On 2011-12-05, Eric Carmichel wrote:
[...] noteworthy responses, particularly regarding my comment that
most binaural recordings that I’ve listened to don’t give a sense of
“open space.” Naturally, we all have a unique HRTF, and recordings or
IRs made with an acoustical test fixture (e.g. KEMAR) probably won’t
match our own HRTF.
Then it's an open research question to you scientific types, where the
discrepancy actually lies. Isn't it? Like, most of the research in here
goes with personalized HRTF's (which don't seem to work either), head
tracking (which does seem to work, but not perfectly), the obvious
theoretical stuff which nobody has even tried (e.g. auditory parallax
while headtracking directions as well), and whatnot which we have't yet
thought up (that's your job). :)
Recordings made with KEMAR (Knowles Electronic Manikin for Acoustic
Research) have the microphones deeply seated in this fixture. Such
recordings will have a “naturally occurring” resonant peak around 3
kHz because of the KEMAR’s pseudo ear canal (which, for KEMAR, is just
a straight tube, with or without Zwislocki couplers).
In the "we don't know yet" department, nobody's ever proven KEMAR-like
modelling is correct. I mean, it relies on a one-dimensional
approximation of the auditory canal, which might not hold in full at the
highest frequencies. Especially if you happen to believe ultrasonics
have something to give us, like many audiophiles do, especially in
spatial reproduction. What if the audiotory canal itself stops being
one-dimensional at HF and becomes a 2D waveguide in crossection? It
isn't as though transverse vibrational modes couldn't be transmitted
through the tympanic membrane, or as though they couldn't then
theoretically, differentially affect the cochlear microcilia, as a minor
transverse induced mode of excitation...
There I'd like to bring up something we already know about natural
excitation of the microcilia: we already know those neural cells nor
those which follow them in the auditory nerve or the lower auditory
nuclei won't depolarize at a rate exceeding some 1kHz or so. Nor do they
have much S/N ratio an sich; nor are they fully uncorrelated.
Thus we already know the afferent auditory pathway cannot *possibly*
carry all of the auditory information we already know it must, if we
think about it simplistically. Thus, the first two things *I'd* really
like to know about the peripheral auditory system are: 1) how precisely
is temporal coding used in excess of the cochlear place coding which we
already know, and 2) how does the feedback modulation via the efferent
auditory pathway participate in that coding, and the place coding, in
whole?
The first part, temporal coding, as the precise instance of neural
firing after the resonant but also place-wise stochastically (fully so?)
rectifying cochlear membrane *must* somehow carry information which is
particularly relevant to dichotic hearing. That is almost self-evident
when you compare the resonance characteristics of coclea in vitro
against the dichotic listening experiments of old. No amount of envelope
detection could get you to such low angular resolution in
dichotic/binaural hearing, ever, unless it was mostly being derived from
the exact, continuous time onset of the neural pulses. Possibly all over
the board. (Which then also makes supersonics relevant, at least in
theory.)
Now that I've read some basics of cochlear implant tech, I don't see how
such considerations are taken into account. Thus, Eric, since you seem
to be worried about the effects of real life background noise on CI's,
maybe you could go double the mile by trying out a CI analysis algorithm
which hybridises your typical Shannonesque noise band vocoder with a
selective application of pure, rectified, time-domain information,
straigh from the sampler? Perhaps a sampler with vastly more bandwidth
than you commonly use for CI purposes?
The style of headphones we use may destroy the ear canal’s natural
resonant peak, particularly if the headphones are of the insert type.
Absolutely. Been there, done that. That's why I always go with open,
gently grabbing designs. And even they kill the outer ear and upper body
response which is so crucial to a real HRTF. (The "R", "related", is
because it's not just about the head, but always about the upper torso
as well.
And to wit, I've never seen anybody study what *really* happens to the
response when you twist your head while keeping your torso/body intact.
We can do that, we do do that, yet no head tracker even that I've seen
takes any notice of the fact. Once again, it could be a valid research
area.)
Otherwise, we may have to use a peaking filter to re-create an
open-ear type of response.
Of course they try to as best they can. But can they really emulate all
of the relevant degrees of freedom which this sort of thing might
require? Do we even know which the relevant de
