JP5006426B2 - Entropy coding to adapt coding between level mode and run length / level mode - Google Patents

Description

  The present invention relates to adaptive entropy coding of audio data. For example, the audio encoder switches between direct level Huffman coding of quantized audio data and run length and level arithmetic coding of quantized audio data.

  With the introduction of compact discs, digital wireless telephone networks, and the distribution of audio over the Internet, digital audio has become commonplace. Engineers use various techniques to efficiently process digital audio while maintaining digital audio quality. Understanding these techniques helps to understand how audio information is represented and processed in a computer.

I. Representation of audio information in a computer A computer processes audio information as a series of numbers representing audio information. For example, a single number can represent an audio sample, which is the amplitude value (ie, loudness) at a particular point in time. Several factors affect the quality of the audio information, including sample depth, sampling rate, and channel mode.

  Sample depth (or accuracy) indicates the range of numbers used to represent a sample. The more that value available for a sample, the higher the quality. This is because the number can capture more subtle variations in amplitude. For example, an 8-bit sample has 256 usable values, while a 16-bit sample has 65,536 usable values.

  The sampling rate (usually measured in samples per second) also affects quality. The higher the sampling rate, the higher the quality. This is because it can represent more frequencies for the sound. Some common sampling rates are: 8,000 samples per second, 11,025 samples per second, 22,050 samples per second, 32,000 samples per second, 44,100 samples per second, 48,000 samples per second, and 96 per second 1,000 samples.

  Table 1 shows several audio formats with different quality levels, as well as the corresponding raw bit rate costs.

  As Table 1 shows, the cost of high quality audio information such as CD audio is a high bit rate. High quality audio information consumes large amounts of computer storage and transmission capacity. However, businesses and consumers increasingly rely on computers to create, distribute, and play high-quality audio content.

II. Audio compression and decompression Many computers and computer networks lack the resources to process raw digital audio. Compression (also called encoding or coding) reduces the cost of storing and transmitting audio information by converting the information to a lower bit rate form. Compression should be lossless (lossless) (not loss of quality) or lossy (lossy) (quality is reduced, but bit rate reduction over lossless compression is more significant) It is possible. Decompression (also called decryption) retrieves a reconstructed version of the original information from the compressed form.

  In general, the purpose of audio compression is to digitally represent an audio signal so as to provide maximum signal quality with as few bits as possible. Conventional audio encoder / decoder [“codec”] systems use subband / transform coding, quantization, rate control, and variable length coding to achieve compression. Quantization and other lossy compression techniques introduce noise that can be heard into the audio signal. The audibility of noise depends on the amount of noise present and the amount of noise perceived by the listener. The first factor is primarily related to objective quality, while the second factor depends on the human perception of the sound. Thus, conventional audio encoders have compressed data that has been quantized using variable length coding without loss in order to further reduce the bit rate.

A. Lossy compression and decompression of audio data Traditionally, audio encoders use a variety of different lossy compression technique techniques. This lossy compression technique typically requires frequency conversion, perceptual modeling / weighting, and quantization. Corresponding decompression requires inverse quantization, inverse weighting, and inverse frequency transform.

  Frequency conversion techniques convert data into a form that makes it easier to separate perceptually important information from information that is not perceptually important. Second, less important information is subject to more lossy compression, while more important information is preserved to provide the best perceived quality for a given bit rate. Can be. A frequency converter typically receives audio samples and converts them into frequency domain data, sometimes called frequency coefficients or spectral coefficients.

  Most energy in natural sounds such as conversation and music is concentrated in the low frequency range. This means that, statistically, the higher frequency range will have more or zero frequency coefficients, reflecting the lack of energy in the higher frequency range.

  Perceptual modeling means processing audio data according to a model of the human auditory system so as to improve the perceived quality of the reconstructed audio signal at a given bit rate. For example, an auditory model typically considers the range of human hearing and critical bands. Using the results of perceptual modeling, the encoder can detect noise (eg, quantization noise) in audio data despite the goal of minimizing noise audibility for a given bit rate. ). Encoders sometimes have to introduce noise (eg, quantization noise) to reduce bit rate, but weighting allows the encoder to add more noise to bands where noise is not so audible. It is possible to put less noise in a band where noise can be heard well.

  Quantization maps each range of each input value to multiple single values to introduce irreversible information loss or quantization noise, but the encoder adjusts the output quality and bit rate It is also possible to do. Sometimes, the encoder achieves quantization in conjunction with a speed controller that adjusts the quantization to adjust the bit rate and / or quality. There are various types of quantization, including adaptive and non-adaptive quantization, scalar quantization and vector quantization, uniform quantization and non-uniform quantization. Perceptual weighting can be thought of as a form of non-uniform quantization.

  By inverse quantization and inverse weighting, the weighted and quantized frequency coefficient data is reconstructed to an approximate value of the original frequency coefficient data. An inverse frequency transformer then converts the reconstructed frequency coefficient data into reconstructed time domain audio samples.

B. Lossless compression and decompression of audio data Traditionally, audio encoders use one or more of a variety of different lossless compression techniques. In general, lossless compression techniques include run length coding, Huffman coding, and arithmetic coding. Corresponding decompression techniques include run length decoding, Huffman decoding, and arithmetic decoding.

  Run-length encoding is a simple well-known compression technique used for camera video, text, and other types of content. In general, run length coding replaces a sequence of consecutive symbols having the same value (ie, a run) with the value and length of the sequence. In run length decoding, a sequence of consecutive symbols is reconstructed from run values and run lengths. A number of variants of run length encoding / decoding have been developed. For more information on run-length encoding / decoding and some of the variants of run-length encoding / decoding, see, for example, Non-Patent Document 1, Non-Patent Document 2, Mayrs et al. See, U.S. Pat. No. 6,037,028 issued to Gill et al., And U.S. Pat.

  Run level coding is similar to run length coding in that runs of consecutive symbols having the same value are replaced with the run length. Run values are significant values in the data (eg, 0), and runs are separated by one or more levels having different values (eg, non-zero values).

  Run length coding (eg, run value and run length) or run level coding results can be Huffman coded to further reduce the bit rate. When this is done, the Huffman encoded data is Huffman decoded prior to run length decoding.

  Huffman coding is another well-known compression technique used for camera video, text, and other types of content. In general, the Huffman code table associates variable length Huffman codes with unique symbol values (or unique combinations of values). Shorter codes are assigned more probable symbol values, and longer codes are assigned less probable symbol values. This probability is calculated for typical examples of several types of content. Alternatively, this probability is calculated for data that has just been encoded, or data that is to be encoded, in which case the Huffman code adapts to the change in probability for a unique symbol value. Compared to static Huffman coding, adaptive Huffman coding usually reduces the bit rate of the compressed data by incorporating a more accurate probability for the data, but additional information specifying the Huffman code is also present. May need to be transmitted.

  To encode a symbol, the Huffman encoder replaces the symbol value with a variable length Huffman code associated with the symbol value in the Huffman code table. To decode, the Huffman decoder replaces the Huffman code with a symbol value associated with the Huffman code.

In scalar Huffman coding, the Huffman code table associates a single Huffman code with a single value, eg, a direct level of quantized data values. In vector Huffman coding, the Huffman code table associates a single Huffman code with a combination of values, eg, a group of direct levels of quantized data values in a particular order. Vector Huffman coding can lead to better bit rate reduction than scalar Huffman coding (eg, by allowing the encoder to use the probability in binary Huffman codes as a fraction). is there. On the other hand, the codebook for vector Huffman coding is when a single code represents a large group of symbols, or when a symbol has a large range of possible values (by a large number of possible combinations): It can be extremely large. For example, if the alphabet size is 256 (as a value between 0 and 255 per symbol) and the number of symbols per vector is 4, the number of possible combinations is 256 ⁴ = 4,294,967,296. is there. This consumes memory and processing resources when the codebook is calculated and finds the Huffman code (uses up), and consumes transmission resources when the codebook is transmitted (uses up).

  A number of variants of Huffman encoding / decoding have been developed. See, for example, Non-Patent Document 1 and Non-Patent Document 2, for more information on Huffman encoding / decoding and some of the variants of Huffman encoding / decoding.

  U.S. Patent No. 6,053,096 issued to Chen et al. Describes multi-level run length encoding of audio data. The frequency conversion results in a series of frequency coefficient values. For those portions of the frequency spectrum where the salient value is zero, the multi-level run length encoder statistically correlates zero value runs to adjacent non-zero values and assigns variable length codewords. The encoder uses a specialized codebook that is generated with respect to the probability of receiving an input run in which spectral coefficients having zero values are followed by non-zero coefficients. A corresponding decoder associates a variable length codeword with a run of zero-valued coefficients and adjacent non-zero-valued coefficients.

  U.S. Patent No. 6,053,059 issued to Chen et al. Describes variable-variable length encoding of audio data. The encoder assigns variable length codes to variable size groups of frequency coefficient values.

U.S. Patent No. 6,053,096 issued to Chen et al. Describes entropy code mode switching for frequency domain audio coding. The frequency domain audio encoder selects between different entropy coding modes according to the characteristics of the input stream. Specifically, the input stream is partitioned into frequency ranges according to statistical criteria derived from a typical input to be encoded or a statistical analysis of the actual input. Each range is assigned an entropy encoder that is optimized to encode the type of data in that range. During encoding and decoding, the mode selector applies the exact method to the individual frequency ranges. Partition boundaries are predetermined and allow the decoder to implicitly know which decoding method is applied to the encoded data. Alternatively, an adaptive configuration can be used in which a flag is set at the boundary in the output stream to indicate a change in encoding mode for subsequent data. For example, a partition boundary separates primarily zero quantized frequency coefficients from non-zero quantized coefficients, and then applies an encoder optimized for such data .
For further details regarding the Chen patent, see the patent itself.

