248 lines
8.3 KiB
TeX
Executable File
248 lines
8.3 KiB
TeX
Executable File
% -*- mode: latex; TeX-master: "Vorbis_I_spec"; -*-
|
|
%!TEX root = Vorbis_I_spec.tex
|
|
% $Id$
|
|
\section{Bitpacking Convention} \label{vorbis:spec:bitpacking}
|
|
|
|
\subsection{Overview}
|
|
|
|
The Vorbis codec uses relatively unstructured raw packets containing
|
|
arbitrary-width binary integer fields. Logically, these packets are a
|
|
bitstream in which bits are coded one-by-one by the encoder and then
|
|
read one-by-one in the same monotonically increasing order by the
|
|
decoder. Most current binary storage arrangements group bits into a
|
|
native word size of eight bits (octets), sixteen bits, thirty-two bits
|
|
or, less commonly other fixed word sizes. The Vorbis bitpacking
|
|
convention specifies the correct mapping of the logical packet
|
|
bitstream into an actual representation in fixed-width words.
|
|
|
|
|
|
\subsubsection{octets, bytes and words}
|
|
|
|
In most contemporary architectures, a 'byte' is synonymous with an
|
|
'octet', that is, eight bits. This has not always been the case;
|
|
seven, ten, eleven and sixteen bit 'bytes' have been used. For
|
|
purposes of the bitpacking convention, a byte implies the native,
|
|
smallest integer storage representation offered by a platform. On
|
|
modern platforms, this is generally assumed to be eight bits (not
|
|
necessarily because of the processor but because of the
|
|
filesystem/memory architecture. Modern filesystems invariably offer
|
|
bytes as the fundamental atom of storage). A 'word' is an integer
|
|
size that is a grouped multiple of this smallest size.
|
|
|
|
The most ubiquitous architectures today consider a 'byte' to be an
|
|
octet (eight bits) and a word to be a group of two, four or eight
|
|
bytes (16, 32 or 64 bits). Note however that the Vorbis bitpacking
|
|
convention is still well defined for any native byte size; Vorbis uses
|
|
the native bit-width of a given storage system. This document assumes
|
|
that a byte is one octet for purposes of example.
|
|
|
|
\subsubsection{bit order}
|
|
|
|
A byte has a well-defined 'least significant' bit (LSb), which is the
|
|
only bit set when the byte is storing the two's complement integer
|
|
value +1. A byte's 'most significant' bit (MSb) is at the opposite
|
|
end of the byte. Bits in a byte are numbered from zero at the LSb to
|
|
$n$ ($n=7$ in an octet) for the
|
|
MSb.
|
|
|
|
|
|
|
|
\subsubsection{byte order}
|
|
|
|
Words are native groupings of multiple bytes. Several byte orderings
|
|
are possible in a word; the common ones are 3-2-1-0 ('big endian' or
|
|
'most significant byte first' in which the highest-valued byte comes
|
|
first), 0-1-2-3 ('little endian' or 'least significant byte first' in
|
|
which the lowest value byte comes first) and less commonly 3-1-2-0 and
|
|
0-2-1-3 ('mixed endian').
|
|
|
|
The Vorbis bitpacking convention specifies storage and bitstream
|
|
manipulation at the byte, not word, level, thus host word ordering is
|
|
of a concern only during optimization when writing high performance
|
|
code that operates on a word of storage at a time rather than by byte.
|
|
Logically, bytes are always coded and decoded in order from byte zero
|
|
through byte $n$.
|
|
|
|
|
|
|
|
\subsubsection{coding bits into byte sequences}
|
|
|
|
The Vorbis codec has need to code arbitrary bit-width integers, from
|
|
zero to 32 bits wide, into packets. These integer fields are not
|
|
aligned to the boundaries of the byte representation; the next field
|
|
is written at the bit position at which the previous field ends.
|
|
|
|
The encoder logically packs integers by writing the LSb of a binary
|
|
integer to the logical bitstream first, followed by next least
|
|
significant bit, etc, until the requested number of bits have been
|
|
coded. When packing the bits into bytes, the encoder begins by
|
|
placing the LSb of the integer to be written into the least
|
|
significant unused bit position of the destination byte, followed by
|
|
the next-least significant bit of the source integer and so on up to
|
|
the requested number of bits. When all bits of the destination byte
|
|
have been filled, encoding continues by zeroing all bits of the next
|
|
byte and writing the next bit into the bit position 0 of that byte.
|
|
Decoding follows the same process as encoding, but by reading bits
|
|
from the byte stream and reassembling them into integers.
|
|
|
|
|
|
|
|
\subsubsection{signedness}
|
|
|
|
The signedness of a specific number resulting from decode is to be
|
|
interpreted by the decoder given decode context. That is, the three
|
|
bit binary pattern 'b111' can be taken to represent either 'seven' as
|
|
an unsigned integer, or '-1' as a signed, two's complement integer.
|
|
The encoder and decoder are responsible for knowing if fields are to
|
|
be treated as signed or unsigned.
|
|
|
|
|
|
|
|
\subsubsection{coding example}
|
|
|
|
Code the 4 bit integer value '12' [b1100] into an empty bytestream.
