1 2 % -*- mode: latex; TeX-master: "Vorbis_I_spec"; -*- 3 %!TEX root = Vorbis_I_spec.tex 4 % $Id$ 5 \section{Codec Setup and Packet Decode} \label{vorbis:spec:codec} 6 7 \subsection{Overview} 8 9 This document serves as the top-level reference document for the 10 bit-by-bit decode specification of Vorbis I. This document assumes a 11 high-level understanding of the Vorbis decode process, which is 12 provided in \xref{vorbis:spec:intro}. \xref{vorbis:spec:bitpacking} covers reading and writing bit fields from 13 and to bitstream packets. 14 15 16 17 \subsection{Header decode and decode setup} 18 19 A Vorbis bitstream begins with three header packets. The header 20 packets are, in order, the identification header, the comments header, 21 and the setup header. All are required for decode compliance. An 22 end-of-packet condition during decoding the first or third header 23 packet renders the stream undecodable. End-of-packet decoding the 24 comment header is a non-fatal error condition. 25 26 \subsubsection{Common header decode} 27 28 Each header packet begins with the same header fields. 29 30 31 \begin{Verbatim}[commandchars=\\\{\}] 32 1) [packet_type] : 8 bit value 33 2) 0x76, 0x6f, 0x72, 0x62, 0x69, 0x73: the characters 'v','o','r','b','i','s' as six octets 34 \end{Verbatim} 35 36 Decode continues according to packet type; the identification header 37 is type 1, the comment header type 3 and the setup header type 5 38 (these types are all odd as a packet with a leading single bit of '0' 39 is an audio packet). The packets must occur in the order of 40 identification, comment, setup. 41 42 43 44 \subsubsection{Identification header} 45 46 The identification header is a short header of only a few fields used 47 to declare the stream definitively as Vorbis, and provide a few externally 48 relevant pieces of information about the audio stream. The 49 identification header is coded as follows: 50 51 \begin{Verbatim}[commandchars=\\\{\}] 52 1) [vorbis_version] = read 32 bits as unsigned integer 53 2) [audio_channels] = read 8 bit integer as unsigned 54 3) [audio_sample_rate] = read 32 bits as unsigned integer 55 4) [bitrate_maximum] = read 32 bits as signed integer 56 5) [bitrate_nominal] = read 32 bits as signed integer 57 6) [bitrate_minimum] = read 32 bits as signed integer 58 7) [blocksize_0] = 2 exponent (read 4 bits as unsigned integer) 59 8) [blocksize_1] = 2 exponent (read 4 bits as unsigned integer) 60 9) [framing_flag] = read one bit 61 \end{Verbatim} 62 63 \varname{[vorbis_version]} is to read '0' in order to be compatible 64 with this document. Both \varname{[audio_channels]} and 65 \varname{[audio_sample_rate]} must read greater than zero. Allowed final 66 blocksize values are 64, 128, 256, 512, 1024, 2048, 4096 and 8192 in 67 Vorbis I. \varname{[blocksize_0]} must be less than or equal to 68 \varname{[blocksize_1]}. The framing bit must be nonzero. Failure to 69 meet any of these conditions renders a stream undecodable. 70 71 The bitrate fields above are used only as hints. The nominal bitrate 72 field especially may be considerably off in purely VBR streams. The 73 fields are meaningful only when greater than zero. 74 75 \begin{itemize} 76 \item All three fields set to the same value implies a fixed rate, or tightly bounded, nearly fixed-rate bitstream 77 \item Only nominal set implies a VBR or ABR stream that averages the nominal bitrate 78 \item Maximum and or minimum set implies a VBR bitstream that obeys the bitrate limits 79 \item None set indicates the encoder does not care to speculate. 80 \end{itemize} 81 82 83 84 85 \subsubsection{Comment header} 86 Comment header decode and data specification is covered in 87 \xref{vorbis:spec:comment}. 