  Arithmetic coding is another well-known compression technique used for camera video and other types of content. In cases where there is a statistical correlation between several individual input symbols, arithmetic coding is sometimes applied where the optimal number of bits to encode a given input symbol is a fractional number of bits. Used in examples. Arithmetic coding generally involves representing an input sequence as a single number within a given range. Usually, the number is a fraction between 0 and 1. Symbols in the input sequence are associated with a range indicating a portion of space between 0 and 1. This range is calculated based on the probability that a particular symbol occurs in the input sequence. The fraction used to represent the input sequence is constructed in relation to that range. Therefore, the probability distribution of the input symbols is important in the arithmetic coding scheme.

  In context-based arithmetic coding, different probability distributions of input symbols are associated with different contexts. The probability distribution used to encode the input sequence changes as the context changes. The context can be calculated by measuring different factors that are expected to affect the probability of a particular input symbol occurring in the input sequence. See Non-Patent Document 3 for more information on arithmetic coding / decoding and some of the variants of arithmetic coding.

  Various codec systems and codec standards, including versions of Microsoft Corporation's Windows Media [“WMA”] encoder and decoder, use lossless compression and decompression. Other codec systems include Motion Picture Experts Group, Audio Layer 3 [“MP3”] standard, Motion Picture Experts Group 2, Advanced Audio Coding [“AAC”] standard, and Dolby (provided by Dolby, AC3) Or is prescribed. For further information, please refer to the respective standards or technical publications.

US Pat. No. 6,304,928 US Pat. No. 5,883,633 US Pat. No. 6,233,017 US Pat. No. 6,233,162 US Pat. No. 6,377,930 US Pat. No. 6,300,888

Bell et al., Text Compression, Prentice Hall PTR, pages 105-107, 1990 Gibson et al., Digital Compression for Multimedia, Morgan Kaufmann, pages 17-62, 1998 Nelson, The Data Compression Book, "Huffman One Better: Arithmetic Coding," Chapter 5, pp. 123-65 (1992)

  Whatever the advantages of conventional techniques and systems for lossless compression of audio data, conventional techniques and systems do not have the advantages of the present invention.

  In summary, the detailed description is directed to various techniques and tools for adaptive entropy coding and adaptive entropy decoding of audio data. The various techniques and tools can be used in combination or independently.

  In one aspect, the encoder encodes the first portion of the audio data sequence directly in variable-dimension vector Huffman encoding mode and switches to run level encoding mode at the switch point. Switching, the second part is encoded in a run level encoding mode (eg, context-based arithmetic encoding, Huffman encoding, vector Huffman encoding). For example, the first part consists mainly of non-zero quantized audio coefficients, and the second part consists mainly of zero-valued quantized audio coefficients. The switching point can be predetermined (eg, by testing the efficiency of encoding a sequence using the switching point) or can be determined adaptively. The encoder can send a flag indicating the switching point in the encoded bitstream.

  In an alternative aspect, the decoder decodes the first part of the encoded sequence directly in the variable dimension vector Huffman decoding mode, switches to the run level decoding mode at the switching point, and the second part Are decoded in a run level decoding mode (eg, context-based arithmetic decoding, Huffman decoding, vector Huffman decoding). Prior to performing the switch, the decoder can receive a flag indicating the switch point.

  In another aspect, an encoder or decoder encodes or decodes a first portion of a sequence directly in a context-based arithmetic mode, switches to run level mode at a switch point, and Encode or decode in run level mode. The run level mode can be a context-based arithmetic mode.

  In another aspect, an encoder selects a first code table from a set of multiple code tables based on the number of symbols in the first vector, and the first vector is a code from the first code table. It expresses by. The first code table may include codes to represent vectors that are likely to have that number of symbols and escape codes that represent vectors that are less likely. The encoder also encodes second vectors having different numbers of symbols. For example, the first vector has a greater number of symbols than the second vector and has a higher probability of occurrence than the second vector. To encode the second vector, the encoder can select a second different code table based on the number of symbols in the second vector. If the second vector has one symbol, the encoder can represent the second vector using a tableless encoding technique.

  In another aspect, a decoder receives a first code and decodes the first vector by looking up the first code in a first code table. If the first code is an escape code, the decoder receives and decodes a second code that does not exist in the first table. If the first code is not an escape code, the decoder looks up the symbol (s) for the first vector in the first table and includes the symbol (s) in the decoded data stream. . The number of symbols in the first vector is a criterion as to whether the first code is an escape code. The decoder can decode the second code by looking up the second code in the second table. If the second code is an escape code, the decoder receives and decodes a third code representing the first vector that is not present in the second table. If the second code is not an escape code, the decoder looks up the symbol for the first vector in the second table and includes the symbol in the decoded data stream.

  In another aspect, the encoder encodes the audio data coefficients using a tableless encoding technique. If the coefficient is within the first value range, the encoder encodes the coefficient with a code that is a 1-bit code followed by an 8-bit encoded value. For other value ranges, the encoder may code its coefficients with a 2-bit code followed by a 16-bit encoded value, a 3-bit code followed by a 24-bit encoded value, or different Encoding is performed using a code in which a 3-bit code is followed by a 31-bit encoded value.

  In another aspect, in a vector Huffman coding scheme, an encoder determines a Huffman code to use to encode a vector from a group of Huffman codes and uses the Huffman code to encode the vector. The determination of the sign is based on the sum of the values of the audio data symbols in the vector. If the Huffman code is an escape code, the Huffman code indicates that the n-dimensional vector should be encoded as x n / x-dimensional vectors using at least one different code table. The encoder can compare the sum to a threshold that depends on the number of symbols in the vector. For example, the threshold is 6 for 4 symbols, 16 for 2 symbols, or 100 for 1 symbol.

  In another aspect, an encoder receives a sequence of audio data and encodes at least a portion of the sequence using context-based arithmetic coding. A decoder receives the encoded sequence of audio data coefficients and decodes at least a portion of the encoded sequence using context-based arithmetic decoding.

  In another aspect, the encoder encodes audio data coefficients using context-based arithmetic coding. One or more contexts have an associated probability distribution that represents the probability of the coefficients. The encoder adaptively identifies the context for the current coefficient based at least in part on the mode of representation of the current coefficient and uses the context to encode the current coefficient. For example, if the mode of expression is direct, the encoder will adaptively identify the context based at least in part on the direct level of the previous coefficient (eg, the two coefficients immediately preceding the current coefficient). To do. If the mode of representation is run level, the encoder adaptively adapts the context based at least in part on the run length, the percentage of the zero value coefficient, before the zero value coefficient in the audio input sequence. Identify. The mode of representation is run level, and the encoder determines a context based at least in part on the current run length of the zero value coefficient, the previous run length of the zero value coefficient, and the direct level of the previous coefficient, Adaptively identify.

  In another aspect, an encoder or decoder uses direct encoding or direct decoding to maintain a count of consecutive coefficients equal to a predominant value (eg, 0) to maintain audio data The first part is encoded or decoded. If the count exceeds the threshold, the encoder or decoder encodes or decodes the second portion of the audio data using run level encoding or run level decoding. The threshold can be static or can be determined adaptively. The threshold can depend on the size of the block of coefficients. For example, the threshold can be 4 for a block of 256 coefficients or 8 for a block of 512 coefficients.

  In another aspect, an encoder or decoder encodes or decodes a first portion of a sequence using a first code table and uses a second code table to generate a second portion of the sequence. Is encoded or decoded. The first table is used when there is a high probability of longer runs of consecutive coefficients equal to a significant value (eg, 0), and the second table is used for shorter runs of equal coefficients of consecutive coefficients. Used when likelihood is high. The table used can be indicated by a signal bit.

  The features and advantages of the adaptive entropy encoding technique and adaptive entropy decoding technique will become apparent from the following detailed description of various embodiments, which proceeds with reference to the accompanying drawings.

