Ambisonic decoding
Encyclopedia
This is really the Ambisonic Speaker Decoder page. The Ambisonic B-format WXYZ signals define what you should hear. How these signals are presented to you by the speakers for best results, depends on how many speakers you have and where they are. Ambisonics treats directions where no speakers are placed with as much importance as speaker positions. The last thing we want, is to be aware that the sound is coming from 5 or 7 speakers.
We present some simple decoding equations known to give good results for common speaker arrangements.
But Ambisonic Speaker Decoders can use much more information about the position of your speakers including their exact position and distance from the listener. Because human beings use different mechanisms to locate sound, Classic Ambisonic Decoders modify the speaker feeds at each frequency to present the best information using Shelf Filters.
The complexities of Shelf Filters and Distance Compensation are explained in "Ambisonic Surround Decoders" and "SHELF FILTERS for Ambisonic Decoders" in the External links.
There are specialised decoders for large audiences in large spaces.
Hardware decoders have been commercially available since the late 1970s; currently,
Ambisonics is standard in surround products offered by
Meridian Audio, Ltd.
. Ad hoc software decoders are also available
(see External links).
More to come.
There are four main types of decoder.
speakers to be arranged in diametrically opposed pairs.
decoder is a Square with the listener in the centre. At least four speakers are required. Triangles do not work, exhibiting large "holes" between the speakers. Regular Hexagons perform better than Squares especially to the sides.
For the simplest (two dimensional) case (no height information), and spacing
the loudspeakers equally in a circle, we derive the loudspeaker
signals from the B-format W, X and Y channels:
where
is the direction of the speaker under consideration.
The most useful of these is the Square 4.0 decoder. This works well with the majority of ITU 5.1 systems; especially if the listener moves forward so he is in the middle of the square. Centre Front is not used. In most 5.1 systems, the centre speaker is very different from the others drawing attention to itself and spoiling the illusion.
If a centre speaker feed must be used, it should be at a very low level. Centre = X / 10 has been used successfully.
Alternatively, a 5.1 set of speaker feeds can be generated by decoding for a Regular Pentagon
(and a 7.1 feed by using a Regular Heptagon) but the speakers really need to be equally spaced in a circle.
More sophisticated 5.1 decodes allow the listener to move back from the centre but so far, these have inferior performance to the simple decodes above.
Readers should note that the coordinate system
used in Ambisonics follows the right hand rule convention with positive
X pointing forwards, positive Y pointing to the left and positive Z pointing
upwards. Horizontal angles run anticlockwise from due front and vertical
angles are positive above the horizontal, negative below.
1992 AES
conference held in Vienna. This
design is covered by and can be licensed from
Trifield Productions (see External links). The technology
provides one approach to the decoding of Ambisonic signals to irregular
loudspeaker arrays (such as ITU
)
commonly used for 5.1 surround sound
replay.
treat the sound's direction of incidence as a parameter that can be estimated through
signal analysis. A large body of research into human spatial hearing suggests
that our auditory cortex applies similar techniques in its auditory scene analysis
, which explains why these methods work.
The major benefits of parametric decoding is a greatly increased angular resolution and the separation of analysis and synthesis into separate processing steps. This separation allows B-format recordings to be rendered using any panning
technique, including delay panning, VBAP and HRTF-based synthesis.
Parametric decoding was pioneered by Lake DSP in the late '90s and independently suggested by Farina and Ugolotti in 1999 . Later work in this domain includes the DirAC method and the Harpex method .
We present some simple decoding equations known to give good results for common speaker arrangements.
But Ambisonic Speaker Decoders can use much more information about the position of your speakers including their exact position and distance from the listener. Because human beings use different mechanisms to locate sound, Classic Ambisonic Decoders modify the speaker feeds at each frequency to present the best information using Shelf Filters.
The complexities of Shelf Filters and Distance Compensation are explained in "Ambisonic Surround Decoders" and "SHELF FILTERS for Ambisonic Decoders" in the External links.
There are specialised decoders for large audiences in large spaces.
Hardware decoders have been commercially available since the late 1970s; currently,
Ambisonics is standard in surround products offered by
Meridian Audio, Ltd.
Meridian Audio, Ltd.
Boothroyd Stuart Meridian is a British manufacturer of high-performance, high-fidelity audio and video components and systems. Founded in 1977 by Bob Stuart and Allen Boothroyd, its products have proved popular among music and movie enthusiasts, and more recently in the lifestyle technology and...
. Ad hoc software decoders are also available
(see External links).
More to come.
There are four main types of decoder.
Diametric decoders
This design is intended for a domestic, small room setting, and allowsspeakers to be arranged in diametrically opposed pairs.
Regular Polygon decoders
This design is intended for a domestic, small room setting. The speakers are equidistant from the listener and lie equally spaced on the circumference of a circle. The simplest Regular PolygonRegular polygon
A regular polygon is a polygon that is equiangular and equilateral . Regular polygons may be convex or star.-General properties:...
decoder is a Square with the listener in the centre. At least four speakers are required. Triangles do not work, exhibiting large "holes" between the speakers. Regular Hexagons perform better than Squares especially to the sides.
