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Please also note that in Ambisonic convention positive x (x =1 in cartesian
coordinates) is infront and positive y (y =1 in cartesian coordinates) is
90° left to the listener. The counting according to that convention goes
anti-clockwise. Usually sounds are encoded that way for ambisonic and as
far as I can see, the source-code for bformenc works that way too. It is a
matter of definition where to put x= 0° in space, though. The maths of the
code works the same, no matter where place x= 0° in real space.
Sound has to be decoded according to the same convention as the encoding was
done. As one does not sometimes know, according to wich convention the sound
has been encoded, if there are several flavors, decoding should follow the
usual convention.
The manual for beformenc does not follow this usual convention.
For beformdec the stereo-setup decode is a good example to see that counting
is starting infront of the listener with 0°. Counting there should be anti-
clockwise, to align with the general convention. In the manual there is no
trace, if counting is clockwise or anti-clockwise, though it has to align
with the way, the position of sound was encoded.
I wonder if the bformdec source is independent of any convention, as long
the decode is consistent with the way the sound has been encoded. I'll leave
this proof to a mathematician.
In practice, besides the above, the bformdec-opcode is not usable for me:
The employed "Spherical Harmonic" decoding equation works well by definition
only for a large number of speakers, which might only be given for the
8-chnl - circle setup. John ffitch has added the "Controlled opposites"
equations as far as I can remember. This is useful for the square setup, but
fails for asymmetric speaker placement as 5.1 and the asymmetric and
distorted cube given in the decoding presets of bformdec.
see
http://www.york.ac.uk/inst/mustech/3d_audio/secondor.html
and
http://www.muse.demon.co.uk/ref/speakers.html
Jan Jacob
Anthony Kozar-2 wrote:
>
> Counterclockwise from the positive X axis is certainly standard so I would
> just change the manual.
>
> You could try contacting Samuel Groner as I do not think he is on this
> list.
> I will send you his email address privately.
>
> Anthony
>
> Victor Lazzarini wrote on 2/19/08 9:50 AM:
>
>> As far as I can see, the expected format for angles in
>> bformenc is counterclockwise, starting from the right (0),
>> as the X direction is cos(A)cos(E) and Y is sin(A)cos(E).
>> [...]
>> The manual says angles are *clockwise*, but as far as I can see
>> the code uses the conventions above. I expect the manual to
>> be wrong. Can anyone confirm/correct this?
>>
>> Now, as far as bformdec is concerned, I see a similar thing. The
>> coefficients for say, 8-channel ring are for angles (described
>> correctly in the manual) increasing counterclockwise from the
>> right. But as the manual does not say anything, we are left wondering.
>> So can this be confirmed and the manual updated?
>>
>> Also, have these opcodes been fixed? I have fixed them locally,
>> but did not commit, as they are not my code. Should I do so?
>
>
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