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diff --git a/doc/01-introduction.tex b/doc/01-introduction.tex new file mode 100644 index 0000000..d7767df --- /dev/null +++ b/doc/01-introduction.tex @@ -0,0 +1,528 @@ +% -*- mode: latex; TeX-master: "Vorbis_I_spec"; -*- +%!TEX root = Vorbis_I_spec.tex +\section{Introduction and Description} \label{vorbis:spec:intro} + +\subsection{Overview} + +This document provides a high level description of the Vorbis codec's +construction. A bit-by-bit specification appears beginning in +\xref{vorbis:spec:codec}. +The later sections assume a high-level +understanding of the Vorbis decode process, which is +provided here. + +\subsubsection{Application} +Vorbis is a general purpose perceptual audio CODEC intended to allow +maximum encoder flexibility, thus allowing it to scale competitively +over an exceptionally wide range of bitrates. At the high +quality/bitrate end of the scale (CD or DAT rate stereo, 16/24 bits) +it is in the same league as MPEG-2 and MPC. Similarly, the 1.0 +encoder can encode high-quality CD and DAT rate stereo at below 48kbps +without resampling to a lower rate. Vorbis is also intended for +lower and higher sample rates (from 8kHz telephony to 192kHz digital +masters) and a range of channel representations (monaural, +polyphonic, stereo, quadraphonic, 5.1, ambisonic, or up to 255 +discrete channels). + + +\subsubsection{Classification} +Vorbis I is a forward-adaptive monolithic transform CODEC based on the +Modified Discrete Cosine Transform. The codec is structured to allow +addition of a hybrid wavelet filterbank in Vorbis II to offer better +transient response and reproduction using a transform better suited to +localized time events. + + +\subsubsection{Assumptions} + +The Vorbis CODEC design assumes a complex, psychoacoustically-aware +encoder and simple, low-complexity decoder. Vorbis decode is +computationally simpler than mp3, although it does require more +working memory as Vorbis has no static probability model; the vector +codebooks used in the first stage of decoding from the bitstream are +packed in their entirety into the Vorbis bitstream headers. In +packed form, these codebooks occupy only a few kilobytes; the extent +to which they are pre-decoded into a cache is the dominant factor in +decoder memory usage. + + +Vorbis provides none of its own framing, synchronization or protection +against errors; it is solely a method of accepting input audio, +dividing it into individual frames and compressing these frames into +raw, unformatted 'packets'. The decoder then accepts these raw +packets in sequence, decodes them, synthesizes audio frames from +them, and reassembles the frames into a facsimile of the original +audio stream. Vorbis is a free-form variable bit rate (VBR) codec and packets have no +minimum size, maximum size, or fixed/expected size. Packets +are designed that they may be truncated (or padded) and remain +decodable; this is not to be considered an error condition and is used +extensively in bitrate management in peeling. Both the transport +mechanism and decoder must allow that a packet may be any size, or +end before or after packet decode expects. + +Vorbis packets are thus intended to be used with a transport mechanism +that provides free-form framing, sync, positioning and error correction +in accordance with these design assumptions, such as Ogg (for file +transport) or RTP (for network multicast). For purposes of a few +examples in this document, we will assume that Vorbis is to be +embedded in an Ogg stream specifically, although this is by no means a +requirement or fundamental assumption in the Vorbis design. + +The specification for embedding Vorbis into +an Ogg transport stream is in \xref{vorbis:over:ogg}. + + + +\subsubsection{Codec Setup and Probability Model} + +Vorbis' heritage is as a research CODEC and its current design +reflects a desire to allow multiple decades of continuous encoder +improvement before running out of room within the codec specification. +For these reasons, configurable aspects of codec setup intentionally +lean toward the extreme of forward adaptive. + +The single most controversial design decision in Vorbis (and the most +unusual for a Vorbis developer to keep in mind) is that the entire +probability model of the codec, the Huffman and VQ codebooks, is +packed into the bitstream header along with extensive CODEC setup +parameters (often several hundred fields). This makes it impossible, +as it would be with MPEG audio layers, to embed a simple frame type +flag in each audio packet, or begin decode at any frame in the stream +without having previously fetched the codec setup header. + + +\begin{note} +Vorbis \emph{can} initiate decode at any arbitrary packet within a +bitstream so long as