Chris@1: % -*- mode: latex; TeX-master: "Vorbis_I_spec"; -*-
Chris@1: %!TEX root = Vorbis_I_spec.tex
Chris@1: % $Id$
Chris@1: \section{Residue setup and decode} \label{vorbis:spec:residue}
Chris@1: 
Chris@1: \subsection{Overview}
Chris@1: 
Chris@1: A residue vector represents the fine detail of the audio spectrum of
Chris@1: one channel in an audio frame after the encoder subtracts the floor
Chris@1: curve and performs any channel coupling.  A residue vector may
Chris@1: represent spectral lines, spectral magnitude, spectral phase or
Chris@1: hybrids as mixed by channel coupling.  The exact semantic content of
Chris@1: the vector does not matter to the residue abstraction.
Chris@1: 
Chris@1: Whatever the exact qualities, the Vorbis residue abstraction codes the
Chris@1: residue vectors into the bitstream packet, and then reconstructs the
Chris@1: vectors during decode.  Vorbis makes use of three different encoding
Chris@1: variants (numbered 0, 1 and 2) of the same basic vector encoding
Chris@1: abstraction.
Chris@1: 
Chris@1: 
Chris@1: 
Chris@1: \subsection{Residue format}
Chris@1: 
Chris@1: Residue format partitions each vector in the vector bundle into chunks,
Chris@1: classifies each chunk, encodes the chunk classifications and finally
Chris@1: encodes the chunks themselves using the the specific VQ arrangement
Chris@1: defined for each selected classification.
Chris@1: The exact interleaving and partitioning vary by residue encoding number,
Chris@1: however the high-level process used to classify and encode the residue
Chris@1: vector is the same in all three variants.
Chris@1: 
Chris@1: A set of coded residue vectors are all of the same length.  High level
Chris@1: coding structure, ignoring for the moment exactly how a partition is
Chris@1: encoded and simply trusting that it is, is as follows:
Chris@1: 
Chris@1: \begin{itemize}
Chris@1: \item Each vector is partitioned into multiple equal sized chunks
Chris@1: according to configuration specified.  If we have a vector size of
Chris@1: \emph{n}, a partition size \emph{residue\_partition\_size}, and a total
Chris@1: of \emph{ch} residue vectors, the total number of partitioned chunks
Chris@1: coded is \emph{n}/\emph{residue\_partition\_size}*\emph{ch}.  It is
Chris@1: important to note that the integer division truncates.  In the below
Chris@1: example, we assume an example \emph{residue\_partition\_size} of 8.
Chris@1: 
Chris@1: \item Each partition in each vector has a classification number that
Chris@1: specifies which of multiple configured VQ codebook setups are used to
Chris@1: decode that partition.  The classification numbers of each partition
Chris@1: can be thought of as forming a vector in their own right, as in the
Chris@1: illustration below.  Just as the residue vectors are coded in grouped
Chris@1: partitions to increase encoding efficiency, the classification vector
Chris@1: is also partitioned into chunks.  The integer elements of each scalar
Chris@1: in a classification chunk are built into a single scalar that
Chris@1: represents the classification numbers in that chunk.  In the below
Chris@1: example, the classification codeword encodes two classification
Chris@1: numbers.
Chris@1: 
Chris@1: \item The values in a residue vector may be encoded monolithically in a
Chris@1: single pass through the residue vector, but more often efficient
Chris@1: codebook design dictates that each vector is encoded as the additive
Chris@1: sum of several passes through the residue vector using more than one
Chris@1: VQ codebook.  Thus, each residue value potentially accumulates values
Chris@1: from multiple decode passes.  The classification value associated with
Chris@1: a partition is the same in each pass, thus the classification codeword
Chris@1: is coded only in the first pass.