1 is a block diagram illustrating a suitable computing environment in which the described embodiments can be implemented. FIG. 2 is a block diagram illustrating an audio encoder that can implement the described embodiments. FIG. 6 is a block diagram illustrating an audio decoder capable of implementing the described embodiments. 2 is a flow diagram illustrating a generalized multi-mode audio encoding technique. 2 is a flow diagram illustrating a multi-mode audio encoding technique with adaptive switch point calculation. 3 is a flow diagram illustrating a generalized multi-mode audio decoding technique. 2 is a flow diagram illustrating a generalized variable dimensional vector Huffman coding technique. 2 is a flow diagram illustrating a detailed technique for encoding audio data using variable dimension vector Huffman coding. FIG. 6 is a flow diagram illustrating a technique for direct signal level variable dimensional vector Huffman coding in an manner that adaptively determines a switch point for the encoder to change to run length and signal level coding. Figure 3 is a flow diagram illustrating a generalized variable dimensional vector Huffman decoding technique. Figure 3 is a flow diagram illustrating a detailed technique for decoding a vector encoded using variable dimensional vector Huffman encoding. FIG. 6 is a flow diagram illustrating a technique for direct signal level variable dimensional vector Huffman decoding in an adaptive manner of determining switch points for a decoder to change to run length and signal level decoding. FIG. 6 is a diagram illustrating a probability distribution for non-run length levels in a context-based arithmetic coding scheme. FIG. 6 is a diagram illustrating a probability distribution for non-run length levels in a context-based arithmetic coding scheme. FIG. 6 is a diagram illustrating a probability distribution for non-run length levels in a context-based arithmetic coding scheme. FIG. 6 is a diagram illustrating a probability distribution for non-run length levels in a context-based arithmetic coding scheme. FIG. 6 shows probability distributions for different run lengths in a context-based arithmetic coding scheme. FIG. 6 shows probability distributions for different run lengths in a context-based arithmetic coding scheme. FIG. 6 shows probability distributions for different run lengths in a context-based arithmetic coding scheme. FIG. 6 shows probability distributions for different run lengths in a context-based arithmetic coding scheme. FIG. 6 shows probability distributions for different run lengths in a context-based arithmetic coding scheme. FIG. 6 shows probability distributions for different run lengths in a context-based arithmetic coding scheme. FIG. 6 shows probability distributions for different run lengths in a context-based arithmetic coding scheme. FIG. 6 shows probability distributions for different run lengths in a context-based arithmetic coding scheme. FIG. 6 illustrates a probability distribution for run-length encoded levels in a context-based arithmetic encoding scheme. FIG. 6 illustrates a probability distribution for run-length encoded levels in a context-based arithmetic encoding scheme. FIG. 6 illustrates a probability distribution for run-length encoded levels in a context-based arithmetic encoding scheme. FIG. 6 illustrates a probability distribution for run-length encoded levels in a context-based arithmetic encoding scheme. FIG. 6 illustrates a probability distribution for run-length encoded levels in a context-based arithmetic encoding scheme. FIG. 6 illustrates a probability distribution for run-length encoded levels in a context-based arithmetic encoding scheme. FIG. 6 illustrates a probability distribution for run-length encoded levels in a context-based arithmetic encoding scheme. FIG. 6 illustrates a probability distribution for run-length encoded levels in a context-based arithmetic encoding scheme. FIG. 6 is a flow diagram illustrating a technique for direct context-based arithmetic coding of coefficients in a manner in which switching points for changing to run length and run level coding are adaptively determined by an encoder. FIG. 6 is a flow diagram illustrating a context-based arithmetic decoding technique in an aspect that adaptively determines a switch point for the decoder to switch to run length and signal level decoding.

  In the described embodiment, the audio encoder performs several adaptive entropy encoding techniques. Adaptive entropy coding techniques that reduce bit rate and / or improve quality improve the performance of the encoder. The decoder performs a corresponding entropy decoding technique. Although these techniques are described in several places as part of a single integrated system, these techniques can also be applied separately and possibly in combination with other techniques.

  The audio encoder and audio decoder process discrete audio signals. In the described embodiment, the audio signal is a quantized coefficient from the frequency converted audio signal. Alternatively, the encoder and decoder process another type of discrete audio signal or a discrete signal representing video or another type of information.

  In some embodiments, the audio encoder adaptively switches between direct signal levels encoding and run length and signal level encoding. The encoder encodes the signal level directly using scalar Huffman coding, vector Huffman coding, arithmetic coding, or another technique. In run length / level coding (also called run level coding), each run length represents zero or more zero runs and each signal level represents a non-zero value. In the run level event space, the encoder encodes run lengths and levels in the event space using Huffman coding, arithmetic coding, or another technique. The decoder performs corresponding adaptive switching during decoding. Adaptive switching is performed when the number of thresholds at the zero value level is reached. Alternatively, the encoder and decoder switch based on further criteria or other criteria.

  In some embodiments, the audio encoder uses variable dimensional vector Huffman coding. Variable dimensional vector Huffman coding is where the encoder represents a more probable combination of symbols using a larger dimensional vector and a less probable combination of symbols using a smaller dimensional vector or scalar It is possible to use the Huffman code represented by The decoder performs corresponding variable dimension Huffman decoding.

  In some embodiments, the audio encoder uses context-based arithmetic coding. The context used by the encoder allows efficient compression for individual types of audio data. The decoder performs corresponding context-based arithmetic decoding.

  In the described embodiment, the audio encoder and audio decoder perform various techniques. The operations relating to these techniques are usually described in a specific order for presentation, but it should be understood that this description encompasses some permutation of the order of operations. For further simplicity, flowcharts typically do not show the various ways in which a particular technique can be used in conjunction with other techniques.

I. Computing Environment FIG. 1 illustrates a generalized example of a suitable computing environment (100) in which the described embodiments can be implemented. The computing environment (100) is not intended to suggest any limitation as to the scope of use or functionality of the invention. This is because the present invention can be implemented in a wide variety of general purpose or special purpose computing environments.

  With reference to FIG. 1, the computing environment (100) includes at least one processing unit (110) and memory (120). In FIG. 1, this most basic configuration (130) is contained within a dashed line. The processing unit (110) executes computer-executable instructions and can be a real or virtual processor. In a multiprocessing system, multiple processing units execute computer-executable instructions to increase processing power. The memory (120) may be volatile memory (eg, registers, cache, RAM), non-volatile memory (eg, ROM, EEPROM, flash memory, etc.), or some combination of volatile and non-volatile memory. Is possible. The memory (120) stores software (180) that implements an audio encoder / decoder that performs adaptive entropy encoding / decoding of audio data.

  A computing environment may have additional features. For example, the computing environment (100) includes a storage (140), one or more input devices (150), one or more output devices (160), and one or more communication connections (170). Including. An interconnection mechanism (not shown) such as a bus, controller, or network connects the components of the computing environment (100) to each other. Typically, operating system software (not shown) provides an operating environment for other software running in the computing environment (100) and coordinates the activities of the components of the computing environment (100). Let

  The storage (140) can be removable or non-removable and is used to store magnetic disks, magnetic tapes or cassettes, CD-ROMs, CD-RWs, DVDs, or information. And any other medium that can be accessed within the computing environment (100). The storage (140) stores instructions for software (180) implementing an audio encoder / decoder that performs adaptive entropy encoding / decoding of audio data.

  The input device (150) is a touch input device such as a keyboard, mouse, pen, or trackball, is a scanning device, a network adapter, or another device that provides input to the computing environment (100) Is possible. For audio, the input device (150) is a sound card, or a similar device that accepts audio input in analog or digital form, or a CD-ROM that provides audio samples to the computing environment. It can be a reader. The output device (160) can be a display, printer, speaker, CD / DVD writer, network adapter, or another device that provides output from the computing environment (100).

  A communication connection (170) enables communication to another computing entity via a communication medium. Communication media conveys information, such as computer-executable instructions, compressed audio information, or other data in a modulated data signal. A modulated data signal is a signal that has one or more of its characteristics set or changed in such a manner as to encode information in the signal. By way of example, and not limitation, communication media includes wired or wireless techniques implemented using electrical, optical, RF, infrared, acoustic, or other carrier waves.

  The invention can be described in the general context of computer-readable media. Computer readable media can be any available media that can be accessed within a computing environment. By way of example, and not limitation, within computing environment (100), computer-readable media include memory (120), storage (140), communication media, and any combination of the above.

  The invention may be described in the general context of computer-executable instructions contained within program modules and executed in a computing environment on a target real or virtual processor. Generally, program modules include routines, programs, libraries, objects, classes, components, data structures, etc. that perform particular tasks or implement particular abstract data types. The functions of the program modules can be combined as desired in various embodiments or divided among the program modules. Computer-executable instructions for program modules may be executed within a local computing environment or in a distributed computing environment.

  For presentation purposes, the detailed description includes "analyze", "send", "compare", and "check" to describe computer behavior in a computing environment. Use terms like These terms are high-level abstractions of operations performed by computers and should not be confused with operations performed by humans. The actual computer operations corresponding to these terms vary depending on the implementation.

II. Generalized Audio Encoder and Audio Decoder FIG. 2 is a block diagram illustrating a generalized audio encoder (200) that can implement the described embodiments. The encoder (200) performs adaptive entropy encoding of audio data. FIG. 3 is a block diagram illustrating a generalized audio decoder (300) that may implement the described embodiments. The decoder (300) decodes the encoded audio data.

  The relationship between the modules within the encoder and the relationship between the modules within the decoder shows the flow of information in the exemplary encoder and exemplary decoder, and other relationships are for simplicity. Not shown. Depending on the implementation and the type of compression desired, an encoder or decoder module may be added, omitted, split into multiple modules, combined with other modules, and / or It can be replaced with a similar module. In alternative embodiments, encoders or decoders with different modules and / or other configurations perform adaptive entropy encoding and adaptive entropy decoding of audio data.

A. Generalized Audio Encoder Generalized audio encoder (200) includes a selector (208), a multi-channel pre-processor (210), a partitioner / tile configurer (220). ), Frequency converter (230), perception modeler (240), weighter (242), multi-channel transformer (250), quantizer (260), entropy encoder ( 270), controller (280), mixed / pure lossless encoder (272) and associated entropy encoder (274), and bitstream multiplexer ["MUX"] (290). Some of the modules of the encoder (200) are described below. For descriptions of other modules of the encoder (200) in some embodiments, please refer to the applications mentioned in the Related Application Data section.

  The encoder (200) receives time-sequential input audio samples (205) at some sampling depth and rate in pulse code modulated ["PCM"] format. The input audio samples (205) can be multi-channel audio (eg, stereo mode, surround) or mono. The encoder (200) compresses audio samples (205) and multiplexes the information generated by the various modules of the encoder (200) to form a Windows Media Audio [“WMA”] format or A bit stream (295) is output in a format such as Advanced Streaming Format [[ASF]]. Alternatively, the encoder (200) functions with other input and / or output formats.