88 89 90 \subsubsection{Setup header} 91 92 Vorbis codec setup is configurable to an extreme degree: 93 94 \begin{center} 95 \includegraphics[width=\textwidth]{components} 96 \captionof{figure}{decoder pipeline configuration} 97 \end{center} 98 99 100 The setup header contains the bulk of the codec setup information 101 needed for decode. The setup header contains, in order, the lists of 102 codebook configurations, time-domain transform configurations 103 (placeholders in Vorbis I), floor configurations, residue 104 configurations, channel mapping configurations and mode 105 configurations. It finishes with a framing bit of '1'. Header decode 106 proceeds in the following order: 107 108 \paragraph{Codebooks} 109 110 \begin{enumerate} 111 \item \varname{[vorbis_codebook_count]} = read eight bits as unsigned integer and add one 112 \item Decode \varname{[vorbis_codebook_count]} codebooks in order as defined 113 in \xref{vorbis:spec:codebook}. Save each configuration, in 114 order, in an array of 115 codebook configurations \varname{[vorbis_codebook_configurations]}. 116 \end{enumerate} 117 118 119 120 \paragraph{Time domain transforms} 121 122 These hooks are placeholders in Vorbis I. Nevertheless, the 123 configuration placeholder values must be read to maintain bitstream 124 sync. 125 126 \begin{enumerate} 127 \item \varname{[vorbis_time_count]} = read 6 bits as unsigned integer and add one 128 \item read \varname{[vorbis_time_count]} 16 bit values; each value should be zero. If any value is nonzero, this is an error condition and the stream is undecodable. 129 \end{enumerate} 130 131 132 133 \paragraph{Floors} 134 135 Vorbis uses two floor types; header decode is handed to the decode 136 abstraction of the appropriate type. 137 138 \begin{enumerate} 139 \item \varname{[vorbis_floor_count]} = read 6 bits as unsigned integer and add one 140 \item For each \varname{[i]} of \varname{[vorbis_floor_count]} floor numbers: 141 \begin{enumerate} 142 \item read the floor type: vector \varname{[vorbis_floor_types]} element \varname{[i]} = 143 read 16 bits as unsigned integer 144 \item If the floor type is zero, decode the floor 145 configuration as defined in \xref{vorbis:spec:floor0}; save 146 this 147 configuration in slot \varname{[i]} of the floor configuration array \varname{[vorbis_floor_configurations]}. 148 \item If the floor type is one, 149 decode the floor configuration as defined in \xref{vorbis:spec:floor1}; save this configuration in slot \varname{[i]} of the floor configuration array \varname{[vorbis_floor_configurations]}. 150 \item If the the floor type is greater than one, this stream is undecodable; ERROR CONDITION 151 \end{enumerate} 152 153 \end{enumerate} 154 155 156 157 \paragraph{Residues} 158 159 Vorbis uses three residue types; header decode of each type is identical. 160 161 162 \begin{enumerate} 163 \item \varname{[vorbis_residue_count]} = read 6 bits as unsigned integer and add one 164 165 \item For each of \varname{[vorbis_residue_count]} residue numbers: 166 \begin{enumerate} 167 \item read the residue type; vector \varname{[vorbis_residue_types]} element \varname{[i]} = read 16 bits as unsigned integer 168 \item If the residue type is zero, 169 one or two, decode the residue configuration as defined in \xref{vorbis:spec:residue}; save this configuration in slot \varname{[i]} of the residue configuration array \varname{[vorbis_residue_configurations]}. 170 \item If the the residue type is greater than two, this stream is undecodable; ERROR CONDITION 171 \end{enumerate} 172 173 \end{enumerate} 174 175 176 177 \paragraph{Mappings} 178 179 Mappings are used to set up specific pipelines for encoding 180 multichannel audio with varying channel mapping applications. Vorbis I 181 uses a single mapping type (0), with implicit PCM channel mappings. 182 183 % FIXME/TODO: LaTeX cannot nest enumerate that deeply, so I have to use 184 % itemize at the innermost level. However, it would be much better to 185 % rewrite this pseudocode using listings or algoritmicx or some other 186 % package geared towards this. 187 \begin{enumerate} 188 \item \varname{[vorbis_mapping_count]} = read 6 bits as unsigned integer and add one 189 \item For each \varname{[i]} of \varname{[vorbis_mapping_count]} mapping numbers: 190 \begin{enumerate} 191 \item read the mapping type: 16 bits as unsigned integer. There's no reason to save the mapping type in Vorbis I. 192 \item If the mapping type is nonzero, the stream is undecodable 193 \item If the mapping type is zero: 194 \begin{enumerate} 195 \item read 1 bit as a boolean flag 196 \begin{enumerate} 197 \item if set, \varname{[vorbis_mapping_submaps]} = read 4 bits as unsigned integer and add one 198 \item if