After request, Ralph Glasgal gave me permission to post some private
mail on sursound.
This is all highly interesting, IMO...
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Thanks for sending me this directly. ...
The idea is to deliver to the ear what it would have heard in a concert
hall. What I was discussing here is just the frontal direct sound plus the
very very early reflections from the rear half circle. RACE in front
removes the crosstalk and so allows the full value of time and level
differences to reach the ears in front. But the pinna region is not fully
accommodated with just two front central speakers. So I blur this frontal
bias with a pinna signal from the rear and this allows the brain to localize
to the far sides using the lower frequency cues. But this full range rear
signal turns out to increase envelopment as well as frontal stage width.
Yes, if you crosstalk cancel any signal you lower the IACC but we are not
talking here about long time hall reverb or diffuse fields. If you turn off
RACE in the rear the only thing that you are doing is making these new
discrete rear vectors have low and indeed erroneous values of ITD and ILD
and worse have the same static values for all frontal source positions.
Again this rear sound has nothing to do with recorded hall ambience.
I think it is fair to say that if you want realism you need to deliver to
the ear what it hears in everyday real life. So using a pair of rear
speakers this way is just one step to delivering a normal sound field to a
listener in the home from standard recorded media. With six speakers you
can have a full circle of sound in the horizontal plane, lots of
envelopment, with the third pair delivering rear hall ambience with good
IACC values if you crosstalk cancel it. For 2.0 files you can have surround
hall ambience using speakers and a convolver to put the frontal enveloping
direct sound field in a large concert hall and get height reflections if you
are into that.
Again this envelopment effect has nothing to do with any reflections
included in the recording. These are likely much later than what is
involved here. In a concert hall you have directional reflections coming
from nearby seats and heads for each instrument in the orchestra and so the
rear speakers mimic this effect. Ambiophonics also treats discrete recorded
reflections as if they were additional direct sound sources and so delivers
them to the ears with reasonable values of ILD and ITD as well as better
pinna angles. I would not use the term decorrelation. This is just
eliminating localization cue distortion caused by having speakers at 30
degrees or 110 in the case of 5.1.
I hope this helps.
Ralph
-----Original Message-----
From: Stefan Schreiber [mailto:st...@mail.telepac.pt]
Sent: Saturday, May 14, 2016 1:47 PM
To: Surround Sound discussion group
Cc: glas...@ambiophonics.org
Subject: Re: [Sursound] Envelopment
Peter Lennox wrote:
Hi Ralph! - long time since I saw you on this site - nice to hear from you.
Just a suggestion: it seems to me that what you are doing is affecting
inter aural cross-correlation (IACC) - and, as this is known to affect the
perception of 'spaciousness' - and spaciousness is related to 'envelopment'.
I wonder: have you come across the paper "Fixing the Phantom Center" by
Earl Vickers? - he describes a method of using all-pass filters to control
IACC, that can make imagery slightly less 'pinpoint-y' (and what exactly is
a point source anyhow?) but more stable for off-centre listeners. Although
this is a technique used for stereo, I've a hunch it actually does something
related to what you're doing.
Effectively, I think you are decorrelating the rear stereo feeds, and thus
preventing image-perception to the rear, and bringing out the ambience
information.
I would have assumed the same.
However, it is hard to see where any decorrelation is supposed to take
place!
For some time now, I have been using Ambiophonics and the RACE program
to generate a sense of envelopment by using two speakers behind the
listening position at an angle of about 20 degrees and only two meters or so
away from the main listening family size area.. This rear speaker pair is
fed a duplicate of the front pair and is just crosstalk cancelled using
somewhat different parameters than used for the front 2.0 pair. Neither the
level nor the delay of the rear pair is critical within reason which is a
rather remarkable result.
(cited from Ralph G.'s posting)
X-talk cancellation (per se) doesn't seem to take out any directional
information.
There must be s.th. more interesting here...
Is it that you bring out reverb but also some side/rear-side/rear
reflections, which have been "mixed" into the front 2.0 pair?
Maybe not in some < correct > way - but it is also not exactly correct to
present these parts from the front. So you could speak of some
improvement(s)?
Another problem for this (admittedly crude) speculation is hat stereo
recordings can be done in such different ways...
So my interpretation could be quite wrong, as well.
I just wanted to say that Ralph's described effect and observations can't be
related to (meaningful or complete) decorrelation. And I don't see where
IACC figures are supposed to change a lot.
Best regards
Stefan
I've accidentally done this with some crosstalk filters (set up wrongly) -
the centre imagery was depressed but there was an enormous and compelling
increase in the perception of depth of field (perhaps related to Neher,
Brooks and Rumsey's ensemble depth). Now, you say you're using slightly
different settings for your rear cross-talk pair - I wonder if you're doing
something similar?
cheers
Dr. Peter Lennox
Senior Fellow of the Higher Education Academy Senior Lecturer in
Perception College of Arts University of Derby
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