audacity/lib-src/libogg/doc/xml/06-floor0.xml

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<!DOCTYPE section PUBLIC "-//OASIS//DTD DocBook XML V4.2//EN"
                "http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd" [

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<section id="vorbis-spec-floor0">
<sectioninfo>
<releaseinfo>
  $Id: 06-floor0.xml,v 1.1.1.1 2004-11-13 16:51:21 mbrubeck Exp $
</releaseinfo>
</sectioninfo>  
<title>Floor type 0 setup and decode</title>


<section>
<title>Overview</title>

<para>
Vorbis floor type zero uses Line Spectral Pair (LSP, also alternately
known as Line Spectral Frequency or LSF) representation to encode a
smooth spectral envelope curve as the frequency response of the LSP
filter.  This representation is equivalent to a traditional all-pole
infinite impulse response filter as would be used in linear predictive
coding; LSP representation may be converted to LPC representation and
vice-versa.</para>

</section>

<section>
<title>Floor 0 format</title>

<para>
Floor zero configuration consists of six integer fields and a list of
VQ codebooks for use in coding/decoding the LSP filter coefficient
values used by each frame. </para>

<section><title>header decode</title>

<para>
Configuration information for instances of floor zero decodes from the
codec setup header (third packet).  configuration decode proceeds as
follows:</para>

<screen>
  1) [floor0_order] = read an unsigned integer of 8 bits
  2) [floor0_rate] = read an unsigned integer of 16 bits
  3) [floor0_bark_map_size] = read an unsigned integer of 16 bits
  4) [floor0_amplitude_bits] = read an unsigned integer of six bits
  5) [floor0_amplitude_offset] = read an unsigned integer of eight bits
  6) [floor0_number_of_books] = read an unsigned integer of four bits and add 1
  7) if any of [floor0_order], [floor0_rate], [floor0_bark_map_size], [floor0_amplitude_bits],
     [floor0_amplitude_offset] or [floor0_number_of_books] are less than zero, the stream is not decodable
  8) array [floor0_book_list] = read a list of [floor0_number_of_books] unsigned integers of eight bits each;
</screen>

<para>
An end-of-packet condition during any of these bitstream reads renders
this stream undecodable.  In addition, any element of the array
<varname>[floor0_book_list]</varname> that is greater than the maximum codebook
number for this bitstream is an error condition that also renders the
stream undecodable.</para>

</section>

<section id="vorbis-spec-floor0-decode">
<title>packet decode</title>

<para>
Extracting a floor0 curve from an audio packet consists of first
decoding the curve amplitude and <varname>[floor0_order]</varname> LSP
coefficient values from the bitstream, and then computing the floor
curve, which is defined as the frequency response of the decoded LSP
filter.</para>

<para>
Packet decode proceeds as follows:</para>
<screen>
  1) [amplitude] = read an unsigned integer of [floor0_amplitude_bits] bits
  2) if ( [amplitude] is greater than zero ) {
       3) [coefficients] is an empty, zero length vector

       4) [booknumber] = read an unsigned integer of <link linkend="vorbis-spec-ilog">ilog</link>( [floor0_number_of_books] ) bits
       5) if ( [booknumber] is greater than the highest number decode codebook ) then packet is undecodable
       6) [last] = zero;
       7) vector [temp_vector] = read vector from bitstream using codebook number [booknumber] in VQ context.
       8) add the scalar value [last] to each scalar in vector [temp_vector]
       9) [last] = the value of the last scalar in vector [temp_vector]
      10) concatenate [temp_vector] onto the end of the [coefficients] vector
      11) if (length of vector [coefficients] is less than [floor0_order], continue at step 6

     }

 12) done.
 
</screen>

<para>
Take note of the following properties of decode:
<itemizedlist>
 <listitem><simpara>An <varname>[amplitude]</varname> value of zero must result in a return code that indicates this channel is unused in this frame (the output of the channel will be all-zeroes in synthesis).  Several later stages of decode don't occur for an unused channel.</simpara></listitem>
 <listitem><simpara>An end-of-packet condition during decode should be considered a
nominal occruence; if end-of-packet is reached during any read
operation above, floor decode is to return 'unused' status as if the
<varname>[amplitude]</varname> value had read zero at the beginning of decode.</simpara></listitem>

 <listitem><simpara>The book number used for decode
can, in fact, be stored in the bitstream in <link linkend="vorbis-spec-ilog">ilog</link>( <varname>[floor0_number_of_books]</varname> -
1 ) bits.  Nevertheless, the above specification is correct and values
greater than the maximum possible book value are reserved.</simpara></listitem>

 <listitem><simpara>The number of scalars read into the vector <varname>[coefficients]</varname>
may be greater than <varname>[floor0_order]</varname>, the number actually
required for curve computation.  For example, if the VQ codebook used
for the floor currently being decoded has a
<varname>[codebook_dimensions]</varname> value of three and
<varname>[floor0_order]</varname> is ten, the only way to fill all the needed
scalars in <varname>[coefficients]</varname> is to to read a total of twelve
scalars as four vectors of three scalars each.  This is not an error
condition, and care must be taken not to allow a buffer overflow in
decode. The extra values are not used and may be ignored or discarded.</simpara></listitem>
</itemizedlist>
</para>

</section>

<section id="vorbis-spec-floor0-synth">
<title>curve computation</title>

<para>
Given an <varname>[amplitude]</varname> integer and <varname>[coefficients]</varname>
vector from packet decode as well as the [floor0_order],
[floor0_rate], [floor0_bark_map_size], [floor0_amplitude_bits] and
[floor0_amplitude_offset] values from floor setup, and an output
vector size <varname>[n]</varname> specified by the decode process, we compute a
floor output vector.</para>

