APPARATUSES AND METHODS FOR ENCODING AND DECODING A MULTICHANNEL AUDIO SIGNAL

An apparatus and a method for encoding an input audio signal are disclosed. The input audio signal comprises a plurality of input audio channels. Metadata associated with a plurality of KLT eigenchannels allows reconstructing the plurality of input audio channels on the basis of the plurality of eigenchannels. A subset of the plurality of eigenchannels is encoded. The metadata is encoded and provided in a quantized form. The plurality of input audio channels is transformed into the plurality of eigenchannels on the basis of the metadata in the quantized form.

TECHNICAL FIELD

The invention relates to the field of audio signal processing. More specifically, the invention relates to apparatuses and methods for encoding and decoding a multichannel audio signal on the basis of the Karhunen-Loève Transform (KLT).

BACKGROUND

In the field of multichannel spatial audio coding the two following challenges will most likely become more prominent in the future: (i) the processing an input audio signal with an arbitrary number of recorded audio channels and (ii) the handling of a plurality of arbitrarily placed microphones, in particular with respect to angles. One reason for this development is the current trend of providing more and more advanced audio recording devices, such as the Eigenmike. Moreover, another current trend is the use of various conventional recording devices at the same time for producing a multichannel audio signal. Thus, there is a need for a generic audio coding scheme that is able to meet the challenges mentioned above.

Currently, activities in multichannel audio coding for streaming and storage purposes are gaining popularity due to the many possible new applications in the field of immersive sound, such as applications for cinemas, virtual reality, telepresence and the like. Exemplary current multichannel audio codecs are Dolby Atmos using a multichannel object based coding, MPEG-H 3D Audio, which incorporates channel objects and Ambisonics-based coding. These current existing multichannel codecs, however, are still limited to some specific numbers of audio channel, such as 5.1, 7.1 or 22.2 channels, as required by industrial standards, such as ITU-R BS.2159-4.

An approach for processing an input audio signal with an arbitrary number of recorded audio channels is based on the Karhunen-Loève Transform (KLT), as disclosed in Yang et al., “High-Fidelity Multichannel Audio Coding with Karhunen-Loève Transform”, IEEE Trans. on Speech and Audio Proc., Vol. 11, No. 4, July 2003. Conventional KLT-based audio coding approaches have the drawback that generally a high metadata bitrate is required for allowing reconstructing the original audio signal with a sufficient perceptual quality on the basis of the compressed audio signal. This is because there is a trade-off between the audio quality and the metadata bitrate, wherein a higher metadata bitrate implies a better audio quality and vice versa. Thus, lowering the metadata bitrate will eventually affect the compressed audio quality.

Thus, there is a need for an improved KLT-based apparatus and method for encoding an multichannel audio signal, which in comparison to conventional apparatuses and methods provides an improved audio quality for similar or lower metadata bitrates.

SUMMARY

It is an object of the invention to provide an improved KLT-based apparatus and method for encoding a multichannel audio signal, which in comparison to conventional apparatuses and methods provides an improved audio quality for similar or lower metadata bitrates.

The foregoing and other objects are achieved by the subject matter of the independent claims. Further implementation forms are apparent from the dependent claims, the description and the figures.

According to a first aspect the invention relates to an apparatus for encoding an input audio signal, wherein the input audio signal is a multichannel audio signal, i.e. comprises a plurality of input audio channels. The apparatus comprises a pre-processor based on the Karhunen-Loève transformation (KLT), i.e. a KLT-based pre-processor. The KLT-based pre-processor is configured to transform the plurality of input audio channels into a plurality of eigenchannels and to provide metadata associated with the plurality of eigenchannels, wherein the metadata allows reconstructing the plurality of input audio channels on the basis of the plurality of eigenchannels. The apparatus further comprises an eigenchannel encoder configured to encode a subset of the plurality of eigenchannels, and a metadata encoding unit configured to encode the metadata and to provide the metadata in a quantized form. The metadata encoding unit is configured to feed the metadata in the quantized form back to the KLT-based pre-processor and the KLT-based pre-processor is configured to transform the plurality of input audio channels into the plurality of eigenchannels on the basis of the metadata in the quantized form.

In a first implementation form of the apparatus according to the first aspect as such, the metadata comprises one or more of: a covariance matrix of the plurality of input audio channels and an eigenvector of the covariance matrix.

In a second implementation form of the apparatus according to the first aspect as such or its first implementation form, the metadata encoding unit comprises a metadata encoder and a metadata decoder, wherein the metadata encoder is configured to encode the metadata and wherein the metadata decoder is configured to provide the metadata in the quantized form by decoding the encoded metadata.

In a third implementation form of the apparatus according to the first aspect as such or its first implementation form, the metadata encoding unit comprises a metadata encoder, wherein the metadata encoder is configured to encode the metadata and to provide the metadata in the quantized form.

In a fourth implementation form of the apparatus according to the first aspect as such or any one of the first to third implementation form thereof, the metadata encoding unit is a lossy encoding unit.

