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2% -*- mode: latex; TeX-master: "Vorbis_I_spec"; -*-
3%!TEX root = Vorbis_I_spec.tex
4\section{Codec Setup and Packet Decode} \label{vorbis:spec:codec}
5
6\subsection{Overview}
7
8This document serves as the top-level reference document for the
9bit-by-bit decode specification of Vorbis I. This document assumes a
10high-level understanding of the Vorbis decode process, which is
11provided in \xref{vorbis:spec:intro}. \xref{vorbis:spec:bitpacking} covers reading and writing bit fields from
12and to bitstream packets.
13
14
15
16\subsection{Header decode and decode setup}
17
18A Vorbis bitstream begins with three header packets. The header
19packets are, in order, the identification header, the comments header,
20and the setup header. All are required for decode compliance. An
21end-of-packet condition during decoding the first or third header
22packet renders the stream undecodable. End-of-packet decoding the
23comment header is a non-fatal error condition.
24
25\subsubsection{Common header decode}
26
27Each header packet begins with the same header fields.
28
29
30\begin{Verbatim}[commandchars=\\\{\}]
31 1) [packet\_type] : 8 bit value
32 2) 0x76, 0x6f, 0x72, 0x62, 0x69, 0x73: the characters 'v','o','r','b','i','s' as six octets
33\end{Verbatim}
34
35Decode continues according to packet type; the identification header
36is type 1, the comment header type 3 and the setup header type 5
37(these types are all odd as a packet with a leading single bit of '0'
38is an audio packet). The packets must occur in the order of
39identification, comment, setup.
40
41
42
43\subsubsection{Identification header}
44
45The identification header is a short header of only a few fields used
46to declare the stream definitively as Vorbis, and provide a few externally
47relevant pieces of information about the audio stream. The
48identification header is coded as follows:
49
50\begin{Verbatim}[commandchars=\\\{\}]
51 1) [vorbis\_version] = read 32 bits as unsigned integer
52 2) [audio\_channels] = read 8 bit integer as unsigned
53 3) [audio\_sample\_rate] = read 32 bits as unsigned integer
54 4) [bitrate\_maximum] = read 32 bits as signed integer
55 5) [bitrate\_nominal] = read 32 bits as signed integer
56 6) [bitrate\_minimum] = read 32 bits as signed integer
57 7) [blocksize\_0] = 2 exponent (read 4 bits as unsigned integer)
58 8) [blocksize\_1] = 2 exponent (read 4 bits as unsigned integer)
59 9) [framing\_flag] = read one bit
60\end{Verbatim}
61
62\varname{[vorbis\_version]} is to read '0' in order to be compatible
63with this document. Both \varname{[audio\_channels]} and
64\varname{[audio\_sample\_rate]} must read greater than zero. Allowed final
65blocksize values are 64, 128, 256, 512, 1024, 2048, 4096 and 8192 in
66Vorbis I. \varname{[blocksize\_0]} must be less than or equal to
67\varname{[blocksize\_1]}. The framing bit must be nonzero. Failure to
68meet any of these conditions renders a stream undecodable.
69
70The bitrate fields above are used only as hints. The nominal bitrate
71field especially may be considerably off in purely VBR streams. The
72fields are meaningful only when greater than zero.
73
74\begin{itemize}
75 \item All three fields set to the same value implies a fixed rate, or tightly bounded, nearly fixed-rate bitstream
76 \item Only nominal set implies a VBR or ABR stream that averages the nominal bitrate
77 \item Maximum and or minimum set implies a VBR bitstream that obeys the bitrate limits
78 \item None set indicates the encoder does not care to speculate.
79\end{itemize}
80
81
82
83
84\subsubsection{Comment header}
85Comment header decode and data specification is covered in
86\xref{vorbis:spec:comment}.
