Hi,
Thanks for finding that. I'll have to dig out the papers and see if I
can find more info, though I remember being hopelessly confused last
time I looked at this, several years back.
In the meantime I'll carry on just using the straight UHJ to B-Format
equations. These work very well on 'normal' stereo recordings:
instant ambisonic surround. How accurate is impossible to say,
especially as none of these recordings have ever been 'surround', and
the original event is totally inaccessible or even never existed.
It seems to work best on recordings made with a crossed pair, or even
pan-potted mono. Hardly surprising really.
Ciao,
Dave
Date: Sat, 7 May 2011 13:54:01 -0600
From: Martin Leese <martin.le...@stanfordalumni.org>
Dave Hunt <davehuntau...@btinternet.com> wrote:
One of these complexities is presumably indicated by the phrase "Note
that two-channel UHJ requires the player to use different shelf
filters than for three- and four-channel UHJ (and B-Format)" at
http://en.wikipedia.org/wiki/Ambisonic_UHJ_format.
Any clues as to why this should be so ?? If we get back something
close to three-channel B-Format, why should it be filtered
differently from a straight three-channel B-Format ?? The encoding/
decoding has no frequency conscious components. Admittedly, the gains
of the filters may need to differ between 2D and 3D decoding.
See the thread on this mailing list from
December 2008 with the Subject "decoding
an UHJ-B stereo file to Ambisonics Quad".
The thread included this post from Geoffrey
Barton:
<quote>
Date: Tue Dec 30 12:16:42 EST 2008
Subject: [Sursound] decoding an UHJ-B stereo file to Ambisonics Quad
Sorry to shatter your simple world, but the shelf filters do have to
be different for 2 channel UHJ!
As the BHJ-> 'WXY' decoding has magically produced three channels from
two, there is information missing. This manifests itself as phase-
shifts between the derived WXY signals.
If you optimise the shelves to maximise rv at lf and ev at hf using
these decoded signals, the resulting values are not the same as for b-
format. You also have to watch the way MAG quoted these values as
sometimes he quoted the 'kernel' values for b-format rather than the
'normal' b-format.
Geoffrey
</quote>
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