Hi Jacob,
refering to your mail, it seems to me that there is a certain anount
of confusion here.
At first: Of course you can insert a 1st order signal into a 2nd order
one, but it is of course not 100% correct to reproduce FOA via a 2nd
order decoder.
In this sense, it is not really accurate what you wrote:
A first order signal
for example can be reproduced without drawbacks (and actually without
any additional processing) at second order. However, one should not term
this "upmixing", since the higher order components ("B-Format") of the
Ambisoncs signals will simply be left at zero (since this information is
not available).
Reason:
Decoding the “zero-stuffed” FOA —> SOA signal via a 2nd order decoder
gives a different result than decoding FOA via a 1st order decoder.
Secondly, nobody in this thread has called this “upmixing”. At least not me.
Nonlinear decoders: Just to give an idea, these decoders usually
perform some DOA (direction of arrival) analysis, usually separating
in time/frequency bins.
There is ample and free literature available about DirAC, HarpeX and
COMPASS, so I suggest to read a bit.
Otherwise you wrote about linear decoding, not upmixing. (Now without
quotation marks.)
Best,
Stefan Schreiber
----- Mensagem de Jacob Hollebon <j.holle...@soton.ac.uk> ---------
Data: Sat, 24 Oct 2020 18:02:13 +0000
De: Jacob Hollebon <j.holle...@soton.ac.uk>
Assunto: Re: [Sursound] ORTF-3D With Higher-order Ambisonics
Para: sursound@music.vt.edu
Hi everyone,
Long time reader of this list, but my first contribution, so I hope it helps!
‘Nevertheless, there is one aspect to distinguish when looking solely at
the reproduction part of an Ambisonics pipeline. A first order signal
for example can be reproduced without drawbacks (and actually without
any additional processing) at second order. However, one should not term
this "upmixing", since the higher order components ("B-Format") of the
Ambisoncs signals will simply be left at zero (since this information is
not available).
Such decoded reproduction signals could nevertheless utilize the higher
dentistry of for example a second order loudspeaker grid. If this
actually yields any technical or perceptual benefits I don't know, since
I am not so familiar with loudspeaker reproduction.
Regarding binaural rendering "directly in the SH domain", there is no
reason or benefit to render at a higher order than the source material.
For the case that the binaural renderer follows the "virtual
loudspeaker" approach, the only limiting factor of the reproduction
resolution (following the resolution of the source material) would be
the utilized HRTF data set.’
This is a very interesting problem. You can absolutely do this, but
depending on your decoder you can have issues e.g see Solvang -
‘Spectral Impairment for Two-Dimensional Higher Order Ambisonics’.
Assuming you are using mode matching, you have a loudspeaker array
that can control higher orders of the soundfield, but you zero these
high orders in your signal. Therefore, the decoder choses the
min-norm energy solution that can reproduce this odd target
soundfield, and in doing so sets these altered higher orders in the
reproduced field to zero. This can create a drop in energy further
away/at higher frequencies from the central reproduction point due
to the N=kr rule. Normally, you would have reproduction
approximately correct up to a radius N=kr, then spatial aliasing
kicks in. But here, you have an odd low energy, or ‘silent layer’
between the correctly reproduced sphere and the aliasing. So
actually, it’s better to ‘down-sample’ your loudspeaker array to
first order. I believe this is analogous to Berhnschutz’s findings
in ‘Binaural Reproduction of Plane Waves With Reduced Modal Order’,
where it is shown you are better off down-sampling a HRTF before
direct rendering in the SH domain. You’ll have the same issue with
virtual loudspeaker rendering too as the relationships are the same
mathematically.
Jacob Hollebon
Postgraduate Research Student
VAAE, Acoustics Group
ISVR, Building 15, Room 1053
University of Southampton
Email: j.holle...@soton.ac.uk<mailto:j.holle...@soton.ac.uk>
Website: http://www.southampton.ac.uk/vaae
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