unset, \varname{[vorbis_mapping_submaps]} = 1 199 \end{enumerate} 200 201 202 \item read 1 bit as a boolean flag 203 \begin{enumerate} 204 \item if set, square polar channel mapping is in use: 205 \begin{itemize} 206 \item \varname{[vorbis_mapping_coupling_steps]} = read 8 bits as unsigned integer and add one 207 \item for \varname{[j]} each of \varname{[vorbis_mapping_coupling_steps]} steps: 208 \begin{itemize} 209 \item vector \varname{[vorbis_mapping_magnitude]} element \varname{[j]}= read \link{vorbis:spec:ilog}{ilog}(\varname{[audio_channels]} - 1) bits as unsigned integer 210 \item vector \varname{[vorbis_mapping_angle]} element \varname{[j]}= read \link{vorbis:spec:ilog}{ilog}(\varname{[audio_channels]} - 1) bits as unsigned integer 211 \item the numbers read in the above two steps are channel numbers representing the channel to treat as magnitude and the channel to treat as angle, respectively. If for any coupling step the angle channel number equals the magnitude channel number, the magnitude channel number is greater than \varname{[audio_channels]}-1, or the angle channel is greater than \varname{[audio_channels]}-1, the stream is undecodable. 212 \end{itemize} 213 214 215 \end{itemize} 216 217 218 \item if unset, \varname{[vorbis_mapping_coupling_steps]} = 0 219 \end{enumerate} 220 221 222 \item read 2 bits (reserved field); if the value is nonzero, the stream is undecodable 223 \item if \varname{[vorbis_mapping_submaps]} is greater than one, we read channel multiplex settings. For each \varname{[j]} of \varname{[audio_channels]} channels: 224 \begin{enumerate} 225 \item vector \varname{[vorbis_mapping_mux]} element \varname{[j]} = read 4 bits as unsigned integer 226 \item if the value is greater than the highest numbered submap (\varname{[vorbis_mapping_submaps]} - 1), this in an error condition rendering the stream undecodable 227 \end{enumerate} 228 229 \item for each submap \varname{[j]} of \varname{[vorbis_mapping_submaps]} submaps, read the floor and residue numbers for use in decoding that submap: 230 \begin{enumerate} 231 \item read and discard 8 bits (the unused time configuration placeholder) 232 \item read 8 bits as unsigned integer for the floor number; save in vector \varname{[vorbis_mapping_submap_floor]} element \varname{[j]} 233 \item verify the floor number is not greater than the highest number floor configured for the bitstream. If it is, the bitstream is undecodable 234 \item read 8 bits as unsigned integer for the residue number; save in vector \varname{[vorbis_mapping_submap_residue]} element \varname{[j]} 235 \item verify the residue number is not greater than the highest number residue configured for the bitstream. If it is, the bitstream is undecodable 236 \end{enumerate} 237 238 \item save this mapping configuration in slot \varname{[i]} of the mapping configuration array \varname{[vorbis_mapping_configurations]}. 239 \end{enumerate} 240 241 \end{enumerate} 242 243 \end{enumerate} 244 245 246 247 \paragraph{Modes} 248 249 \begin{enumerate} 250 \item \varname{[vorbis_mode_count]} = read 6 bits as unsigned integer and add one 251 \item For each of \varname{[vorbis_mode_count]} mode numbers: 252 \begin{enumerate} 253 \item \varname{[vorbis_mode_blockflag]} = read 1 bit 254 \item \varname{[vorbis_mode_windowtype]} = read 16 bits as unsigned integer 255 \item \varname{[vorbis_mode_transformtype]} = read 16 bits as unsigned integer 256 \item \varname{[vorbis_mode_mapping]} = read 8 bits as unsigned integer 257 \item verify ranges; zero is the only legal value in Vorbis I for 258 \varname{[vorbis_mode_windowtype]} 259 and \varname{[vorbis_mode_transformtype]}. \varname{[vorbis_mode_mapping]} must not be greater than the highest number mapping in use. Any illegal values render the stream undecodable. 260 \item save this mode configuration in slot \varname{[i]} of the mode configuration array 261 \varname{[vorbis_mode_configurations]}. 262 \end{enumerate} 263 264 \item read 1 bit as a framing flag. If unset, a framing error occurred and the stream is not 265 decodable. 266 \end{enumerate} 267 268 After reading mode descriptions, setup header decode is complete. 