<para>
If the value <varname>[amplitude]</varname> is zero, the return value is a
length <varname>[n]</varname> vector with all-zero scalars.  Otherwise, begin by
assuming the following definitions for the given vector to be
synthesized:</para>

<informalequation>
 <mediaobject>
  <textobject><phrase>[lsp map equation]</phrase></textobject>
  <textobject role="tex"><phrase>
   <![CDATA[
   \begin{math}
     \mathrm{map}_i = \left\{
       \begin{array}{ll}
          \min (  
            \mathtt{floor0\_bark\_map\_size} - 1,
            foobar
          ) & \textrm{for } i \in [0,n-1] \\
          -1 & \textrm{for } i = n
        \end{array}
      \right.
    \end {math}
    
    where
    
    \begin{math}
    foobar = 
      \left\lfloor
        \mathrm{bark}\left(\frac{\mathtt{floor0\_rate} \cdot i}{2n}\right) \cdot \frac{\mathtt{floor0\_bark\_map\_size}} {\mathrm{bark}(.5 \cdot \mathtt{floor0\_rate})} 
      \right\rfloor
    \end{math}
                  
    and
                
    \begin{math}
      \mathrm{bark}(x) = 13.1 \arctan (.00074x) + 2.24 \arctan (.0000000158x^2)+.0001x
    \end{math}
    ]]>
   </phrase></textobject>
  <imageobject><imagedata  fileref="lspmap.png"/></imageobject>
 </mediaobject>
</informalequation>

<para>
The above is used to synthesize the LSP curve on a Bark-scale frequency
axis, then map the result to a linear-scale frequency axis.
Similarly, the below calculation synthesizes the output LSP curve <varname>[output]</varname> on a log
(dB) amplitude scale, mapping it to linear amplitude in the last step:</para>

<orderedlist>
 <listitem><simpara> <varname>[i]</varname> = 0 </simpara></listitem>
 <listitem><para>if ( <varname>[floor0_order]</varname> is odd ) {
  <orderedlist>
   <listitem><para>calculate <varname>[p]</varname> and <varname>[q]</varname> according to:
        <informalequation>
         <mediaobject>
          <textobject><phrase>[equation for odd lsp]</phrase></textobject>
	  <textobject role="tex"><phrase>
	   <![CDATA[
           \begin{eqnarray*}
             p & = & (1 - \cos^2\omega)\prod_{j=0}^{(\mathtt{order}-3)/2} 4 (\cos c_{2j+1} - \cos \omega)^2 \\
             q & = & \frac{1}{4} \prod_{j=0}^{(\mathtt{order}-1)/2} 4 (\cos c_{2j+1} - \cos \omega)^2
           \end{eqnarray*}
	   ]]>
          </phrase></textobject>
	  <imageobject><imagedata fileref="oddlsp.png"/></imageobject>
	 </mediaobject>
        </informalequation>
   </para></listitem>
  </orderedlist>
  } else <varname>[floor0_order]</varname> is even {
  <orderedlist>
   <listitem><para>calculate <varname>[p]</varname> and <varname>[q]</varname> according to:
        <informalequation>
         <mediaobject>
	  <textobject><phrase>[equation for even lsp]</phrase></textobject>
	  <textobject role="tex"><phrase>
	   <![CDATA[
           \begin{eqnarray*}
             p & = & \frac{(1 - \cos^2\omega)}{2} \prod_{j=0}^{(\mathtt{order}-2)/2} 4 (\cos c_{2j} - \cos \omega)^2 \\
             q & = & \frac{(1 + \cos^2\omega)}{2} \prod_{j=0}^{(\mathtt{order}-2)/2} 4 (\cos c_{2j} - \cos \omega)^2
           \end{eqnarray*}
	   ]]>
          </phrase></textobject>
	  <imageobject><imagedata fileref="evenlsp.png"/></imageobject>
	 </mediaobject>
        </informalequation>
   </para></listitem>
  </orderedlist> 
  }
 </para></listitem>
 <listitem><para>calculate <varname>[linear_floor_value]</varname> according to:
     <informalequation>
      <mediaobject>
       <textobject><phrase>[expression for floorval]</phrase></textobject>
       <textobject role="tex"><phrase>
	 <![CDATA[
         \begin{math}
           \exp \left( .11512925 \left(\frac{\mathtt{amplitude} \cdot \mathtt{floor0\_amplitute\_offset}}{(2^{\mathtt{floor0\_amplitude\_bits}}-1)\sqrt{p+q}}
                  - \mathtt{floor0\_amplitude\_offset} \right) \right)
         \end{math}
	 ]]>
       </phrase></textobject>
       <imageobject><imagedata fileref="floorval.png"/></imageobject>
      </mediaobject>
     </informalequation>
 </para></listitem>
 <listitem><simpara><varname>[iteration_condition]</varname> = map element <varname>[i]</varname></simpara></listitem>
 <listitem><simpara><varname>[output]</varname> element <varname>[i]</varname> = <varname>[linear_floor_value]</varname></simpara></listitem>
 <listitem><simpara>increment <varname>[i]</varname></simpara></listitem>
 <listitem><simpara>if ( map element <varname>[i]</varname> is equal to <varname>[iteration_condition]</varname> ) continue at step 5</simpara></listitem>
 <listitem><simpara>if ( <varname>[i]</varname> is less than <varname>[n]</varname> ) continue at step 2</simpara></listitem>
 <listitem><simpara>done</simpara></listitem>
</orderedlist>

</section>

</section>

</section>