Add docs for OpenAL Soft and standard decoder presets
Chris Robinson
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Overview
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========
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3D7.1 is a custom speaker layout designed by Simon Goodwin at Codemasters[1].
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Typical surround sound setups, like quad, 5.1, 6.1, and 7.1, only produce audio
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on a 2D horizontal plane with no verticality, which means the envelopment of
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"surround" sound is limited to left, right, front, and back panning. Sounds
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that should come from above or below will still only play in 2D since there is
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no height difference in the speaker array.
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To work around this, 3D7.1 was designed so that some speakers are placed higher
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than the listener while others are lower, in a particular configuration that
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tries to provide balanced output and maintain some compatibility with existing
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audio content and software. Software that recognizes this setup, or can be
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configured for it, can then take advantage of the height difference and
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increase the perception of verticality for true 3D audio. The result is that
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sounds can be perceived as coming from left, right, front, and back, as well as
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up and down.
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[1] http://www.codemasters.com/research/3D_sound_for_3D_games.pdf
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Hardware Setup
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==============
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Setting up 3D7.1 requires an audio device capable of raw 8-channel or 7.1
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output, along with a 7.1 speaker kit. The speakers should be hooked up to the
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device in the usual way, with front-left and front-right output going to the
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front-left and front-right speakers, etc. The placement of the speakers should
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be set up according to the table below. Azimuth is the horizontal angle in
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degrees, with 0 directly in front and positive values go /left/, and elevation
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is the vertical angle in degrees, with 0 at head level and positive values go
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/up/.
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------------------------------------------------------------
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- Speaker label | Azimuth | Elevation | New label -
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------------------------------------------------------------
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- Front left | 51 | 24 | Upper front left -
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- Front right | -51 | 24 | Upper front right -
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- Front center | 0 | 0 | Front center -
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- Subwoofer/LFE | N/A | N/A | Subwoofer/LFE -
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- Side left | 129 | -24 | Lower back left -
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- Side right | -129 | -24 | Lower back right -
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- Back left | 180 | 55 | Upper back center -
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- Back right | 0 | -55 | Lower front center -
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------------------------------------------------------------
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Note that this speaker layout *IS NOT* compatible with standard 7.1 content.
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Audio that should be played from the back will come out at the wrong location
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since the back speakers are placed in the lower front and upper back positions.
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However, this speaker layout *IS* more or less compatible with standard 5.1
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content. Though slightly tilted, to a listener sitting a bit further back from
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the center, the front and side speakers will be close enough to their intended
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locations that the output won't be too off.
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Software Setup
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==============
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To enable 3D7.1 on OpenAL Soft, first make sure the audio device is configured
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for 7.1 output. Then in the alsoft-config utility, under the Renderer tab,
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select the 3D7.1.ambdec preset for the 7.1 Surround decoder configuration. And
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that's it. Any applications using OpenAL Soft can take advantage of fully 3D
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audio, and multi-channel sounds will be properly remixed for the speaker
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layout.
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Playback can be improved by (copying and) modifying the 3D7.1.ambdec preset,
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changing the specified speaker distances to match the the real distance (in
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meters) from the center of the speaker array, then enable High Quality Mode in
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alsoft-config. That will improve the quality when the speakers are not all
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equidistant.
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Note that care must be taken that the audio device is not treated as a "true"
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7.1 device by non-3D7.1-capable applications. In particular, the audio server
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should not try to upmix stereo and 5.1 content to "fill out" the back speakers,
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and non-3D7.1 apps should be set to either stereo or 5.1 output.
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As such, if your system is capable of it, it may be useful to define a virtual
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5.1 device that maps the front, side, and LFE channels to the main device for
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output and disables upmixing, then use that virtual 5.1 device for apps that do
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normal stereo or surround sound output, and use the main device for apps that
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understand 3D7.1 output.
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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 quadrophonic and hexagonal
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to cubic and other periphonic (with height) layouts.
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Although it was developed over 30 years ago, various factors held ambisonics
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back 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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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 directional
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accuracy over what 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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is done to an ambisonic buffer, which is later decoded for output utilizing the
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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, then deriving a
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frontal stereo mix with some of the rear sounds filtered in with it. Although
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the result is not as good as 3-channel (2D) B-Format, it has the distinct
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advantage of only using 2 channels and being compatible with stereo output.
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This means it will sound just fine when played as-is through a normal stereo
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device, or it may optionally be fed to a properly configured surround sound
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receiver which can extract the encoded information and restore some of the
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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, doing so with a high degree of spatial detail
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requires a fair amount of resources, in both memory and processing time. And
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even with it, mixing sounds with HRTF directly will still be better for
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positional accuracy.