269 270 271 272 273 274 275 276 277 \subsection{Audio packet decode and synthesis} 278 279 Following the three header packets, all packets in a Vorbis I stream 280 are audio. The first step of audio packet decode is to read and 281 verify the packet type. \emph{A non-audio packet when audio is expected 282 indicates stream corruption or a non-compliant stream. The decoder 283 must ignore the packet and not attempt decoding it to audio}. 284 285 286 \subsubsection{packet type, mode and window decode} 287 288 \begin{enumerate} 289 \item read 1 bit \varname{[packet_type]}; check that packet type is 0 (audio) 290 \item read \link{vorbis:spec:ilog}{ilog}([vorbis_mode_count]-1) bits 291 \varname{[mode_number]} 292 \item decode blocksize \varname{[n]} is equal to \varname{[blocksize_0]} if 293 \varname{[vorbis_mode_blockflag]} is 0, else \varname{[n]} is equal to \varname{[blocksize_1]}. 294 \item perform window selection and setup; this window is used later by the inverse MDCT: 295 \begin{enumerate} 296 \item if this is a long window (the \varname{[vorbis_mode_blockflag]} flag of this mode is 297 set): 298 \begin{enumerate} 299 \item read 1 bit for \varname{[previous_window_flag]} 300 \item read 1 bit for \varname{[next_window_flag]} 301 \item if \varname{[previous_window_flag]} is not set, the left half 302 of the window will be a hybrid window for lapping with a 303 short block. See \xref{vorbis:spec:window} for an illustration of overlapping 304 dissimilar 305 windows. Else, the left half window will have normal long 306 shape. 307 \item if \varname{[next_window_flag]} is not set, the right half of 308 the window will be a hybrid window for lapping with a short 309 block. See \xref{vorbis:spec:window} for an 310 illustration of overlapping dissimilar 311 windows. Else, the left right window will have normal long 312 shape. 313 \end{enumerate} 314 315 \item if this is a short window, the window is always the same 316 short-window shape. 317 \end{enumerate} 318 319 \end{enumerate} 320 321 Vorbis windows all use the slope function $y=\sin(\frac{\pi}{2} * \sin^2((x+0.5)/n * \pi))$, 322 where $n$ is window size and $x$ ranges $0 \ldots n-1$, but dissimilar 323 lapping requirements can affect overall shape. Window generation 324 proceeds as follows: 325 326 \begin{enumerate} 327 \item \varname{[window_center]} = \varname{[n]} / 2 328 \item if (\varname{[vorbis_mode_blockflag]} is set and \varname{[previous_window_flag]} is 329 not set) then 330 \begin{enumerate} 331 \item \varname{[left_window_start]} = \varname{[n]}/4 - 332 \varname{[blocksize_0]}/4 333 \item \varname{[left_window_end]} = \varname{[n]}/4 + \varname{[blocksize_0]}/4 334 \item \varname{[left_n]} = \varname{[blocksize_0]}/2 335 \end{enumerate} 336 else 337 \begin{enumerate} 338 \item \varname{[left_window_start]} = 0 339 \item \varname{[left_window_end]} = \varname{[window_center]} 340 \item \varname{[left_n]} = \varname{[n]}/2 341 \end{enumerate} 342 343 \item if (\varname{[vorbis_mode_blockflag]} is set and \varname{[next_window_flag]} is not 344 set) then 345 \begin{enumerate} 346 \item \varname{[right_window_start]} = \varname{[n]*3}/4 - 347 \varname{[blocksize_0]}/4 348 \item \varname{[right_window_end]} = \varname{[n]*3}/4 + 349 \varname{[blocksize_0]}/4 350 \item \varname{[right_n]} = \varname{[blocksize_0]}/2 351 \end{enumerate} 352 else 353 \begin{enumerate} 354 \item \varname{[right_window_start]} = \varname{[window_center]} 355 \item \varname{[right_window_end]} = \varname{[n]} 356 \item \varname{[right_n]} = \varname{[n]}/2 357 \end{enumerate} 358 359 \item window from range 0 ... \varname{[left_window_start]}-1 inclusive is zero 360 \item for \varname{[i]} in range \varname{[left_window_start]} ... 361 \varname{[left_window_end]}-1, window(\varname{[i]}) = $\sin(\frac{\pi}{2} * \sin^2($ (\varname{[i]}-\varname{[left_window_start]}+0.5) / \varname{[left_n]} $* \frac{\pi}{2})$ ) 362 \item window from range \varname{[left_window_end]} ... \varname{[right_window_start]}-1 363 inclusive is one\item for \varname{[i]} in range \varname{[right_window_start]} ... \varname{[right_window_end]}-1, window(\varname{[i]}) = $\sin(\frac{\pi}{2} * \sin^2($ (\varname{[i]}-\varname{[right_window_start]}+0.5) / \varname{[right_n]} $ * \frac{\pi}{2} + \frac{\pi}{2})$ ) 364 \item window from range \varname{[right_window_start]} ... \varname{[n]}-1 is 365 zero 366 \end{enumerate} 367 368 An end-of-packet condition up to this point should be considered an 369 error that discards this packet from the stream. An end of packet 370 condition past this point is to be considered a possible nominal 371 occurrence. 