[vorbis] About Vorbis Encoder

[Leena]

   This is as per xiph.org

"""""""""The first advantage of a polar representation is that it
effectively separates the spatial audio information into a 'point image'
(magnitude) at a given frequency and located somewhere in the sound field,
and a 'diffuse image' (angle) that fills a large amount of space
simultaneously. Even if we preserve only the magnitude (point) data, a
detailed and carefully chosen floor function in each channel provides us
with a free, fine-grained, frequency relative intensity stereo*. Angle
information represents diffuse sound fields, such as reverberation that
fills the entire space simultaneously.""""""

<p>As per your explanation --------->
Hence the magnitude represents the strongest common value while the
angle represents the difference (diffuse sounds) of the other channel.

So is that as in spatial audio, the audio signal is repesented with point
 in polar form )
( that represents actual intenstity ) and angle ( that repesentes diffuse
image ie the difference
between the actual aduio signal and its reverberation , echo ) , Vorbis
repersent L , R channels
in angle and mag form ( polar .) So we don't have to do with any spatial
information of audio signal. It is given only as example ....
Just we are using polar repesentation as that is used in spatial audio
represetation .

Do correct me If I am wrong .

Thanks

Leena

<p><p><p><p>-----Original Message-----
From: owner-vorbis at xiph.org [mailto:owner-vorbis at xiph.org]On Behalf Of
Stephen So
Sent: Friday, February 27, 2004 3:48 PM
To: vorbis at xiph.org
Subject: Re: [vorbis] About Vorbis Encoder

<p>I'm no expert in Vorbis internals but I have been trying to learn more
to do my own tunings.  I can tell you what I know and hopefully someone
more knowledgeable can correct me if I make an error. :)

Currently, the Vorbis encoder use a mix of lossless stereo and point
stereo for 2 channel encoding.  If there are more channels, each channel
is coded independently, hence they are uncoupled.

As you have read in the stereo documentation, Vorbis uses square polar
mapping where L and R channels are converted to a magnitude and angle.
In lossless stereo coupling, assuming A=L, and B=R  (the rc3 source code
is clearer on this),

1.  if A is larger than B, set mag to A and angle to A-B (if B is
positive) or B-A (if B is negative).
2. if B is larger than A, set mag to B and angle to A-B (if B is
positive) or B-A (if B is negative)

Hence the magnitude represents the strongest common value while the
angle represents the difference (diffuse sounds) of the other channel.
This is similar to intensity stereo in MP3 where L and R channels are
mapped to L and L-R.  In point stereo, the angle is set to 0, thus is no
diffuse information and the sound appears centered.  The beauty of this
is that both point stereo and lossless stereo can be decoupled using the
same procedure (the one in the stereo docs)

Currently, Vorbis mostly uses lossless stereo coupling for quality
levels of 6 and above while 5 and below uses a mix of point and
lossless.  In the /lib/modes/psych_44.h file (which contains all the
important tuning parameters of Vorbis), you will notice close to the
bottom, a lot of adj_stereo structure initialisations.  The first row
are prepoint thresholds which govern whether to use point stereo or
not.  If the quantised residue is less than the threshold, they are
coupled using point stereo.  Otherwise, they are losslessly coupled.

To see all this in the source code, go to psy.c and the function
_vp_couple and you will see an if statement determining whether to use
point stereo or lossless.  To see the decoupling routine, go to
mapping0.c and it should be in the inverse mapping function down toward
the end. :)

That's all I know. :)

Best regards,

Steve.

Leena wrote:

>Hello !!
>
>  Can anyone explain me the coupling mechanism used in vorbis . I have read
>the
>spects and enocder code also to clear the 'square polar mapping' concept .
>But still
>I haven't understood it well .
>
>
>Leena
>
>
>
>-----Original Message-----
>From: Leena [mailto:leena.padgaonkar at patni.com]
>Sent: Monday, February 23, 2004 6:48 PM
>To: 'vorbis at xiph.org'
>Subject: RE: [vorbis] About Vorbis Encoder
>
>
>Thanks Steve a lot !! Unless one is clear with encoding process ,only
>deocder
>reading is not going to help to understand Ogg vorbis  in details.
>
>Leena
>
>
>-----Original Message-----
>From: owner-vorbis at xiph.org [mailto:owner-vorbis at xiph.org]On Behalf Of
>Stephen So
>Sent: Monday, February 23, 2004 6:38 PM
>To: vorbis at xiph.org
>Subject: Re: [vorbis] About Vorbis Encoder
>
>
>The Vorbis I specifications only provides details from a decoder point
>of view, which I assume gives some freedom in the implementation of the
>encoder.  Currently the best way to understand the encoder is to
>download the source and try and understand what's going on.  A good
>source file, from which to observe the sequence of Vorbis encoding, is
>mapping0.c in the function called mapping0_forward.
>
>Good luck :)
>
>Best regards,
>
>Steve.
>
>Leena wrote:
>
>
>
>>Hello !!
>>
>>
>>  Can I get the vorbis -I encoder spects ..? I have Vorbis I
>>
>>
>specifications
>
>
>>. That talks mostly about vorbis
>>Decoder .
>>
>>
>>Leena
>>
>>--- >8 ----
>>List archives:  http://www.xiph.org/archives/
>>Ogg project homepage: http://www.xiph.org/ogg/
>>To unsubscribe from this list, send a message to 'vorbis-request at xiph.org'
>>containing only the word 'unsubscribe' in the body.  No subject is needed.
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>>
>>
>>
>>
>>
>
>--
>--------------------------------------------------
>Stephen So, BEng(Hons)
>
>PhD Student,
>Signal Processing Laboratory,
>School of Microelectronic Engineering,
>Faculty of Engineering and Information Technology,
>Griffith University, Nathan Campus,
>Brisbane, QLD, Australia, 4111.
>
>Phone: +61-7-3875 3754
>E-mail: s.so at griffith.edu.au
>--------------------------------------------------
>
>--- >8 ----
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>Ogg project homepage: http://www.xiph.org/ogg/
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>
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>
>
>


--
--------------------------------------------------
Stephen So, BEng(Hons)

PhD Student,
Signal Processing Laboratory,
School of Microelectronic Engineering,
Faculty of Engineering and Information Technology,
Griffith University, Nathan Campus,
Brisbane, QLD, Australia, 4111.

Phone: +61-7-3875 3754
E-mail: s.so at griffith.edu.au
--------------------------------------------------

--- >8 ----
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