels are still independent of any speaker layout, and aim to further improve the spatial detail for playback. Today, the processing power available on even low-end computers means real-time Ambisonics processing is possible. Not only can decoders be implemented in software, but so can encoders, synthesizing a soundfield using multiple panned sources, thus taking advantage of what ambisonics offers in a virtual audio environment. How Does It Help? ================= Positional sound has come a long way from pan-pot stereo (aka pair-wise). Although useful at the time, the issues became readily apparent when trying to extend it for surround sound. Pan-pot doesn't work as well for depth (front- back) or vertical panning, it has a rather small "sweet spot" (the area the head needs to be in to perceive the sound in its intended direction), and it misses key distance-related details of sound waves. Ambisonics takes a different approach. It uses all available speakers to help localize a sound, and it also takes into account how the brain localizes low frequency sounds compared to high frequency ones -- a so-called psychoacoustic model. It may seem counter-intuitive (if a sound is coming from the front-left, surely just play it on the front-left speaker?), but to properly model a sound coming from where a speaker doesn't exist, more needs to be done to construct a proper sound wave that's perceived to come from the intended direction. Doing this creates a larger sweet spot, allowing the perceived sound direction to remain correct over a larger area around the center of the speakers. In addition, Ambisonics can encode the near-field effect of sounds, effectively capturing the sound distance. The near-field effect is a subtle low-frequency boost as a result of wave-front curvature, and properly compensating for this occuring with the output speakers (as well as emulating it with a synthesized soundfield) can create an improved sense of distance for sounds that move near or far. How Is It Used? =============== As a 3D audio API, OpenAL is tasked with playing 3D sound as best it can with the speaker setup the user has. Since the OpenAL API does not explicitly handle the output channel configuration, it has a lot of leeway in how to deal with the audio before it's played back for the user to hear. Consequently, OpenAL Soft (or any other OpenAL implementation that wishes to) can render using Ambisonics and decode the ambisonic mix for a high level of accuracy over what simple pan-pot could provide. This is effectively what the high-quality mode option does, when given an appropriate decoder configuation for the playback channel layout. 3D rendering and effect mixing is done to an ambisonic buffer, which is later decoded for output utilizing the benefits available to ambisonic processing. The basic, non-high-quality, renderer uses similar principles, however it skips the frequency-dependent processing (so low frequency sounds are treated the same as high frequency sounds) and does some creative manipulation of the involved math to skip the intermediate ambisonic buffer, rendering more directly to the output while still taking advantage of all the available speakers to reconstruct the sound wave. This method trades away some playback quality for less memory and processor usage. In addition to providing good support for surround sound playback, Ambisonics also has benefits with stereo output. 2-channel UHJ is a stereo-compatible format that encodes some surround sound information using a wide-band 90-degree phase shift filter. It works by taking a B-Format signal, and deriving a frontal stereo mix with the rear sounds attenuated and filtered in with it. Although the result is not as good as 3-channel (2D) B-Format, it has the distinct advantage of only using 2 channels and being compatible with stereo output. This means it will sound just fine when played as-is through a normal stereo device, or it may optionally be fed to a properly configured surround sound receiver which can extract the encoded information and restore some of the original surround sound signal. What Are Its Limitations? ========================= As good as Ambisonics is, it's not a magic bullet that can overcome all problems. One of the bigger issues it has is dealing with irregular speaker setups, such as 5.1 surround sound. The problem mainly lies in the imbalanced speaker positioning -- there are three speakers within the front 60-degree area (meaning only 30-degree gaps in between each of the three speakers), while only two speakers cover the back 140-degree area, leaving 80-degree gaps on the sides. It should be noted that this problem is inherent to the speaker layout itself; there isn't much that can be done to get an optimal surround sound response, with ambisonics or not. It will do the best it can, but there are trade-offs between detail and accuracy. Another issue lies with HRTF. While it's certainly possible to play an ambisonic mix using HRTF and retain a sense of 3D sound, doing so with a high degree of spatial detail requires a fair amount of resources, in both memory and processing time. And even with it, mixing sounds with HRTF directly will still be better for positional accuracy.