372 373 374 375 \subsubsection{floor curve decode} 376 377 From this point on, we assume out decode context is using mode number 378 \varname{[mode_number]} from configuration array 379 \varname{[vorbis_mode_configurations]} and the map number 380 \varname{[vorbis_mode_mapping]} (specified by the current mode) taken 381 from the mapping configuration array 382 \varname{[vorbis_mapping_configurations]}. 383 384 Floor curves are decoded one-by-one in channel order. 385 386 For each floor \varname{[i]} of \varname{[audio_channels]} 387 \begin{enumerate} 388 \item \varname{[submap_number]} = element \varname{[i]} of vector [vorbis_mapping_mux] 389 \item \varname{[floor_number]} = element \varname{[submap_number]} of vector 390 [vorbis_submap_floor] 391 \item if the floor type of this 392 floor (vector \varname{[vorbis_floor_types]} element 393 \varname{[floor_number]}) is zero then decode the floor for 394 channel \varname{[i]} according to the 395 \xref{vorbis:spec:floor0-decode} 396 \item if the type of this floor 397 is one then decode the floor for channel \varname{[i]} according 398 to the \xref{vorbis:spec:floor1-decode} 399 \item save the needed decoded floor information for channel for later synthesis 400 \item if the decoded floor returned 'unused', set vector \varname{[no_residue]} element 401 \varname{[i]} to true, else set vector \varname{[no_residue]} element \varname{[i]} to 402 false 403 \end{enumerate} 404 405 406 An end-of-packet condition during floor decode shall result in packet 407 decode zeroing all channel output vectors and skipping to the 408 add/overlap output stage. 409 410 411 412 \subsubsection{nonzero vector propagate} 413 414 A possible result of floor decode is that a specific vector is marked 415 'unused' which indicates that that final output vector is all-zero 416 values (and the floor is zero). The residue for that vector is not 417 coded in the stream, save for one complication. If some vectors are 418 used and some are not, channel coupling could result in mixing a 419 zeroed and nonzeroed vector to produce two nonzeroed vectors. 420 421 for each \varname{[i]} from 0 ... \varname{[vorbis_mapping_coupling_steps]}-1 422 423 \begin{enumerate} 424 \item if either \varname{[no_residue]} entry for channel 425 (\varname{[vorbis_mapping_magnitude]} element \varname{[i]}) 426 or channel 427 (\varname{[vorbis_mapping_angle]} element \varname{[i]}) 428 are set to false, then both must be set to false. Note that an 'unused' 429 floor has no decoded floor information; it is important that this is 430 remembered at floor curve synthesis time. 431 \end{enumerate} 432 433 434 435 436 \subsubsection{residue decode} 437 438 Unlike floors, which are decoded in channel order, the residue vectors 439 are decoded in submap order. 440 441 for each submap \varname{[i]} in order from 0 ... \varname{[vorbis_mapping_submaps]}-1 442 443 \begin{enumerate} 444 \item \varname{[ch]} = 0 445 \item for each channel \varname{[j]} in order from 0 ... \varname{[audio_channels]} - 1 446 \begin{enumerate} 447 \item if channel \varname{[j]} in submap \varname{[i]} (vector \varname{[vorbis_mapping_mux]} element \varname{[j]} is equal to \varname{[i]}) 448 \begin{enumerate} 449 \item if vector \varname{[no_residue]} element \varname{[j]} is true 450 \begin{enumerate} 451 \item vector \varname{[do_not_decode_flag]} element \varname{[ch]} is set 452 \end{enumerate} 453 else 454 \begin{enumerate} 455 \item vector \varname{[do_not_decode_flag]} element \varname{[ch]} is unset 456 \end{enumerate} 457 458 \item increment \varname{[ch]} 459 \end{enumerate} 460 461 \end{enumerate} 462 \item \varname{[residue_number]} = vector \varname{[vorbis_mapping_submap_residue]} element \varname{[i]} 463 \item \varname{[residue_type]} = vector \varname{[vorbis_residue_types]} element \varname{[residue_number]} 464 \item decode \varname{[ch]} vectors using residue \varname{[residue_number]}, according to type \varname{[residue_type]}, also passing vector \varname{[do_not_decode_flag]} to indicate which vectors in the bundle should not be decoded. Correct per-vector decode length is \varname{[n]}/2. 