[vorbis] About Vorbis Encoder

Fri Feb 27 02:52:31 PST 2004

Thanks Stephen !!

  I was  little bit confused as they have stated that polar representation
has
seperated spatial audio in point image and diffuse image . From their doc I
was
not clear about how mag and angle are compuated .
  I was getting totally on wrong path while reading the code. As per your
seggestion
I will read the follwing code to clear the coupling Idea .

<p>      mag_memo=_vp_quantize_couple_memo(vb,
                                        &ci->psy_g_param,
                                        psy_look,
                                        info,
                                        gmdct);

      mag_sort=_vp_quantize_couple_sort(vb,
                                        psy_look,
                                        info,
                                        mag_memo);

<p>                        if(info->coupling_steps){
        _vp_couple(k,
                   &ci->psy_g_param,
                   psy_look,
                       info,
                   vb->pcm,
                   mag_memo,
                   mag_sort,
                   ilogmaskch,
                   nonzero,
                   ci->psy_g_param.sliding_lowpass[vb->W][k]);
      }

Thanks & regards

Leena

<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.
>>Unsubscribe messages sent to the list will be ignored/filtered.
>>
>>
>>
>>
>>
>
>--
>--------------------------------------------------
>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 ----
>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'
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>Unsubscribe messages sent to the list will be ignored/filtered.
>
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>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/
To unsubscribe from this list, send a message to 'vorbis-request at xiph.org'
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Unsubscribe messages sent to the list will be ignored/filtered.

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Ogg project homepage: http://www.xiph.org/ogg/
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Unsubscribe messages sent to the list will be ignored/filtered.