In a fifth implementation form of the apparatus according to the first aspect as such or any one of the first to fourth implementation form thereof, wherein the KLT-based pre-processor is configured to transform the plurality of input audio channels into the plurality of eigenchannels on the basis of the metadata in the quantized form by performing a matrix multiplication.

In a sixth implementation form of the apparatus according to the first aspect as such or any one of the first to fifth implementation form thereof, the input audio signal comprises a plurality of frequency bands and the apparatus is configured to encode the input audio signal separately in the different frequency bands.

In a seventh implementation form of the apparatus according to the first aspect as such or any one of the first to sixth implementation form thereof, the KLT-based pre-processor is configured to transform the plurality of input audio channels into the plurality of eigenchannels on the basis of the metadata in the quantized form by optimizing a perceptual performance measure.

In an eighth implementation form of the apparatus according to the first aspect as such or any one of the first to seventh implementation form thereof, the apparatus is configured to encode the input audio signal in a frame-wise manner and the metadata encoding unit is configured to encode the metadata only every N-th frame, wherein N is an integer greater than 1.

According to the second aspect the invention relates to a method for encoding an input audio signal, wherein the input audio signal comprises a plurality of input audio channels. The method comprises providing by a KLT-based pre-processor, which is configured to transform the plurality of input audio channels into a plurality of eigenchannels, metadata associated with the plurality of eigenchannels, wherein the metadata allows reconstructing the plurality of input audio channels on the basis of the plurality of eigenchannels, encoding the metadata and providing the metadata in a quantized form, feeding the metadata in the quantized form back to the KLT-based pre-processor, transforming the plurality of input audio channels into the plurality of eigenchannels on the basis of the metadata in the quantized form, and encoding a subset of the plurality of eigenchannels.

The encoding method according to the second aspect of the invention can be performed by the encoding apparatus according to the first aspect of the invention. Further features of the encoding method according to the second aspect of the invention result directly from the functionality of the encoding apparatus according to the first aspect of the invention and its different implementation forms.

According to a third aspect the invention relates to a computer program comprising program code for performing the encoding method according to the second aspect of the invention when executed on a computer.

The invention can be implemented in hardware and/or software.

In the various figures, identical reference signs will be used for identical or at least functionally equivalent features.

DETAILED DESCRIPTION OF EMBODIMENTS

In the following description, reference is made to the accompanying drawings, which form part of the disclosure, and in which are shown, by way of illustration, specific aspects in which the invention may be placed. It will be appreciated that the invention may be placed in other aspects and that structural or logical changes may be made without departing from the scope of the invention. The following detailed description, therefore, is not to be taken in a limiting sense, as the scope of the invention is defined by the appended claims.

For instance, it will be appreciated that a disclosure in connection with a described method will generally also hold true for a corresponding device or system configured to perform the method and vice versa. For example, if a specific method step is described, a corresponding device may include a unit to perform the described method step, even if such unit is not explicitly described or illustrated in the figures.

Moreover, in the following detailed description as well as in the claims, embodiments with functional blocks or processing units are described, which are connected with each other or exchange signals. It will be appreciated that the invention also covers embodiments which include additional functional blocks or processing units that are arranged between the functional blocks or processing units of the embodiments described below.

Finally, it is understood that the features of the various exemplary aspects described herein may be combined with each other, unless specifically noted otherwise.

FIG. 1shows a schematic diagram of a conventional audio coding system100comprising an apparatus110for encoding a multichannel audio signal and an apparatus120for decoding the encoded multichannel audio signal. The encoding apparatus110and the decoding apparatus120implement a KLT-based audio coding approach. Further details about this approach are described in Yang et al., “High-Fidelity Multichannel Audio Coding with Karhunen-Loève Transform”, IEEE Trans. on Speech and Audio Proc., Vol. 11, No. 4, July 2003, which is hereby incorporated by reference in its entirety.

FIG. 2shows a schematic diagram of a KLT-based audio coding system200including an encoding apparatus210according to an embodiment. The apparatus210is configured to encode an input audio signal having Q input audio channels. To this end, the encoding apparatus210comprises a KLT-based pre-processor211configured to transform the Q input audio channels into P eigenchannels (also referred to as transform coefficients) and to provide metadata associated with the P eigenchannels, which allows reconstructing the Q input audio channels on the basis of the P eigenchannels. The number of P-channels is expected to be much lower than Q.

Moreover, the encoding apparatus210comprises an eigenchannel encoder213configured to encode the P eigenchannels and a metadata encoding unit215configured to encode the metadata and to provide the metadata in a quantized form. The metadata encoding unit215is configured to feed the metadata in the quantized form back to the KLT-based pre-processor211. The KLT-based pre-processor211is configured to transform the plurality of input audio channels into the plurality of eigenchannels on the basis of the metadata in the quantized form. Accordingly, the KLT-based pre-processor211is enabled to use the metadata in the quantized form rather than the original, unquantized metadata to transform the plurality of input audio channels into the plurality of eigenchannels. This improves the coding accuracy. Therefore a higher compression ratio can be achieved for a given desired audio quality level of the compressed audio, or the audio quality can be improved for a given compression ratio or bitrate of the compressed audio. In short, the compression scheme is improved.

