Ambiophonics - (Fig1) loudspeaker-binaural reproduction
of 2-channel LP/CD/MP3/SACD, etc., which processes a stereo (2.0)
pair of signals and presents them to a centered listener by an
Ambiodipole (closely-spaced speaker pair) in front. Optionally,
image width may be augmented by duplicating the signal pair
in a rear Ambiodipole. Additionally, to restore a natural level
of ambience that has been deliberately reduced in commercial
stereo releases, two or more Ambiostats (surround speakers) driven by
hall impulse response convolvers DSP provide 2D or even 3D
surround reverberation based on two media channels (including
most legacy stereo recordings on CD, LP, games or broadcasts).
Ambio - colloquial usage, as stereo is for stereophonics.
Panorambiophonics - (PanAmbio 4.0/4.1, see Fig2 ) Uses
independent front and back Ambiodipole pairs of speakers to
reproduce 360° sound fields from 4.0/5.1 media. Records
and replays two Ambiophonics stages – a front stage typically
of direct and frontal reflected sounds, and a stage behind the
listener for reflected and ambience sounds, along with any
direct sources around the sides and in back. In contrast to 5.1,
PanAmbio is isotropic (equal in all directions) in the horizontal
plane. It renders accurate 360° localization ±5° even at the extreme sides and avoids localizing to 5.1’s speakers at 110°.
Perambiophonics - (PerAmbio 6.0/6.1) Full 3D
reproduction which is spherically isotropic, as with live, natural
human hearing. TriAmbio adds a third, elevated Ambiodipole
for reproducing height using special 6.0/6.1 recordings.
Stereo, stereophonic – 2-channel audio reproduction
attributable to work in the 1930s at Bell Labs and EMI.
“5.1” – international surround standard ITU-R775 for 5 to
7 speaker channels plus Low Frequency Effect (LFE, aka “0.1”)
for home theaters, evolved from multi-channel cinema sound
since the 1939 movie “Fantasia.” PanAmbio is 5.1-compatible;
PerAmbio and TriAmbio are 6.1/7.1-compatible.
Binaural hearing - (normal, everyday hearing mechanism) – this term is also often used to describe lifelike reproduction recorded with a
surrogate “dummy head” with microphones at its ear positions,
then played back using headphones. When reproduced directly into
the listener’s ear canals such as with intra-aural ear receivers
(“ear-buds”), the dummy head has generic pinna that might not
work with an individual listener. Pinna-less microphones such
as the PanAmbiophone, reproduced over loudspeakers, involve
listeners’ own pinna, and so satisfy life-like perception for all.
ILD, ITD – interaural level & time (phase) difference in
binaural ear signals from a sound source, registered by auditory
sub-brain structures, which contribute to the perceptual qualities
of localization, spatiality, timbre. About equally splitting the
audible spectrum (but with much overlay), ITD is the controlling mechanism for sounds
with frequencies in the five octaves below 700Hz and ILD for
the five octaves above 700Hz (depends on listener’s head size).
Localization (imaging) – the ability of humans (using
IDD, ILD and pinna binaural cues) to identify within a sphere of possible arrival
angles the direction of origin of sounds, both those that arrive
along a line from the source (direct sounds) or of temporally
(echo) distinguishable reflected (indirect) sounds. Acuity within an
ellipse in front of the listener is about ±1° horizontally and ±10°
vertically.
LEV (listener envelopment) – a sense of being surrounded
in 2-dimensions (or better yet immersed in the full-sphere 3D of
human hearing); essential to human perception of the space
containing the listener and sources of sound. This quality of
space is largely missing in conventional stereo reproduction,
but is much improved using Ambiophonics, where the ambience
sounds especially may be localized within direction-dependent
regions of maximum LEV (see Fig1). PanAmbio surround is
capable of regenerating more LEV than 5.1 surround (see Fig2).
Spatial resolution – the degree, in contrast to either front-only
stereo or to 5.1, by which 2D ambio, panambio surround sound,
or full-sphere 3D preserve individual voices amid the ensemble and allow listeners to perceive separation of voices and events, as in
live hearing. Heroics by stereo mixers to maintain audible
“layers” (singer discernable from instruments, etc.) are needed
far less given the greater spatial resolution of surround sound.