87
88
89\subsubsection{Setup header}
90
91Vorbis codec setup is configurable to an extreme degree:
92
93\begin{center}
94\includegraphics[width=\textwidth]{components}
95\captionof{figure}{decoder pipeline configuration}
96\end{center}
97
98
99The setup header contains the bulk of the codec setup information
100needed for decode. The setup header contains, in order, the lists of
101codebook configurations, time-domain transform configurations
102(placeholders in Vorbis I), floor configurations, residue
103configurations, channel mapping configurations and mode
104configurations. It finishes with a framing bit of '1'. Header decode
105proceeds in the following order:
106
107\paragraph{Codebooks}
108
109\begin{enumerate}
110\item \varname{[vorbis\_codebook\_count]} = read eight bits as unsigned integer and add one
111\item Decode \varname{[vorbis\_codebook\_count]} codebooks in order as defined
112in \xref{vorbis:spec:codebook}. Save each configuration, in
113order, in an array of
114codebook configurations \varname{[vorbis\_codebook\_configurations]}.
115\end{enumerate}
116
117
118
119\paragraph{Time domain transforms}
120
121These hooks are placeholders in Vorbis I. Nevertheless, the
122configuration placeholder values must be read to maintain bitstream
123sync.
124
125\begin{enumerate}
126\item \varname{[vorbis\_time\_count]} = read 6 bits as unsigned integer and add one
127\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.
128\end{enumerate}
129
130
131
132\paragraph{Floors}
133
134Vorbis uses two floor types; header decode is handed to the decode
135abstraction of the appropriate type.
136
137\begin{enumerate}
138 \item \varname{[vorbis\_floor\_count]} = read 6 bits as unsigned integer and add one
139 \item For each \varname{[i]} of \varname{[vorbis\_floor\_count]} floor numbers:
140 \begin{enumerate}
141 \item read the floor type: vector \varname{[vorbis\_floor\_types]} element \varname{[i]} =
142read 16 bits as unsigned integer
143 \item If the floor type is zero, decode the floor
144configuration as defined in \xref{vorbis:spec:floor0}; save
145this
146configuration in slot \varname{[i]} of the floor configuration array \varname{[vorbis\_floor\_configurations]}.
147 \item If the floor type is one,
148decode 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]}.
149 \item If the the floor type is greater than one, this stream is undecodable; ERROR CONDITION
150 \end{enumerate}
151
152\end{enumerate}
153
154
155
156\paragraph{Residues}
157
158Vorbis uses three residue types; header decode of each type is identical.
159
160
161\begin{enumerate}
162\item \varname{[vorbis\_residue\_count]} = read 6 bits as unsigned integer and add one
163
164\item For each of \varname{[vorbis\_residue\_count]} residue numbers:
165 \begin{enumerate}
166 \item read the residue type; vector \varname{[vorbis\_residue\_types]} element \varname{[i]} = read 16 bits as unsigned integer
167 \item If the residue type is zero,
168one 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]}.
169 \item If the the residue type is greater than two, this stream is undecodable; ERROR CONDITION
170 \end{enumerate}
171
172\end{enumerate}
173
174
175
176\paragraph{Mappings}
177
178Mappings are used to set up specific pipelines for encoding
179multichannel audio with varying channel mapping applications. Vorbis I
180uses a single mapping type (0), with implicit PCM channel mappings.
181
182% FIXME/TODO: LaTeX cannot nest enumerate that deeply, so I have to use
183% itemize at the innermost level. However, it would be much better to
184% rewrite this pseudocode using listings or algoritmicx or some other
185% package geared towards this.
186\begin{enumerate}
187 \item \varname{[vorbis\_mapping\_count]} = read 6 bits as unsigned integer and add one
188 \item For each \varname{[i]} of \varname{[vorbis\_mapping\_count]} mapping numbers:
189 \begin{enumerate}
190 \item read the mapping type: 16 bits as unsigned integer. There's no reason to save the mapping type in Vorbis I.
