Ambiophonics (not to be confused with Ambisonics) is a method in the public domain that employs digital signal
processing (DSP) and two loudspeakers directly in front of the listener in
order to improve reproduction of stereophonic and 5.1 surround
sound for music, movies, and games in home
theaters, gaming PCs, workstations, or studio monitoring applications. First
implemented using mechanical means in 1986 [1][2], today a number of hardware
and VST plug-in makers offer Ambiophonic DSP [3]. Ambiophonics eliminates
crosstalk inherent in the conventional “stereo triangle” speaker placement, and
thereby generates a speaker-binaural soundfield that emulates headphone-binaural, and creates for the listener improved
perception of “reality” of recorded auditory scenes. A second speaker pair can
be added in back in order to enable 360° surround sound reproduction.
Additional surround speakers may be used for hall ambience, including height,
if desired.
Ambiophonics,
stereophonics, and human hearing
In stereophonics, the reproduced sound is
distorted by crosstalk, where signals from either speaker reach not only the
intended ear, but the opposite ear, causing comb filtering that distorts timbre of
central voices, and creating false “early reflections” due to the delay of
sound reaching the opposite ear. In addition, auditory images are bounded
between left (L) and right (R) speakers, usually positioned at ±30° with
respect to the listener, thereby including 60°, only 1/6 of the horizontal
circle, with the listener at the center. (It should be noted that human hearing
can locate sound from directions not only in a 360° circle, but a full sphere.)
Ambiophonics eliminates speaker crosstalk and
its deleterious effects. Using ambiophonics, auditory images can extend in
theory all the way to the sides, at ±90° left and right and including the front
hemi-circle of 180°, depending on listening acoustics and to what degree the
recording has captured the interaural level differences (ILD) and the
interaural time differences (ITD) that characterize two-eared human hearing.
Most existing two channel discs (LPs as well as CDs) include ILD and ITD data
that cannot be reproduced by the stereo loudspeaker “triangle” due to inherent
crosstalk. When reproduced using ambiophonics, such existing recordings’ true
qualities are revealed, with natural solo voices and wider images, up to 150°
in practice.
It is also possible to make new recordings
using binaurally-based main microphones, such as an ambiophone,[3] which is
optimized for Ambiophonic reproduction (stereo-compatible) since it captures
and preserves the same ILD and ITD that one would experience with one’s own
ears at the recording session. Along with life-like spatial qualities, more
correct timbre (tone color) of sounds is preserved. Use of ORTF, Jecklin Disk, and sphere microphones without pinna (outer ear) can produce similar results. (Note that
microphone techniques such as these that are binaural-based but without pinna
also produce compatible results using conventional speaker-stereo, 5.1
surround, and mp3 players.)
[edit]Roots & research
Ambiophonics is an amalgam of new research
and previously known psychoacoustic principles and binaural technologies.
This knowledge has enabled audio recording and reproduction that approaches the
realistic soundfield at the ears of the listener that is comparable to what one
would perceive in a concert hall, movie scene, or game environment. This level
of high-fidelity was not realizable until human hearing and acoustics
principles were thoroughly researched, and affordable PCs with sufficient
processing speed became available. At the Casa Della Musica at the University
of Parma, Italy, or at the listening lab at Filmaker Technology, Pennsylvania
USA, ambiophonics, ambisonics, stereophonics, 5.1 2D surround, and hybrid
full-sphere 3D systems can be compared for the abilities of these methods to
convey the spatiality and tone color of real perception. Developers have
provided many scientific papers and downloadable tools for implementing
ambiophonics free of charge for personal use.[4]
[edit]Results & limitations
By repositioning speakers closer together,
and using digital
signal processing (DSP)
such as free RACE (Recursive Ambiophonic Crosstalk Elimination) or similar
software,[5] ambiophonic reproduction is able to generate wide auditory images
from most ordinary CDs/LPs/DVDs or MP3s of music, movies, or games and,
depending upon the recording, restore the life-like localization, spatiality,
and tone color they have captured. For most test subjects, results are
dramatic, suggesting that Ambiophonics has the potential to revitalize interest
in high-fidelity sound reproduction, both in stereo and surround.
Additionally, ambiophonics provides for the
optional use of concert-hall or other ambience impulse response convolution to generate hall ambience
signals for virtually any number and any placement of surround speakers.[6][7]
But ambiophonics is not for theaters, auditoriums, or any large groups.
Ambiophonics can usually accommodate more than one listener since one can move
back and forth along the line bisecting the speakers. Precisely because of the
higher level of envelopment along this line, the loss of realism when one moves
away from the center line is more dramatic in the case of Ambiophonics than
stereo. The listening area can be enlarged with ambience convolution, whereby
surround speakers mimic the contributions of concert-hall walls.
Ambiophonics methods can be implemented in
ordinary laptops, PCs, soundcards, hi-fi amplifiers, and even modest
loudspeakers with consistent phase response, especially in any crossover
regions. Neither true-binaural (dummy head with pinna) recordings nor head
tracking are required, as with headphone-binaural listening. Commercial
products now implement ambiophonics DSP, although tools for use on PCs are also
available online.[4]
[edit]Surround sound
In practice in its simplest two-speaker
implementation, ambiophonic reproduction unlocks auditory cues for images of up
to 150° horizontally (azimuth), depending on the binaural cues captured in
existing stereo recordings. Multi-channel recordings made with ambiophone-like
microphone arrays to make 5.1-compatible DVD/SACD recordings can be reproduced
using just four speakers (a center speaker is obviated in ambiophonic layouts).
Allowing for the human hearing “cone of confusion” at each side, a full 360°
degree circle of perceived sound localization has been measured within ±5° of
actual source azimuth, reproducing life-like spatial envelopment and timbre
(contributed by accurate directional provenance of early reflections) of
multi-channel music, movies, and game content.[3][8][9]
Especially in the case of stereo content
where ambience has been purposely reduced (because a natural level coming from
front 60°-only is perceived as too much), additional signals for surround
speakers can be produced using a measured hall impulse response, convolved in a
PC with the two front channel signals. For full ambiophonic replay, one PC can
provide the DSP for 4-channel crosstalk-cancellation and four or more (up to 16
depending on the PC) surround speakers.[10]
The development of ambiophonics is the work
of several researchers and companies including Ralph Glasgal, founder of the
Ambiophonic Institute; Dr. Angelo Farina, University of Parma; Robin Miller,
Filmaker Technology; Waves Audio; Dr. Roger West, Soundlab; Dr. Radomir
Bozovic, TacT Audio; and Prof. Edgar Choueiri, Princeton University.