465 \item \varname{[ch]} = 0 466 \item for each channel \varname{[j]} in order from 0 ... \varname{[audio_channels]} 467 \begin{enumerate} 468 \item if channel \varname{[j]} is in submap \varname{[i]} (vector \varname{[vorbis_mapping_mux]} element \varname{[j]} is equal to \varname{[i]}) 469 \begin{enumerate} 470 \item residue vector for channel \varname{[j]} is set to decoded residue vector \varname{[ch]} 471 \item increment \varname{[ch]} 472 \end{enumerate} 473 474 \end{enumerate} 475 476 \end{enumerate} 477 478 479 480 \subsubsection{inverse coupling} 481 482 for each \varname{[i]} from \varname{[vorbis_mapping_coupling_steps]}-1 descending to 0 483 484 \begin{enumerate} 485 \item \varname{[magnitude_vector]} = the residue vector for channel 486 (vector \varname{[vorbis_mapping_magnitude]} element \varname{[i]}) 487 \item \varname{[angle_vector]} = the residue vector for channel (vector 488 \varname{[vorbis_mapping_angle]} element \varname{[i]}) 489 \item for each scalar value \varname{[M]} in vector \varname{[magnitude_vector]} and the corresponding scalar value \varname{[A]} in vector \varname{[angle_vector]}: 490 \begin{enumerate} 491 \item if (\varname{[M]} is greater than zero) 492 \begin{enumerate} 493 \item if (\varname{[A]} is greater than zero) 494 \begin{enumerate} 495 \item \varname{[new_M]} = \varname{[M]} 496 \item \varname{[new_A]} = \varname{[M]}-\varname{[A]} 497 \end{enumerate} 498 else 499 \begin{enumerate} 500 \item \varname{[new_A]} = \varname{[M]} 501 \item \varname{[new_M]} = \varname{[M]}+\varname{[A]} 502 \end{enumerate} 503 504 \end{enumerate} 505 else 506 \begin{enumerate} 507 \item if (\varname{[A]} is greater than zero) 508 \begin{enumerate} 509 \item \varname{[new_M]} = \varname{[M]} 510 \item \varname{[new_A]} = \varname{[M]}+\varname{[A]} 511 \end{enumerate} 512 else 513 \begin{enumerate} 514 \item \varname{[new_A]} = \varname{[M]} 515 \item \varname{[new_M]} = \varname{[M]}-\varname{[A]} 516 \end{enumerate} 517 518 \end{enumerate} 519 520 \item set scalar value \varname{[M]} in vector \varname{[magnitude_vector]} to \varname{[new_M]} 521 \item set scalar value \varname{[A]} in vector \varname{[angle_vector]} to \varname{[new_A]} 522 \end{enumerate} 523 524 \end{enumerate} 525 526 527 528 529 \subsubsection{dot product} 530 531 For each channel, synthesize the floor curve from the decoded floor 532 information, according to packet type. Note that the vector synthesis 533 length for floor computation is \varname{[n]}/2. 534 535 For each channel, multiply each element of the floor curve by each 536 element of that channel's residue vector. The result is the dot 537 product of the floor and residue vectors for each channel; the produced 538 vectors are the length \varname{[n]}/2 audio spectrum for each 539 channel. 540 541 % TODO/FIXME: The following two paragraphs have identical twins 542 % in section 1 (under "compute floor/residue dot product") 543 One point is worth mentioning about this dot product; a common mistake 544 in a fixed point implementation might be to assume that a 32 bit 545 fixed-point representation for floor and residue and direct 546 multiplication of the vectors is sufficient for acceptable spectral 547 depth in all cases because it happens to mostly work with the current 548 Xiph.Org reference encoder. 549 550 However, floor vector values can span \~140dB (\~24 bits unsigned), and 551 the audio spectrum vector should represent a minimum of 120dB (\~21 552 bits with sign), even when output is to a 16 bit PCM device. For the 553 residue vector to represent full scale if the floor is nailed to 554 $-140$dB, it must be able to span 0 to $+140$dB. For the residue vector 555 to reach full scale if the floor is nailed at 0dB, it must be able to 556 represent $-140$dB to $+0$dB. Thus, in order to handle full range 557 dynamics, a residue vector may span $-140$dB to $+140$dB entirely within 558 spec. A 280dB range is approximately 48 bits with sign; thus the 559 residue vector must be able to represent a 48 bit range and the dot 560 product must be able to handle an effective 48 bit times 24 bit 561 multiplication. This range may be achieved using large (64 bit or 562 larger) integers, or implementing a movable binary point 563 representation. 