191 \item If the mapping type is nonzero, the stream is undecodable
192 \item If the mapping type is zero:
193 \begin{enumerate}
194 \item read 1 bit as a boolean flag
195 \begin{enumerate}
196 \item if set, \varname{[vorbis\_mapping\_submaps]} = read 4 bits as unsigned integer and add one
197 \item if unset, \varname{[vorbis\_mapping\_submaps]} = 1
198 \end{enumerate}
199
200
201 \item read 1 bit as a boolean flag
202 \begin{enumerate}
203 \item if set, square polar channel mapping is in use:
204 \begin{itemize}
205 \item \varname{[vorbis\_mapping\_coupling\_steps]} = read 8 bits as unsigned integer and add one
206 \item for \varname{[j]} each of \varname{[vorbis\_mapping\_coupling\_steps]} steps:
207 \begin{itemize}
208 \item vector \varname{[vorbis\_mapping\_magnitude]} element \varname{[j]}= read \link{vorbis:spec:ilog}{ilog}(\varname{[audio\_channels]} - 1) bits as unsigned integer
209 \item vector \varname{[vorbis\_mapping\_angle]} element \varname{[j]}= read \link{vorbis:spec:ilog}{ilog}(\varname{[audio\_channels]} - 1) bits as unsigned integer
210 \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.
211 \end{itemize}
212
213
214 \end{itemize}
215
216
217 \item if unset, \varname{[vorbis\_mapping\_coupling\_steps]} = 0
218 \end{enumerate}
219
220
221 \item read 2 bits (reserved field); if the value is nonzero, the stream is undecodable
222 \item if \varname{[vorbis\_mapping\_submaps]} is greater than one, we read channel multiplex settings. For each \varname{[j]} of \varname{[audio\_channels]} channels:
223 \begin{enumerate}
224 \item vector \varname{[vorbis\_mapping\_mux]} element \varname{[j]} = read 4 bits as unsigned integer
225 \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
226 \end{enumerate}
227
228 \item for each submap \varname{[j]} of \varname{[vorbis\_mapping\_submaps]} submaps, read the floor and residue numbers for use in decoding that submap:
229 \begin{enumerate}
230 \item read and discard 8 bits (the unused time configuration placeholder)
231 \item read 8 bits as unsigned integer for the floor number; save in vector \varname{[vorbis\_mapping\_submap\_floor]} element \varname{[j]}
232 \item verify the floor number is not greater than the highest number floor configured for the bitstream. If it is, the bitstream is undecodable
233 \item read 8 bits as unsigned integer for the residue number; save in vector \varname{[vorbis\_mapping\_submap\_residue]} element \varname{[j]}
234 \item verify the residue number is not greater than the highest number residue configured for the bitstream. If it is, the bitstream is undecodable
235 \end{enumerate}
236
237 \item save this mapping configuration in slot \varname{[i]} of the mapping configuration array \varname{[vorbis\_mapping\_configurations]}.
238 \end{enumerate}
239
240 \end{enumerate}
241
242\end{enumerate}
243
244
245
246\paragraph{Modes}
247
248\begin{enumerate}
249 \item \varname{[vorbis\_mode\_count]} = read 6 bits as unsigned integer and add one
250 \item For each of \varname{[vorbis\_mode\_count]} mode numbers:
251 \begin{enumerate}
252 \item \varname{[vorbis\_mode\_blockflag]} = read 1 bit
253 \item \varname{[vorbis\_mode\_windowtype]} = read 16 bits as unsigned integer
254 \item \varname{[vorbis\_mode\_transformtype]} = read 16 bits as unsigned integer
255 \item \varname{[vorbis\_mode\_mapping]} = read 8 bits as unsigned integer
256 \item verify ranges; zero is the only legal value in Vorbis I for
257\varname{[vorbis\_mode\_windowtype]}
258and \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.
259 \item save this mode configuration in slot \varname{[i]} of the mode configuration array
260\varname{[vorbis\_mode\_configurations]}.
261 \end{enumerate}
262
263\item read 1 bit as a framing flag. If unset, a framing error occurred and the stream is not
264decodable.
265\end{enumerate}
266
267After reading mode descriptions, setup header decode is complete.
268
269
270
271
272
273
274
275
276\subsection{Audio packet decode and synthesis}
277
278Following the three header packets, all packets in a Vorbis I stream
279are audio. The first step of audio packet decode is to read and
280verify the packet type. \emph{A non-audio packet when audio is expected
281indicates stream corruption or a non-compliant stream. The decoder
282must ignore the packet and not attempt decoding it to audio}.
