[LAU] first order ITU 5.1 preset for ambdec or second order stereo panner

Tue Apr 7 17:19:39 EDT 2009

Hello Fons,

Thanks a lot for your lengthy explanation and for sharing the preset.
I'm not that much concerned about how the stereo image will transfer
to the ambisonic soundfield but more about preservinge the general
textures and local movement of the stereo sounds. I'm panning quite a
few stereo tracks in Ardour and to split all of them to mono tracks
and have separate automations for their azimuts would be a bit of a
pain at this stage. I tried the first order preset and found the
second order much better for my purposes. So based on the code of your
first order stereo panner LADSPA plugin and the second order mono
panner I added a stereo second order panner. I can post the patch here
although I'm not sure if doing this might go against your intentions
with the AMB plugins (for my purposes it worked great).

Cheers

Hector

On Monday, April 6, 2009, Fons Adriaensen <fons at kokkinizita.net> wrote:
> Hello Hector,
>
>> Hello (Fons),
>> I was wondering if anyone has a ITU 5.1 preset for ambdec. The one
>> included is for second order only and I would like  to use the stereo
>> panner included in the LADPSA AMB plugins by Fons. A second order
>> stereo panner would be great too.
>
> I delayed replying to this because the answers are
> not simple.
>
> I do have a 1st order ITU 5.1 preset, it's included
> at the end of this message. But you should consider
> the following. If you are panning individual sources
> into first order AMB then the results on a 5.1 rig
> could be inferior to what is possible using normal
> 5.1 panning. It depends a bit on what you expect -
> the 1st order images will always be more 'diffuse'
> than 'real' 5.1, OTOH, the imaging quality will be
> more constant as a function of position, and sound
> will appear less to come 'from the speakers'. First
> order to ITU 5.1 works well with signals recorded
> with an AMB microphone, but for panned material you
> want 2nd order.
>
> Regarding the stereo panner, it is equivalent to
> two mono panners, just with a different interface.
>
> But panning stereo into 2nd or higher order AMB may
> not give the result you expect. It is *not* a correct
> mapping from stereo to AMB - this is impossibles as
> essential information is lost when encoding to stereo
> (if it were possible then higher order AMB could be
> encoded without loss into two channels...).
>
> What happens is that you encode two source directions
> (the stereo speakers) correctly, and all the rest is
> just a linear combination of these. The AMB reproduction
> system will create two 'virtual speakers' for your stereo
> signal. The higher the order the more accurate this will
> be, but the acoustic result is just stereo. A central
> sound will be as 'fuzzy' as would be just using normal
> stereo reproduction. Even if your AMB rig has a speaker
> near to the central position it just won't be used, or
> very little, because you didn't pan anything there.
>
> How much this matters depends on how you use it.
> It will not be worse than stereo, but not better
> either while with higher order it _should_ be better.
> For first order the errors are relatively small
> for a normal stereo angle of 60 degrees. As order
> goes up, the maximum stereo angle that will give
> near to correct encoding gets smaller. Going above
> that will just lead to the 'stereo over ambisonics'
> situation explained above.
>
> For higher order, if you stereo signal is itself
> the result of panning individual sources, you should
> just skip the stereo step, and pan the same sources
> directly into AMB.
>
> Ciao,
>
>
> # AmbDec configuration
> # Written by MakeDec-0.7.0 at Tue Apr  7 00:04:23 2009
>
> /description      5.0 First order decoder
>
> /version          1
>
> /dec/hor_order    1
> /dec/ver_order    0
> /dec/freq_bands   2
> /dec/speakers     5
> /dec/coeff_scale  fuma
>
> /opt/input_scale  fuma
> /opt/nfeff_comp   input
> /opt/delay_comp   off
> /opt/level_comp   off
> /opt/xover_freq     300
> /opt/xover_ratio    0.0
>
> /speakers/{
> #            id      dist     azim     elev     conn
> #-----------------------------------------------------------------------
> add_spkr    LS     1.500  110.000    0.000    alsa_pcm:playback_1
> add_spkr    LF     1.500   30.000    0.000    alsa_pcm:playback_2
> add_spkr    CE     1.500    0.000    0.000    alsa_pcm:playback_3
> add_spkr    RF     1.500  -30.000    0.000    alsa_pcm:playback_4
> add_spkr    RS     1.500 -110.000    0.000    alsa_pcm:playback_5
> /}
>
> /lfmatrix/{
> order_gain     1.00000  1.00000
> #            W        X        Y
> #-----------------------------------
> add_row    0.42898 -0.32754  0.26524
> add_row    0.22140  0.24055  0.37042
> add_row    0.08957  0.21362  0.00000
> add_row    0.22140  0.24055 -0.37042
> add_row    0.42898 -0.32754 -0.26524
> /}
>
> /hfmatrix/{
> order_gain     1.00000  1.37000
> #            W        X        Y
> #-----------------------------------
> add_row    0.67181 -0.19974  0.22776
> add_row    0.40690  0.17925  0.23360
> add_row    0.23133  0.29002  0.00000
> add_row    0.40690  0.17925 -0.23360
> add_row    0.67181 -0.19974 -0.22776
> /}
>
>
> /end
>
>
> --
> FA
>
> Io lo dico sempre: l'Italia è troppo stretta e lunga.
>
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