  Initially, the selector (208) selects from a plurality of encoding modes for the audio sample (205). In FIG. 2, the selector (208) switches between two modes: a mixed / pure lossless coding mode and a lossy coding mode. The lossless coding mode includes a composite / pure lossless encoder (272) and is typically used for high quality (and high bit rate) compression. The lossy coding mode includes components such as a weighter (242) and a quantizer (260) and is typically used for compression of adjustable quality (and regulated bit rate). The selection decision in the selector (208) depends on user input (eg, the user selecting lossless encoding to create a high quality audio copy) or other criteria. In other situations (eg, when lossy compression cannot provide sufficient quality, or excessive bits are generated), the encoder (200) may combine from lossy encoding for a frame or set of frames. / It is possible to switch to pure lossless coding.

  A frequency converter (230) receives the audio samples (205) and converts the audio samples (205) into data in the frequency domain. The frequency converter (230) outputs a block of frequency coefficient data to the weighter (242), and outputs secondary information such as a block size to the MUX (290). The frequency converter (230) outputs both the frequency coefficient and the side information to the perceptual modeler (240).

  A perceptual modeler (240) models the characteristics of the human auditory system to improve the perceived quality of an audio signal that is reconstructed at a given bit rate. In general, the perceptual modeler (240) provides information to a weighter (242) that can be used to generate weighting factors for the audio data after processing the audio data according to the auditory model. The perceptual modeler (240) uses any of a variety of auditory models and sends excitation pattern information, or other information, to the weighter (242).

  As a quantization band weighter, the weighter (242) generates a weighting factor for the quantization matrix based on the information received from the perceptual modeler (240) and converts the weighting factor to the frequency converter (230). Applies to data received from The weighter (242) outputs secondary information such as a set of weighting coefficients to the MUX (290). Next, as a channel weighter, the weighter (242) uses a channel-specific weighting factor based on information received from the perceptual modeler (240) and based on the quality of the locally reconstructed signal. Generate. This scalar weight allows the reconstructed channel to have a nearly uniform quality. The weighter (242) outputs a weighted block of coefficient data to the multi-channel converter (250) and outputs side information such as a set of channel weight coefficients to the MUX (290). Alternatively, the encoder (200) uses another form of weighting or omits the weighting.

  For multi-channel audio data, multiple channels are often correlated for noise-shaped frequency coefficient data generated by the weighter (242). To take advantage of this correlation, the multi-channel converter (250) can apply multi-channel conversion to the audio data. Multi-channel converter (250) generates side information to MUX (290) indicating, for example, the multi-channel conversion used and the multi-channel converted portion of the frame.

  The quantizer (260) quantizes the output of the multi-channel converter (250), generates quantized coefficient data to the entropy encoder (270), and performs a quantization step to the MUX (290). Generate side information including size. Quantization introduces irreversible loss of information, but also allows the encoder (200) to adjust the quality and bit rate of the output bitstream (295) in conjunction with the controller (280). In some embodiments, the quantizer (260) is an adaptive, uniform, scalar quantizer. In alternative embodiments, the quantizer is a non-uniform quantizer, a vector quantizer, and / or a non-adaptive quantizer, or an adaptive, uniform, scalar quantization different Use form.

  The entropy encoder (270) compresses the quantized coefficient data received from the quantizer (260) without loss. In some embodiments, the entropy encoder (270) uses adaptive entropy coding as described in the following sections. The entropy encoder (270) can calculate the number of bits spent encoding audio information and send the information to the speed / quality controller (280).

  The controller (280) cooperates with the quantizer (260) to adjust the bit rate and / or quality of the output of the encoder (200). The controller (280) receives information from other modules of the encoder (200) and processes the received information to determine the desired quantization factor given the current condition. The controller (280) outputs the quantization factors to the quantizer (260) using a goal that satisfies the quality constraint and / or the bit rate constraint.

  A composite lossless / pure lossless encoder (272) and associated entropy encoder (274) compress audio data during the composite / pure lossless encoding mode. The encoder (200) uses a composite / pure lossless coding mode for the entire sequence, or switches between coding modes on a frame-by-frame basis or other criteria.

  MUX (290) multiplexes the side information received from the other modules of audio encoder (200) along with the entropy encoded data received from entropy encoder (270). MUX (290) outputs the information in WMA format or in another format recognized by the audio decoder. The MUX (290) includes a virtual buffer that stores the bitstream (295) output by the encoder (200). The current fullness of the buffer, the rate of change of the buffer fullness, and other characteristics of the buffer are used by the controller (280) to suit the quality and / or bit rate for the particular application. Can be adjusted (eg, to a constant quality / variable bit rate, or to a constant bit rate / variable quality, or below).

B. Generalized Audio Decoder Referring to FIG. 3, a generalized audio decoder (300) includes a bitstream demultiplexer ["DEMUX"] (310) and one or more decoders (320). A composite / pure lossless decoder (322), a tile configuration decoder (330), an inverse multi / channel converter (340), an inverse quantizer / inverse weighter (350), It includes an inverse frequency converter (360), an overlapper / adder (370), and a multi-channel post processor (380). The decoder (300) is somewhat simpler than the encoder (300) because it does not include modules for speed / quality control or perceptual modeling. Some of the modules of the decoder (300) are described below. For descriptions of other modules of the decoder (300) in some embodiments, please refer to the applications mentioned in the Related Application Data section.

  The decoder (300) receives a bitstream (305) of compressed audio information in WMA format or another format. Bitstream (305) includes entropy encoded data as well as side information from which decoder (300) reconstructs audio samples (395).

  The DEMUX (310) parses the information in the bitstream (305) and sends the information to each module of the decoder (300). The DEMUX (310) includes one or more buffers to compensate for short-term variations in bit rate due to audio complexity variations, network jitter, and / or other factors.

  One or more entropy decoders (320) decompress the entropy code received from the DEMUX (310) without loss. For simplicity, one entropy decoder is shown in FIG. 3, but different entropy decoders may be used for the lossless and lossy encoding modes, or within each mode. Also, for simplicity, FIG. 3 does not show mode selection logic. The entropy decoder (320) typically applies the inverse of the entropy coding technique used in the encoder (200). When decoding the data compressed in the lossy encoding mode, the entropy decoder (320) generates quantized frequency coefficient data.

  A composite / pure lossless decoder (322) and associated entropy decoder (320) decompress the audio data encoded for the composite / pure lossless coding mode without loss. The decoder (300) uses a specific decoding mode for the entire sequence, or switches decoding modes on a frame-by-frame basis or on other criteria.

  An inverse multi-channel transformer (340) is entropy-decoded quantized frequency coefficient data from the entropy decoder (320), and, for example, the multi-channel transform used and the transformed portion of the frame Sub-information is received from the DEMUX (310) indicating

  The inverse quantizer / inverse weighter (350) receives the quantized coefficients as well as the quantization matrix from the DEMUX (310) and receives the quantized frequency coefficient data from the inverse multi-channel transformer (340). An inverse quantizer / weighter (350) decompresses the received quantized coefficient / matrix information as necessary, and then performs inverse quantization and inverse weighting.

  An inverse frequency transformer (360) receives the frequency coefficient data output by the inverse quantizer / inverse weighter (350) as well as side information from the DEMUX (310). The inverse frequency transformer (360) applies the inverse of the frequency transform used in the encoder and outputs the block to the overlay / adder (370).

  The superimposer / adder (370) receives the decoded information from the inverse frequency converter (360) and / or the composite / pure lossless decoder (322). An overlapper / adder (370) overlays and adds audio data as needed, and interleaves frames or other sequences of audio data encoded in different modes.

III. Adaptive Entropy Encoding / Decoding Mode Switching
If the input sequence includes multiple occurrences of a single value (eg, 0), run level encoding is often more effective than direct level encoding. However, run level coding is not effective over the entire frequency range, since non-zero quantized transform coefficients are common in audio data input sequences, especially at lower frequencies. Furthermore, for higher quality audio, non-zero quantized transform coefficients become more common even at higher frequencies. (For higher quality audio, the quantization level is usually smaller.) Thus, in some embodiments, an encoder, such as the encoder (200) of FIG. 2, may be part of an audio data input sequence. Implement a multi-mode encoding technique that can use run-level coding for and use direct level coding for another part of the sequence. A decoder such as the decoder (300) of FIG. 3 implements the corresponding multi-mode decoding technique.

A. Adaptive Entropy Coding Mode Switching With reference to FIG. 4, in a multi-mode coding technique 400, the encoder first directly codes the signal level in the input stream (410). For example, the encoder performs variable dimension Huffman coding, context-based arithmetic coding, or another entropy coding technique directly on the signal level.

  At the switching point during encoding, the encoder switches the encoding scheme (420). The encoder can switch coding schemes at predetermined switching points, or can analyze input data to determine an appropriate point to switch coding schemes. For example, the encoder can parse the input sequence to find the best point to switch to run-level encoding and send the switch point in the output bitstream to the decoder. Alternatively, the encoder adaptively calculates the switching point by counting consecutive zeros (or alternatively another significant value) in the input data, and a specific threshold number of consecutive zeros When counted, it can be switched to run level encoding. The decoder can also calculate the switch point in the same way, so the switch point does not need to be included in the bitstream. Alternatively, the encoder and decoder use some other criteria to determine the switch point.

  After the switch point, the encoder encodes the remaining signal level using run level encoding (430). For example, the encoder performs Huffman coding, context-based arithmetic coding, or another entropy coding technique on run lengths and signal levels. The encoder may use the same technique (eg, context-based arithmetic coding) before and after the switch point, or may use a different technique.

  Furthermore, although FIG. 4 and various other illustrations of the application show a single switching point, additional switching points can be used to split the input data into more than two parts. . For example, additional adaptive switching points can be set for higher thresholds of consecutive zeros. This allows different coding schemes to be applied to different parts. Alternatively, the encoder can try various split points in the sequence to evaluate the coding efficiency for the different split configurations, at the cost of signaling the different configurations to the decoder.