283
284
285\subsubsection{packet type, mode and window decode}
286
287\begin{enumerate}
288 \item read 1 bit \varname{[packet\_type]}; check that packet type is 0 (audio)
289 \item read \link{vorbis:spec:ilog}{ilog}([vorbis\_mode\_count]-1) bits
290\varname{[mode\_number]}
291 \item decode blocksize \varname{[n]} is equal to \varname{[blocksize\_0]} if
292\varname{[vorbis\_mode\_blockflag]} is 0, else \varname{[n]} is equal to \varname{[blocksize\_1]}.
293 \item perform window selection and setup; this window is used later by the inverse MDCT:
294 \begin{enumerate}
295 \item if this is a long window (the \varname{[vorbis\_mode\_blockflag]} flag of this mode is
296set):
297 \begin{enumerate}
298 \item read 1 bit for \varname{[previous\_window\_flag]}
299 \item read 1 bit for \varname{[next\_window\_flag]}
300 \item if \varname{[previous\_window\_flag]} is not set, the left half
301 of the window will be a hybrid window for lapping with a
302 short block. See \xref{vorbis:spec:window} for an illustration of overlapping
303dissimilar
304 windows. Else, the left half window will have normal long
305 shape.
306 \item if \varname{[next\_window\_flag]} is not set, the right half of
307 the window will be a hybrid window for lapping with a short
308 block. See \xref{vorbis:spec:window} for an
309illustration of overlapping dissimilar
310 windows. Else, the left right window will have normal long
311 shape.
312 \end{enumerate}
313
314 \item if this is a short window, the window is always the same
315 short-window shape.
316 \end{enumerate}
317
318\end{enumerate}
319
320Vorbis windows all use the slope function $y=\sin(\frac{\pi}{2} * \sin^2((x+0.5)/n * \pi))$,
321where $n$ is window size and $x$ ranges $0 \ldots n-1$, but dissimilar
322lapping requirements can affect overall shape. Window generation
323proceeds as follows:
324
325\begin{enumerate}
326 \item \varname{[window\_center]} = \varname{[n]} / 2
327 \item if (\varname{[vorbis\_mode\_blockflag]} is set and \varname{[previous\_window\_flag]} is
328not set) then
329 \begin{enumerate}
330 \item \varname{[left\_window\_start]} = \varname{[n]}/4 -
331\varname{[blocksize\_0]}/4
332 \item \varname{[left\_window\_end]} = \varname{[n]}/4 + \varname{[blocksize\_0]}/4
333 \item \varname{[left\_n]} = \varname{[blocksize\_0]}/2
334 \end{enumerate}
335 else
336 \begin{enumerate}
337 \item \varname{[left\_window\_start]} = 0
338 \item \varname{[left\_window\_end]} = \varname{[window\_center]}
339 \item \varname{[left\_n]} = \varname{[n]}/2
340 \end{enumerate}
341
342 \item if (\varname{[vorbis\_mode\_blockflag]} is set and \varname{[next\_window\_flag]} is not
343set) then
344 \begin{enumerate}
345 \item \varname{[right\_window\_start]} = \varname{[n]*3}/4 -
346\varname{[blocksize\_0]}/4
347 \item \varname{[right\_window\_end]} = \varname{[n]*3}/4 +
348\varname{[blocksize\_0]}/4
349 \item \varname{[right\_n]} = \varname{[blocksize\_0]}/2
350 \end{enumerate}
351 else
352 \begin{enumerate}
353 \item \varname{[right\_window\_start]} = \varname{[window\_center]}
354 \item \varname{[right\_window\_end]} = \varname{[n]}
355 \item \varname{[right\_n]} = \varname{[n]}/2
356 \end{enumerate}
357
358 \item window from range 0 ... \varname{[left\_window\_start]}-1 inclusive is zero
359 \item for \varname{[i]} in range \varname{[left\_window\_start]} ...