Chris@1: 
Chris@1: \end{itemize}
Chris@1: 
Chris@1: 
Chris@1: \begin{center}
Chris@1: \includegraphics[width=\textwidth]{residue-pack}
Chris@1: \captionof{figure}{illustration of residue vector format}
Chris@1: \end{center}
Chris@1: 
Chris@1: 
Chris@1: 
Chris@1: \subsection{residue 0}
Chris@1: 
Chris@1: Residue 0 and 1 differ only in the way the values within a residue
Chris@1: partition are interleaved during partition encoding (visually treated
Chris@1: as a black box--or cyan box or brown box--in the above figure).
Chris@1: 
Chris@1: Residue encoding 0 interleaves VQ encoding according to the
Chris@1: dimension of the codebook used to encode a partition in a specific
Chris@1: pass.  The dimension of the codebook need not be the same in multiple
Chris@1: passes, however the partition size must be an even multiple of the
Chris@1: codebook dimension.
Chris@1: 
Chris@1: As an example, assume a partition vector of size eight, to be encoded
Chris@1: by residue 0 using codebook sizes of 8, 4, 2 and 1:
Chris@1: 
Chris@1: \begin{programlisting}
Chris@1: 
Chris@1:             original residue vector: [ 0 1 2 3 4 5 6 7 ]
Chris@1: 
Chris@1: codebook dimensions = 8  encoded as: [ 0 1 2 3 4 5 6 7 ]
Chris@1: 
Chris@1: codebook dimensions = 4  encoded as: [ 0 2 4 6 ], [ 1 3 5 7 ]
Chris@1: 
Chris@1: codebook dimensions = 2  encoded as: [ 0 4 ], [ 1 5 ], [ 2 6 ], [ 3 7 ]
Chris@1: 
Chris@1: codebook dimensions = 1  encoded as: [ 0 ], [ 1 ], [ 2 ], [ 3 ], [ 4 ], [ 5 ], [ 6 ], [ 7 ]
Chris@1: 
Chris@1: \end{programlisting}
Chris@1: 
Chris@1: It is worth mentioning at this point that no configurable value in the
Chris@1: residue coding setup is restricted to a power of two.
Chris@1: 
Chris@1: 
Chris@1: 
Chris@1: \subsection{residue 1}
Chris@1: 
Chris@1: Residue 1 does not interleave VQ encoding.  It represents partition
Chris@1: vector scalars in order.  As with residue 0, however, partition length
Chris@1: must be an integer multiple of the codebook dimension, although
Chris@1: dimension may vary from pass to pass.
Chris@1: 
Chris@1: As an example, assume a partition vector of size eight, to be encoded
Chris@1: by residue 0 using codebook sizes of 8, 4, 2 and 1:
Chris@1: 
Chris@1: \begin{programlisting}
Chris@1: 
Chris@1:             original residue vector: [ 0 1 2 3 4 5 6 7 ]
Chris@1: 
Chris@1: codebook dimensions = 8  encoded as: [ 0 1 2 3 4 5 6 7 ]
Chris@1: 
Chris@1: codebook dimensions = 4  encoded as: [ 0 1 2 3 ], [ 4 5 6 7 ]
Chris@1: 
Chris@1: codebook dimensions = 2  encoded as: [ 0 1 ], [ 2 3 ], [ 4 5 ], [ 6 7 ]
Chris@1: 
Chris@1: codebook dimensions = 1  encoded as: [ 0 ], [ 1 ], [ 2 ], [ 3 ], [ 4 ], [ 5 ], [ 6 ], [ 7 ]
Chris@1: 
Chris@1: \end{programlisting}
Chris@1: 
Chris@1: 
Chris@1: 
Chris@1: \subsection{residue 2}
Chris@1: 
Chris@1: Residue type two can be thought of as a variant of residue type 1.
Chris@1: Rather than encoding multiple passed-in vectors as in residue type 1,
Chris@1: the \emph{ch} passed in vectors of length \emph{n} are first
Chris@1: interleaved and flattened into a single vector of length
Chris@1: \emph{ch}*\emph{n}.  Encoding then proceeds as in type 1. Decoding is
Chris@1: as in type 1 with decode interleave reversed. If operating on a single
Chris@1: vector to begin with, residue type 1 and type 2 are equivalent.
