1 % -*- mode: latex; TeX-master: "Vorbis_I_spec"; -*- 2 %!TEX root = Vorbis_I_spec.tex 3 % $Id$ 4 \section{Bitpacking Convention} \label{vorbis:spec:bitpacking} 5 6 \subsection{Overview} 7 8 The Vorbis codec uses relatively unstructured raw packets containing 9 arbitrary-width binary integer fields. Logically, these packets are a 10 bitstream in which bits are coded one-by-one by the encoder and then 11 read one-by-one in the same monotonically increasing order by the 12 decoder. Most current binary storage arrangements group bits into a 13 native word size of eight bits (octets), sixteen bits, thirty-two bits 14 or, less commonly other fixed word sizes. The Vorbis bitpacking 15 convention specifies the correct mapping of the logical packet 16 bitstream into an actual representation in fixed-width words. 17 18 19 \subsubsection{octets, bytes and words} 20 21 In most contemporary architectures, a 'byte' is synonymous with an 22 'octet', that is, eight bits. This has not always been the case; 23 seven, ten, eleven and sixteen bit 'bytes' have been used. For 24 purposes of the bitpacking convention, a byte implies the native, 25 smallest integer storage representation offered by a platform. On 26 modern platforms, this is generally assumed to be eight bits (not 27 necessarily because of the processor but because of the 28 filesystem/memory architecture. Modern filesystems invariably offer 29 bytes as the fundamental atom of storage). A 'word' is an integer 30 size that is a grouped multiple of this smallest size. 31 32 The most ubiquitous architectures today consider a 'byte' to be an 33 octet (eight bits) and a word to be a group of two, four or eight 34 bytes (16, 32 or 64 bits). Note however that the Vorbis bitpacking 35 convention is still well defined for any native byte size; Vorbis uses 36 the native bit-width of a given storage system. This document assumes 37 that a byte is one octet for purposes of example. 38 39 \subsubsection{bit order} 40 41 A byte has a well-defined 'least significant' bit (LSb), which is the 42 only bit set when the byte is storing the two's complement integer 43 value +1. A byte's 'most significant' bit (MSb) is at the opposite 44 end of the byte. Bits in a byte are numbered from zero at the LSb to 45 $n$ ($n=7$ in an octet) for the 46 MSb. 47 48 49 50 \subsubsection{byte order} 51 52 Words are native groupings of multiple bytes. Several byte orderings 53 are possible in a word; the common ones are 3-2-1-0 ('big endian' or 54 'most significant byte first' in which the highest-valued byte comes 55 first), 0-1-2-3 ('little endian' or 'least significant byte first' in 56 which the lowest value byte comes first) and less commonly 3-1-2-0 and 57 0-2-1-3 ('mixed endian'). 58 59 The Vorbis bitpacking convention specifies storage and bitstream 60 manipulation at the byte, not word, level, thus host word ordering is 61 of a concern only during optimization when writing high performance 62 code that operates on a word of storage at a time rather than by byte. 63 Logically, bytes are always coded and decoded in order from byte zero 64 through byte $n$. 65 66 67 68 \subsubsection{coding bits into byte sequences} 69 70 The Vorbis codec has need to code arbitrary bit-width integers, from 71 zero to 32 bits wide, into packets. These integer fields are not 72 aligned to the boundaries of the byte representation; the next field 73 is written at the bit position at which the previous field ends. 74 75 The encoder logically packs integers by writing the LSb of a binary 76 integer to the logical bitstream first, followed by next least 77 significant bit, etc, until the requested number of bits have been 78 coded. When packing the bits into bytes, the encoder begins by 79 placing the LSb of the integer to be written into the least 80 significant unused bit position of the destination byte, followed by 81 the next-least significant bit of the source integer and so on up to 82 the requested number of bits. When all bits of the destination byte 83 have been filled, encoding continues by zeroing all bits of the next 84 byte and writing the next bit into the bit position 0 of that byte. 85 Decoding follows the same process as encoding, but by reading bits 86 from the byte stream and reassembling them into integers. 