the codec has been initialized/setup with the +setup headers. +\end{note} + +Thus, Vorbis headers are both required for decode to begin and +relatively large as bitstream headers go. The header size is +unbounded, although for streaming a rule-of-thumb of 4kB or less is +recommended (and Xiph.Org's Vorbis encoder follows this suggestion). + +Our own design work indicates the primary liability of the +required header is in mindshare; it is an unusual design and thus +causes some amount of complaint among engineers as this runs against +current design trends (and also points out limitations in some +existing software/interface designs, such as Windows' ACM codec +framework). However, we find that it does not fundamentally limit +Vorbis' suitable application space. + + +\subsubsection{Format Specification} +The Vorbis format is well-defined by its decode specification; any +encoder that produces packets that are correctly decoded by the +reference Vorbis decoder described below may be considered a proper +Vorbis encoder. A decoder must faithfully and completely implement +the specification defined below (except where noted) to be considered +a proper Vorbis decoder. + +\subsubsection{Hardware Profile} +Although Vorbis decode is computationally simple, it may still run +into specific limitations of an embedded design. For this reason, +embedded designs are allowed to deviate in limited ways from the +`full' decode specification yet still be certified compliant. These +optional omissions are labelled in the spec where relevant. + + +\subsection{Decoder Configuration} + +Decoder setup consists of configuration of multiple, self-contained +component abstractions that perform specific functions in the decode +pipeline. Each different component instance of a specific type is +semantically interchangeable; decoder configuration consists both of +internal component configuration, as well as arrangement of specific +instances into a decode pipeline. Componentry arrangement is roughly +as follows: + +\begin{center} +\includegraphics[width=\textwidth]{components} +\captionof{figure}{decoder pipeline configuration} +\end{center} + +\subsubsection{Global Config} +Global codec configuration consists of a few audio related fields +(sample rate, channels), Vorbis version (always '0' in Vorbis I), +bitrate hints, and the lists of component instances. All other +configuration is in the context of specific components. + +\subsubsection{Mode} + +Each Vorbis frame is coded according to a master 'mode'. A bitstream +may use one or many modes. + +The mode mechanism is used to encode a frame according to one of +multiple possible methods with the intention of choosing a method best +suited to that frame. Different modes are, e.g. how frame size +is changed from frame to frame. The mode number of a frame serves as a +top level configuration switch for all other specific aspects of frame +decode. + +A 'mode' configuration consists of a frame size setting, window type +(always 0, the Vorbis window, in Vorbis I), transform type (always +type 0, the MDCT, in Vorbis I) and a mapping number. The mapping +number specifies which mapping configuration instance to use for +low-level packet decode and synthesis. + + +\subsubsection{Mapping} + +A mapping contains a channel coupling description and a list of +'submaps' that bundle sets of channel vectors together for grouped +encoding and decoding. These submaps are not references to external +components; the submap list is internal and specific to a mapping. + +A 'submap' is a configuration/grouping that applies to a subset of +floor and residue vectors within a mapping. The submap functions as a +last layer of indirection such that specific special floor or residue +settings can be applied not only to all the vectors in a given mode, +but also specific vectors in a specific mode. Each submap specifies +the proper floor and residue instance number to use for decoding that +submap's spectral floor and spectral residue vectors. + +As an example: + +Assume a Vorbis stream that contains six channels in the standard 5.1 +format. The sixth channel, as is normal in 5.1, is bass only. +Therefore it would be wasteful to encode a full-spectrum version of it +as with the other channels. The submapping mechanism can be used to +apply a full range floor and residue encoding to channels 0 through 4, +and a bass-only representation to the bass channel, thus saving space. +In this example, channels 0-4 belong to submap 0 (which indicates use +of a full-range floor) and channel 5 belongs to submap 1, which uses a +bass-only representation. + + +\subsubsection{Floor} + +Vorbis encodes a spectral 'floor' vector for each PCM channel. This +vector is a low-resolution representation of the audio spectrum for +the given channel in