  FIG. 5 illustrates a multi-mode encoding technique (500) with adaptive switch point calculation according to one implementation. The adaptive switching point depends on the count of consecutive zero value coefficients. The input data is the signal level of each quantized transform coefficient that proceeds from the lowest frequency coefficient to the highest frequency coefficient. In practice, the position of the switching point depends on the signal being compressed and the bit rate / quality of the encoding. Alternatively, the input data is another form and / or organization of audio data.

  Initially, the encoder initializes several variables. Specifically, the encoder sets the run count variable to 0 (510) and the encoding state variable to “direct” (512).

  The encoder receives the next coefficient QC as input (520). The encoder then checks (530) whether the coefficient QC is zero. If the coefficient QC is not zero, the encoder resets the run count (538). Otherwise (ie, if the coefficient QC is zero), the encoder increments the run count variable (532) to see if the current run count exceeds the run count threshold. (534). The run count threshold is static or depends on factors such as the size of the block of coefficients (eg, 4 run count threshold for a sequence of 256 coefficients, 512 coefficients For example, a run count threshold of 8), or some other form of adaptation. If the run count exceeds the threshold, the encoder switches the encoding state to run-level encoding ["RLE"] (536).

  The encoder then encodes the coefficient QC, if appropriate (540). (In some cases, the group coefficients are encoded together using techniques such as vector Huffman coding. In such cases, the encoder defers encoding the coefficient QC. can do.)

  Next, the encoder checks if the encoding mode should be switched (550). Specifically, the encoder checks the encoding state. If the coding state is no longer direct (eg, the encoder switches the coding state to RLE as a result of the zero coefficient reaching the threshold number), the encoder will perform a run level coding of the coefficient. Is started (560). (Still, in the case where groups of coefficients are encoded together, the encoder can defer switching decisions until a convenient breakpoint is reached for a group of coefficients.)

  If the encoder does not switch encoding modes, the encoder checks to see if it has finished encoding the coefficients (570). If so, the encoder ends. If not, the encoder inputs the next coefficient (520) and continues the encoding process.

B. Adaptive Entropy Decoding Mode Switching Referring to FIG. 6, in a multi-mode decoding technique (600), the decoder directly decodes the encoded signal level (610). For example, the decoder performs variable dimension Huffman decoding, context-based arithmetic decoding, or another entropy decoding technique directly on the encoded signal level.

  At the switch point during decoding, the decoder switches (620) the decoding scheme. If the switch point is predetermined, the decoder can receive data in the form of a flag or other notification mechanism that explicitly tells the decoder when to switch the decoding scheme. Alternatively, the decoder can adaptively calculate when to switch decoding schemes based on the input data it receives. When the decoder calculates the switching point, it uses the same calculation technique used by the encoder to ensure that the decoding scheme switches at the correct point. For example, the decoder adaptively determines the switch point by counting consecutive zeros (or alternatively, another significant value). In one implementation, the decoder uses a technique corresponding to the encoder technique shown in FIG. Alternatively, the decoder uses some other criteria to determine the switch point.

  After the switch point, the decoder decodes (630) the remaining run level encoded signal levels. For example, the decoder performs a Huffman decoding technique, a context-based arithmetic decoding technique, or another entropy decoding technique on the encoded run length and signal level. The decoder may use the same technique (eg, context-based arithmetic decoding) before and after the switch point, or may use a different technique.

IV. Variable Dimension Huffman Coding and Variable Dimension Huffman Decoding Symbols such as direct signal levels can be encoded using scalar Huffman coding, but such techniques are optimal for encoding symbols. If the number of bits is a fraction, there is a limit. Scalar Huffman coding is also limited by the fact that scalar Huffman codes cannot take into account the statistical correlation between symbols. Vector Huffman coding results in better bit rate reduction than scalar Huffman coding (eg, by allowing the encoder to exploit the probability in binary Huffman codes as a fraction). Also, in general, higher dimensional vectors result in better bit rate reduction than smaller dimensional vectors. However, if a code is assigned to each possible symbol combination, the codebook size increases exponentially as the vector dimension increases. For example, in a 32-bit system, the number of possible combinations for a four-dimensional vector is (2 ³² ) ⁴ . Also, the search time taken to match vectors to find a Huffman code increases dramatically as the codebook size increases.

  In some embodiments, to reduce the codebook size, an encoder such as the encoder (200) of FIG. 2 uses a variable dimension vector Huffman encoding technique. Rather than assigning a codebook code to each possible n-dimensional combination, a code is assigned to a limited number of the most probable n-dimensional vectors. If no code is assigned to a particular n-dimensional vector, the n-dimensional vector is instead instead as a smaller-dimensional vector (eg, two n / 2-dimensional vectors), as a scalar with a Huffman code, or Encoded as a scalar that uses some tableless technique to represent discrete values. A decoder, such as the decoder (300) of FIG. 3, finds the code for the vector and reconstructs the vector by finding the associated value.

  For example, for a four-dimensional vector that allows 256 values per symbol, the encoder encodes the 500 most probable four-dimensional vector using a Huffman code and uses an escape code to indicate other vectors. To do. The encoder splits such other vectors into two-dimensional vectors. The encoder encodes the 500 most probable two-dimensional vector using a Huffman code and uses an escape code to indicate the other vector, which is divided into scalar Huffman codes Is encoded using Therefore, the encoder uses 501 + 501 + 256 codes.

  With respect to determining which vector or which scalar is represented by the Huffman code in the table, and with respect to assigning the Huffman code itself to the table, the codebook structure is static, previously encoded data It is possible to adapt to the data to be encoded.

A. Variable Dimension Vector Huffman Coding Referring to FIG. 7, the encoder uses a variable-dimension vector Huffman [“VDVH”] coding technique (700). For example, the encoder uses this technique (700) to directly encode the signal level for the frequency coefficient of the audio data. Alternatively, the encoder uses this technique (700) to encode another form of audio data. For simplicity, FIG. 7 does not show the codebook structure. The codebook structure can be static, adapt to previously encoded data, or adapt to the data to be encoded.

  The encoder obtains a next vector of n symbols (710). For example, the encoder obtains the next four symbols sequentially.

  The encoder checks (720) whether the codebook contains a code for the vector. If so, the encoder encodes (730) the vector using a single Huffman code. For example, to determine how to encode an n-dimensional vector, the encoder looks up an n-dimensional vector code table for codes associated with that vector. Encoders use Huffman codes for the most probable n-dimensional vectors because larger dimensional vectors typically provide greater bit rate savings. However, to limit the size of the table, only a part of the n-dimensional vector has an associated code.

  If the codebook does not include a code for the vector, the encoder divides the vector into smaller vectors and / or scalars (740) and encodes the smaller vectors and / or scalars. For example, the encoder splits a vector of n symbols into x n / x symbol vectors. For each n / x symbol vector, the encoder recursively repeats the encoding technique and the n / x symbol vector, or the vector / scalar comprising the n / symbol vector, is a Huffman code or (scalar In order to represent discrete values, it ends when encoded using a tableless technique.

  The encoder then checks (750) if there are more vectors to be encoded. If not, the encoder ends. If so, the encoder obtains the next vector of n symbols (710).

1. Exemplary Implementation FIG. 8 shows a detailed technique (800) for encoding a vector using VDVH encoding in one implementation. In this technique (800), the encoder sums the integer values of symbols in a vector of symbols and encodes the vector using a single Huffman code or makes the vector smaller Decide whether to split into vector / scalar. This substantially limits the codebook size and speeds up the code search.

The codebook table for an n-dimensional [“n-dim”] vector contains Huffman codes for L ₁ n-dimensional vectors. The codebook table also includes an escape code. The L ₁ codes are for each vector whose sum of vector components (which is an integer) is below a certain threshold T ₁ . For example, suppose n is 4 and the threshold T _{1 for a} 4D vector is 6. The codebook table for a four-dimensional vector includes an escape code and 126 codes, where the sum of the components of each possible vector (eg, the absolute value of the component) is less than 6 − (0,0,0, 0), (0, 0, 0, 1), etc. Limiting the table size based on the vector component sum is generally useful because the most probable vector is the vector with the smaller component sum.

  If the codebook table for an n-dimensional vector does not have a Huffman code for a particular n-dimensional vector, the encoder adds an escape code to the output bitstream and has a smaller dimension than other codebook tables. The plurality of vectors or scalars are looked up and the n-dimensional vector is encoded as a vector of smaller dimensions or a plurality of scalars. For example, the smaller dimension is n / 2 unless n / 2 is 1, and if n / 2 is 1, the n-dimensional vector is divided into multiple scalars. Alternatively, n-dimensional vectors are divided in some other way.

The codebook table for the smaller dimension vectors includes Huffman codes for L ₂ smaller dimension vectors, as well as escape codes. The L ₂ codes are codes for each vector whose sum of vector components is below a certain threshold T ₂ for a table of smaller dimensions. For example, assume that the smaller dimension is 2 and the threshold T _{2 for a} 2D vector is 16. The codebook table for a two-dimensional vector is an escape code and 136 codes, one for each possible vector whose sum of components (eg, absolute values of components) is less than 16,-(0,0), (0, 1), etc.

  If the codebook table for a smaller dimension vector does not have a Huffman code for a particular smaller dimension vector, the encoder adds an escape code to the output bitstream and uses the other codebook table. The vector as a smaller dimension vector or scalar. This process is repeated until the scalar level. For example, the division is performed in powers of 2 up to the scalar level. Alternatively, the vector is split in some other way.