Chris@1: 
Chris@1: \begin{center}
Chris@1: \includegraphics[width=\textwidth]{residue2}
Chris@1: \captionof{figure}{illustration of residue type 2}
Chris@1: \end{center}
Chris@1: 
Chris@1: 
Chris@1: \subsection{Residue decode}
Chris@1: 
Chris@1: \subsubsection{header decode}
Chris@1: 
Chris@1: Header decode for all three residue types is identical.
Chris@1: \begin{programlisting}
Chris@1:   1) [residue\_begin] = read 24 bits as unsigned integer
Chris@1:   2) [residue\_end] = read 24 bits as unsigned integer
Chris@1:   3) [residue\_partition\_size] = read 24 bits as unsigned integer and add one
Chris@1:   4) [residue\_classifications] = read 6 bits as unsigned integer and add one
Chris@1:   5) [residue\_classbook] = read 8 bits as unsigned integer
Chris@1: \end{programlisting}
Chris@1: 
Chris@1: \varname{[residue\_begin]} and
Chris@1: \varname{[residue\_end]} select the specific sub-portion of
Chris@1: each vector that is actually coded; it implements akin to a bandpass
Chris@1: where, for coding purposes, the vector effectively begins at element
Chris@1: \varname{[residue\_begin]} and ends at
Chris@1: \varname{[residue\_end]}.  Preceding and following values in
Chris@1: the unpacked vectors are zeroed.  Note that for residue type 2, these
Chris@1: values as well as \varname{[residue\_partition\_size]}apply to
Chris@1: the interleaved vector, not the individual vectors before interleave.
Chris@1: \varname{[residue\_partition\_size]} is as explained above,
Chris@1: \varname{[residue\_classifications]} is the number of possible
Chris@1: classification to which a partition can belong and
Chris@1: \varname{[residue\_classbook]} is the codebook number used to
Chris@1: code classification codewords.  The number of dimensions in book
Chris@1: \varname{[residue\_classbook]} determines how many
Chris@1: classification values are grouped into a single classification
Chris@1: codeword.  Note that the number of entries and dimensions in book
Chris@1: \varname{[residue\_classbook]}, along with
Chris@1: \varname{[residue\_classifications]}, overdetermines to
Chris@1: possible number of classification codewords.  
Chris@1: If \varname{[residue\_classifications]}\^{}\varname{[residue\_classbook]}.dimensions
Chris@1: exceeds \varname{[residue\_classbook]}.entries, the
Chris@1: bitstream should be regarded to be undecodable.
Chris@1: 
Chris@1: Next we read a bitmap pattern that specifies which partition classes
Chris@1: code values in which passes.
Chris@1: 
Chris@1: \begin{programlisting}
Chris@1:   1) iterate [i] over the range 0 ... [residue\_classifications]-1 {
Chris@1: 
Chris@1:        2) [high\_bits] = 0
Chris@1:        3) [low\_bits] = read 3 bits as unsigned integer
Chris@1:        4) [bitflag] = read one bit as boolean
Chris@1:        5) if ( [bitflag] is set ) then [high\_bits] = read five bits as unsigned integer
Chris@1:        6) vector [residue\_cascade] element [i] = [high\_bits] * 8 + [low\_bits]
Chris@1:      }
Chris@1:   7) done
Chris@1: \end{programlisting}
Chris@1: 
Chris@1: Finally, we read in a list of book numbers, each corresponding to
Chris@1: specific bit set in the cascade bitmap.  We loop over the possible
Chris@1: codebook classifications and the maximum possible number of encoding
Chris@1: stages (8 in Vorbis I, as constrained by the elements of the cascade
Chris@1: bitmap being eight bits):
Chris@1: 
Chris@1: \begin{programlisting}
Chris@1:   1) iterate [i] over the range 0 ... [residue\_classifications]-1 {
Chris@1: 
Chris@1:        2) iterate [j] over the range 0 ... 7 {
Chris@1: 
Chris@1:             3) if ( vector [residue\_cascade] element [i] bit [j] is set ) {
Chris@1: 
Chris@1:                  4) array [residue\_books] element [i][j] = read 8 bits as unsigned integer
Chris@1: 
Chris@1:                } else {
Chris@1: 
Chris@1:                  5) array [residue\_books] element [i][j] = unused
Chris@1: 
Chris@1:                }
Chris@1:           }
Chris@1:       }
Chris@1: 
Chris@1:   6) done
Chris@1: \end{programlisting}
Chris@1: 
Chris@1: An end-of-packet condition at any point in header decode renders the
Chris@1: stream undecodable.  In addition, any codebook number greater than the
Chris@1: maximum numbered codebook set up in this stream also renders the
Chris@1: stream undecodable. All codebooks in array [residue\_books] are
Chris@1: required to have a value mapping.  The presence of codebook in array
Chris@1: [residue\_books] without a value mapping (maptype equals zero) renders
Chris@1: the stream undecodable.