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# AmbDec configuration
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# Written by Ambisonic Decoder Toolbox, version 8.0
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/description 3D7_2h1v_allrad_5200_rE_max_1_band
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/version 3
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/dec/chan_mask 1bf
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/dec/freq_bands 1
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/dec/speakers 7
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/dec/coeff_scale fuma
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/opt/input_scale fuma
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/opt/nfeff_comp output
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/opt/delay_comp on
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/opt/level_comp on
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/opt/xover_freq 400.000000
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/opt/xover_ratio 0.000000
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/speakers/{
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# id dist azim elev conn
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#-----------------------------------------------------------------------
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add_spkr LF 1.500000 51.000000 24.000000
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add_spkr RF 1.500000 -51.000000 24.000000
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add_spkr CE 1.500000 0.000000 0.000000
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add_spkr LB 1.500000 180.000000 55.000000
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add_spkr RB 1.500000 0.000000 -55.000000
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add_spkr LS 1.500000 129.000000 -24.000000
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add_spkr RS 1.500000 -129.000000 -24.000000
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/}
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/matrix/{
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order_gain 1.000000 0.774597 0.400000 0.000000
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add_row 0.325031 0.357638 0.206500 0.234037 0.202440 0.135692 0.116927 -0.098768
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add_row 0.325036 -0.357619 0.206537 0.234033 -0.202427 -0.135680 0.116934 -0.098768
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add_row 0.080073 -0.000010 -0.000296 0.155843 -0.000016 -0.000011 -0.000623 0.163306
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add_row 0.353556 0.000002 0.408453 -0.288377 -0.000004 -0.000003 -0.221039 0.077297
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add_row 0.325297 0.000008 -0.414018 0.232789 0.000004 0.000003 -0.232940 0.018311
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add_row 0.353558 0.352704 -0.203542 -0.290124 -0.191868 -0.134582 0.110616 -0.038294
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add_row 0.353556 -0.352691 -0.203576 -0.290115 0.191871 0.134585 0.110612 -0.038293
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/}
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/end
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# AmbDec configuration
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# Written by Ambisonic Decoder Toolbox, version 8.0
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/description Hexagon_2h0p_pinv_match_rV_max_rE_2_band
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/version 3
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/dec/chan_mask 11b
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/dec/freq_bands 2
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/dec/speakers 6
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/dec/coeff_scale fuma
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/opt/input_scale fuma
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/opt/nfeff_comp input
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/opt/delay_comp on
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/opt/level_comp on
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/opt/xover_freq 400.000000
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/opt/xover_ratio 0.000000
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/speakers/{
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# id dist azim elev conn
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#-----------------------------------------------------------------------
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add_spkr LF 1.000000 30.000000 0.000000
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add_spkr RF 1.000000 -30.000000 0.000000
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add_spkr RS 1.000000 -90.000000 0.000000
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add_spkr RB 1.000000 -150.000000 0.000000
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add_spkr LB 1.000000 150.000000 0.000000
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add_spkr LS 1.000000 90.000000 0.000000
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/}
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/lfmatrix/{
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order_gain 1.000000 1.000000 1.000000 0.000000
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add_row 0.235702 0.166667 0.288675 0.288675 0.166667
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add_row 0.235702 -0.166667 0.288675 -0.288675 0.166667
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add_row 0.235702 -0.333333 0.000000 -0.000000 -0.333333
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add_row 0.235702 -0.166667 -0.288675 0.288675 0.166667
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add_row 0.235702 0.166667 -0.288675 -0.288675 0.166667
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add_row 0.235702 0.333333 0.000000 -0.000000 -0.333333
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/}
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/hfmatrix/{
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order_gain 1.414214 1.224745 0.707107 0.000000
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add_row 0.235702 0.166667 0.288675 0.288675 0.166667
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add_row 0.235702 -0.166667 0.288675 -0.288675 0.166667
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add_row 0.235702 -0.333333 0.000000 -0.000000 -0.333333
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add_row 0.235702 -0.166667 -0.288675 0.288675 0.166667
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add_row 0.235702 0.166667 -0.288675 -0.288675 0.166667
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add_row 0.235702 0.333333 0.000000 -0.000000 -0.333333
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/}
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/end
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Ambisonic decoder configuration presets are provided here for common surround
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sound speaker layouts. The presets are prepared to work with OpenAL Soft's high
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quality decoder. By default all of the speaker distances within a preset are
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set to the same value, which results in no effect from distance compensation.
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If this doesn't match your physical speaker setup, it may be worth copying the
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preset and modifying the distance values to match (note that modifying the
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azimuth and elevation values in the presets will not have any effect; the
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specified angles do not change the decoder behavior).
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Details of the individual presets are as follows.