87 88 89 90 \subsubsection{signedness} 91 92 The signedness of a specific number resulting from decode is to be 93 interpreted by the decoder given decode context. That is, the three 94 bit binary pattern 'b111' can be taken to represent either 'seven' as 95 an unsigned integer, or '-1' as a signed, two's complement integer. 96 The encoder and decoder are responsible for knowing if fields are to 97 be treated as signed or unsigned. 98 99 100 101 \subsubsection{coding example} 102 103 Code the 4 bit integer value '12' [b1100] into an empty bytestream. 104 Bytestream result: 105 106 \begin{Verbatim}[commandchars=\\\{\}] 107 | 108 V 109 110 7 6 5 4 3 2 1 0 111 byte 0 [0 0 0 0 1 1 0 0] <- 112 byte 1 [ ] 113 byte 2 [ ] 114 byte 3 [ ] 115 ... 116 byte n [ ] bytestream length == 1 byte 117 118 \end{Verbatim} 119 120 121 Continue by coding the 3 bit integer value '-1' [b111]: 122 123 \begin{Verbatim}[commandchars=\\\{\}] 124 | 125 V 126 127 7 6 5 4 3 2 1 0 128 byte 0 [0 1 1 1 1 1 0 0] <- 129 byte 1 [ ] 130 byte 2 [ ] 131 byte 3 [ ] 132 ... 133 byte n [ ] bytestream length == 1 byte 134 \end{Verbatim} 135 136 137 Continue by coding the 7 bit integer value '17' [b0010001]: 138 139 \begin{Verbatim}[commandchars=\\\{\}] 140 | 141 V 142 143 7 6 5 4 3 2 1 0 144 byte 0 [1 1 1 1 1 1 0 0] 145 byte 1 [0 0 0 0 1 0 0 0] <- 146 byte 2 [ ] 147 byte 3 [ ] 148 ... 149 byte n [ ] bytestream length == 2 bytes 150 bit cursor == 6 151 \end{Verbatim} 152 153 154 Continue by coding the 13 bit integer value '6969' [b110 11001110 01]: 155 156 \begin{Verbatim}[commandchars=\\\{\}] 157 | 158 V 159 160 7 6 5 4 3 2 1 0 161 byte 0 [1 1 1 1 1 1 0 0] 162 byte 1 [0 1 0 0 1 0 0 0] 163 byte 2 [1 1 0 0 1 1 1 0] 164 byte 3 [0 0 0 0 0 1 1 0] <- 165 ... 166 byte n [ ] bytestream length == 4 bytes 167 168 \end{Verbatim} 169 170 171 172 173 \subsubsection{decoding example} 174 175 Reading from the beginning of the bytestream encoded in the above example: 176 177 \begin{Verbatim}[commandchars=\\\{\}] 178 | 179 V 180 181 7 6 5 4 3 2 1 0 182 byte 0 [1 1 1 1 1 1 0 0] <- 183 byte 1 [0 1 0 0 1 0 0 0] 184 byte 2 [1 1 0 0 1 1 1 0] 185 byte 3 [0 0 0 0 0 1 1 0] bytestream length == 4 bytes 186 187 \end{Verbatim} 188 189 190 We read two, two-bit integer fields, resulting in the returned numbers 191 'b00' and 'b11'. Two things are worth noting here: 192 193 \begin{itemize} 194 \item Although these four bits were originally written as a single 195 four-bit integer, reading some other combination of bit-widths from the 196 bitstream is well defined. There are no artificial alignment 197 boundaries maintained in the bitstream. 198 199 \item The second value is the 200 two-bit-wide integer 'b11'. This value may be interpreted either as 201 the unsigned value '3', or the signed value '-1'. Signedness is 202 dependent on decode context. 203 \end{itemize} 204 205 206 207 208 \subsubsection{end-of-packet alignment} 209 210 The typical use of bitpacking is to produce many independent 211 byte-aligned packets which are embedded into a larger byte-aligned 212 container structure, such as an Ogg transport bitstream. Externally, 213 each bytestream (encoded bitstream) must begin and end on a byte 214 boundary. Often, the encoded bitstream is not an integer number of 215 bytes, and so there is unused (uncoded) space in the last byte of a 216 packet. 217 218 Unused space in the last byte of a bytestream is always zeroed during 219 the coding process. Thus, should this unused space be read, it will 220 return binary zeroes. 221 222 Attempting to read past the end of an encoded packet results in an 223 'end-of-packet' condition. End-of-packet is not to be considered an 224 error; it is merely a state indicating that there is insufficient 225 remaining data to fulfill the desired read size. Vorbis uses truncated 226 packets as a normal mode of operation, and as such, decoders must 227 handle reading past the end of a packet as a typical mode of 228 operation. Any further read operations after an 'end-of-packet' 229 condition shall also return 'end-of-packet'. 230 231 232 233 \subsubsection{reading zero bits} 234 235 Reading a zero-bit-wide integer returns the value '0' and does not 236 increment the stream cursor. Reading to the end of the packet (but 237 not past, such that an 'end-of-packet' condition has not triggered) 238 and then reading a zero bit integer shall succeed, returning 0, and 239 not trigger an end-of-packet condition. Reading a zero-bit-wide 240 integer after a previous read sets 'end-of-packet' shall also fail 241 with 'end-of-packet'. 242 243 244 245 246 247 248