360\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]} $* \frac{\pi}{2})$ )
361 \item window from range \varname{[left\_window\_end]} ... \varname{[right\_window\_start]}-1
362inclusive 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})$ )
363\item window from range \varname{[right\_window\_start]} ... \varname{[n]}-1 is
364zero
365\end{enumerate}
366
367An end-of-packet condition up to this point should be considered an
368error that discards this packet from the stream. An end of packet
369condition past this point is to be considered a possible nominal
370occurrence.
371
372
373
374\subsubsection{floor curve decode}
375
376From this point on, we assume out decode context is using mode number
377\varname{[mode\_number]} from configuration array
378\varname{[vorbis\_mode\_configurations]} and the map number
379\varname{[vorbis\_mode\_mapping]} (specified by the current mode) taken
380from the mapping configuration array
381\varname{[vorbis\_mapping\_configurations]}.
382
383Floor curves are decoded one-by-one in channel order.
384
385For each floor \varname{[i]} of \varname{[audio\_channels]}
386 \begin{enumerate}
387 \item \varname{[submap\_number]} = element \varname{[i]} of vector [vorbis\_mapping\_mux]
388 \item \varname{[floor\_number]} = element \varname{[submap\_number]} of vector
389[vorbis\_submap\_floor]
390 \item if the floor type of this
391floor (vector \varname{[vorbis\_floor\_types]} element
392\varname{[floor\_number]}) is zero then decode the floor for
393channel \varname{[i]} according to the
394\xref{vorbis:spec:floor0-decode}
395 \item if the type of this floor
396is one then decode the floor for channel \varname{[i]} according
397to the \xref{vorbis:spec:floor1-decode}
398 \item save the needed decoded floor information for channel for later synthesis
399 \item if the decoded floor returned 'unused', set vector \varname{[no\_residue]} element
400\varname{[i]} to true, else set vector \varname{[no\_residue]} element \varname{[i]} to
401false
402 \end{enumerate}
403
404
405An end-of-packet condition during floor decode shall result in packet
406decode zeroing all channel output vectors and skipping to the
407add/overlap output stage.
408
409
410
411\subsubsection{nonzero vector propagate}
412
413A possible result of floor decode is that a specific vector is marked
414'unused' which indicates that that final output vector is all-zero
415values (and the floor is zero). The residue for that vector is not
416coded in the stream, save for one complication. If some vectors are
417used and some are not, channel coupling could result in mixing a
418zeroed and nonzeroed vector to produce two nonzeroed vectors.
419
420for each \varname{[i]} from 0 ... \varname{[vorbis\_mapping\_coupling\_steps]}-1
421
422\begin{enumerate}
423 \item if either \varname{[no\_residue]} entry for channel
424(\varname{[vorbis\_mapping\_magnitude]} element \varname{[i]})
425or channel
426(\varname{[vorbis\_mapping\_angle]} element \varname{[i]})
427are set to false, then both must be set to false. Note that an 'unused'
428floor has no decoded floor information; it is important that this is
429remembered at floor curve synthesis time.
430\end{enumerate}
431
432
433
434
435\subsubsection{residue decode}
436
437Unlike floors, which are decoded in channel order, the residue vectors
438are decoded in submap order.
439
440for each submap \varname{[i]} in order from 0 ... \varname{[vorbis\_mapping\_submaps]}-1
441
442\begin{enumerate}
443 \item \varname{[ch]} = 0
444 \item for each channel \varname{[j]} in order from 0 ... \varname{[audio\_channels]} - 1
445 \begin{enumerate}
446 \item if channel \varname{[j]} in submap \varname{[i]} (vector \varname{[vorbis\_mapping\_mux]} element \varname{[j]} is equal to \varname{[i]})
447 \begin{enumerate}
448 \item if vector \varname{[no\_residue]} element \varname{[j]} is true
449 \begin{enumerate}
450 \item vector \varname{[do\_not\_decode\_flag]} element \varname{[ch]} is set
451 \end{enumerate}
452 else
453 \begin{enumerate}
454 \item vector \varname{[do\_not\_decode\_flag]} element \varname{[ch]} is unset
455 \end{enumerate}
456
457 \item increment \varname{[ch]}
458 \end{enumerate}
459
460 \end{enumerate}
461 \item \varname{[residue\_number]} = vector \varname{[vorbis\_mapping\_submap\_residue]} element \varname{[i]}
462 \item \varname{[residue\_type]} = vector \varname{[vorbis\_residue\_types]} element \varname{[residue\_number]}
463 \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.