In an embodiment, the metadata comprises the covariance matrix of the plurality of input audio channels or at least the non-redundant elements thereof and/or the eigenvectors of the covariance matrix.

As will be appreciated, the encoding apparatus210implements a kind of serial or staged encoding process, as has been indicated inFIG. 2by the four stages identified by the encircled numerals1to4.

In stage1the metadata provided by the KLT-based pre-processor211is fed to the metadata encoding unit215. In the embodiment shown inFIG. 2the metadata encoding unit215comprises a metadata encoder216and a metadata decoder217. The metadata encoder216provides a metadata bitstream, which is ready to be stored or transmitted to the metadata decoder125of the decoding apparatus120.

In stage2the metadata bitstream is fed to the metadata decoder217, which outputs in response thereto the metadata in a quantized form.

In stage3the metadata in the quantized form is fed back to the KLT-based pre-processor211.

In stage4the KLT-based pre-processor211transforms the Q input audio channels into the P eigenchannels on the basis of the metadata in the quantized form provided by the metadata decoder217. In an embodiment, the KLT-based pre-processor211is configured to transform the Q input audio channels into the P eigenchannels on the basis of the metadata in the quantized form by performing a matrix multiplication based on the covariance matrix. The KLT-based pre-processor211is configured to provide these P eigenchannels, which have been obtained on the basis of the original Q input audio channels and the quantized metadata, to the eigenchannel encoder213.

FIG. 3shows a schematic diagram of the KLT-based audio coding system200including the encoding apparatus210according to another embodiment. The encoding apparatus210shown inFIG. 3differs from the encoding apparatus210shown inFIG. 2in that the metadata encoding unit215comprises a modified metadata encoder216′, which is configured to encode the metadata and to provide the metadata in the quantized form. To this end, the modified metadata encoder216′ of the encoding apparatus210shown inFIG. 3comprises a quantizer216′aand a bitstream generator216′b.In other words, in the embodiment shown inFIG. 3the quantized metadata is a byproduct of the metadata encoding process without the need for a metadata decoder.

The innovation allows providing a synergistic effect between the metadata encoding unit215and the eigenchannel encoder213, which allows for an improved error compensation mechanism at the encoder side. This is because the invention shifts the quantization error, which cannot be masked perceptually by the metadata encoding unit215, to the P eigenchannels, which can be considered as audio channels and processed in an error correcting manner using a perceptual auditory mask. Thus, in an embodiment, the KLT-based pre-processor211is configured to transform the plurality of input audio channels into the plurality of eigenchannels on the basis of the metadata in the quantized form by optimizing a perceptual performance measure. Moreover, in an embodiment, the metadata encoding unit215is a lossy encoding unit.

In an embodiment, the input audio signal comprises a plurality of frequency bands and the encoding apparatus210is configured to encode the input audio signal separately in the different frequency bands.

In an embodiment, the encoding apparatus210is configured to encode the input audio signal in a frame-wise manner and the metadata encoding unit215is configured to encode the metadata only every N-th frame, wherein N is an integer greater than 1.

FIG. 4shows a schematic diagram illustrating a method400for encoding a multichannel audio signal according to an embodiment. The method400comprises the following steps: providing401by the KLT-based pre-processor211, which is configured to transform the plurality of input audio channels into a plurality of eigenchannels, metadata associated with the plurality of eigenchannels, wherein the metadata allows reconstructing the plurality of input audio channels on the basis of the plurality of eigenchannels, encoding403the metadata and providing the metadata in a quantized form, feeding405the metadata in the quantized form back to the KLT-based pre-processor211, transforming406the plurality of input audio channels into the plurality of eigenchannels on the basis of the metadata in the quantized form; and encoding407a subset of the plurality of eigenchannels.

While a particular feature or aspect of the disclosure may have been disclosed with respect to only one of several implementations or embodiments, such feature or aspect may be combined with one or more other features or aspects of the other implementations or embodiments as may be desired and advantageous for any given or particular application. Furthermore, to the extent that the terms “include”, “have”, “with”, or other variants thereof are used in either the detailed description or the claims, such terms are intended to be inclusive in a manner similar to the term “comprise”. Also, the terms “exemplary”, “for example” and “e.g.” are merely meant as an example, rather than the best or optimal. The terms “coupled” and “connected”, along with derivatives may have been used. It should be understood that these terms may have been used to indicate that two elements cooperate or interact with each other regardless whether they are in direct physical or electrical contact, or they are not in direct contact with each other.

Many alternatives, modifications, and variations will be apparent to those skilled in the art in light of the above teachings. Of course, those skilled in the art readily recognize that there are numerous applications of the invention beyond those described herein. While the invention has been described with reference to one or more particular embodiments, those skilled in the art recognize that many changes may be made thereto without departing from the scope of the invention. It is therefore to be understood that within the scope of the appended claims and their equivalents, the invention may be practiced otherwise than as specifically described herein.