At the scalar level, the codebook table contains L ₃ scalars as well as escape codes. L ₃ codes are for each scalar below a certain threshold T ₃ (assuming a small probability of probability is high). For example, assume that the threshold T ₃ for a scalar is 100. The codebook table for scalars contains 100 codes and escape codes. A scalar is encoded using an escape code and a value from a tableless technique (eg, literal) if it does not have an associated code in the scalar code table. Using all of the numerical examples given in this section, the table contains a total of 126 + 1 + 136 + 1 + 100 + 1 = 365 codes.

  The thresholds for table dimension size, vector partition factor, and vector component sum depend on the implementation. Other implementations use different vector sizes, different division factors, and / or different thresholds. Alternatively, the encoder performs a vector size / codebook table switch using criteria other than the sum of vector components when performing VDVH encoding.

  Referring to FIG. 8, the encoder first obtains an n-dimensional vector (810). An n-dimensional vector includes n symbols, each symbol having a value representing a quantized level, for example, with respect to the frequency coefficient of the audio data.

  The encoder sums the vector components (812) and compares the sum to a threshold for the n-dimensional vector (820). If the sum is less than or equal to the threshold, the encoder encodes the n-dimensional vector using the Huffman code from the code table (822) and continues until the encoding is complete (824). If the sum is greater than the threshold, the encoder sends an escape code (826) and splits the n-dimensional vector into two smaller vectors having n / 2 dimensions (830).

  The encoder obtains the next n / 2-dimensional vector (840) and sums the components of the n / 2-dimensional vector (842). The encoder checks the sum against a threshold associated with the n / 2-dimensional vector (850). If the sum is less than or equal to the threshold, the encoder encodes the n / 2-dimensional vector using the Huffman code from the code table for the n / 2-dimensional vector (852); If the encoder has not finished encoding the n / 2-dimensional vector (854), the next n / 2-dimensional vector is obtained (840). If the sum is greater than the threshold for the n / 2-dimensional vector, the encoder sends another escape code (856).

  The encoder generally follows this pattern when processing the vectors and either encodes each vector or divides it into smaller dimensional vectors. If the encoder divides the vector into two scalar (one-dimensional) components (860), the encoder obtains the next scalar (870) and compares the value of that scalar with a threshold value associated with the scalar value. (880). If the scalar value is less than or equal to the threshold (880), the encoder encodes the scalar using the Huffman code from the code table (882) for the scalar. If the scalar value is greater than the threshold, the encoder encodes the scalar using a tableless technique (884). Next, if the encoder has not finished processing the scalar (886), it obtains the next scalar (870).

  Alternatively, the encoder uses a table with a different dimension size when performing VDVH encoding, splits the vector in some way other than by a power of 2, and / or uses a criterion other than the sum of vector components To switch the vector size / codebook table.

2. Adaptive Switching FIG. 9 illustrates a technique for VDVH encoding of direct signal level coefficients in a manner in which the encoder adaptively determines switching points for switching to run length and signal level encoding according to one implementation. (900). The adaptive switching point depends on the count of consecutive zero value coefficients. The input data is the signal level for the quantized transform coefficients that progress from the lowest frequency coefficient to the highest frequency coefficient. Alternatively, the input data is another form and / or organization of audio data.

  Initially, the encoder initializes several variables. Specifically, the encoder sets the run count variable to 0 (910), sets the current vector variable to empty (912), and sets the encoding state variable directly to the variable dimensional vector Huffman [ “DVDVH”] is set (914).

  The encoder receives the next coefficient QC as input (920). The encoder then checks 930 whether the coefficient is zero. If the coefficient QC is not zero, the encoder resets the run count (938) and adds the coefficient QC to the current vector (940). Otherwise (ie, if the coefficient QC is zero), the encoder increments the run count variable (932) to see if the current run count exceeds the run count threshold. (934). The run count threshold is static or depends on factors such as the size of the block of coefficients (eg, four zeros in an input sequence of 256 coefficients) or in some other way It is possible to adapt. For example, the threshold value can be incremented or decremented with or without associated coefficients in the input sequence. If the run count exceeds the threshold, the encoder switches the encoding state to run level encoding ["RLE"] (936) and the coefficient QC is added as a component of the current vector ( 940).

  By adding the coefficient QC to the current vector, the vector dimension is incremented. The encoder determines whether the current vector is ready to be encoded by comparing the number of components in the current vector with the maximum dimension of the current vector (950). If so, the encoder encodes the current vector using DVDVH encoding (960). If the current vector is less than the maximum dimension, but the coefficient QC is the end of the sequence, the encoder embeds the current vector and encodes the current vector using DVDVH encoding (960). The maximum dimension depends on the implementation. In one implementation, the maximum dimension is 8. However, the maximum dimension can be increased or decreased depending on the amount of resources available to create, store, or transmit a codebook, for example.

  After encoding the vector, the encoder checks the encoding state (970). If the encoding state is no longer DVDVH (eg, if the encoder has switched the encoding state to RLE as a result of exceeding the zero coefficient threshold number), the encoder will Coding of coefficients is started (980). Run level coding can be done in several ways, including, for example, Huffman coding, vector Huffman coding, or context-based arithmetic coding. In some embodiments, run level encoding is used to encode data where one table is likely to have a shorter run and another table is likely to be a longer run. This is done using Huffman coding with two Huffman code tables used to encode high data. The encoder tries each table and selects a code from one table with a signal bit indicating which table the encoder used.

  If the encoding state has not switched or the current vector is not ready for encoding, the encoder determines whether there are more coefficients to be encoded (990). If so, the encoder inputs the next coefficient (920) and continues the encoding process.

B. Variable Dimension Vector Huffman Decoding FIG. 10 illustrates a VDVH decoding technique (1000) corresponding to the VDVH encoding technique (700) shown in FIG. For example, the decoder uses this technique (1000) to directly decode the encoded signal levels for the frequency coefficients of the audio data. Alternatively, the decoder uses this technique to decode another form of audio data.

  The decoder obtains the next Huffman code for the n-dimensional vector Huffman code table (1010). For example, the decoder obtains the next Huffman code for four symbols in the sequence.

  The decoder checks if the Huffman code is an escape code for the n-dimensional vector Huffman code table (1020). If it is not an escape code, the decoder obtains n symbols represented by the Huffman code (1030). For example, the decoder obtains four symbols associated with the Huffman code in a four-dimensional vector Huffman codebook.

  If the code is an escape code, the n-dimensional codebook does not include a code for that vector, and the decoder obtains a Huffman code for the smaller vector and / or scalar (1040). For example, the decoder obtains a code for x n / x symbol vectors. For each n / x symbol vector, the decoder recursively repeats the decoding technique and ends when the n / x symbol vector or the vectors / scalars that make up the n / x symbol vector are decoded. To do.

  The decoder then checks if there are more codes for the n-dimensional vector Huffman code table to be decoded (1050). If not, the decoder ends. If so, the decoder obtains the next such Huffman code (1010).

1. Exemplary Implementation FIG. 11 shows a detailed technique (1100) for decoding a vector encoded using VDVH encoding in one implementation. This decoding technique (1100) corresponds to the encoding technique (800) shown in FIG.

  Referring to FIG. 11, the decoder obtains the next code for the n-dimensional vector Huffman code table (1110). The decoder checks if the code is an escape code for an n-dimensional vector Huffman code table (1120). If not an escape code, the decoder obtains n symbols represented by that code in the n-dimensional vector table (1122). The decoder continues until it finishes processing the encoded data (1124).

  If the code is an escape code for an n-dimensional vector Huffman code table, the decoder uses the n / 2-dimensional vector Huffman code table to decode the n-dimensional vector as two n / 2-dimensional vectors. . Specifically, the decoder obtains the next code for the n / 2-dimensional vector Huffman code table (1130). The decoder checks if the code is an escape code for an n / 2-dimensional vector Huffman code table (1140). If not, the decoder obtains n / 2 symbols represented by the code in the n / 2-dimensional vector Huffman code table (1142). The decoder continues to process codes for the n / 2-dimensional vector Huffman code table until the processing of such codes is complete (1144).

  If the code is an escape code for an n / 2-dimensional vector Huffman code table, the decoder decodes the n / 2-dimensional vector as two n / 4-dimensional vectors. At this time, the two n / 4-dimensional vectors can be scalars.

  The decoder generally decodes a larger dimension vector as two smaller dimension vectors until an escape code is detected, until the vector to be decoded is a scalar (one-dimensional vector). Follow. At that point, the decoder obtains the next code for the scalar Huffman code table (1150). The decoder checks if the code is an escape code for a scalar Huffman code table (1160). If not, the decoder obtains the scalar represented by the code in the scalar Huffman code table (1162). The decoder continues to process the code for the scalar until the processing of such code is complete (1164). If the code was an escape code for a scalar Huffman code table, the scalar is encoded using a tableless technique and the decoder obtains its value (1170).

  Alternatively, when performing VDVH decoding, the decoder uses a table with a different dimension size and / or splits the vector in some other way other than by a power of two.

2. Adaptive Switching FIG. 12 illustrates decoding of a vector encoded using VDVH coding in a manner in which the decoder adaptively determines switching points for switching to run length and signal level decoding. FIG. 6 shows a technique (1200) according to one implementation for doing so. The adaptive switch point relies on a counter of consecutive zero value coefficients in the data that is the signal level for the quantized transform coefficients that go from the lowest frequency coefficient to the highest frequency coefficient. Alternatively, the data is another form and / or organization of audio data.

  Initially, the decoder initializes several variables. Specifically, the decoder sets the run count to 0 (1210) and sets the decoding state to DVDVH (1212).

  The decoder decodes (1220) the next vector by looking up the code for that vector in the Huffman code table. For example, the decoder performs the decoding technique (1100) shown in FIG. The decoder then updates (1230) the run count based on the decoded vector (specifically, resets the run count using the number of zero values in the decoded vector). , Increment, or otherwise adjust).

