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