the current frame, generally used akin to a +whitening filter. It is named a 'floor' because the Xiph.Org +reference encoder has historically used it as a unit-baseline for +spectral resolution. + +A floor encoding may be of two types. Floor 0 uses a packed LSP +representation on a dB amplitude scale and Bark frequency scale. +Floor 1 represents the curve as a piecewise linear interpolated +representation on a dB amplitude scale and linear frequency scale. +The two floors are semantically interchangeable in +encoding/decoding. However, floor type 1 provides more stable +inter-frame behavior, and so is the preferred choice in all +coupled-stereo and high bitrate modes. Floor 1 is also considerably +less expensive to decode than floor 0. + +Floor 0 is not to be considered deprecated, but it is of limited +modern use. No known Vorbis encoder past Xiph.Org's own beta 4 makes +use of floor 0. + +The values coded/decoded by a floor are both compactly formatted and +make use of entropy coding to save space. For this reason, a floor +configuration generally refers to multiple codebooks in the codebook +component list. Entropy coding is thus provided as an abstraction, +and each floor instance may choose from any and all available +codebooks when coding/decoding. + + +\subsubsection{Residue} +The spectral residue is the fine structure of the audio spectrum +once the floor curve has been subtracted out. In simplest terms, it +is coded in the bitstream using cascaded (multi-pass) vector +quantization according to one of three specific packing/coding +algorithms numbered 0 through 2. The packing algorithm details are +configured by residue instance. As with the floor components, the +final VQ/entropy encoding is provided by external codebook instances +and each residue instance may choose from any and all available +codebooks. + +\subsubsection{Codebooks} + +Codebooks are a self-contained abstraction that perform entropy +decoding and, optionally, use the entropy-decoded integer value as an +offset into an index of output value vectors, returning the indicated +vector of values. + +The entropy coding in a Vorbis I codebook is provided by a standard +Huffman binary tree representation. This tree is tightly packed using +one of several methods, depending on whether codeword lengths are +ordered or unordered, or the tree is sparse. + +The codebook vector index is similarly packed according to index +characteristic. Most commonly, the vector index is encoded as a +single list of values of possible values that are then permuted into +a list of n-dimensional rows (lattice VQ). + + + +\subsection{High-level Decode Process} + +\subsubsection{Decode Setup} + +Before decoding can begin, a decoder must initialize using the +bitstream headers matching the stream to be decoded. Vorbis uses +three header packets; all are required, in-order, by this +specification. Once set up, decode may begin at any audio packet +belonging to the Vorbis stream. In Vorbis I, all packets after the +three initial headers are audio packets. + +The header packets are, in order, the identification +header, the comments header, and the setup header. + +\paragraph{Identification Header} +The identification header identifies the bitstream as Vorbis, Vorbis +version, and the simple audio characteristics of the stream such as +sample rate and number of channels. + +\paragraph{Comment Header} +The comment header includes user text comments (``tags'') and a vendor +string for the application/library that produced the bitstream. The +encoding and proper use of the comment header is described in \xref{vorbis:spec:comment}. + +\paragraph{Setup Header} +The setup header includes extensive CODEC setup information as well as +the complete VQ and Huffman codebooks needed for decode. + + +\subsubsection{Decode Procedure} + +The decoding and synthesis procedure for all audio packets is +fundamentally the same. +\begin{enumerate} +\item decode packet type flag +\item decode mode number +\item decode window shape (long windows only) +\item decode floor +\item decode residue into residue vectors +\item inverse channel coupling of residue vectors +\item generate floor curve from decoded floor data +\item compute dot product of floor and residue, producing audio spectrum vector +\item inverse monolithic transform of audio spectrum vector, always an MDCT in Vorbis I +\item overlap/add left-hand output of transform with right-hand output of previous frame +\item store right hand-data from transform of current frame for future lapping +\item if not first frame, return results of overlap/add as audio result of current frame +\end{enumerate} + +Note that clever rearrangement of the synthesis arithmetic is +possible; as an example, one can take advantage of symmetries in the +MDCT to store the