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								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 quadraphonic 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.


								When given an appropriate decoder configuration for the channel layout, the

								ambisonic mix can be decoded utilizing the benefits available to ambisonic

								processing, including frequency-dependent processing and near-field effects.

								Without a decoder configuration, the ambisonic mix can still be decoded for

								good stereo or surround sound output, although without near-field effects as

								there's no speaker distance information.


								In addition to surround sound output, 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. This is

								generated by taking the ambisonic mix and deriving a front-stereo mix with

								with the rear sounds 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.