For the simplest (two dimensional) case (no height information), and spacing
the loudspeakers equally in a circle, we derive the loudspeaker
signals from the B-format W, X and Y channels:
where
is the direction of the speaker under consideration.The most useful of these is the Square 4.0 decoder. This works well with the majority of ITU 5.1 systems; especially if the listener moves forward so he is in the middle of the square. Centre Front is not used. In most 5.1 systems, the centre speaker is very different from the others drawing attention to itself and spoiling the illusion.
If a centre speaker feed must be used, it should be at a very low level. Centre = X / 10 has been used successfully.
Alternatively, a 5.1 set of speaker feeds can be generated by decoding for a Regular Pentagon
Pentagon
In geometry, a pentagon is any five-sided polygon. A pentagon may be simple or self-intersecting. The sum of the internal angles in a simple pentagon is 540°. A pentagram is an example of a self-intersecting pentagon.- Regular pentagons :In a regular pentagon, all sides are equal in length and...
(and a 7.1 feed by using a Regular Heptagon) but the speakers really need to be equally spaced in a circle.
More sophisticated 5.1 decodes allow the listener to move back from the centre but so far, these have inferior performance to the simple decodes above.
Readers should note that the coordinate system
Coordinate system
In geometry, a coordinate system is a system which uses one or more numbers, or coordinates, to uniquely determine the position of a point or other geometric element. The order of the coordinates is significant and they are sometimes identified by their position in an ordered tuple and sometimes by...
used in Ambisonics follows the right hand rule convention with positive
X pointing forwards, positive Y pointing to the left and positive Z pointing
upwards. Horizontal angles run anticlockwise from due front and vertical
angles are positive above the horizontal, negative below.
"Vienna" decoders
These are so named because the paper introducing them was presented at the1992 AES
Audio Engineering Society
Established in 1948, the Audio Engineering Society draws its membership from amongst engineers, scientists, other individuals with an interest or involvement in the professional audio industry. The membership largely comprises engineers developing devices or products for audio, and persons working...
conference held in Vienna. This
design is covered by and can be licensed from
Trifield Productions (see External links). The technology
provides one approach to the decoding of Ambisonic signals to irregular
loudspeaker arrays (such as ITU
International Telecommunication Union
The International Telecommunication Union is the specialized agency of the United Nations which is responsible for information and communication technologies...
)
commonly used for 5.1 surround sound
Surround sound
Surround sound encompasses a range of techniques such as for enriching the sound reproduction quality of an audio source with audio channels reproduced via additional, discrete speakers. Surround sound is characterized by a listener location or sweet spot where the audio effects work best, and...
replay.
Parametric decoders
The idea behind parametric decoding is totreat the sound's direction of incidence as a parameter that can be estimated through
signal analysis. A large body of research into human spatial hearing suggests
that our auditory cortex applies similar techniques in its auditory scene analysis
Auditory scene analysis
In psychophysics, auditory scene analysis is the process by which the human auditory system organizes sound into perceptually meaningful elements. The term was coined by psychologist Albert Bregman...
, which explains why these methods work.
The major benefits of parametric decoding is a greatly increased angular resolution and the separation of analysis and synthesis into separate processing steps. This separation allows B-format recordings to be rendered using any panning
Panning (audio)
Panning is the spread of a sound signal into a new stereo or multi-channel sound field. A typical physical recording console pan control is a knob with a pointer which can be placed from the 8 o'clock dial position fully left to the 4 o'clock position fully right...
technique, including delay panning, VBAP and HRTF-based synthesis.
Parametric decoding was pioneered by Lake DSP in the late '90s and independently suggested by Farina and Ugolotti in 1999 . Later work in this domain includes the DirAC method and the Harpex method .
See also
- AmbisonicsAmbisonicsAmbisonics is a series of recording and replay techniques using multichannel mixing technology that can be used live or in the studio. By encoding and decoding sound information on a number of channels, a 2-dimensional or 3-dimensional sound field can be presented...
- Ambisonic UHJ format: Decoding UHJ
- Meridian Audio, Ltd.Meridian Audio, Ltd.Boothroyd Stuart Meridian is a British manufacturer of high-performance, high-fidelity audio and video components and systems. Founded in 1977 by Bob Stuart and Allen Boothroyd, its products have proved popular among music and movie enthusiasts, and more recently in the lifestyle technology and...
, manufacturer of hardware decoders
External links
- Ambisonic Surround Sound FAQ (Sections 17 and 18 for hardware decoders)
- Ambisonia website Bruce Wiggins's WAD decoders for 4.0, 6.0 and 8.0 are nearly Classic Ambisonic Decoders and easy to use plugins for Windows Media PlayerWindows Media PlayerWindows Media Player is a media player and media library application developed by Microsoft that is used for playing audio, video and viewing images on personal computers running the Microsoft Windows operating system, as well as on Pocket PC and Windows Mobile-based devices...
. - B2X Plug-Ins B2D, B2G and B2Stereo software decoders, in VST and Audio Unit formats, for Mac OS X
- Shelf Filters and Distance Compensation "Ambisonic Surround Decoder" and "SHELF FILTERS for Ambisonic Decoders" explain these important features of Classic Ambisonic Decoders for those designing software decoders
- Trifield Productions (for "Vienna" decoder licensing)
- Harpex Ltd (for stand-alone and plug-in versions of the Harpex method)