right-hand transform data of a partial MDCT for a +50\% inter-frame buffer space savings, and then complete the transform +later before overlap/add with the next frame. This optimization +produces entirely equivalent output and is naturally perfectly legal. +The decoder must be \emph{entirely mathematically equivalent} to the +specification, it need not be a literal semantic implementation. + +\paragraph{Packet type decode} + +Vorbis I uses four packet types. The first three packet types mark each +of the three Vorbis headers described above. The fourth packet type +marks an audio packet. All other packet types are reserved; packets +marked with a reserved type should be ignored. + +Following the three header packets, all packets in a Vorbis I stream +are audio. The first step of audio packet decode is to read and +verify the packet type; \emph{a non-audio packet when audio is expected +indicates stream corruption or a non-compliant stream. The decoder +must ignore the packet and not attempt decoding it to +audio}. + + + + +\paragraph{Mode decode} +Vorbis allows an encoder to set up multiple, numbered packet 'modes', +as described earlier, all of which may be used in a given Vorbis +stream. The mode is encoded as an integer used as a direct offset into +the mode instance index. + + +\paragraph{Window shape decode (long windows only)} \label{vorbis:spec:window} + +Vorbis frames may be one of two PCM sample sizes specified during +codec setup. In Vorbis I, legal frame sizes are powers of two from 64 +to 8192 samples. Aside from coupling, Vorbis handles channels as +independent vectors and these frame sizes are in samples per channel. + +Vorbis uses an overlapping transform, namely the MDCT, to blend one +frame into the next, avoiding most inter-frame block boundary +artifacts. The MDCT output of one frame is windowed according to MDCT +requirements, overlapped 50\% with the output of the previous frame and +added. The window shape assures seamless reconstruction. + +This is easy to visualize in the case of equal sized-windows: + +\begin{center} +\includegraphics[width=\textwidth]{window1} +\captionof{figure}{overlap of two equal-sized windows} +\end{center} + +And slightly more complex in the case of overlapping unequal sized +windows: + +\begin{center} +\includegraphics[width=\textwidth]{window2} +\captionof{figure}{overlap of a long and a short window} +\end{center} + +In the unequal-sized window case, the window shape of the long window +must be modified for seamless lapping as above. It is possible to +correctly infer window shape to be applied to the current window from +knowing the sizes of the current, previous and next window. It is +legal for a decoder to use this method. However, in the case of a long +window (short windows require no modification), Vorbis also codes two +flag bits to specify pre- and post- window shape. Although not +strictly necessary for function, this minor redundancy allows a packet +to be fully decoded to the point of lapping entirely independently of +any other packet, allowing easier abstraction of decode layers as well +as allowing a greater level of easy parallelism in encode and +decode. + +A description of valid window functions for use with an inverse MDCT +can be found in \cite{Sporer/Brandenburg/Edler}. Vorbis windows +all use the slope function +\[ y = \sin(.5*\pi \, \sin^2((x+.5)/n*\pi)) . \] + + + +\paragraph{floor decode} +Each floor is encoded/decoded in channel order, however each floor +belongs to a 'submap' that specifies which floor configuration to +use. All floors are decoded before residue decode begins. + + +\paragraph{residue decode} + +Although the number of residue vectors equals the number of channels, +channel coupling may mean that the raw residue vectors extracted +during decode do not map directly to specific channels. When channel +coupling is in use, some vectors will correspond to coupled magnitude +or angle. The coupling relationships are described in the codec setup +and may differ from frame to frame, due to different mode numbers. + +Vorbis codes residue vectors in groups by submap; the coding is done +in submap order from submap 0 through n-1. This differs from floors +which are coded using a configuration provided by submap number, but +are coded individually in channel order. + + + +\paragraph{inverse channel coupling} + +A detailed discussion of stereo in the Vorbis codec can be found in +the document \href{stereo.html}{Stereo Channel Coupling in the +Vorbis CODEC}. Vorbis is not limited to only stereo coupling, but +the stereo document also gives a good overview of the generic coupling +mechanism. + +Vorbis coupling applies to pairs of residue vectors at a time; +decoupling is done in-place a pair at a time in the order and using +the vectors