564 565 566 567 \subsubsection{inverse MDCT} 568 569 Convert the audio spectrum vector of each channel back into time 570 domain PCM audio via an inverse Modified Discrete Cosine Transform 571 (MDCT). A detailed description of the MDCT is available in \cite{Sporer/Brandenburg/Edler}. The window 572 function used for the MDCT is the function described earlier. 573 574 575 576 \subsubsection{overlap_add} 577 578 Windowed MDCT output is overlapped and added with the right hand data 579 of the previous window such that the 3/4 point of the previous window 580 is aligned with the 1/4 point of the current window (as illustrated in 581 \xref{vorbis:spec:window}). The overlapped portion 582 produced from overlapping the previous and current frame data is 583 finished data to be returned by the decoder. This data spans from the 584 center of the previous window to the center of the current window. In 585 the case of same-sized windows, the amount of data to return is 586 one-half block consisting of and only of the overlapped portions. When 587 overlapping a short and long window, much of the returned range does not 588 actually overlap. This does not damage transform orthogonality. Pay 589 attention however to returning the correct data range; the amount of 590 data to be returned is: 591 592 \begin{programlisting} 593 window_blocksize(previous_window)/4+window_blocksize(current_window)/4 594 \end{programlisting} 595 596 from the center (element windowsize/2) of the previous window to the 597 center (element windowsize/2-1, inclusive) of the current window. 598 599 Data is not returned from the first frame; it must be used to 'prime' 600 the decode engine. The encoder accounts for this priming when 601 calculating PCM offsets; after the first frame, the proper PCM output 602 offset is '0' (as no data has been returned yet). 603 604 605 606 \subsubsection{output channel order} 607 608 Vorbis I specifies only a channel mapping type 0. In mapping type 0, 609 channel mapping is implicitly defined as follows for standard audio 610 applications. As of revision 16781 (20100113), the specification adds 611 defined channel locations for 6.1 and 7.1 surround. Ordering/location 612 for greater-than-eight channels remains 'left to the implementation'. 613 614 These channel orderings refer to order within the encoded stream. It 615 is naturally possible for a decoder to produce output with channels in 616 any order. Any such decoder should explicitly document channel 617 reordering behavior. 618 619 \begin{description} %[style=nextline] 620 \item[one channel] 621 the stream is monophonic 622 623 \item[two channels] 624 the stream is stereo. channel order: left, right 625 626 \item[three channels] 627 the stream is a 1d-surround encoding. channel order: left, 628 center, right 629 630 \item[four channels] 631 the stream is quadraphonic surround. channel order: front left, 632 front right, rear left, rear right 633 634 \item[five channels] 635 the stream is five-channel surround. channel order: front left, 636 center, front right, rear left, rear right 637 638 \item[six channels] 639 the stream is 5.1 surround. channel order: front left, center, 640 front right, rear left, rear right, LFE 641 642 \item[seven channels] 643 the stream is 6.1 surround. channel order: front left, center, 644 front right, side left, side right, rear center, LFE 645 646 \item[eight channels] 647 the stream is 7.1 surround. channel order: front left, center, 648 front right, side left, side right, rear left, rear right, 649 LFE 650 651 \item[greater than eight channels] 652 channel use and order is defined by the application 653 654 \end{description} 655 656 Applications using Vorbis for dedicated purposes may define channel 657 mapping as seen fit. Future channel mappings (such as three and four 658 channel \href{http://www.ambisonic.net/}{Ambisonics}) will 659 make use of channel mappings other than mapping 0. 660 661 662