  The decoder checks (1240) whether the run count exceeds a threshold. The run count threshold is static or depends on factors such as the size of the block of coefficients (eg, four zeros in an input sequence of 256 coefficients), or in some other way It is possible to adapt. If the run count exceeds the threshold, the decoder begins decoding coefficients that have been encoded using run level decoding (1250). Run level decoding may be performed in several ways including, for example, Huffman decoding, vector Huffman decoding, or context-based arithmetic decoding.

  In some embodiments, run level decoding is used where one table is used to decode data that is more likely to be a shorter run and another table is likely to be a longer run. This is done using Huffman decoding with two possible Huffman code tables used to decode high data. When the decoder receives a code, the signal bit in the code indicates which table the encoder used, and the decoder looks up the code in the appropriate table.

  If the run count does not exceed the threshold, the decoder continues to process the vector until decoding is complete (1260).

V. Context-Based Arithmetic Coding and Context-Based Arithmetic Decoding In some embodiments, an encoder such as the encoder (200) of FIG. 2 may be connected to a context-based arithmetic [" CBA "] encoding is used to encode the sequence of audio data. In CBA coding, different probability distributions for input symbols are associated with different contexts. The probability distribution used to encode the input sequence changes when the context changes. The context can be calculated by measuring various factors that are expected to affect the probability of a particular input symbol appearing in the input sequence. A decoder, such as the decoder (300) of FIG. 3, performs the corresponding arithmetic decoding.

  When encoding a coefficient directly (ie, as a direct level), the encoder calculates the context using a factor that includes the value of the previous coefficient in the sequence. When encoding a coefficient using run-level encoding, the encoder uses a factor that includes the current run length and the previous run length in addition to the previous coefficient value. calculate. The encoder uses the probability distribution associated with the calculated context to determine the appropriate arithmetic code for the data. Thus, by using various factors in calculating the context, the encoder adapts the context to adapt to the data and to the mode of representation of that data (ie, directly, runlevel). Can be determined.

  In alternative embodiments, the encoder may use additional factors, omit some factors, or use the aforementioned factors in other combinations.

A. Exemplary Context Implementations Tables 2-5 and FIGS. 13A-13D, 14A-14H, and 15A-15H are shown in the exemplary implementation when performing CBA encoding and CBA decoding, respectively. Shows the context and probability distribution used. Alternatively, CBA encoding and CBA decoding use different contexts and / or different probability distributions.

  The following description focuses on context calculations at the encoder in the exemplary implementation, but the decoder uses the previously decoded audio data to perform the corresponding context calculations during decoding. .

  As described above, the encoder can encode the coefficients using CBA encoding, whether it is encoding only the direct level or the run length and direct level. However, in one implementation, the technique for calculating the context varies depending on whether the encoder is encoding only the direct level or the run length and the direct level. Further, when encoding run lengths and direct levels, the encoder uses different contexts depending on whether it is encoding run lengths or direct levels.

The encoder uses a 4-context system to compute the context during direct level arithmetic coding using a causal context. The encoder calculates the context for the current level L [n] based on the value of the previous level (L [n−1]) and the level immediately preceding the previous level ([L [n−2]]) To do. This context calculation is
1) an assumption that if the previous level is low, the current level is likely to be low, and
2) the assumption that the previous two levels are likely to be better predictors of the current level than the other levels;
Based on. Table 2 shows the context associated with the previous two levels of values in a four context system. 13A-13D show the probability distributions for the current level for those contexts.

  The probability distributions of FIGS. 13A-13D show that if the previous two levels are zero or close to zero, the current level tends to be zero or more likely to be zero. .

  The encoder can also use CBA encoding when performing level run length encoding. When encoding run lengths, the factor used by the encoder to calculate the context includes the percentage of zeros in the input sequence (running total over part or all of the sequence up to that point) ), And the run length before the zero (R [n-1]). The encoder calculates a zero percentage index based on the percentage of zeros in the input sequence as shown below in Table 3.

  The encoder uses the zero percentage indicator along with the length of the previous run to calculate the context for encoding the current run length as shown in Table 4. 14A-14H show probability distributions for run length values associated with those contexts.

  For example, in an input sequence where 91% of the levels are zero (resulting in a zero percentage index of 0), if the run length before zero was 15, then the context is 4. The probability distributions of FIGS. 14A-14H show that the higher the percentage of zeros in the input sequence, the greater the likelihood of a longer run length. Also, the probability distribution is within the range of a given zero percentage indicator, and the run length following a run length of zero is likely to be shorter than the run length following a run length greater than zero. It shows a trend.

  When encoding levels in run level data, the factors used by the encoder to calculate the context include the current run length (R [n]), the previous run length ( R [n-1]) and the values of the previous two levels (L [n-1] and L [n-2]) are included. This context calculation is based on the observation that the current level depends on the two previous levels unless the interval between levels (ie, run length) is too large. Also, if the previous level is low, and if the previous run is short, the current level is more likely to be low. If the previous run is long, the previous level has less impact on the current level.

  The context associated with the current run length value, the previous run length value, and the previous two levels of values is shown below in Table 5. Figures 15A-15H show probability distributions for the levels associated with those contexts.

  For example, in an input sequence where the length of the current run of zero is 1, the length of the previous run of zero is 2, and the previous level is 1, the context is 1. The probability distributions of FIGS. 15A-15H are more likely to be zero or close to zero if the previous level is low, and if the current run length and the previous run length are short. high.

B. Adaptive Switching FIG. 16 illustrates one implementation for direct CBA coding of coefficient at the signal level in a manner that adaptively determines switching points for the encoder to switch to run length and signal level coding. Shows the technique (1600). The adaptive switching point depends on the count of consecutive zero value coefficients. The input data is the signal level for the quantized transform coefficients that progress from the lowest frequency coefficient to the highest frequency coefficient. Alternatively, the input data is another form and / or organization of audio data.

  Initially, the encoder initializes several variables. Specifically, the encoder sets the run count variable to 0 (1610) and sets the encoding state variable to direct context-based arithmetic (DCBA) (1612).

  The encoder receives the next coefficient QC as input (1620). The encoder then checks (1630) whether the coefficient is zero. If the coefficient QC is not zero, the encoder resets the run count (1638) and encodes the coefficient using DCBA encoding (1640).

  Otherwise (ie, if the coefficient QC is zero), the encoder increments the run count variable (1632) to see if the current run count exceeds the run count threshold. (1634). The run count threshold is static or depends on factors such as the size of the block of coefficients (eg, four zeros in an input sequence of 256 coefficients), or in some other way It is possible to adapt. For example, the threshold can be raised or lowered with or without the number of coefficients in the input sequence. If the run count exceeds the threshold, the encoder switches the encoding state to run level encoding ["RLE"] (1636). The encoder then encodes the coefficients using DCBA encoding (1640).

  After encoding the coefficients, the encoder checks the encoding state (1650). If the coding state is no longer DCBA (for example, if the encoder has switched the coding state to RLE as a result of exceeding the zero coefficient threshold number), the encoder uses the coefficients as run length and level. Begin encoding (1660). Run level encoding can be done in several ways, including, for example, Huffman coding, vector Huffman coding, or CBA coding (possibly with a different context than the previous CBA coding described above) Can do. In some embodiments, run level encoding is used to encode data where one table is likely to have a shorter run and another table is likely to be a longer run. This is done using Huffman coding with two Huffman code tables used to encode high data. The encoder tries each table and selects a code from one table with a signal bit indicating which table the encoder used.

  If the encoding state has not switched, the encoder determines whether there are additional coefficients to be encoded (1670). If so, the encoder inputs the next coefficient (1620) and continues the encoding process.

C. Context-Based Arithmetic Decoding FIG. 17 was encoded using CBA coding in a manner that the decoder adaptively determines the switch point for switching to run length and signal level decoding. FIG. 7 illustrates a technique (1700) according to one implementation for decoding coefficients. The adaptive switch point depends on the count of consecutive zero value coefficients in the data, which is the signal level for the quantized transform coefficients that progress from the lowest frequency coefficient to the highest frequency coefficient. Alternatively, the data is another form and / or organization of audio data.

  Initially, the encoder initializes several variables. Specifically, the decoder sets the run count to 0 (1710) and the decoding state directly to context-based arithmetic (DCBA) (1712).

  The decoder references the number used by the encoder to represent the coefficient in arithmetic coding and extracts the value of the coefficient from that number, thereby decoding the next quantized coefficient using DCBA (1720). Next, the decoder updates the run count based on the decoded coefficient (1730) (specifically, resets the run count based on whether the decoded coefficient is zero). Or increment).

  The decoder checks (1740) whether the run count exceeds a threshold. The run count threshold is static or depends on factors such as the size of the block of coefficients (eg, four zeros in an input sequence of 256 coefficients), or in some other way It is possible to adapt. If the run count exceeds the threshold, the decoder begins decoding 1750 the coefficients that were encoded using run level decoding. Run level decoding is performed in several ways including, for example, Huffman decoding, vector Huffman decoding, or CBA decoding (possibly with a different context than the previous CBA decoding described above). It is possible.

In some embodiments, run level decoding is used where one table is used to decode data that is more likely to be a shorter run and another table is likely to be a longer run. This is done using Huffman coding with two possible Huffman code tables used to decode high data. When the decoder receives a code, the signal bit in the code indicates which table the encoder used, and the decoder looks up the code in the appropriate table.
If the run count does not exceed the threshold, the decoder continues to process the coefficients until decoding is complete (1760).

