Coded data generation or conversion – Digital code to digital code converters
Reexamination Certificate
2003-08-18
2004-08-10
Jeanglaude, Jean Bruner (Department: 2819)
Coded data generation or conversion
Digital code to digital code converters
C341S200000
Reexamination Certificate
active
06774820
ABSTRACT:
FIELD OF INVENTION
The invention relates to the encoding and decoding of digital signal streams, particularly digital audio streams, with reference to matrixing multichannel signals.
BACKGROUND TO THE INVENTION
Lossless compression is now an established means of reducing the data rate required for storing or transmitting a digital audio signal. One method of reducing the data rate of a multichannel signal is to apply matrixing so that dominant information is concentrated in some of the transmitted channels while the other channels carry relatively little information. For example, two-channel audio may have nearly the same waveform in the left and right channels if conveying a central sound image, in which case it is more efficient to encode the sum and difference of the two channels. This process is described in some detail in WO-A 96/37048, including the use of a cascade of ‘primitive matrix quantisers’ to achieve the matrixing in a perfectly invertible or lossless manner.
The process disclosed in WO-A 96/37048 also envisages the use of matrix quantisers to apply a matrix to a multichannel original digital signal in order to derive matrixed digital signals representing speaker feeds more suitable for general domestic listening. These matrixed signals may be recorded on a carrier such as a DVD, and the ordinary player will simply feed each matrixed signal to a loudspeaker. The advanced player, however, may invert the effect of the matrix quantisers and thus reconstruct the original digital signal exactly in order to reproduce it in an alternative manner.
In a commercial application of DVD-Audio there is a requirement to combine the above two concepts so that a transmission system using lossless compression may also provide both a matrixed signal and an original signal. In this application the required matrixed signal has two channels whereas the original signal has more than two channels, thus additional information must be provided to allow the multichannel signal to be recovered; however, the additional information should not impose a computational overhead for decoders that wish to decode the two-channel matrixed signal only.
Currently, digital audio is often transmitted with 24 bits, and popular Digital Signal Processing (DSP) chips designed for audio such as the Motorola 56000 series also easily handle a 24-bit word. However the processing described in WO-A 96/37048 can generate numbers requiring a word width greater than the original signal. Because the use of ‘double-precision’ computation is prohibitively expensive, a method is needed to allow the processing to be substantially carried out while not requiring an increased word width.
Finally the consumer, having bought equipment designed to provide lossless reproduction, would like reassurance that the signal recovered is indeed lossless. Conventional parity and CRC checks within the encoded stream will show errors due to data corruption within the stream, but they will not expose errors due to matrixing or other algorithmic mismatch between an encoder and a decoder.
SUMMARY OF THE INVENTION
According to a first aspect of the invention, there is provided a stream divided into two substreams, the first substream providing information relating to a ‘downmix’ signal obtained by matrixing and containing fewer channels than an original multichannel digital signal, and the second substream providing additional information allowing the original multichannel digital signal to be losslessly recovered by a decoder. In the context where both substreams are conveyed using lossless compression, a decoder that decodes only the downmix signal needs to decompress the first substream only and can therefore use fewer computational resources than are required to decode the multichannel digital signal.
In a variant of this first aspect, the first substream may be replaced by a plurality of substreams. allowing a plurality of different matrixed presentations to be selected. Again however, the last substream will contain additional information that allows a complete original multichannel digital signal to be reproduced losslessly.
In a preferred implementation of the first aspect an encoder furnishes the downmix signal using a cascade of one or more primitive matrix quantisers, each of which implements an n by n matrix, followed by selection of the m channels required for the downmix.
A multichannel decoder will take the signals from both substreams and apply a cascade of inverse primitive matrices in order to recover the original multichannel signal. It might be considered natural to order the channels that are input to the decoder's cascade so that the channels from the first substream are placed at the beginning. However this may result in incorrect channel ordering at the output of the decoder's cascade, so preferably a channel permutation is specified by the encoder and implemented by the decoder to recover the correct channel order.
Preferably, any truncation or rounding within the matrixing should be computed using dither. In this case, for lossless coding, the dither signal must be made available to the decoder in order that it may invert the computations performed by the encoder and thus recover the original signal losslessly. The dither may be computed using an ‘autodither’ method as envisaged in WO-A 96/37048; but in the context of a lossless compression scheme, autodither can be avoided by providing a dither seed in the encoded stream that allows a decoder to synchronize its dithering process to that which was used by the encoder.
Therefore according to a second aspect of the invention, there is provided a lossless compression system including a dither seed in the encoded bitstream. The dither seed is used to synchronise a pseudo-random sequence generator in the decoder with a functionally identical generator in an encoder.
In an important application of the invention, the downmix has two channels, and is most conveniently derived by the application of two primitive matrix quantisers to the original multichannel digital signal. In embodiments that implement the second aspect of the invention, dither is required by each quantiser; moreover different dither should be provided for the two quantisers and the preferred probability distribution function (PDF) for each dither is triangular. An efficient way to furnish two such triangular PDF (TPDF) dither signals, which is referred to herein as ‘diamond dither’, is to add and subtract two independent rectangular PDF (RPDF) signals. For further details and generalisation to more channels, see R. Wannamaker, “Efficient Generation of Multichannel Dither Signals”, AES 103rd Convention, New York, 1997, preprint no. 4533.
Accordingly, in a preferred implementation of the second aspect, the encoder uses a single sequence generator to furnish two independent RPDF dither signals, and the sum and difference of these signals is used to provide the dither required by two primitive matrix quantisers used to derive a two-channel downmix.
WO-A 96/37048 describes the use of primitive matrix quantisers within a lossless compression system. and above we have referred to a preferred implementation of the first aspect, which also uses primitive matrix quantisers in order to place the information required for a ‘downmix’ signal into a separate substream.
Accordingly, in a third aspect of the invention there are provided encoders and decoders containing uncommitted primitive matrix quantisers, the encoder having logic that accepts a downmix specified as a matrix of coefficients, allocates a number of primitive matrix quantisers to furnish the downmix and optionally allocates a further number to provide matrixing to reduce the data rate. The encoder furnishes a stream containing specifications of the primitive matrix quantisers to be used, and optionally may include the addition of dither. In a preferred implementation, the dither is generated as two RPDF dither sequences, and the encoder specifies a coefficient for each dither sequence. Diamond dither is thus obtainable by specifying two coeffici
Craven Peter Graham
Law Malcolm James
Stuart John Robert
Dolby Laboratories Licensing Corporation
Gallagher & Lathrop
Jeanglaude Jean Bruner
Lathrop, Esq. David N.
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