Antonio:
I totally agree with your comments. It sounds like you and I are thinking
along the same lines.
Here's the way I see it:
Actually, the holophone has a very good design if one is only listening via
the two forward speakers (using only the signals from the front left and
right mikes). It is basically a binaural setup, similar to the SASS, and
will produce a very nice sound field using only two speakers. And it might
produce quite acceptable 5.1, but this is not truly surround and don't let
anyone convince you otherwise.
I will limit my discussion to a four channel listening setup where there ar=
e
front right, front left, rear right, and rear left speakers. The addition o=
f
a front center speaker would add too much confusion to our thinking, at
least right now.
The problem arises when one adds two rear channels that are extremely
"correlated" to the two front channels. These correlated signals coming out
of the rear speakers will interact with the front speaker signals "at the
central listening position" and cause things to go a bit haywire, primarily
due to complex phase interactions. The rear signals would degrade the
frontal signals and vice versa.
This is why, in most 5.1 productions, the rear channels contain mostly
uncorrelated material added later in the studio. In this case, the rear
speaker signals have no effect whatsover on the front speaker signals and
front imaging remains excellent and is not compromised by the rear channel
waveforms. Usually, some form of gentle ambience is placed in the rear
channels, to add a sense of spaciousness but to not detract from a stable
frontal sound experience.
If one desires to record all surround channels simultaneously as we desire
for surround nature recording, and possibly portray an accurate 360 degree
soundscape, then problems of correlation of front versus rear become a
primary issue.
One possible solution is to put a delay on the rear channel signals in the
studio after the recording is made. This will have the effect of forcing al=
l
correlated material to the front because the human brain gives precedence t=
o
the signals arriving first at the ears. Uncorrelated "ambience" is all that
should then be experienced from the rear speakers. Of course, this in no wa=
y
reproduces a natural listening field where primary sound sources occur all
the way around the listener.
Another more elegant solution is to de-correlate the front signals from the
rear signals by using a large barrier between the front mikes and the rear
mikes when the recording is made. This barrier will reduce correlation and
reduce interference between the front signals and the rear signals (be awar=
e
that we are concerned only with wave interference that occurs at the centra=
l
listening position).
This is rough explanation of why barriers between the front and rear are
important, at least when one is trying to record surround as it occurs in
nature. It all relates to the interactions between waveforms that occur at
the listening position, at the listener's ears, in the indoor listening
situation. It is difficult enough to generate a really nice experience usin=
g
just two frontal speakers. Adding additional speakers is asking for trouble=
.
This was one of the major problems with quadraphonics
(www.audiosignal.co.uk/Resources/
What's%20wrong%20with%20quadraphonics%20A4.pdf).
This is only a partial explanation and I have not addressed at all the
problems that occur when the listener turns his/her head toward either side=
,
thereby changing what is defined as "front". In other words, if the listene=
r
turns his head (or head and body) 90 degrees to the right, then the front
right speaker now becomes "front left" and the right rear speaker now
becomes "front right". For imaging to remain stable, signals from this new
speaker pair must not be compromised by signals from the remaining speaker
pair.
I have been envisioning a quad setup that really does produce a realistic
360 degree soundscape, allowing the listener to actually turn in any
direction and still hear a stable soundfield. This can only be accomplished
if there are adequate barriers between not only the front and rear channels=
,
but also between the right side pair and the left side pair. In other words=
,
there actually would be no "front" or "rear" defined in this kind of setup.
"Front" would be the direction that one faces.
I truly believe this can be accomplished by recording simulataneously only
four channels, and through the careful use of "barriers" between microphone=
s
that serve to reduce correlations and interference between any front mike
pair and the corresponding rear pair (in this kind of symmetric 4-mike
setup, there would be four possible mike pair combinations).
This is all very complicated, to be sure. But the notion of waveform
interference at the listener's ears is critical to any understanding of the
challenge. When folks first started working with Quad, they were very naive
in thinking about these kinds of issues. That is why most quad experiences
were entirely constructed in the studio, where discrete signals could be
made to come out of only one speaker or a speaker pair, thereby eliminating
interference from the other two speakers.
I have thought up a basic four channel miking design that combines the best
principles of binaural with equal attention to the matter of de-correlating
mike pairs as much as possible. The result is a kind of "quad-SASS", but
with the addition of somewhat larger barriers between any of the four mike
pairs. I tested a version of this at the Nature Sounds Society workshop in
May and the results were excellent, though they have not scrutinized in
detail.