specified in the current mapping configuration. The +decoupling operation is the same for all pairs, converting square +polar representation (where one vector is magnitude and the second +angle) back to Cartesian representation. + +After decoupling, in order, each pair of vectors on the coupling list, +the resulting residue vectors represent the fine spectral detail +of each output channel. + + + +\paragraph{generate floor curve} + +The decoder may choose to generate the floor curve at any appropriate +time. It is reasonable to generate the output curve when the floor +data is decoded from the raw packet, or it can be generated after +inverse coupling and applied to the spectral residue directly, +combining generation and the dot product into one step and eliminating +some working space. + +Both floor 0 and floor 1 generate a linear-range, linear-domain output +vector to be multiplied (dot product) by the linear-range, +linear-domain spectral residue. + + + +\paragraph{compute floor/residue dot product} + +This step is straightforward; for each output channel, the decoder +multiplies the floor curve and residue vectors element by element, +producing the finished audio spectrum of each channel. + +% TODO/FIXME: The following two paragraphs have identical twins +% in section 4 (under "dot product") +One point is worth mentioning about this dot product; a common mistake +in a fixed point implementation might be to assume that a 32 bit +fixed-point representation for floor and residue and direct +multiplication of the vectors is sufficient for acceptable spectral +depth in all cases because it happens to mostly work with the current +Xiph.Org reference encoder. + +However, floor vector values can span \~{}140dB (\~{}24 bits unsigned), and +the audio spectrum vector should represent a minimum of 120dB (\~{}21 +bits with sign), even when output is to a 16 bit PCM device. For the +residue vector to represent full scale if the floor is nailed to +$-140$dB, it must be able to span 0 to $+140$dB. For the residue vector +to reach full scale if the floor is nailed at 0dB, it must be able to +represent $-140$dB to $+0$dB. Thus, in order to handle full range +dynamics, a residue vector may span $-140$dB to $+140$dB entirely within +spec. A 280dB range is approximately 48 bits with sign; thus the +residue vector must be able to represent a 48 bit range and the dot +product must be able to handle an effective 48 bit times 24 bit +multiplication. This range may be achieved using large (64 bit or +larger) integers, or implementing a movable binary point +representation. + + + +\paragraph{inverse monolithic transform (MDCT)} + +The audio spectrum is converted back into time domain PCM audio via an +inverse Modified Discrete Cosine Transform (MDCT). A detailed +description of the MDCT is available in \cite{Sporer/Brandenburg/Edler}. + +Note that the PCM produced directly from the MDCT is not yet finished +audio; it must be lapped with surrounding frames using an appropriate +window (such as the Vorbis window) before the MDCT can be considered +orthogonal. + + + +\paragraph{overlap/add data} +Windowed MDCT output is overlapped and added with the right hand data +of the previous window such that the 3/4 point of the previous window +is aligned with the 1/4 point of the current window (as illustrated in +the window overlap diagram). At this point, the audio data between the +center of the previous frame and the center of the current frame is +now finished and ready to be returned. + + +\paragraph{cache right hand data} +The decoder must cache the right hand portion of the current frame to +be lapped with the left hand portion of the next frame. + + + +\paragraph{return finished audio data} + +The overlapped portion produced from overlapping the previous and +current frame data is finished data to be returned by the decoder. +This data spans from the center of the previous window to the center +of the current window. In the case of same-sized windows, the amount +of data to return is one-half block consisting of and only of the +overlapped portions. When overlapping a short and long window, much of +the returned range is not actually overlap. This does not damage +transform orthogonality. Pay attention however to returning the +correct data range; the amount of data to be returned is: + +\begin{Verbatim}[commandchars=\\\{\}] +window\_blocksize(previous\_window)/4+window\_blocksize(current\_window)/4 +\end{Verbatim} + +from the center of the previous window to the center of the current +window. + +Data is not returned from the first frame; it must be used to 'prime' +the decode engine. The encoder accounts for this priming when +calculating PCM offsets; after the first frame, the proper PCM output +offset is '0' (as no data has been returned yet). |