Chris@1: 
Chris@1: 
Chris@1: 
Chris@1: \subsubsection{packet decode}
Chris@1: 
Chris@1: Format 0 and 1 packet decode is identical except for specific
Chris@1: partition interleave.  Format 2 packet decode can be built out of the
Chris@1: format 1 decode process.  Thus we describe first the decode
Chris@1: infrastructure identical to all three formats.
Chris@1: 
Chris@1: In addition to configuration information, the residue decode process
Chris@1: is passed the number of vectors in the submap bundle and a vector of
Chris@1: flags indicating if any of the vectors are not to be decoded.  If the
Chris@1: passed in number of vectors is 3 and vector number 1 is marked 'do not
Chris@1: decode', decode skips vector 1 during the decode loop.  However, even
Chris@1: 'do not decode' vectors are allocated and zeroed.
Chris@1: 
Chris@1: Depending on the values of \varname{[residue\_begin]} and
Chris@1: \varname{[residue\_end]}, it is obvious that the encoded
Chris@1: portion of a residue vector may be the entire possible residue vector
Chris@1: or some other strict subset of the actual residue vector size with
Chris@1: zero padding at either uncoded end.  However, it is also possible to
Chris@1: set \varname{[residue\_begin]} and
Chris@1: \varname{[residue\_end]} to specify a range partially or
Chris@1: wholly beyond the maximum vector size.  Before beginning residue
Chris@1: decode, limit \varname{[residue\_begin]} and
Chris@1: \varname{[residue\_end]} to the maximum possible vector size
Chris@1: as follows.  We assume that the number of vectors being encoded,
Chris@1: \varname{[ch]} is provided by the higher level decoding
Chris@1: process.
Chris@1: 
Chris@1: \begin{programlisting}
Chris@1:   1) [actual\_size] = current blocksize/2;
Chris@1:   2) if residue encoding is format 2
Chris@1:        3) [actual\_size] = [actual\_size] * [ch];
Chris@1:   4) [limit\_residue\_begin] = maximum of ([residue\_begin],[actual\_size]);
Chris@1:   5) [limit\_residue\_end] = maximum of ([residue\_end],[actual\_size]);
Chris@1: \end{programlisting}
Chris@1: 
Chris@1: The following convenience values are conceptually useful to clarifying
Chris@1: the decode process:
Chris@1: 
Chris@1: \begin{programlisting}
Chris@1:   1) [classwords\_per\_codeword] = [codebook\_dimensions] value of codebook [residue\_classbook]
Chris@1:   2) [n\_to\_read] = [limit\_residue\_end] - [limit\_residue\_begin]
Chris@1:   3) [partitions\_to\_read] = [n\_to\_read] / [residue\_partition\_size]
Chris@1: \end{programlisting}
Chris@1: 
Chris@1: Packet decode proceeds as follows, matching the description offered earlier in the document.