|
|
Bytestream result:
|
|
|
|
\begin{Verbatim}[commandchars=\\\{\}]
|
|
|
|
|
V
|
|
|
|
7 6 5 4 3 2 1 0
|
|
byte 0 [0 0 0 0 1 1 0 0] <-
|
|
byte 1 [ ]
|
|
byte 2 [ ]
|
|
byte 3 [ ]
|
|
...
|
|
byte n [ ] bytestream length == 1 byte
|
|
|
|
\end{Verbatim}
|
|
|
|
|
|
Continue by coding the 3 bit integer value '-1' [b111]:
|
|
|
|
\begin{Verbatim}[commandchars=\\\{\}]
|
|
|
|
|
V
|
|
|
|
7 6 5 4 3 2 1 0
|
|
byte 0 [0 1 1 1 1 1 0 0] <-
|
|
byte 1 [ ]
|
|
byte 2 [ ]
|
|
byte 3 [ ]
|
|
...
|
|
byte n [ ] bytestream length == 1 byte
|
|
\end{Verbatim}
|
|
|
|
|
|
Continue by coding the 7 bit integer value '17' [b0010001]:
|
|
|
|
\begin{Verbatim}[commandchars=\\\{\}]
|
|
|
|
|
V
|
|
|
|
7 6 5 4 3 2 1 0
|
|
byte 0 [1 1 1 1 1 1 0 0]
|
|
byte 1 [0 0 0 0 1 0 0 0] <-
|
|
byte 2 [ ]
|
|
byte 3 [ ]
|
|
...
|
|
byte n [ ] bytestream length == 2 bytes
|
|
bit cursor == 6
|
|
\end{Verbatim}
|
|
|
|
|
|
Continue by coding the 13 bit integer value '6969' [b110 11001110 01]:
|
|
|
|
\begin{Verbatim}[commandchars=\\\{\}]
|
|
|
|
|
V
|
|
|
|
7 6 5 4 3 2 1 0
|
|
byte 0 [1 1 1 1 1 1 0 0]
|
|
byte 1 [0 1 0 0 1 0 0 0]
|
|
byte 2 [1 1 0 0 1 1 1 0]
|
|
byte 3 [0 0 0 0 0 1 1 0] <-
|
|
...
|
|
byte n [ ] bytestream length == 4 bytes
|
|
|
|
\end{Verbatim}
|
|
|
|
|
|
|
|
|
|
\subsubsection{decoding example}
|
|
|
|
Reading from the beginning of the bytestream encoded in the above example:
|
|
|
|
\begin{Verbatim}[commandchars=\\\{\}]
|
|
|
|
|
V
|
|
|
|
7 6 5 4 3 2 1 0
|
|
byte 0 [1 1 1 1 1 1 0 0] <-
|
|
byte 1 [0 1 0 0 1 0 0 0]
|
|
byte 2 [1 1 0 0 1 1 1 0]
|
|
byte 3 [0 0 0 0 0 1 1 0] bytestream length == 4 bytes
|
|
|
|
\end{Verbatim}
|
|
|
|
|
|
We read two, two-bit integer fields, resulting in the returned numbers
|
|
'b00' and 'b11'. Two things are worth noting here:
|
|
|
|
\begin{itemize}
|
|
\item Although these four bits were originally written as a single
|
|
four-bit integer, reading some other combination of bit-widths from the
|
|
bitstream is well defined. There are no artificial alignment
|
|
boundaries maintained in the bitstream.
|
|
|
|
\item The second value is the
|
|
two-bit-wide integer 'b11'. This value may be interpreted either as
|
|
the unsigned value '3', or the signed value '-1'. Signedness is
|
|
dependent on decode context.
|
|
\end{itemize}
|
|
|
|
|
|
|
|
|
|
\subsubsection{end-of-packet alignment}
|
|
|
|
The typical use of bitpacking is to produce many independent
|
|
byte-aligned packets which are embedded into a larger byte-aligned
|
|
container structure, such as an Ogg transport bitstream. Externally,
|
|
each bytestream (encoded bitstream) must begin and end on a byte
|
|
boundary. Often, the encoded bitstream is not an integer number of
|
|
bytes, and so there is unused (uncoded) space in the last byte of a
|
|
packet.
|
|
|
|
Unused space in the last byte of a bytestream is always zeroed during
|
|
the coding process. Thus, should this unused space be read, it will
|
|
return binary zeroes.
|
|
|
|
Attempting to read past the end of an encoded packet results in an
|
|
'end-of-packet' condition. End-of-packet is not to be considered an
|
|
error; it is merely a state indicating that there is insufficient
|
|
remaining data to fulfill the desired read size. Vorbis uses truncated
|
|
packets as a normal mode of operation, and as such, decoders must
|
|
handle reading past the end of a packet as a typical mode of
|
|
operation. Any further read operations after an 'end-of-packet'
|
|
condition shall also return 'end-of-packet'.
|
|
|
|
|
|
|
|
\subsubsection{reading zero bits}
|
|
|
|
Reading a zero-bit-wide integer returns the value '0' and does not
|
|
increment the stream cursor. Reading to the end of the packet (but
|
|
not past, such that an 'end-of-packet' condition has not triggered)
|
|
and then reading a zero bit integer shall succeed, returning 0, and
|
|
not trigger an end-of-packet condition. Reading a zero-bit-wide
|
|
integer after a previous read sets 'end-of-packet' shall also fail
|
|
with 'end-of-packet'.
|
|
|
|
|
|
|
|
|
|
|
|
|