464 \item \varname{[ch]} = 0
465 \item for each channel \varname{[j]} in order from 0 ... \varname{[audio\_channels]}
466 \begin{enumerate}
467 \item if channel \varname{[j]} is in submap \varname{[i]} (vector \varname{[vorbis\_mapping\_mux]} element \varname{[j]} is equal to \varname{[i]})
468 \begin{enumerate}
469 \item residue vector for channel \varname{[j]} is set to decoded residue vector \varname{[ch]}
470 \item increment \varname{[ch]}
471 \end{enumerate}
472
473 \end{enumerate}
474
475\end{enumerate}
476
477
478
479\subsubsection{inverse coupling}
480
481for each \varname{[i]} from \varname{[vorbis\_mapping\_coupling\_steps]}-1 descending to 0
482
483\begin{enumerate}
484 \item \varname{[magnitude\_vector]} = the residue vector for channel
485(vector \varname{[vorbis\_mapping\_magnitude]} element \varname{[i]})
486 \item \varname{[angle\_vector]} = the residue vector for channel (vector
487\varname{[vorbis\_mapping\_angle]} element \varname{[i]})
488 \item for each scalar value \varname{[M]} in vector \varname{[magnitude\_vector]} and the corresponding scalar value \varname{[A]} in vector \varname{[angle\_vector]}:
489 \begin{enumerate}
490 \item if (\varname{[M]} is greater than zero)
491 \begin{enumerate}
492 \item if (\varname{[A]} is greater than zero)
493 \begin{enumerate}
494 \item \varname{[new\_M]} = \varname{[M]}
495 \item \varname{[new\_A]} = \varname{[M]}-\varname{[A]}
496 \end{enumerate}
497 else
498 \begin{enumerate}
499 \item \varname{[new\_A]} = \varname{[M]}
500 \item \varname{[new\_M]} = \varname{[M]}+\varname{[A]}
501 \end{enumerate}
502
503 \end{enumerate}
504 else
505 \begin{enumerate}
506 \item if (\varname{[A]} is greater than zero)
507 \begin{enumerate}
508 \item \varname{[new\_M]} = \varname{[M]}
509 \item \varname{[new\_A]} = \varname{[M]}+\varname{[A]}
510 \end{enumerate}
511 else
512 \begin{enumerate}
513 \item \varname{[new\_A]} = \varname{[M]}
514 \item \varname{[new\_M]} = \varname{[M]}-\varname{[A]}
515 \end{enumerate}
516
517 \end{enumerate}
518
519 \item set scalar value \varname{[M]} in vector \varname{[magnitude\_vector]} to \varname{[new\_M]}
520 \item set scalar value \varname{[A]} in vector \varname{[angle\_vector]} to \varname{[new\_A]}
521 \end{enumerate}
522
523\end{enumerate}
524
525
526
527
528\subsubsection{dot product}
529
530For each channel, synthesize the floor curve from the decoded floor
531information, according to packet type. Note that the vector synthesis
532length for floor computation is \varname{[n]}/2.
533
534For each channel, multiply each element of the floor curve by each
535element of that channel's residue vector. The result is the dot
536product of the floor and residue vectors for each channel; the produced
537vectors are the length \varname{[n]}/2 audio spectrum for each
538channel.
539
540% TODO/FIXME: The following two paragraphs have identical twins
541% in section 1 (under "compute floor/residue dot product")
542One point is worth mentioning about this dot product; a common mistake
543in a fixed point implementation might be to assume that a 32 bit
544fixed-point representation for floor and residue and direct
545multiplication of the vectors is sufficient for acceptable spectral
546depth in all cases because it happens to mostly work with the current
547Xiph.Org reference encoder.
