Coded data generation or conversion – Digital code to digital code converters – Adaptive coding
Reexamination Certificate
1999-07-08
2001-06-12
Williams, Howard L. (Department: 2819)
Coded data generation or conversion
Digital code to digital code converters
Adaptive coding
C704S501000, C704S229000, C348S384100
Reexamination Certificate
active
06246345
ABSTRACT:
TECHNICAL FIELD
The present invention relates generally to encoding and decoding signals. The present invention may be used advantageously for split-band encoding and decoding in which frequency-subband signals are separately coded. The present invention is particularly useful in perceptual audio coding systems.
BACKGROUND ART
There is a continuing interest to encode digital audio signals in a form that imposes low information capacity requirements on transmission channels and storage media yet can convey the encoded audio signals with a high level of subjective quality. Perceptual coding systems attempt to achieve these conflicting goals by using a process that encodes and quantizes the audio signals in a manner that uses larger spectral components within the audio signal to mask or render inaudible the resultant quantizing noise. Generally, it is advantageous to control the shape and amplitude of the quantizing noise spectrum so that it lies just below the psychoacoustic masking threshold of the signal to be encoded.
A perceptual encoding process may be performed by a so called split-band encoder that applies a bank of analysis filters to the audio signal to obtain subband signals having bandwidths that are commensurate with the critical bands of the human auditory system, estimates the masking threshold of the audio signal by applying a perceptual model to the subband signals or to some other measure of audio signal spectral content, establishes quantization step sizes for quantizing the subband signals that are just small enough so that the resultant quantizing noise lies just below the estimated masking threshold of the audio signal, quantizes the subband signals according to the established quantization step sizes, and assembles into an encoded signal a plurality of symbols that represent the quantized subband signals. A complementary perceptual decoding process may be performed by a split-band decoder that extracts the symbols from the encoded signal and recovers the quantized subband signals therefrom, obtains dequantized representations of the quantized subband signals, and applies a bank of synthesis filters to the dequantized representations to generate an audio signal that is, ideally, perceptually indistinguishable from the original audio signal.
The coding processes in these coding systems often use a uniform length symbol to represent the quantized signal elements or components in each subband signal. Unfortunately, the use of uniform length symbols imposes a higher information capacity than is necessary. The required information capacity can be reduced by using non-uniform length symbols to represent the quantized components in each subband signal.
One technique for providing non-uniform length symbols is Huffman encoding of quantized subband-signal component. Typically, Huffman code tables are designed using “training signals” that have been selected to represent the signals to be encoded in actual applications. Huffman coding can provide very good coding gain if the average probability density function (PDF) of the training signals are reasonably close to the PDF of the actual signal to be encoded, and if the PDF is not flat.
If the PDF of the actual signal to be encoded is not close to the average PDF of the training signals, Huffman coding will not realize a coding gain but may incur a coding penalty, increasing the information capacity requirements of the encoded signal. This problem can be minimized by using multiple code books corresponding to different signal PDFs; however, additional storage space is required to store the code books and additional processing is required to encode the signal according to each code book and then pick the one that provides the best results.
There remains a need for a coding technique that can represent blocks of quantized subband-signal components using non-uniform length symbols within each subband, that is not dependent upon any particular PDF of component values, and can be performed efficiently using minimal computational and memory resources.
DISCLOSURE OF INVENTION
It is an object of the present invention to provide for the advantages that can be realized by using non-uniform length symbols to represent quantized signal components such as subband-signal components within a respective frequency subband in a split-band coding system.
The present invention achieves this object using a technique that does not depend upon any particular PDF of component values to achieve good coding gain and can be performed efficiently using minimal computational and memory resources. In some applications, coding systems may advantageously use features of the present invention in conjunction with other techniques like Huffman coding.
According to the teachings of one aspect of the present invention, a method for encoding an input signal comprises receiving the input signal and generating a subband-signal block of subband-signal components representing a frequency subband of the input signal; comparing magnitudes of the components in the subband-signal block with a threshold, placing each component into one of two or more classes according to component magnitude, and obtaining a gain factor; applying the gain factor to the components placed into one of the classes to modify the magnitudes of some of the components in the subband-signal block; quantizing the components in the subband-signal block; and assembling into an encoded signal control information conveying classification of the components and non-uniform length symbols representing the quantized subband-signal components.
According to the teachings of another aspect of the present invention, a method for decoding an encoded signal comprises receiving the encoded signal and obtaining therefrom control information and non-uniform length symbols, and obtaining from the non-uniform length symbols quantized subband-signal components representing a frequency subband of an input signal; dequantizing the subband-signal components to obtain subband-signal dequantized components; applying a gain factor to modify magnitudes of some of the dequantized components according to the control information; and generating an output signal in response to the subband-signal dequantized components.
These methods may be embodied in a medium as a program of instructions that can be executed by a device to carry out the present invention.
According to the teachings of another aspect of the present invention, an apparatus for encoding an input signal comprises an analysis filter having an input that receives the input signal and having an output through which is provided a subband-signal block of subband-signal components representing a frequency subband of the input signal; a subband-signal block analyzer coupled to the analysis filter that compares magnitudes of the components in the subband-signal block with a threshold, places each component into one of two or more classes according to component magnitude, and obtains a gain factor, a subband-signal component processor coupled to the subband-signal block analyzer that applies the gain factor to the components placed into one of the classes to modify the magnitudes of some of the components in the subband-signal block; a first quantizer coupled to the subband-signal processor that quantizes the components in the subband-signal block having magnitudes modified according to the gain factor; and a formatter coupled to the first quantizer that assembles non-uniform length symbols representing the quantized subband-signal components and control information conveying classification of the components into an encoded signal.
According to the teachings of yet another aspect of the present invention in an apparatus for decoding an encoded signal, the apparatus comprises a deformatter that receives the encoded signal and obtains therefrom control information and non-uniform length symbols, and obtains from the non-uniform length symbols quantized subband-signal components; a first dequantizer coupled to the deformatter that dequantizes some of the subband-signal c
Davidson Grant Allen
Robinson Charles Quito
Truman Michael Mead
Dolby Laboratories Licensing Corporation
Gallagher & Lathrop
Jeanglaude Jean Barnes
Lathrop David N.
Williams Howard L.
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