Chris@1: \begin{programlisting}
Chris@1:   1) allocate and zero all vectors that will be returned.
Chris@1:   2) if ([n\_to\_read] is zero), stop; there is no residue to decode.
Chris@1:   3) iterate [pass] over the range 0 ... 7 {
Chris@1: 
Chris@1:        4) [partition\_count] = 0
Chris@1: 
Chris@1:        5) while [partition\_count] is less than [partitions\_to\_read]
Chris@1: 
Chris@1:             6) if ([pass] is zero) {
Chris@1: 
Chris@1:                  7) iterate [j] over the range 0 .. [ch]-1 {
Chris@1: 
Chris@1:                       8) if vector [j] is not marked 'do not decode' {
Chris@1: 
Chris@1:                            9) [temp] = read from packet using codebook [residue\_classbook] in scalar context
Chris@1:                           10) iterate [i] descending over the range [classwords\_per\_codeword]-1 ... 0 {
Chris@1: 
Chris@1:                                11) array [classifications] element [j],([i]+[partition\_count]) =
Chris@1:                                    [temp] integer modulo [residue\_classifications]
Chris@1:                                12) [temp] = [temp] / [residue\_classifications] using integer division
Chris@1: 
Chris@1:                               }
Chris@1: 
Chris@1:                          }
Chris@1: 
Chris@1:                     }
Chris@1: 
Chris@1:                }
Chris@1: 
Chris@1:            13) iterate [i] over the range 0 .. ([classwords\_per\_codeword] - 1) while [partition\_count]
Chris@1:                is also less than [partitions\_to\_read] {
Chris@1: 
Chris@1:                  14) iterate [j] over the range 0 .. [ch]-1 {
Chris@1: 
Chris@1:                       15) if vector [j] is not marked 'do not decode' {
Chris@1: 
Chris@1:                            16) [vqclass] = array [classifications] element [j],[partition\_count]
Chris@1:                            17) [vqbook] = array [residue\_books] element [vqclass],[pass]
Chris@1:                            18) if ([vqbook] is not 'unused') {
Chris@1: 
Chris@1:                                 19) decode partition into output vector number [j], starting at scalar
Chris@1:                                     offset [limit\_residue\_begin]+[partition\_count]*[residue\_partition\_size] using
Chris@1:                                     codebook number [vqbook] in VQ context
Chris@1:                           }
Chris@1:                      }
Chris@1: 
Chris@1:                  20) increment [partition\_count] by one
Chris@1: 
Chris@1:                }
Chris@1:           }
Chris@1:      }
Chris@1: 
Chris@1:  21) done
Chris@1: 
Chris@1: \end{programlisting}
Chris@1: 
Chris@1: An end-of-packet condition during packet decode is to be considered a
Chris@1: nominal occurrence.  Decode returns the result of vector decode up to
Chris@1: that point.