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square.ambdec
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Specifies a basic square speaker setup for Quadrophonic output, with identical
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width and depth. Front speakers are placed at +45 and -45 degrees, and back
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speakers are placed at +135 and -135 degrees.
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rectangle.ambdec
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Specifies a narrower speaker setup for Quadrophonic output, with a little less
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width but a little more depth over a basic square setup. Front speakers are
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placed at +30 and -30 degrees, providing a bit more compatibility for existing
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stereo content, with back speakers at +150 and -150 degrees.
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hexagon.ambdec
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Specifies a flat-front hexagonal speaker setup for 7.1 Surround output. The
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front left and right speakers are placed at +30 and -30 degrees, the side
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speakers are placed at +90 and -90 degrees, and the back speakers are placed at
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+150 and -150 degrees. Although this is for 7.1 output, no front-center speaker
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is defined for the decoder, meaning that speaker will be silent for 3D sound
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(however it may still be used with AL_SOFT_direct_channels or ALC_EXT_DEDICATED
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output). A "proper" 7.1 decoder may be provided in the future, but due to the
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nature of the speaker configuration will have trade-offs.
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3D7.1.ambdec
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Specifies a 3D7.1 speaker setup for 7.1 Surround output. Although it's for 7.1
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output, the speakers for such a configuration need to be placed in different
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positions for proper results. Please see docs/3D7.1.txt for more information.
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# AmbDec configuration
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# Written by Ambisonic Decoder Toolbox, version 8.0
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/description Rectangle_1h0p_pinv_match_rV_max_rE_2_band
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/version 3
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/dec/chan_mask b
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/dec/freq_bands 2
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/dec/speakers 4
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/dec/coeff_scale fuma
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/opt/input_scale fuma
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/opt/nfeff_comp input
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/opt/delay_comp on
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/opt/level_comp on
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/opt/xover_freq 400.000000
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/opt/xover_ratio 0.000000
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/speakers/{
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# id dist azim elev conn
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#-----------------------------------------------------------------------
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add_spkr LF 1.000000 30.000000 0.000000
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add_spkr RF 1.000000 -30.000000 0.000000
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add_spkr RB 1.000000 -150.000000 0.000000
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add_spkr LB 1.000000 150.000000 0.000000
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/}
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/lfmatrix/{
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order_gain 1.000000 1.000000 0.000000 0.000000
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add_row 0.353553 0.500000 0.288675
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add_row 0.353553 -0.500000 0.288675
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add_row 0.353553 -0.500000 -0.288675
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add_row 0.353553 0.500000 -0.288675
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/}
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/hfmatrix/{
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order_gain 1.414214 1.000000 0.000000 0.000000
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add_row 0.353553 0.500000 0.288675
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add_row 0.353553 -0.500000 0.288675
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add_row 0.353553 -0.500000 -0.288675
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add_row 0.353553 0.500000 -0.288675
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/}
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/end
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# AmbDec configuration
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# Written by Ambisonic Decoder Toolbox, version 8.0
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/description Square_1h0p_pinv_match_rV_max_rE_2_band
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/version 3
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/dec/chan_mask b
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/dec/freq_bands 2
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/dec/speakers 4
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/dec/coeff_scale fuma
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/opt/input_scale fuma
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/opt/nfeff_comp input
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/opt/delay_comp on
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/opt/level_comp on
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/opt/xover_freq 400.000000
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/opt/xover_ratio 0.000000
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||||
/speakers/{
|
||||
# id dist azim elev conn
|
||||
#-----------------------------------------------------------------------
|
||||
add_spkr LF 1.000000 45.000000 0.000000
|
||||
add_spkr RF 1.000000 -45.000000 0.000000
|
||||
add_spkr RB 1.000000 -135.000000 0.000000
|
||||
add_spkr LB 1.000000 135.000000 0.000000
|
||||
/}
|
||||
|
||||
/lfmatrix/{
|
||||
order_gain 1.000000 1.000000 0.000000 0.000000
|
||||
add_row 0.353553 0.353553 0.353553
|
||||
add_row 0.353553 -0.353553 0.353553
|
||||
add_row 0.353553 -0.353553 -0.353553
|
||||
add_row 0.353553 0.353553 -0.353553
|
||||
/}
|
||||
|
||||
/hfmatrix/{
|
||||
order_gain 1.414214 1.000000 0.000000 0.000000
|
||||
add_row 0.353553 0.353553 0.353553
|
||||
add_row 0.353553 -0.353553 0.353553
|
||||
add_row 0.353553 -0.353553 -0.353553
|
||||
add_row 0.353553 0.353553 -0.353553
|
||||
/}
|
||||
|
||||
/end
|
||||
Loading…
Reference in New Issue