We made a test recording where a person walked and talked all the way aroun=
d
the mike array, circumscribing a complete 360 degree circle. The results
were amazing. We found when doing playback in the studio using a quad
speaker array, that a listener who faced forward throughout was able to
accurately track the person's movement all the way around the 360 degree
soundfield. But what was really impressive is that the listener could also
continually turn and face the walker, and also track his movement all the
way around with great accuracy. For the soundfield to remain stable in this
kind of situation, where the listener turns and faces in different
directions, is exactly what we're after. In typical 5.1 it is ALWAYS assume=
d
that the listener will face forward, and the soundfield generally goes
completely bonkers if someone turns to face the sides.
As nature recordists, we desire surround that is truly encompassing and tha=
t
allows a listener to face in any direction he pleases. In fact, imagine a
circular listening room where speakers are hidden and where there is no
defined FRONT. The soundscape begins to play and the listener faces toward
different singing birds, appreciating each in turn and never worrying one
whit about front, or side, or rear. This is the way we experience sound in
nature and this is what we ultimately want to hear indoors. And this is
where my brain has been set to work. My question is how to record so that
one can re-immerse oneself in a fully encompassing soundscape that closely
resembles what one actually heard.
I truly believe this is possible using just four omnidirectional mikes
coupled with some well thought-out barriers. And this would be mirrored by =
a
four speaker quad setup in the home. Maybe I'm nuts, but this is what I
believe right now. I am not arguing for a rebirth of Quadrophonics, at leas=
t
not as it was originally conceived. I'm arguing for a truly natural
listening experience that can be accomplished with just four mikes and four
speakers.
And the Holophone design won't do this for me. It does not employ the prope=
r
use of barriers and the mike spacings and positionings do not follow solid
binaural psychoacoustical rules. Maybe the Holophone will produce a
reasonable 5.1 experience. But we nature folks want way more than that.
Remember that 5.1 was designed for watching movies, where most of the sound
is frontal and where the rear speakers only convey uncorrelated ambience,
and where the listeners are seated and facing forward. 5.1 is NOT AT ALL
designed to produce a true surround experience!
We nature recordists are better off devising our own mike systems where we
can use quiet mikes and get a more stable 360 degree listening experience, =
a
listening experience that wows us almost as much as the natural event did i=
n
the first place.
Antonio, keep me abreast of your work. My only suggestion is to keep mike
elements ear-spaced, or about 6.75" apart, which will help preserve
important binaural cues.
Lang
Lang Elliott, you wrote,
>Also, while the design looks really neat, there are
some inherent problems
>in placing the rear mikes on a "head" without any
barrier between the rear
>and the front mikes. This will result in phase
problems between the fronts
>and rears. Maybe the result sounds okay when played
in 5.1, but the design
>is not clearly a great one.
--- Dan Dugan <> escribi=F3:
I don't understand why, if not having a barrier
between front and
rear causes phase problems, not having a barrier
between left and
right doesn't cause phase problems! I don't think you
need that
barrier.
-Dan Dugan
Hi all
I don't know what kind of mic is using the Holophone
and how is the position of each mic. But I think the
Holophone should work good without the barriers cause
is made for TV, Film-makers, etc.
But if we're talking about 4 ch, I think the barrier
depends on the needs of the recordist. Probably having
the same separation between fronts and rears and
rights and lefts we can have a truly sense of
soundscape (having working each mic in the same way to
the others; again thinking in 4 ch). In my case, what
I'm trying to do is to get the best sense of location
of each sound recorded (p.e bird songs and calls to
monitoring them) and for me the barriers work great.
But if the idea is to record soundscape for TV or
other things probably the Holophone is a good option
and the barrier is not necesary (thinking that a
person is going to be listen sitting in a chair).
Recently, I did some little experiments with 4 ch and
doing comparisons using different material to separate
the mics, different sizes of material to separate,
different distances between mics and different
positions of mics (horizontally and vertically and
angles). After this "home-made" tests, one of MY most
important things is to have in each mic a good
isolation from the others (at least the necessary for
your needs). In the case of the Holophone the
separation between right front and left front is the
face shape (or E.T face) but the case between fronts
and rears is different, what it means the rear mics
are working in a different way than the fronts but
because is made for TV and broadcasting probably is
fine (sorry about my technical vocabulary, english is
not my first language).
Another thing that is really true is the quality of
the mics. One of the problems that I have is that I
have been using a very noisy mics and I really can
listen this problem.
I think it will be good to test the work of the
barriers in 4 ch (all different options like sizes,
materials, etc) to figure it out if they really work
and what is best.
I hope the expertise can make suggestions
Antonio Celis
=3D=3D=3D=3D=3D
Antonio Celis Murillo
Center for Conservation Biology
University of California Riverside
208 University Laboratory Building
Riverside, CA 92521-0334
(951) 827-5484
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