Reverberation – Physically, the acoustical impulse
response of a reflective room or space. The more reflective or
larger the space, the longer lasting the reverberation.
“Reverb” – jargon connoting artificial reverberation,
sometimes acceptable, but often phony-sounding. Realistic
results are either directly recorded in a reverberant space, or
carefully convolved using a 2D or 3D hall impulse response.
Impulse; Impulse Response (IR) – momentary non-zero
event (e.g. hand-clap, gun-shot, balloon burst, or electronic
"Dirac” pulse); which characterizes electro-acoustic systems –
its Impulse Response (“IR”, e.g. reverberation). Each sample
of a digital recording can be considered a Dirac impulse, which
spawns its own IR by convolution (e.g. hall ambience by DSP).
Convolution – using a digital recording and applying DSP
signal processing to “imprint” an impulse response on a source
signal. For example, multiplying and adding to the samples of
an instrument or voice the acoustical impulse response
of a hall as though the source had been recorded in that hall.
DSP – digital signal processing. Used in Ambiophonics
for crosstalk cancellation and optional ambience convolution.
DSP for “XTC” include single delayed inversion (Wareing),
inverse impulse response (Farina), band-splitting (Choueiri),
and RACE (Glasgal). DSP for generating ambience convolves 2channel
stereo recordings with 2D or 3D hall impulse responses.
Pinna – the outer ear, whose fleshy
convolutions cause peaks and dips in the frequency response depending upon the 3D angle of
arrival of a sound. Together with ear signal difference ILD and
ITD, pinna filtering encodes direction for the conscious brain to
perceive localization. Pinna contradiction
results when reproduction systems (notably conventional stereo
and 5.1 surround) create phantom images the brain interprets as
coming from a fuzzily-perceived direction despite being pinnaencoded
as coming from their actual direction, e.g. a speaker.
Phantom image – localization of a sound, the perceived
direction of which is not real but the result of “conspiring” cues,
such as two loudspeakers playing the same signal, perceived as
coming from a point between the speakers. In conventional
stereo reproduction, the common practice of “panning” a single
microphone of a soloist equally to both channels/speakers is
determined by the brain as coming from the center, although not
convincingly (due to pinna confusion) and exhibiting a raspy
comb-filtering. Phantom images to one side of center
are drawn in non-linear fashion toward one speaker or the other.
Comb filtering – distortion of a sound’s spectral character
(frequency response) due to wave cancellation alternating with
reinforcement when the identical signal arrives simultaneously
from two points (e.g. speakers). Comb filtering due to crosstalk
in stereo affects the frequency response at each ear of important
central voices (e.g. soloist, dialogue), and unfortunately causes
some mixing engineers to compensate by boosting equalization of the
isolated central sound around 2kHz that permanently scars the
recording for headphone or Ambiophonic replay. (The center
“C” channel/loudspeaker solves this condition for 5.1 surround.)
Crosstalk – unintended presence of reproduced sound from
any speaker to the ear on the opposite side. Avoided by using
headphones, speaker crosstalk at either ear is accompanied, for
important central voices, by duplicate sounds, delayed in time,
that interfere with earlier sound from the speaker on the same
side, resulting in raspy-sounding comb-filtering and pinna-confusion
in localizing the sound.
Crosstalk is the principal impetus for Ambiophonics solutions.
Crosstalk cancellation (XTC, CTX) – Ambiophonic
solutions address the problem of crosstalk: if it can’t be avoided
it in the first place, cancel it. Approaches to XTC all attempt to
segregate speaker sounds to the intended ear. Methods over the
years include a physical barrier projecting from the listener’s
nose, adding speakers recessed and therefore delayed in time,
and digital signal processing to introduce cancellation signals
into the audio path, as with Ambiophonics’ RACE.
RACE – (Recursive Ambiophonic Crosstalk Eliminator)
method developed for creating (using DSP) properly timed and
attenuated recursive crosstalk cancellation signals. Crosstalk is cancelled acoustically at listeners’ ears.