VI. Table-less coding
In some embodiments using Huffman coding, an encoder such as the encoder (200) of FIG. 2 uses an escape code for the Huffman code table to table a particular symbol (or combination of symbols). Indicates that there is no associated code. Occasionally, an escape code is a specific symbol (eg, a level not represented in the scalar Huffman code table for the level, a run length not represented in the scalar Huffman code table for the run length. Is used to indicate that it should be encoded without using the code from the Huffman table. In other words, the symbol will be encoded using a “table-less” encoding technique.

  In some embodiments that use arithmetic encoding, escape codes are sometimes used to indicate that a particular symbol should not be arithmetic encoded. The symbols can be encoded using codes from the Huffman table, or can be encoded using "tableless" encoding techniques.

  Some tableless coding techniques represent symbols using a fixed-length code. However, using a fixed-length code can result in an unnecessarily long code.

  Thus, in some embodiments, in a tableless encoding technique, symbols such as quantized transform coefficients are represented using variable length codes if they are not otherwise encoded. A decoder such as the decoder (300) of FIG. 3 performs a decoding technique without a corresponding table.

  For example, Table 6 shows pseudo code for one implementation of an encoding technique without such a table.

Table 6: Pseudocode for tableless encoding technique in one implementation
If (value <2 ⁸ ) {
Send “0”;
Send value using 8 bits;
}
else if (value <2 ¹⁶ ) {
Send “10”;
Send value using 16 bits
}
else if (value <2 ²⁴ ) {
Send “110”;
Send value using 24 bits;
}
else if (value <2 ³¹ ) {
Send “111”;
Send value using 31 bits;
}

The number of bits that the encoder uses to encode a coefficient depends on the value of the coefficient. The encoder indicates the number of bits used to encode the value by sending a 1-bit value, 2-bit value, or 3-bit value, then 8 bits, 16 bits, 24 bits, Or send the value itself encoded using 31 bits. The total number of bits used by the encoder to encode the coefficients ranges from 9 bits for values less than 2 ⁸ to 34 bits for values less than 2 ³¹ but greater than or equal to 2 ^24. is there.

For a series of coefficients, the average bit sent is
P (0 ≦ C <2 ⁸ ) ^* 9 + P (2 ⁸ ≦ C <2 ¹⁶ ) ^* 18 + P (2 ¹⁶ ≦ C <2 ²⁴ ) ^* 27 + P (2 ²⁴ ≦ C <2 ³¹ ) ^* 34
Where P (m ≦ C <n) is the probability of occurrence of coefficient C within the indicated range in the input sequence. Thus, if the large percentage of coefficients is small (eg, less than 2 ¹⁶ ), significant bit savings are possible.

  Alternatively, the encoder and decoder use an encoding / decoding technique without a separate table.

  While the principles of the invention have been described and illustrated in connection with various previous embodiments, it will be appreciated that the configuration and details of the foregoing embodiments may be changed without departing from such principles. It is to be understood that the programs, processes, or methods described herein are not related or limited to any particular type of computing environment, unless explicitly stated otherwise. Various types of general purpose or specialized computing environments can be used with or perform operations in accordance with the teachings described herein. Elements of the above-described embodiments shown in software can be implemented in hardware and vice versa.

  In view of the many possible embodiments to which the principles of the present invention may be applied, all embodiments that may fall within the scope and spirit of the appended claims, as well as equivalent forms, are described herein. Claim as an invention.

100 computing environment 110 processing unit 120 memory 130 basic configuration 140 storage 150 input device 160 output device 170 communication connection 180 software

Claims (12)

In a computer system, a method for decoding data comprising a plurality of encoded vectors, comprising:
Decoding the first code, and decoding the first code comprises:
Receiving the first code;
Looking up the first code in a first code table in a group of code tables;
Determining whether the first code is an escape code from the first code table;
If the first code is the escape code from the first code table, a second code not included in the first code table is received and the second code is decoded And steps to
If the first code is not the escape code from the first code table, look up a first vector having a first number of symbols in the first code table; Including the first number of symbols of a vector in a decoded data stream, wherein the first vector is associated with the first code of the first code table. And comprising steps
The value of the first number is a criterion as to whether the first code is the escape code from the first code table or not the escape code;
The second code is associated with a second vector having a second number of symbols in a second code table;
The method of claim 1, wherein the first number differs from the second number by a factor of two. Decoding the second code comprises:
Looking up the second code in the second code table of the group of code tables;
Determining whether the second code is an escape code from the second code table;
If the second code is the escape code from the second code table, a third code not included in the second code table is received and the third code is decoded And steps to
If the second code is not the escape code from the second code table, look up the second vector having the second number of symbols in the second code table; Including the second number of symbols of the second vector in the decoded data stream;
The method of claim 1, wherein the second code table is different from the first code table. The second code is the escape code from the second code table;
The third code is associated with a third vector having one symbol;
Decoding the third code comprises finding the one symbol using a tableless decoding technique and including the one symbol in the decoded data stream. 3. A method according to claim 2, characterized in that The first number is four;
The first code table includes: (a) a plurality of associations associating a value of the first code with a combination of symbols each having four symbols; and (b) the escape code from the first code table. And
The group of code tables includes the second code table;
The second code table includes: (c) a plurality of associations that associate a value of the second code with a combination of symbols each having two symbols; and (d) an escape code from the second code table; With
The escape code from the second code table comprises: (e) a plurality of associations for associating the value of the third code with individual symbols; and (f) an escape code from the third code table. Show the transition to decoding of individual symbols using a code table,
The method of claim 1, wherein the escape code from the third code table indicates a transition to decoding individual symbols using a tableless decoding technique. A method for encoding data comprising a plurality of vectors in a computer system, comprising:
Encoding a first vector having a first number of symbols, encoding the first vector comprises:
Determining whether a code representing the first vector is included in a first code table in a group of a plurality of code tables;
Including an escape code from the first code table in the encoded data stream if the first code table does not include the code representing the first vector;
Including the code representing the first vector in the encoded data stream when the code representing the first vector is included in the first code table;
The value of the first number is a criterion as to whether or not the code representing the first vector is included in the first code table;
The code representing the first vector is not included in the first code table, and the method includes:
Encoding a second vector having a second number of symbols, encoding the second vector comprises the steps of:
Determining whether a second code table in the group of code tables includes a code representing the second vector;
Including an escape code from the second code table in the encoded data stream if the second code table does not include the code representing the second vector;
Including the code representing the second vector in the encoded data stream when the code representing the second vector is included in the second code table;
The value of the second number is a criterion as to whether or not the code representing the second vector is included in the second code table;
The second code table is different from the first code table,
The method wherein the first number differs from the second number by a factor of two. The code representing the second vector is not included in the second code table,
The method
Encoding a third vector having one symbol using a tableless encoding technique;
6. The method of claim 5, further comprising including an encoded representation of the one symbol in the encoded data stream. The first number is four;
The first code table includes: (a) a plurality of associations associating a value of the first code with a combination of symbols each having four symbols; and (b) the escape code from the first code table. And
The group of code tables includes the second code table;
The second code table includes: (c) a plurality of associations that associate a value of the second code with a combination of symbols each having two symbols; and (d) an escape code from the second code table; With
The escape code from the second code table comprises: (e) a plurality of associations for associating the value of the third code with individual symbols; and (f) an escape code from the third code table. Show the transition to encoding of individual symbols using the code table,
6. The method of claim 5, wherein the escape code from the third code table indicates a transition to encoding individual symbols using a tableless encoding technique.   A computer-readable storage medium storing a program for causing a computer to execute the method according to claim 5. A computer-readable storage medium storing a program for causing a computer to execute a method of decoding data comprising a plurality of encoded vectors,
Decoding the first code, and decoding the first code comprises:
Receiving the first code;
Looking up the first code in a first code table in a group of code tables;
Determining whether the first code is an escape code from the first code table;
If the first code is the escape code from the first code table, a second code not included in the first code table is received and the second code is decoded And steps to
If the first code is not the escape code from the first code table, look up a first vector having a first number of symbols in the first code table; Including the first number of symbols of a vector in a decoded data stream, wherein the first vector is associated with the first code of the first code table. And comprising steps
The value of the first number is a criterion as to whether the first code is the escape code from the first code table or not the escape code;
The second code is associated with a second vector having a second number of symbols in a second code table;
The computer-readable storage medium, wherein the first number differs from the second number by a factor of two. Decoding the second code comprises:
Looking up the second code in the second code table of the group of code tables;
Determining whether the second code is an escape code from the second code table;
If the second code is the escape code from the second code table, a third code not included in the second code table is received and the third code is decoded And steps to
If the second code is not the escape code from the second code table, look up the second vector having the second number of symbols in the second code table; Including the second number of symbols of the second vector in the decoded data stream;
The computer-readable storage medium according to claim 9, wherein the second code table is different from the first code table. The second code is the escape code from the second code table;
The third code is associated with a third vector having one symbol;
Decoding the third code comprises finding the one symbol using a tableless decoding technique and including the one symbol in the decoded data stream. The computer-readable storage medium according to claim 10. The first number is four;
The first code table includes: (a) a plurality of associations associating a value of the first code with a combination of symbols each having four symbols; and (b) the escape code from the first code table. And
The group of code tables includes the second code table;
The second code table includes: (c) a plurality of associations that associate a value of the second code with a combination of symbols each having two symbols; and (d) an escape code from the second code table; With
The escape code from the second code table comprises: (e) a plurality of associations for associating the value of the third code with individual symbols; and (f) an escape code from the third code table. Show the transition to decoding of individual symbols using a code table,
The computer-readable storage medium of claim 9, wherein the escape code from the third code table indicates a transition to decoding individual symbols using a tableless decoding technique.

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