548
549However, floor vector values can span \~140dB (\~24 bits unsigned), and
550the audio spectrum vector should represent a minimum of 120dB (\~21
551bits with sign), even when output is to a 16 bit PCM device. For the
552residue vector to represent full scale if the floor is nailed to
553$-140$dB, it must be able to span 0 to $+140$dB. For the residue vector
554to reach full scale if the floor is nailed at 0dB, it must be able to
555represent $-140$dB to $+0$dB. Thus, in order to handle full range
556dynamics, a residue vector may span $-140$dB to $+140$dB entirely within
557spec. A 280dB range is approximately 48 bits with sign; thus the
558residue vector must be able to represent a 48 bit range and the dot
559product must be able to handle an effective 48 bit times 24 bit
560multiplication. This range may be achieved using large (64 bit or
561larger) integers, or implementing a movable binary point
562representation.
563
564
565
566\subsubsection{inverse MDCT}
567
568Convert the audio spectrum vector of each channel back into time
569domain PCM audio via an inverse Modified Discrete Cosine Transform
570(MDCT). A detailed description of the MDCT is available in \cite{Sporer/Brandenburg/Edler}. The window
571function used for the MDCT is the function described earlier.
572
573
574
575\subsubsection{overlap\_add}
576
577Windowed MDCT output is overlapped and added with the right hand data
578of the previous window such that the 3/4 point of the previous window
579is aligned with the 1/4 point of the current window (as illustrated in
580\xref{vorbis:spec:window}). The overlapped portion
581produced from overlapping the previous and current frame data is
582finished data to be returned by the decoder. This data spans from the
583center of the previous window to the center of the current window. In
584the case of same-sized windows, the amount of data to return is
585one-half block consisting of and only of the overlapped portions. When
586overlapping a short and long window, much of the returned range does not
587actually overlap. This does not damage transform orthogonality. Pay
588attention however to returning the correct data range; the amount of
589data to be returned is:
590
591\begin{programlisting}
592window_blocksize(previous_window)/4+window_blocksize(current_window)/4
593\end{programlisting}
594
595from the center (element windowsize/2) of the previous window to the
596center (element windowsize/2-1, inclusive) of the current window.
597
598Data is not returned from the first frame; it must be used to 'prime'
599the decode engine. The encoder accounts for this priming when
600calculating PCM offsets; after the first frame, the proper PCM output
601offset is '0' (as no data has been returned yet).
602
603
604
605\subsubsection{output channel order}
606
607Vorbis I specifies only a channel mapping type 0. In mapping type 0,
608channel mapping is implicitly defined as follows for standard audio
609applications. As of revision 16781 (20100113), the specification adds
610defined channel locations for 6.1 and 7.1 surround. Ordering/location
611for greater-than-eight channels remains 'left to the implementation'.
612
613These channel orderings refer to order within the encoded stream. It
614is naturally possible for a decoder to produce output with channels in
615any order. Any such decoder should explicitly document channel
616reordering behavior.
617
618\begin{description} %[style=nextline]
619 \item[one channel]
620 the stream is monophonic
621
622\item[two channels]
623 the stream is stereo. channel order: left, right
624
625\item[three channels]
626 the stream is a 1d-surround encoding. channel order: left,
627center, right
628
629\item[four channels]
630 the stream is quadraphonic surround. channel order: front left,
631front right, rear left, rear right
632
633\item[five channels]
634 the stream is five-channel surround. channel order: front left,
635center, front right, rear left, rear right
636
637\item[six channels]
638 the stream is 5.1 surround. channel order: front left, center,
639front right, rear left, rear right, LFE
640
641\item[seven channels]
642 the stream is 6.1 surround. channel order: front left, center,
643front right, side left, side right, rear center, LFE
644
645\item[eight channels]
646 the stream is 7.1 surround. channel order: front left, center,
647front right, side left, side right, rear left, rear right,
648LFE
649
650\item[greater than eight channels]
651 channel use and order is defined by the application
652
653\end{description}
654
655Applications using Vorbis for dedicated purposes may define channel
656mapping as seen fit. Future channel mappings (such as three and four
657channel \href{http://www.ambisonic.net/}{Ambisonics}) will
658make use of channel mappings other than mapping 0.
659
660
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