Chris@1: 
Chris@1: 
Chris@1: 
Chris@1: \subsubsection{format 0 specifics}
Chris@1: 
Chris@1: Format zero decodes partitions exactly as described earlier in the
Chris@1: 'Residue Format: residue 0' section.  The following pseudocode
Chris@1: presents the same algorithm. Assume:
Chris@1: 
Chris@1: \begin{itemize}
Chris@1: \item  \varname{[n]} is the value in \varname{[residue\_partition\_size]}
Chris@1: \item \varname{[v]} is the residue vector
Chris@1: \item \varname{[offset]} is the beginning read offset in [v]
Chris@1: \end{itemize}
Chris@1: 
Chris@1: 
Chris@1: \begin{programlisting}
Chris@1:  1) [step] = [n] / [codebook\_dimensions]
Chris@1:  2) iterate [i] over the range 0 ... [step]-1 {
Chris@1: 
Chris@1:       3) vector [entry\_temp] = read vector from packet using current codebook in VQ context
Chris@1:       4) iterate [j] over the range 0 ... [codebook\_dimensions]-1 {
Chris@1: 
Chris@1:            5) vector [v] element ([offset]+[i]+[j]*[step]) =
Chris@1: 	        vector [v] element ([offset]+[i]+[j]*[step]) +
Chris@1:                 vector [entry\_temp] element [j]
Chris@1: 
Chris@1:          }
Chris@1: 
Chris@1:     }
Chris@1: 
Chris@1:   6) done
Chris@1: 
Chris@1: \end{programlisting}
Chris@1: 
Chris@1: 
Chris@1: 
Chris@1: \subsubsection{format 1 specifics}
Chris@1: 
Chris@1: Format 1 decodes partitions exactly as described earlier in the
Chris@1: 'Residue Format: residue 1' section.  The following pseudocode
Chris@1: presents the same algorithm. Assume:
Chris@1: 
Chris@1: \begin{itemize}
Chris@1: \item  \varname{[n]} is the value in
Chris@1: \varname{[residue\_partition\_size]}
Chris@1: \item \varname{[v]} is the residue vector
Chris@1: \item \varname{[offset]} is the beginning read offset in [v]
Chris@1: \end{itemize}
Chris@1: 
Chris@1: 
Chris@1: \begin{programlisting}
Chris@1:  1) [i] = 0
Chris@1:  2) vector [entry\_temp] = read vector from packet using current codebook in VQ context
Chris@1:  3) iterate [j] over the range 0 ... [codebook\_dimensions]-1 {
Chris@1: 
Chris@1:       4) vector [v] element ([offset]+[i]) =
Chris@1: 	  vector [v] element ([offset]+[i]) +
Chris@1:           vector [entry\_temp] element [j]
Chris@1:       5) increment [i]
Chris@1: 
Chris@1:     }
Chris@1: 
Chris@1:   6) if ( [i] is less than [n] ) continue at step 2
Chris@1:   7) done
Chris@1: \end{programlisting}
Chris@1: 
Chris@1: 
Chris@1: 
Chris@1: \subsubsection{format 2 specifics}
Chris@1: 
Chris@1: Format 2 is reducible to format 1.  It may be implemented as an additional step prior to and an additional post-decode step after a normal format 1 decode.
Chris@1: 
Chris@1: 
Chris@1: Format 2 handles 'do not decode' vectors differently than residue 0 or
Chris@1: 1; if all vectors are marked 'do not decode', no decode occurrs.
Chris@1: However, if at least one vector is to be decoded, all the vectors are
Chris@1: decoded.  We then request normal format 1 to decode a single vector
Chris@1: representing all output channels, rather than a vector for each
Chris@1: channel.  After decode, deinterleave the vector into independent vectors, one for each output channel.  That is:
Chris@1: 
Chris@1: \begin{enumerate}
Chris@1:  \item If all vectors 0 through \emph{ch}-1 are marked 'do not decode', allocate and clear a single vector \varname{[v]}of length \emph{ch*n} and skip step 2 below; proceed directly to the post-decode step.
Chris@1:  \item Rather than performing format 1 decode to produce \emph{ch} vectors of length \emph{n} each, call format 1 decode to produce a single vector \varname{[v]} of length \emph{ch*n}.
Chris@1:  \item Post decode: Deinterleave the single vector \varname{[v]} returned by format 1 decode as described above into \emph{ch} independent vectors, one for each outputchannel, according to:
Chris@1:   \begin{programlisting}
Chris@1:   1) iterate [i] over the range 0 ... [n]-1 {
Chris@1: 
Chris@1:        2) iterate [j] over the range 0 ... [ch]-1 {
Chris@1: 
Chris@1:             3) output vector number [j] element [i] = vector [v] element ([i] * [ch] + [j])
Chris@1: 
Chris@1:           }
Chris@1:      }
Chris@1: 
Chris@1:   4) done
Chris@1:   \end{programlisting}
Chris@1: 
Chris@1: \end{enumerate}
Chris@1: 
Chris@1: 
Chris@1: 
Chris@1: 
Chris@1: 
Chris@1: 
Chris@1: