# Patent

## Title
Time warp contour calculator, audio signal encoder, encoded audio signal representation, methods and computer program

## Abstract

A time warp contour calculator for use in an audio signal decoder receives an encoded warp ratio information, derives a sequence of warp ratio values from the encoded warp ratio information, and obtains warp contour node values starting from a time warp contour start value. Ratios between the time warp contour node values and the time warp contour starting value are determined by the warp ratio values. The time warp contour calculator computes a time warp contour node value of a given time warp contour node, on the basis of a product-formation having a ratio between the time warp contour node values of the intermediate time warp contour node and the time warp contour starting value and a ratio between the time warp contour node values of the given time warp contour node and of the intermediate time warp contour node as factors.

## Background
CROSS-REFERENCE TO RELATED APPLICATION
This application is a U.S. National Phase entry of PCT/EP2009/004756 filed Jul. 1, 2009, and claims priority to U.S. Patent Application No. 61/079,873 filed Jul. 11, 2008, and U.S. Patent Application No. 61/103,820 filed Oct. 8, 2008, each of which is incorporated herein by references hereto.


BACKGROUND OF THE INVENTION
Embodiments according to the invention are related to a time warp contour calculator. Further embodiments according to the invention are related to an audio signal encoder. Further embodiments according to the invention are related to an encoded audio signal representation. Further embodiments according to the inventions are related to methods for providing a decoded audio signal representation and for providing an encoded representation of an audio signal. Still further embodiments according to the invention are related to a computer program.
Some embodiments according to the invention are related to methods for a time warped MDCT transform coder.
In the following, a brief introduction will be given into the field of time warped audio encoding, concepts of which can be applied in conjunction with some of the embodiments of the invention.
In the recent years, techniques have been developed to transform an audio signal into a frequency domain representation, and to efficiently encode this frequency domain representation, for example taking into account perceptual masking thresholds. This concept of audio signal encoding is particularly efficient if the block lengths, for which a set of encoded spectral coefficients are transmitted, are long, and if only a comparatively small number of spectral coefficients are well above the global masking threshold while a large number of spectral coefficients are nearby or below the global masking threshold and can thus be neglected (or coded with minimum code length).
For example, cosine-based or sine-based modulated lapped transforms are often used in applications for source coding due to their energy compaction properties. That is, for harmonic tones with constant fundamental frequencies (pitch), they concentrate the signal energy to a low number of spectral components (sub-bands), which leads to an efficient signal representation.
Generally, the (fundamental) pitch of a signal shall be understood to be the lowest dominant frequency distinguishable from the spectrum of the signal. In the common speech model, the pitch is the frequency of the excitation signal modulated by the human throat. If only one single fundamental frequency would be present, the spectrum would be extremely simple, comprising the fundamental frequency and the overtones only. Such a spectrum could be encoded highly efficiently. For signals with varying pitch, however, the energy corresponding to each harmonic component is spread over several transform coefficients, thus leading to a reduction of coding efficiency.
In order to overcome this reduction of the coding efficiency, the audio signal to be encoded is effectively resampled on a non-uniform temporal grid. In the subsequent processing, the sample positions obtained by the non-uniform resampling are processed as if they would represent values on a uniform temporal grid. This operation is commonly denoted by the phrase “time warping”. The sample times may be advantageously chosen in dependence on the temporal variation of the pitch, such that a pitch variation in the time warped version of the audio signal is smaller than a pitch variation in the original version of the audio signal (before time warping). After time warping of the audio signal, the time warped version of the audio signal is converted into the frequency domain. The pitch-dependent time warping has the effect that the frequency domain representation of the time warped audio signal is typically concentrated into a much smaller number of spectral components than a frequency domain representation of the original (non time warped) audio signal.
At the decoder side, the frequency-domain representation of the time warped audio signal is converted back to the time domain, such that a time-domain representation of the time warped audio signal is available at the decoder side. However, in the time-domain representation of the decoder-sided reconstructed time warped audio signal, the original pitch variations of the encoder-sided input audio signal are not included. Accordingly, yet another time warping by resampling of the decoder-sided reconstructed time domain representation of the time warped audio signal is applied. In order to obtain a good reconstruction of the encoder-sided input audio signal at the decoder, it is desirable that the decoder-sided time warping is at least approximately the inverse operation with respect to the encoder-sided time warping. In order to obtain an appropriate time warping, it is desirable to have an information available at the decoder which allows for an adjustment of the decoder-sided time warping.
As it is typically necessitated to transfer such an information from the audio signal encoder to the audio signal decoder, it is desirable to keep a bit rate needed for this transmission small while still allowing for a reliable reconstruction of the necessitated time warp information at the decoder side.
In view of the above discussion, there is a desire to have a concept which allows for an efficient reconstruction of a time warp information on the basis of an efficiently encoded representation of the time warp information.
SUMMARY
An embodiment may have a time warp contour calculator for use in an audio signal decoder for providing a decoded audio signal representation on the basis of an encoded audio signal representation, wherein the time warp contour calculator is configured to receive an encoded warp ratio information, to derive a sequence of warp ratio values from the encoded warp ratio information, and to obtain warp contour node values starting from a time warp contour start value, wherein ratios between the time warp contour node values and the time warp contour starting value associated with a time warp contour start node are determined by the warp ratio values; and wherein the time warp contour calculator is configured to compute a time warp contour node value of a given time warp contour node, which is spaced from the time warp contour starting node by an intermediate time warp contour node on the basis of a product-formation comprising a ratio between the time warp contour node value of the intermediate time warp contour node and the time warp contour starting value and a ratio between the time warp contour node value of the given time warp contour node and the time-warp contour node value of the intermediate time warp contour node as factors.
According to an embodiment, an audio signal encoder for providing an encoded representation of an audio signal may have: a time warp contour encoder configured to receive a time warp contour information associated with the audio signal, to compute a ratio between subsequent node values of the time warp contour, and to encode the ratio between subsequent node values of the time warp contour; and a time warping signal encoder configured to obtain an encoded representation of a spectrum of the audio signal, taking into account a time warp described by the time warp contour information; wherein the encoded representation of the audio signal comprises the encoded ratios and the encoded representation of the spectrum.
According to another embodiment, an encoded audio signal representation representing an audio signal may have: an encoded frequency domain representation representing one or more time warp resampled audio channels, resampled in accordance with a time warp; and an encoded representation of a time warp contour representing the time warp, wherein the encoded representation of the time warp contour comprises a plurality of encoded time warp ratio values, wherein the time warp ratio values represent ratios between subsequent node values of the time warp contour.
According to another embodiment, a method for providing a decoded audio signal representation on the basis of an encoded audio signal representation may have the steps of: receiving an encoded warp ratio information; deriving a sequence of warp ratio values from the encoded warp ratio information; and obtaining a plurality of time warp contour node values starting from a time warp contour start value, wherein ratios between the time warp contour node values and the time warp contour starting value associated with the time warp contour starting node are determined by the warp ratio values; wherein a time warp contour node value of a given time warp contour node, which is spaced from the time warp contour starting node by an intermediate time warp contour node, is computed on the basis of a product-formation, comprising a ratio between the time warp contour node value of the intermediate time warp contour node and the time warp contour starting value and a ratio between the time warp contour node value of the given time warp contour node and the time warp contour node value of the intermediate time warp contour node as factors.
According to still another embodiment, a method for providing an encoded representation of an audio signal, may have the steps of: receiving a time warp contour information associated with the audio signal; computing a ratio between subsequent node values of the time warp contour; encoding the ratio between subsequent node values of the time warp contour; and obtaining an encoded representation of a spectrum of the audio signal, taking into account a time warp described by the time warp contour information; wherein the encoded representation of the audio signal comprises the encoded ratios and the encoded representation of the spectrum.
Another embodiment may have a computer program for performing the above methods, when the computer program runs on a computer.
An embodiment according to the invention creates a time warp contour calculator for use in an audio signal decoder for providing a decoded audio signal representation on the basis of an encoded audio signal representation. The time warp contour calculator is configured to receive an encoded warp ratio information, to derive a sequence of warp ratio values from the encoded warp ratio information, and to obtain warp contour node values starting from a time warp contour start value. Ratios between the time warp contour node values (i.e. values of time warp contour nodes other than the time warp contour start node) and the time warp contour starting value associated with a time warp contour start node are determined by the warp ratio values. The time warp contour calculator is configured to compute a time warp contour node value of a given time warp contour node, which is spaced from the time warp contour starting node by an intermediate time warp contour node, on the basis of a product formation comprising a ratio between the time warp contour node value of the intermediate time warp contour node and the time warp contour starting value and a ratio between the time warp contour node value of the given time warp contour node and the time warp contour node value of the intermediate time warp contour node as factors.
This embodiment of the invention is based on the key idea that an efficient encoding of a time warp contour can be obtained if ratios between subsequent time contour node values are encoded in the form of an encoded warp ratio information. It has been found that a relative change (i.e. ratio) between (time warp contour) node values of two subsequent time warp contour nodes is a quantity which can be encoded in a bit-efficient form without seriously degrading a reconstruction of the time warp contour. For example, it has been found that ratios between time warp contour node values of subsequent time warp contour nodes typically cover the same range of values irrespective of the absolute value of the time warp contour, such that the encoding of the warp ratio values can be chosen independent from a current absolute value of the time warp contour. The time warp contour node values are computed on the basis of a product formation, such that a time warp contour node value of a new time warp contour node is derived from a node value of a previous time warp contour node by a product formation (i.e. multiplication). In this way, it is insured that a relative difference between time warp contour node values of subsequent time warp contour nodes is within a predetermined range of values, wherein the predetermined range of values is determined by the encoded warp ratio values. Accordingly, it is ensured that the time warp contour does not comprise undesirably large discontinuities (steps), which would result in an audible distortion.
Further, it has been found that complicated curve fitting operations can be avoided by computing time warp contour node values of subsequent time warp contour nodes using a product formation. Accordingly, the decoder complexity can be held comparatively small. In particular, a number of difficult-to-implement mathematical operations (for example, division operations) can be kept sufficiently small.
To summarize the above, the described embodiment according to the invention allows for an efficient and precise reconstruction of the time warp contour, taking advantage of the fact that the relative change of the time warp contour between subsequent time warp contour nodes is typically limited to a small range of values, which can be described with sufficient precision by the encoded time warp ratio information (also briefly designated as warp ratio information herein), even if a small number of bits (e.g. 3 bits, or 4 bits) is used for the encoding of the warp ratio values. The computation of the time warp contour node values is computationally efficient and ensures a psycho-acoustically sufficient continuity of the time warp contour.
In an embodiment, the time warp contour calculator is configured to periodically restart from the time warp contour start value. By performing a periodic restart from the time warp contour starting value, it can be achieved that the range of values of the time warp contour is limited to values in an environment of the time warp contour starting value. Accordingly, the needed complexity of the time warp contour calculator can be kept small and is very well controllable, as the deviation of the time warp contour node values from the time warp contour starting value is limited by the range of values of the warp ratio values and the number of time warp contour nodes between two subsequent restarts. Thus, a numeric underflow or overflow can be reliably prevented, even if the time warp contour calculator comprises a relatively small numeric resolution or numeric range of values (which allows for a simple implementation).
In an embodiment, the time warp contour calculator is configured to map the encoded warp ratio information on the sequence of warp ratio values using a mapping rule, wherein the mapping rule describes a mapping of a plurality of warp ratio codebook indices onto corresponding warp ratio values, and wherein the mapping rule is chosen such that the mapping rule comprises a plurality of pairs of reciprocal warp ratio values, such that a product of two warp ratio values of a pair of reciprocal warp ratio values lies between 0.9997 and 1.0003. Such an encoding of the warp ratio values allows for a precise representation of time warp contours which return to a previous value. It has been found that in some cases it is desirable that a time warp contour deviates from an initial value for a while (for example for a plurality of time warp contour nodes) and then returns to the initial value. Also, it has been found that audible distortions may occur if the value, which the time warp contour finally reaches, deviates form the initial value. Nevertheless, by providing pairs of reciprocal warp ratio values, it can be achieved that a time warp contour returns to its initial value with a very high precision. Accordingly, potential audible artifacts, which could arise from a mismatch between an initial time warp contour node value and a time warp contour node value to which the time warp contour returns after a while, are prevented.
In an embodiment, the time warp contour calculator is configured to map the encoded warp ratio information onto a sequence of warp ratio values using a mapping rule, wherein the mapping rule describes the mapping of a plurality of warp ratio codebook indices onto corresponding warp ratio values, wherein the mapping rule is chosen such that the warp ratio values, onto which the warp ratio codebook indices are mapped, are within a range between 0.97 and 1.03. It has been found that such a choice allows for a sufficiently precise description of the time warp contour while keeping the needed bit rate for the encoding of the warp ratio sufficiently small.
In an embodiment, the time warp contour calculator is configured to map the encoded warp ratio information onto a sequence of warp ratio values using a mapping rule, wherein the mapping rule describes the mapping of a plurality of warp ratio codebook indices onto corresponding warp ratio values, and wherein the mapping rule is chosen asymmetrically, such that a range of ascending warp ratio values is larger than a range of descending warp ratio values. It has been found that such a choice of the mapping rule is well adapted to the characteristics of human speech and of typical pieces of music. Accordingly, an asymmetric choice of the mapping rule allows for an optimal usage of the available bit rate, which is a very important criterion in the field of audio encoding and audio decoding.
In an embodiment, the time warp contour calculator is configured to receive a side information indicating a non-varying (e.g. flat) time warp contour or a varying (e.g. non-flat) time warp contour for a given frame of the encoded audio signal representation, and, in dependence on the side information indicating a non-varying time warp contour or a varying time warp contour, to obtain the time warp contour node values for the given frame on the basis of the encoded warp ratio information, or to set the time warp contour node values for the given frame to the time warp contour start value. In this embodiment, a transfer of any encoded time warp ratio information to the time warp contour calculator can be omitted for frames in which the side information indicates the presence of a non-varying time warp contour. Accordingly, audio frames in which the time warp contour is non-varying (or for which a varying time warp contour cannot be identified), merely comprise an appropriate flag indicating this non-varying time warp contour (or the absence of a varying time warp contour). In contrast, audio frames in which the time warp contour is varying comprise a flag indicating that the time warp contour is not non-varying and, in addition, the encoded time warp ratio information. Thus, while audio frames comprising a varying time warp contour comprise an additional flag, for example one bit, in addition to the encoded time warp ratio information, audio frames in which the time warp contour is non-varying merely comprise a flag (for example one bit), but do not comprise the encoded warp ratio information. As there is typically a significant percentage of frames in which the time warp contour is non-varying (or a varying time warp contour cannot be identified), a number of bits needed for the description of the time warp contour is typically reduced when compared to a solution in which the encoded time warp ratio information is transmitted for every audio frame, even though the bit count of the time warp contour information is even increased (for example, by one bit) in those frames in which the time warp contour is varying.
In an embodiment, the time warp contour calculator is configured to linearly interpolate between the time warp contour node values, to obtain time warp contour values of new time warp contour portions. By performing such an interpolation, an increased accuracy of the reconstruction of the time warp contour can be obtained.
In an embodiment, the time warp contour calculator is configured to iteratively obtain a sequence of time warp contour node values, wherein the time warp contour calculator is configured to obtain a subsequent time warp contour node value from a present time warp contour node value by multiplying the present time warp contour node value with a corresponding time warp ratio value. In this way, an efficient usage can be made of the time warp ratio values. In particular, a time warp contour node value can be obtained from a previous time warp contour node value in a single-step operation.
Another embodiment according to the invention creates an audio signal encoder for providing an encoded representation of an audio signal. The audio signal encoder comprises a time warp contour encoder configured to receive a time warp contour information associated with the audio signal, to compute a ratio between subsequent node values of the time warp contour, and to encode the ratio between subsequent node values of the time warp contour. The audio signal encoder further comprises a time warping signal encoder configured to obtain an encoded representation of a spectrum of the audio signal, taking into account a time warp described by the time warp contour information. The encoded audio representation of the audio signal comprises the encoded ratio (between subsequent node values of the time warp contour) and the encoded representation of the spectrum of the audio signal. The audio signal encoder according to this embodiment provides an encoded representation of the audio signal, which is well-suited for the encoder-sided calculation of a time warp contour, which has been described above. For example, it is typically possible to encode the ratio between subsequent node values of the time warp contour with good precision using a small number of bits. As discussed above, the ratio between subsequent node values of the time warp contour is typically within the same range of values, both for small absolute values of the time warp contour and for large absolute values of the time warp contour. Further, the computation of a ratio between subsequent node values of the time warp contour can be performed with very low computational complexity, thereby facilitating the design of the audio signal encoder.
In an embodiment, the time warp contour encoder is configured to check whether a varying time warp contour is available for a given frame of the audio signal, and to set a flag within the encoded representation of the audio signal to indicate the absence of a varying time warp contour if a varying time warp contour is not available for the given frame of the audio signal. For example, a flag indicating the presence of a varying time warp contour may be deactivated (or reset) in this case. The time warp contour encoder is also configured to omit the inclusion of encoded ratio values into the encoded representation of the audio signal if a varying time warp contour is not available for the given frame of the audio signal. In this way, a bit rate is minimized for audio signals having a significant number of frames for which a varying time warp contour is not available. It should be noted here that a varying time warp contour is typically not available for audio signals, in which there is a non-varying time warp contour, and also for audio signals for which the extraction of a time warp contour fails (or does not bring along a meaningful result). As already discussed above, the usage of a flag indicating the presence or absence of a varying time warp contour, allows for a reduction of the bit rate needed for the encoding of the time warp contour for typical audio signals.
Another embodiment according to the invention creates an encoded audio signal representation representing an audio signal. The encoded audio signal representation comprises an encoded frequency domain representation representing one or more time warp re-sampled audio channels, re-sampled in accordance with a time warp. The encoded audio signal representation also comprises an encoded representation of a time warp contour representing the time warp, wherein the encoded representation of the time warp contour comprises a plurality of encoded time warp ratio values. The time warp ratio values represent ratios between subsequent node values of the time warp contour. Such an encoded audio signal representation carries the time warp information in a particularly efficient way and allows for the usage of the above described efficient time warp contour calculator.
In an embodiment, the encoded audio signal representation comprises, on a per-audio-frame basis, a flag indicating the presence of an encoded representation of a time warp contour for the respective frame.
Another embodiment according to the invention comprises a method for providing an decoded audio signal representation on the basis of an encoded audio signal representation. The method comprises receiving an encoded warp ratio information, deriving a sequence of warp ratio values from the encoded warp ratio information and obtaining a plurality of warp contour node values starting from a warp contour start value. Ratios between time warp contour node values (of time warp contour nodes other than the time warp contour starting node) and the time warp contour starting value associated with the time warp contour starting node are determined by the time warp ratio values. The time warp contour node value of a given time warp contour node, which is spaced from the time warp contour starting node by an intermediate time warp contour node, is computed on the basis of a product-formation, comprising a ratio between the time warp contour node value of the intermediate time warp contour node and the time warp contour starting value and a ratio between the time warp contour node value of the given time warp contour node and the time warp contour node value of the intermediate time warp contour node as factors. This method comprises the same advantages as the above discussed time warp contour calculator and can be supplemented by the same features and functionalities as the time warp contour calculator described herein.
An embodiment of the invention creates a method for providing an encoded representation of an audio signal. The method comprises receiving a time warp contour information associated with the audio signal, computing a ratio between subsequent node values of the time warp contour and encoding the ratio between subsequent node values of the time warp contour. The method also comprises obtaining an encoded representation of a spectrum of the audio signal, taking into account a time warp described by the time warp information. The encoded representation of the audio signal comprises the encoded ratio and the encoded representation of the spectrum. This method comprises the same advantages as the audio signal decoder mentioned above, and can be supplemented by any of the features and functionalities described herein with respect to the audio signal encoder.
Another embodiment according to the invention creates a computer program for performing the methods discussed herein.
Another embodiment according to the invention creates an audio signal decoder comprising the above mentioned time warp contour calculator. The audio signal decoder can be supplemented by any of the features and functionalities described herein.



BRIEF DESCRIPTION OF THE DRAWINGS
Embodiments according to the invention will subsequently be described taking reference to the enclosed figures, in which:
FIG. 1 shows a block schematic diagram of a time warp audio encoder;
FIG. 2 shows a block schematic diagram of a time warp audio decoder;
FIG. 3 shows a block schematic diagram of an audio signal decoder, according to an embodiment of the invention;
FIG. 4 shows a flowchart of a method for providing a decoded audio signal representation, according to an embodiment of the invention;
FIG. 5 shows a detailed extract from a block schematic diagram of an audio signal decoder according to an embodiment of the invention;
FIG. 6 shows a detailed extract of a flowchart of a method for providing a decoded audio signal representation according to an embodiment of the invention;
FIGS. 7a,7b show a graphical representation of a reconstruction of a time warp contour, according to an embodiment of the invention;
FIG. 8 shows another graphical representation of a reconstruction of a time warp contour, according to an embodiment of the invention;
FIGS. 9a and 9b show algorithms for the calculation of the time warp contour;
FIG. 9c shows a table of a mapping from a time warp ratio index to a time warp ratio value;
FIGS. 10a and 10b show representations of algorithms for the calculation of a time contour, a sample position, a transition length, a “first position” and a “last position”;
FIG. 10c shows a representation of algorithms for a window shape calculation;
FIGS. 10d and 10e show a representation of algorithms for an application of a window;
FIG. 10f shows a representation of algorithms for a time-varying resampling;
FIG. 10g shows a graphical representation of algorithms for a post time warping frame processing and for an overlapping and adding;
FIGS. 11a and 11b show a legend;
FIG. 12 shows a graphical representation of a time contour, which can be extracted from a time warp contour;
FIG. 13 shows a detailed block schematic diagram of an apparatus for providing a warp contour, according to an embodiment of the invention;
FIG. 14 shows a block schematic diagram of an audio signal decoder, according to another embodiment of the invention;
FIG. 15 shows a block schematic diagram of another time warp contour calculator according to an embodiment of the invention;
FIGS. 16a, 16b show a graphical representation of a computation of time warp node values, according to an embodiment of the invention;
FIG. 17 shows a block schematic diagram of another audio signal encoder, according to an embodiment of the invention;
FIG. 18 shows a block schematic diagram of another audio signal decoder, according to an embodiment of the invention; and
FIGS. 19a-19f show representations of syntax elements of an audio stream, according to an embodiment of the invention.



DETAILED DESCRIPTION OF THE INVENTION
1. Time Warp Audio Encoder According to FIG. 1
As the present invention is related to time warp audio encoding and time warp audio decoding, a short overview will be given of a prototype time warp audio encoder and a time warp audio decoder, in which the present invention can be applied.
FIG. 1 shows a block schematic diagram of a time warp audio encoder, into which some aspects and embodiments of the invention can be integrated. The audio signal encoder 100 of FIG. 1 is configured to receive an input audio signal 110 and to provide an encoded representation of the input audio signal 110 in a sequence of frames. The audio encoder 100 comprises a sampler 104, which is adapted to sample the audio signal 110 (input signal) to derive signal blocks (sampled representations) 105 used as a basis for a frequency domain transform. The audio encoder 100 further comprises a transform window calculator 106, adapted to derive scaling windows for the sampled representations 105 output from the sampler 104. These are input into a windower 108 which is adapted to apply the scaling windows to the sampled representations 105 derived by the sampler 104. In some embodiments, the audio encoder 100 may additionally comprise a frequency domain transformer 108a, in order to derive a frequency-domain representation (for example in the form of transform coefficients) of the sampled and scaled representations 105. The frequency domain representations may be processed or further transmitted as an encoded representation of the audio signal 110.
The audio encoder 100 further uses a pitch contour 112 of the audio signal 110, which may be provided to the audio encoder 100 or which may be derived by the audio encoder 100. The audio encoder 100 may therefore optionally comprise a pitch estimator for deriving the pitch contour 112. The sampler 104 may operate on a continuous representation of the input audio signal 110. Alternatively, the sampler 104 may operate on an already sampled representation of the input audio signal 110. In the latter case, the sampler 104 may resample the audio signal 110. The sampler 104 may for example be adapted to time warp neighboring overlapping audio blocks such that the overlapping portion has a constant pitch or reduced pitch variation within each of the input blocks after the sampling.
The transform window calculator 106 derives the scaling windows for the audio blocks depending on the time warping performed by the sampler 104. To this end, an optional sampling rate adjustment block 114 may be present in order to define a time warping rule used by the sampler, which is then also provided to the transform window calculator 106. In an alternative embodiment the sampling rate adjustment block 114 may be omitted and the pitch contour 112 may be directly provided to the transform window calculator 106, which may itself perform the appropriate calculations. Furthermore, the sampler 104 may communicate the applied sampling to the transform window calculator 106 in order to enable the calculation of appropriate scaling windows.
The time warping is performed such that a pitch contour of sampled audio blocks time warped and sampled by the sampler 104 is more constant than the pitch contour of the original audio signal 110 within the input block.
2. Time Warp Audio Decoder According to FIG. 2
FIG. 2 shows a block schematic diagram of a time warp audio decoder 200 for processing a first time warped and sampled, or simply time warped representation of a first and second frame of an audio signal having a sequence of frames in which the second frame follows the first frame and for further processing a second time warped representation of the second frame and of a third frame following the second frame in the sequence of frames. The audio decoder 200 comprises a transform window calculator 210 adapted to derive a first scaling window for the first time warped representation 211a using information on a pitch contour 212 of the first and the second frame and to derive a second scaling window for the second time warped representation 211b using information on a pitch contour of the second and the third frame, wherein the scaling windows may have identical numbers of samples and wherein the first number of samples used to fade out the first scaling window may differ from a second number of samples used to fade in the second scaling window. The audio decoder 200 further comprises a windower 216 adapted to apply the first scaling window to the first time warp representation and to apply the second scaling window to the second time warped representation. The audio decoder 200 furthermore comprises a resampler 218 adapted to inversely time warp the first scaled time warped representation to derive a first sampled representation using the information on the pitch contour of the first and the second frame and to inversely time warp the second scaled representation to derive a second sampled representation using the information on the pitch contour of the second and the third frame such that a portion of the first sampled representation corresponding to the second frame comprises a pitch contour which equals, within a predetermined tolerance range, a pitch contour of the portion of the second sampled representation corresponding to the second frame. In order to derive the scaling window, the transform window calculator 210 may either receive the pitch contour 212 directly or receive information on the time warping from an optional sample rate adjustor 220, which receives the pitch contour 212 and which derives a inverse time warping strategy in such a manner that the sample positions on a linear time scale for the samples of the overlapping regions are identical or nearly identical and regularly spaced, so that the pitch becomes the same in the overlapping regions, and optionally the different fading lengths of overlapping window parts before the inverse time warping become the same length after the inverse time warping.
The audio decoder 200 furthermore comprises an optional adder 230, which is adapted to add the portion of the first sampled representation corresponding to the second frame and the portion of the second sampled representation corresponding to the second frame to derive a reconstructed representation of the second frame of the audio signal as an output signal 242. The first time warped representation and the second time warped representation could, in one embodiment, be provided as an input to the audio decoder 200. In a further embodiment, the audio decoder 200 may, optionally, comprise an inverse frequency domain transformer 240, which may derive the first and the second time warped representations from frequency domain representations of the first and second time warped representations provided to the input of the inverse frequency domain transformer 240.
3. Time Warp Audio Signal Decoder According to FIG. 3
In the following, a simplified audio signal decoder will be described. FIG. 3 shows a block schematic diagram of this simplified audio signal decoder 300. The audio signal decoder 300 is configured to receive the encoded audio signal representation 310, and to provide, on the basis thereof, a decoded audio signal representation 312, wherein the encoded audio signal representation 310 comprises a time warp contour evolution information. The audio signal decoder 300 comprises a time warp contour calculator 320 configured to generate time warp contour data 322 on the basis of the time warp contour evolution information, which time warp contour evolution information describes a temporal evolution of the time warp contour, and which time warp contour evolution information is comprised by the encoded audio signal representation 310. When deriving the time warp contour data 322 from the time warp contour evolution information 312, the time warp contour calculator 320 repeatedly restarts from a predetermined time warp contour start value, as will be described in detail in the following. The restart may have the consequence that the time warp contour comprises discontinuities (step-wise changes which are larger than the steps encoded by the time warp contour evolution information 312). The audio signal decoder 300 further comprises a time warp contour data rescaler 330 which is configured to rescale at least a portion of the time warp contour data 322, such that a discontinuity at a restart of the time warp contour calculation is avoided, reduced or eliminated in a rescaled version 332 of the time warp contour.
The audio signal decoder 300 also comprises a warp decoder 340 configured to provide a decoded audio signal representation 312 on the basis of the encoded audio signal representation 310 and using the rescaled version 332 of the time warp contour.
To put the audio signal decoder 300 into the context of time warp audio decoding, it should be noted that the encoded audio signal representation 310 may comprise an encoded representation of the transform coefficients 211 and also an encoded representation of the pitch contour 212 (also designated as time warp contour). The time warp contour calculator 320 and the time warp contour data rescaler 330 may be configured to provide a reconstructed representation of the pitch contour 212 in the form of the rescaled version 332 of the time warp contour. The warp decoder 340 may, for example, take over the functionality of the windowing 216, the resampling 218, the sample rate adjustment 220 and the window shape adjustment 210. Further, the warp decoder 340 may, for example, optionally, comprise the functionality of the inverse transform 240 and of the overlap/add 230, such that the decoded audio signal representation 312 may be equivalent to the output audio signal 232 of the time warp audio decoder 200.
By applying the resealing to the time warp contour data 322, a continuous (or at least approximately continuous) rescaled version 332 of the time warp contour can be obtained, thereby ensuring that a numeric overflow or underflow is avoided even when using an efficient-to-encode relative-variation time warp contour evolution information.
4. Method for Providing a Decoded Audio Signal Representation According to FIG. 4
FIG. 4 shows a flowchart of a method for providing a decoded audio signal representation on the basis of an encoded audio signal representation comprising a time warp contour evolution information, which can be performed by the apparatus 300 according to FIG. 3. The method 400 comprises a first step 410 of generating the time warp contour data, repeatedly restarting from a predetermined time warp contour start value, on the basis of a time warp contour evolution information describing a temporal evolution of the time warp contour.
The method 400 further comprises a step 420 of rescaling at least a portion of the time warp control data, such that a discontinuity at one of the restarts is avoided, reduced or eliminated in a rescaled version of the time warp contour.
The method 400 further comprises a step 430 of providing a decoded audio signal representation on the basis of the encoded audio signal representation using the rescaled version of the time warp contour.
5. Detailed Description of an Embodiment According to the Invention Taking Reference to FIGS. 5-9
In the following, an embodiment according to the invention will be described in detail taking reference to FIGS. 5-9.
FIG. 5 shows a block schematic diagram of an apparatus 500 for providing a time warp control information 512 on the basis of a time warp contour evolution information 510. The apparatus 500 comprises a means 520 for providing a reconstructed time warp contour information 522 on the basis of the time warp contour evolution information 510, and a time warp control information calculator 530 to provide the time warp control information 512 on the basis of the reconstructed time warp contour information 522.
Means 520 for Providing the Reconstructed Time Warp Contour Information
In the following, the structure and functionality of the means 520 will be described. The means 520 comprises a time warp contour calculator 540, which is configured to receive the time warp contour evolution information 510 and to provide, on the basis thereof, a new warp contour portion information 542. For example, a set of time warp contour evolution information may be transmitted to the apparatus 500 for each frame of the audio signal to be reconstructed. Nevertheless, the set of time warp contour evolution information 510 associated with a frame of the audio signal to be reconstructed may be used for the reconstruction of a plurality of frames of the audio signal. Similarly, a plurality of sets of time warp contour evolution information may be used for the reconstruction of the audio content of a single frame of the audio signal, as will be discussed in detail in the following. As a conclusion, it can be stated that in some embodiments, the time warp contour evolution information 510 may be updated at the same rate at which sets of the transform domain coefficient of the audio signal to be reconstructed or updated (one time warp contour portion per frame of the audio signal).
The time warp contour calculator 540 comprises a warp node value calculator 544, which is configured to compute a plurality (or temporal sequence) of warp contour node values on the basis of a plurality (or temporal sequence) of time warp contour ratio values (or time warp ratio indices), wherein the time warp ratio values (or indices) are comprised by the time warp contour evolution information 510. For this purpose, the warp node value calculator 544 is configured to start the provision of the time warp contour node values at a predetermined starting value (for example 1) and to calculate subsequent time warp contour node values using the time warp contour ratio values, as will be discussed below.
Further, the time warp contour calculator 540 optionally comprises an interpolator 548 which is configured to interpolate between subsequent time warp contour node values. Accordingly, the description 542 of the new time warp contour portion is obtained, wherein the new time warp contour portion typically starts from the predetermined starting value used by the warp node value calculator 524. Furthermore, the means 520 is configured to consider additional time warp contour portions, namely a so-called “last time warp contour portion” and a so-called “current time warp contour portion” for the provision of a full time warp contour section. For this purpose, means 520 is configured to store the so-called “last time warp contour portion” and the so-called “current time warp contour portion” in a memory not shown in FIG. 5.
However, the means 520 also comprises a rescaler 550, which is configured to rescale the “last time warp contour portion” and the “current time warp contour portion” to avoid (or reduce, or eliminate) any discontinuities in the full time warp contour section, which is based on the “last time warp contour portion”, the “current time warp contour portion” and the “new time warp contour portion”. For this purpose, the rescaler 550 is configured to receive the stored description of the “last time warp contour portion” and of the “current time warp contour portion” and to jointly rescale the “last time warp contour portion” and the “current time warp contour portion”, to obtain rescaled versions of the “last time warp contour portion” and the “current time warp contour portion”. Details regarding the rescaling performed by the rescaler 550 will be discussed below, taking reference to FIGS. 7a, 7b and 8.
Moreover, the rescaler 550 may also be configured to receive, for example from a memory not shown in FIG. 5, a sum value associated with the “last time warp contour portion” and another sum value associated with the “current time warp contour portion”. These sum values are sometimes designated with “last_warp_sum” and “cur_warp_sum”, respectively. The rescaler 550 is configured to rescale the sum values associated with the time warp contour portions using the same rescale factor which the corresponding time warp contour portions are rescaled with. Accordingly, rescaled sum values are obtained.
In some cases, the means 520 may comprise an updater 560, which is configured to repeatedly update the time warp contour portions input into the rescaler 550 and also the sum values input into the rescaler 550. For example, the updater 560 may be configured to update said information at the frame rate. For example, the “new time warp contour portion” of the present frame cycle may serve as the “current time warp contour portion” in a next frame cycle. Similarly, the rescaled “current time warp contour portion” of the current frame cycle may serve as the “last time warp contour portion” in a next frame cycle. Accordingly, a memory efficient implementation is created, because the “last time warp contour portion” of the current frame cycle may be discarded upon completion of the current frame cycle.
To summarize the above, the means 520 is configured to provide, for each frame cycle (with the exception of some special frame cycles, for example at the beginning of a frame sequence, or at the end of a frame sequence, or in a frame in which time warping is inactive) a description of a time warp contour section comprising a description of a “new time warp contour portion”, of a “rescaled current time warp contour portion” and of a “rescaled last time warp contour portion”. Furthermore, the means 520 may provide, for each frame cycle (with the exception of the above mentioned special frame cycle) a representation of warp contour sum values, for example, comprising a “new time warp contour portion sum value”, a “rescaled current time warp contour sum value” and a “rescaled last time warp contour sum value”.
The time warp control information calculator 530 is configured to calculate the time warp control information 512 on the basis of the reconstructed time warp contour information provided by the means 520. For example, the time warp control information calculator comprises a time contour calculator 570, which is configured to compute a time contour 572 on the basis of the reconstructed time warp control information. Further, the time warp contour information calculator 530 comprises a sample position calculator 574, which is configured to receive the time contour 572 and to provide, on the basis thereof, a sample position information, for example in the form of a sample position vector 576. The sample position vector 576 describes the time warping performed, for example, by the resampler 218.
The time warp control information calculator 530 also comprises a transition length calculator, which is configured to derive a transition length information from the reconstructed time warp control information. The transition length information 582 may, for example, comprise an information describing a left transition length and an information describing a right transition length. The transition length may, for example, depend on a length of time segments described by the “last time warp contour portion”, the “current time warp contour portion” and the “new time warp contour portion”. For example, the transition length may be shortened (when compared to a default transition length) if the temporal extension of a time segment described by the “last time warp contour portion” is shorter than a temporal extension of the time segment described by the “current time warp contour portion”, or if the temporal extension of a time segment described by the “new time warp contour portion” is shorter than the temporal extension of the time segment described by the “current time warp contour portion”. In addition, the time warp control information calculator 530 may further comprise a first and last position calculator 584, which is configured to calculate a so-called “first position” and a so-called “last position” on the basis of the left and right transition length. The “first position” and the “last position” increase the efficiency of the resampler, as regions outside of these positions are identical to zero after windowing and are therefore not needed to be taken into account for the time warping. It should be noted here that the sample position vector 576 comprises, for example, information needed by the time warping performed by the resampler 280. Furthermore, the left and right transition length 582 and the “first position” and “last position” 586 constitute information, which is, for example, needed by the windower 216.
Accordingly, it can be said that the means 520 and the time warp control information calculator 530 may together take over the functionality of the sample rate adjustment 220, of the window shape adjustment 210 and of the sampling position calculation 219.
In the following, the functionality of an audio decoder comprises the means 520 and the time warp control information calculator 530 will be described with reference to FIGS. 6, 7a, 7b, 8, 9a-9c, 10a-10g, 11a, 11b and 12.
FIG. 6 shows a flowchart of a method for decoding an encoded representation of an audio signal, according to an embodiment of the invention. The method 600 comprises providing a reconstructed time warp contour information, wherein providing the reconstructed time warp contour information comprises calculating 610 warp node values, interpolating 620 between the warp node values and rescaling 630 one or more previously calculated warp contour portions and one or more previously calculated warp contour sum values. The method 600 further comprises calculating 640 time warp control information using a “new time warp contour portion” obtained in steps 610 and 620, the rescaled previously calculated time warp contour portions (“current time warp contour portion” and “last time warp contour portion”) and also, optionally, using the rescaled previously calculated warp contour sum values. As a result, a time contour information, and/or a sample position information, and/or a transition length information and/or a first portion and last position information can be obtained in the step 640.
The method 600 further comprises performing 650 time warped signal reconstruction using the time warp control information obtained in step 640. Details regarding the time warp signal reconstruction will be described subsequently.
The method 600 also comprises a step 660 of updating a memory, as will be described below.
Calculation of the Time Warp Contour Portions
In the following, details regarding the calculation of the time warp contour portions will be described, taking reference to FIGS. 7a, 7b, 8, 9a, 9b, 9c. 
It will be assumed that an initial state is present, which is illustrated in a graphical representation 710 of FIG. 7a. As can be seen, a first warp contour portion 716 (warp contour portion 1) and a second warp contour portion 718 (warp contour portion 2) are present. Each of the warp contour portions typically comprises a plurality of discrete warp contour data values, which are typically stored in a memory. The different warp contour data values are associated with time values, wherein a time is shown at an abscissa 712. A magnitude of the warp contour data values is shown at an ordinate 714.
As can be seen, the first warp contour portion has an end value of 1, and the second warp contour portion has a start value of 1, wherein the value of 1 can be considered as a “predetermined value”. It should be noted that the first warp contour portion 716 can be considered as a “last time warp contour portion” (also designated as “last_warp_contour”), while the second warp contour portion 718 can be considered as a “current time warp contour portion” (also referred to as “cur_warp_contour”).
Starting from the initial state, a new warp contour portion is calculated, for example, in the steps 610, 620 of the method 600. Accordingly, warp contour data values of the third warp contour portion (also designated as “warp contour portion 3” or “new time warp contour portion” or “new_warp_contour”) is calculated. The calculation may, for example, be separated in a calculation of warp node values, according to an algorithm 910 shown in FIG. 9a, and an interpolation 620 between the warp node values, according to an algorithm 920 shown in FIG. 9a. Accordingly, a new warp contour portion 722 is obtained, which starts from the predetermined value (for example 1) and which is shown in a graphical representation 720 of FIG. 7a. As can be seen, the first time warp contour portion 716, the second time warp contour portion 718 and the third new time warp contour portion are associated with subsequent and contiguous time intervals. Further, it can be seen that there is a discontinuity 724 between an end point 718b of the second time warp contour portion 718 and a start point 722a of the third time warp contour portion.
It should be noted here that the discontinuity 724 typically comprises a magnitude which is larger than a variation between any two temporally adjacent warp contour data values of the time warp contour within a time warp contour portion. This is due to the fact that the start value 722a of the third time warp contour portion 722 is forced to the predetermined value (e.g. 1), independent from the end value 718b of the second time warp contour portion 718. It should be noted that the discontinuity 724 is therefore larger than the unavoidable variation between two adjacent, discrete warp contour data values.
Nevertheless, this discontinuity between the second time warp contour portion 718 and the third time warp contour portion 722 would be detrimental for the further use of the time warp contour data values.
Accordingly, the first time warp contour portion and the second time warp contour portion are jointly rescaled in the step 630 of the method 600. For example, the time warp contour data values of the first time warp contour portion 716 and the time warp contour data values of the second time warp contour portion 718 are rescaled by multiplication with a rescaling factor (also designated as “norm_fac”). Accordingly, a rescaled version 716′ of the first time warp contour portion 716 is obtained, and also a rescaled version 718′ of the second time warp contour portion 718 is obtained. In contrast, the third time warp contour portion is typically left unaffected in this rescaling step, as can be seen in a graphical representation 730 of FIG. 7a. Rescaling can be performed such that the rescaled end point 718b′ comprises, at least approximately, the same data value as the start point 722a of the third time warp contour portion 722. Accordingly, the resealed version 716′ of the first time warp contour portion, the resealed version 718′ of the second time warp contour portion and the third time warp contour portion 722 together form an (approximately) continuous time warp contour section. In particular, the scaling can be performed such that a difference between the data value of the resealed end point 718b′ and the start point 722a is not larger than a maximum of the difference between any two adjacent data values of the time warp contour portions 716′, 718′,722.
Accordingly, the approximately continuous time warp contour section comprising the resealed time warp contour portions 716′, 718′ and the original time warp contour portion 722 is used for the calculation of the time warp control information, which is performed in the step 640. For example, time warp control information can be computed for an audio frame temporally associated with the second time warp contour portion 718.
However, upon calculation of the time warp control information in the step 640, a time-warped signal reconstruction can be performed in a step 650, which will be explained in more detail below.
Subsequently, it is necessitated to obtain time warp control information for a next audio frame. For this purpose, the resealed version 716′ of the first time warp contour portion may be discarded to save memory, because it is not needed anymore. However, the resealed version 716′ may naturally also be saved for any purpose. Moreover, the resealed version 718′ of the second time warp contour portion takes the place of the “last time warp contour portion” for the new calculation, as can be seen in a graphical representation 740 of FIG. 7b. Further, the third time warp contour portion 722, which took the place of the “new time warp contour portion” in the previous calculation, takes the role of the “current time warp contour portion” for a next calculation. The association is shown in the graphical representation 740.
Subsequent to this update of the memory (step 660 of the method 600), a new time warp contour portion 752 is calculated, as can be seen in the graphical representation 750. For this purpose, steps 610 and 620 of the method 600 may be re-executed with new input data. The fourth time warp contour portion 752 takes over the role of the “new time warp contour portion” for now. As can be seen, there is typically a discontinuity between an end point 722b of the third time warp contour portion and a start point 752a of the fourth time warp contour portion 752. This discontinuity 754 is reduced or eliminated by a subsequent rescaling (step 630 of the method 600) of the rescaled version 718′ of the second time warp contour portion and of the original version of the third time warp contour portion 722. Accordingly, a twice-rescaled version 718′′ of the second time warp contour portion and a once rescaled version 722′ of the third time warp contour portion are obtained, as can be seen from a graphical representation 760 of FIG. 7b. As can be seen, the time warp contour portions 718′′, 722′, 752 form an at least approximately continuous time warp contour section, which can be used for the calculation of time warp control information in a re-execution of the step 640. For example, a time warp control information can be calculated on the basis of the time warp contour portions 718′′, 722′, 752, which time warp control information is associated to an audio signal time frame centered on the second time warp contour portion.
It should be noted that in some cases it is desirable to have an associated warp contour sum value for each of the time warp contour portions. For example, a first warp contour sum value may be associated with the first time warp contour portion, a second warp contour sum value may be associated with the second time warp contour portion, and so on. The warp contour sum values may, for example, be used for the calculation of the time warp control information in the step 640.
For example, the warp contour sum value may represent a sum of the warp contour data values of a respective time warp contour portion. However, as the time warp contour portions are scaled, it is sometimes desirable to also scale the time warp contour sum value, such that the time warp contour sum value follows the characteristic of its associated time warp contour portion. Accordingly, a warp contour sum value associated with the second time warp contour portion 718 may be scaled (for example by the same scaling factor) when the second time warp contour portion 718 is scaled to obtain the scaled version 718′ thereof Similarly, the warp contour sum value associated with the first time warp contour portion 716 may be scaled (for example with the same scaling factor) when the first time warp contour portion 716 is scaled to obtain the scaled version 716′ thereof, if desired.
Further, a re-association (or memory re-allocation) may be performed when proceeding to the consideration of a new time warp contour portion. For example, the warp contour sum value associated with the scaled version 718′ of the second time warp contour portion, which takes the role of a “current time warp contour sum value” for the calculation of the time warp control information associated with the time warp contour portions 716′, 718′, 722 may be considered as a “last time warp sum value” for the calculation of a time warp control information associated with the time warp contour portions 718′′, 722′, 752. Similarly, the warp contour sum value associated with the third time warp contour portion 722 may be considered as a “new warp contour sum value” for the calculation of the time warp control information associated with time warp contour portions 716′, 718′, 722 and may be mapped to act as a “current warp contour sum value” for the calculation of the time warp control information associated with the time warp contour portions 718′′, 722′, 752. Further, the newly calculated warp contour sum value of the fourth time warp contour portion 752 may take the role of the “new warp contour sum value” for the calculation of the time warp control information associated with the time warp contour portions 718′′, 722′, 752.
Example According to FIG. 8
FIG. 8 shows a graphical representation illustrating a problem which is solved by the embodiments according to the invention. A first graphical representation 810 shows a temporal evolution of a reconstructed relative pitch over time, which is obtained in some conventional embodiments. An abscissa 812 describes the time, an ordinate 814 describes the relative pitch. A curve 816 shows the temporal evolution of the relative pitch over time, which could be reconstructed from a relative pitch information. Regarding the reconstruction of the relative pitch contour, it should be noted that for the application of the time warped modified discrete cosine transform (MDCT) only the knowledge of the relative variation of the pitch within the actual frame is necessitated. In order to understand this, reference is made to the calculation steps for obtaining the time contour from the relative pitch contour, which lead to an identical time contour for scaled versions of the same relative pitch contour. Therefore, it is sufficient to only encode the relative instead of an absolute pitch value, which increases the coding efficiency. To further increase the efficiency, the actual quantized value is not the relative pitch but the relative change in pitch, i.e., the ratio of the current relative pitch over the previous relative pitch (as will be discussed in detail in the following). In some frames, where, for example, the signal exhibits no harmonic structure at all, no time warping might be desired. In such cases, an additional flag may optionally indicate a flat pitch contour instead of coding this flat contour with the afore mentioned method. Since in real world signals the amount of such frames is typically high enough, the trade-off between the additional bit added at all times and the bits saved for non-warped frames is in favor of the bit savings.
The start value for the calculation of the pitch variation (relative pitch contour, or time warp contour) can be chosen arbitrary and even differ in the encoder and decoder. Due to the nature of the time warped MDCT (TW-MDCT) different start values of the pitch variation still yield the same sample positions and adapted window shapes to perform the TW-MDCT.
For example, an (audio) encoder gets a pitch contour for every node which is expressed as actual pitch lag in samples in conjunction with an optional voiced/unvoiced specification, which was, for example, obtained by applying a pitch estimation and voiced/unvoiced decision known from speech coding. If for the current node the classification is set to voiced, or no voiced/unvoiced decision is available, the encoder calculates the ratio between the actual pitch lag and quantizes it, or just sets the ratio to 1 if unvoiced. Another example might be that the pitch variation is estimated directly by an appropriate method (for example signal variation estimation).
In the decoder, the start value for the first relative pitch at the start of the coded audio is set to an arbitrary value, for example to 1. Therefore, the decoded relative pitch contour is no longer in the same absolute range of the encoder pitch contour, but a scaled version of it. Still, as described above, the TW-MDCT algorithm leads to the same sample positions and window shapes. Furthermore, the encoder might decide, if the encoded pitch ratios would yield a flat pitch contour, not to send the fully coded contour, but set the activePitchData flag to 0 instead, saving bits in this frame (for example saving numPitchbits * numPitches bits in this frame).
In the following, the problems will be discussed which occur in the absence of the inventive pitch contour renormalization. As mentioned above, for the TW-MDCT, only the relative pitch change within a certain limited time span around the current block is needed for the computation of the time warping and the correct window shape adaptation (see the explanations above). The time warping follows the decoded contour for segments where a pitch change has been detected, and stays constant in all other cases (see the graphical representation 810 of FIG. 8). For the calculation of the window and sampling positions of one block, three consecutive relative pitch contour segments (for example three time warp contour portions) are needed, wherein the third one is the one newly transmitted in the frame (designated as “new time warp contour portion”) and the other two are buffered from the past (for example designated as “last time warp contour portion” and “current time warp contour portion”).
To get an example, reference is made, for example, to the explanations which were made with reference to FIGS. 7a and 7b, and also to the graphical representations 810, 860 of FIG. 8. To calculate, for example, the sampling positions of the window for (or associated with) frame 1, which extends from frame 0 to frame 2, the pitch contours of (or associated with) frame 0, 1 and 2 are needed. In the bit stream, only the pitch information for frame 2 is sent in the current frame, and the two others are taken from the past. As explained herein, the pitch contour can be continued by applying the first decoded relative pitch ratio to the last pitch of frame 1 to obtain the pitch at the first node of frame 2, and so on. It is now possible, due to the nature of the signal, that if the pitch contour is simply continued (i.e., if the newly transmitted part of the contour is attached to the existing two parts without any modification), that a range overflow in the coder's internal number format occurs after a certain time. For example, a signal might start with a segment of strong harmonic characteristics and a high pitch value at the beginning which is decreasing throughout the segment, leading to a decreasing relative pitch. Then, a segment with no pitch information can follow, so that the relative pitch keeps constant. Then again, a harmonic section can start with an absolute pitch that is higher than the last absolute pitch of the previous segment, and again going downwards. However, if one simply continues the relative pitch, it is the same as at the end of the last harmonic segment and will go down further, and so on. If the signal is strong enough and has in its harmonic segments an overall tendency to go either up or down (like shown in the graphical representation 810 of FIG. 8), sooner or later the relative pitch reaches the border of a range of the internal number format. It is well known from speech coding that speech signals indeed exhibit such a characteristic. Therefore it comes as no surprise, that the encoding of a concatenated set of real world signals including speech actually exceeded the range of the float values used for the relative pitch after a relatively short amount of time when using the conventional method described above.
To summarize, for an audio signal segment (or frame) for which a pitch can be determined, an appropriate evolution of the relative pitch contour (or time warp contour) could be determined. For audio signal segments (or audio signal frames) for which a pitch cannot be determined (for example because the audio signal segments are noise-like) the relative pitch contour (or time warp contour) could be kept constant. Accordingly, if there was an imbalance between audio segments with increasing pitch and decreasing pitch, the relative pitch contour (or time warp contour) would either run into a numeric underflow or a numeric overflow.
For example, in the graphical representation 810 a relative pitch contour is shown for the case that there is a plurality of relative pitch contour portions 820a, 820a, 820c, 820d with decreasing pitch and some audio segments 822a, 822b without pitch, but no audio segments with increasing pitch. Accordingly, it can be seen that the relative pitch contour 816 runs into a numeric underflow (at least under very adverse circumstances).
In the following, a solution for this problem will be described. To prevent the above-mentioned problems, in particular the numeric underflow or overflow, a periodic relative pitch contour renormalization has been introduced according to an aspect of the invention. Since the calculation of the warped time contour and the window shapes only rely on the relative change over the aforementioned three relative pitch contour segments (also designated as “time warp contour portions”), as explained herein, it is possible to normalize this contour (for example, the time warp contour, which may be composed of three pieces of “time warp contour portions”) for every frame (for example of the audio signal) anew with the same outcome.
For this, the reference was, for example, chosen to be the last sample of the second contour segment (also designated as “time warp contour portion”), and the contour is now normalized (for example, multiplicatively in the linear domain) in such a way so that this sample has a value of a 1.0 (see the graphical representation 860 of FIG. 8).
The graphical representation 860 of FIG. 8 represents the relative pitch contour normalization. An abscissa 862 shows the time, subdivided in frames (frames 0, 1, 2). An ordinate 864 describes the value of the relative pitch contour.
A relative pitch contour before normalization is designated with 870 and covers two frames (for example frame number 0 and frame number 1). A new relative pitch contour segment (also designated as “time warp contour portion”) starting from the predetermined relative pitch contour starting value (or time warp contour starting value) is designated with 874. As can be seen, the restart of the new relative pitch contour segment 874 from the predetermined relative pitch contour starting value (e.g. 1) brings along a discontinuity between the relative pitch contour segment 870 preceding the restart point-in-time and the new relative pitch contour segment 874, which is designated with 878. This discontinuity would bring along a severe problem for the derivation of any time warp control information from the contour and will possibly result in audio distortions. Therefore, a previously obtained relative pitch contour segment 870 preceding the restart point-in-time restart is rescaled (or normalized), to obtain a rescaled relative pitch contour segment 870′. The normalization is performed such that the last sample of the relative pitch contour segment 870 is scaled to the predetermined relative pitch contour start value (e.g. of 1.0).
Detailed Description of the Algorithm
In the following, some of the algorithms performed by an audio decoder according to an embodiment of the invention will be described in detail. For this purpose, reference will be made to FIGS. 5, 6, 9a, 9b, 9c and 10a-10g. Further, reference is made to the legend of data elements, help elements and constants of FIGS. 11a and 11b. 
Generally speaking, it can be said that the method described here can be used for decoding an audio stream which is encoded according to a time warped modified discrete cosine transform. Thus, when the TW-MDCT is enabled for the audio stream (which may be indicated by a flag, for example referred to as “twMdct” flag, which may be comprised in a specific configuration information), a time warped filter bank and block switching may replace a standard filter bank and block switching. Additionally to the inverse modified discrete cosine transform (IMDCT) the time warped filter bank and block switching contains a time domain to time domain mapping from an arbitrarily spaced time grid to the normal regularly spaced time grid and a corresponding adaptation of window shapes.
In the following, the decoding process will be described. In a first step, the warp contour is decoded. The warp contour may be, for example, encoded using codebook indices of warp contour nodes. The codebook indices of the warp contour nodes are decoded, for example, using the algorithm shown in a graphical representation 910 of FIG. 9a. According to said algorithm, warp ratio values (warp_value_tbl) are derived from warp ratio codebook indices (tw_ratio), for example using a mapping defined by a mapping table 990 of FIG. 9c. As can be seen from the algorithm shown as reference numeral 910, the warp node values may be set to a constant predetermined value, if a flag (tw_data_present) indicates that time warp data is not present. In contrast, if the flag indicates that time warp data is present, a first warp node value can be set to the predetermined time warp contour starting value (e.g. 1). Subsequent warp node values (of a time warp contour portion) can be determined on the basis of a formation of a product of multiple time warp ratio values. For example, a warp node value of a node immediately following the first warp node (i=0) may be equal to a first warp ratio value (if the starting value is 1) or equal to a product of the first warp ratio value and the starting value. Subsequent time warp node values (i=2,3, . . . , num_tw_nodes) are computed by forming a product of multiple time warp ratio values (optionally taking into consideration the starting value, if the starting value differs from 1). Naturally, the order of the product formation is arbitrary. However, it is advantageous to derive a (i+1)-th warp mode value from an i-th warp node value by multiplying the i-th warp node value with a single warp ratio value describing a ratio between two subsequent node values of the time warp contour.
As can be seen from the algorithm shown at reference numeral 910, there may be multiple warp ratio codebook indices for a single time warp contour portion over a single audio frame (wherein there may be a 1-to-1 correspondence between time warp contour portions and audio frames).
To summarize, a plurality of time warp node values can be obtained for a given time warp contour portion (or a given audio frame) in the step 610, for example using the warp node value calculator 544. Subsequently, a linear interpolation can be performed between the time warp node values (warp_node_values[i]). For example, to obtain the time warp contour data values of the “new time warp contour portion” (new_warp_contour) the algorithm shown at reference numeral 920 in FIG. 9a can be used. For example, the number of samples of the new time warp contour portion is equal to half the number of the time domain samples of an inverse modified discrete cosine transform. Regarding this issue, it should be noted that adjacent audio signal frames are typically shifted (at least approximately) by half the number of the time domain samples of the MDCT or IMDCT. In other words, to obtain the sample-wise (N_long samples) new_warp_contour[ ], the warp_node_values[ ] are interpolated linearly between the equally spaced (interp_dist apart) nodes using the algorithm shown at reference numeral 920.
The interpolation may, for example, be performed by the interpolator 548 of the apparatus of FIG. 5, or in the step 620 of the algorithm 600.
Before obtaining the full warp contour for this frame (i.e. for the frame presently under consideration) the buffered values from the past are resealed so that the last warp value of the past_warp_contour[ ] equals 1 (or any other predetermined value, which may be equal to the starting value of the new time warp contour portion).
It should be noted here that the term “past warp contour” may comprise the above-described “last time warp contour portion” and the above-described “current time warp contour portion”. It should also be noted that the “past warp contour” typically comprises a length which is equal to a number of time domain samples of the IMDCT, such that values of the “past warp contour” are designated with indices between 0 and 2*n_long-1. Thus, “past_warp_contour[2*n_long-1]” designates a last warp value of the “past warp contour”. Accordingly, a normalization factor “norm_fac” can be calculated according to an equation shown at reference numeral 930 in FIG. 9a. Thus, the past warp contour (comprising the “last time warp contour portion” and the “current time warp contour portion”) can be multiplicatively rescaled according to the equation shown at reference numeral 932 in FIG. 9a. In addition, the “last warp contour sum value” (last_warp_sum) and the “current warp contour sum value” (cur_warp_sum) can be multiplicatively resealed, as shown in reference numerals 934 and 936 in FIG. 9a. The rescaling can be performed by the rescaler 550 of FIG. 5, or in step 630 of the method 600 of FIG. 6.
It should be noted that the normalization described here, for example at reference numeral 930, then could be modified, for example, by replacing the starting value of “1” by any other desired predetermined value.
By applying the normalization, a “full warp_contour[ ]” also designated as a “time warp contour section” is obtained by concatenating the “past_warp_contour” and the “new_warp_contour”. Thus, three time warp contour portions (“last time warp contour portion”, “current time warp contour portion”, and “new time warp contour portion”) form the “full warp contour”, which may be applied in further steps of the calculation.
In addition, a warp contour sum value (new_warp_sum) is calculated, for example, as a sum over all “new_warp_contour[ ]” values. For example, a new warp contour sum value can be calculated according to the algorithms shown at reference numeral 940 in FIG. 9a. 
Following the above-described calculations, the input information needed by the time warp control information calculator 330 or by the step 640 of the method 600 is available. Accordingly, the calculation 640 of the time warp control information can be performed, for example by the time warp control information calculator 530. Also, the time warped signal reconstruction 650 can be performed by the audio decoder. Both, the calculation 640 and the time-warped signal reconstruction 650 will be explained in more detail below.
However, it is important to note that the present algorithm proceeds iteratively. It is therefore computationally efficient to update a memory. For example, it is possible to discard information about the last time warp contour portion. Further, it is recommendable to use the present “current time warp contour portion” as a “last time warp contour portion” in a next calculation cycle. Further, it is recommendable to use the present “new time warp contour portion” as a “current time warp contour portion” in a next calculation cycle. This assignment can be made using the equation shown at reference numeral 950 in FIG. 9b, (wherein warp_contour[n] describes the present “new time warp contour portion” for 2* n_long<n<3·n_long).
Appropriate assignments can be seen at reference numerals 952 and 954 in FIG. 9b. 
In other words, memory buffers used for decoding the next frame can be updated according to the equations shown at reference numerals 950, 952 and 954.
It should be noted that the update according to the equations 950, 952 and 954 does not provide a reasonable result, if the appropriate information is not being generated for a previous frame. Accordingly, before decoding the first frame or if the last frame was encoded with a different type of coder (for example a LPC domain coder) in the context of a switched coder, the memory states may be set according to the equations shown at reference numerals 960, 962 and 964 of FIG. 9b. 
Calculation of Time Warp Control Information
In the following, it will be briefly described how the time warp control information can be calculated on the basis of the time warp contour (comprising, for example, three time warp contour portions) and on the basis of the warp contour sum values.
For example, it is desired to reconstruct a time contour using the time warp contour. For this purpose, an algorithm can be used which is shown at reference numerals 1010, 1012 in FIG. 10a. As can be seen, the time contour maps an index i (0≦i≦3·n_long) onto a corresponding time contour value. An example of such a mapping is shown in FIG. 12.
Based on the calculation of the time contour, it is typically necessitated to calculate a sample position (sample_pos[ ]), which describes positions of time warped samples on a linear time scale. Such a calculation can be performed using an algorithm, which is shown at reference numeral 1030 in FIG. 10b. In the algorithm 1030, helper functions can be used, which are shown at reference numerals 1020 and 1022 in FIG. 10a. Accordingly, an information about the sample time can be obtained.
Furthermore, some lengths of time warped transitions (warped_trans_len_left; warped_trans_len_right) are calculated, for example using an algorithm 1032 shown in FIG. 10b. Optionally, the time warp transition lengths can be adapted dependent on a type of window or a transform length, for example using an algorithm shown at reference numeral 1034 in FIG. 10b. Furthermore, a so-called “first position” and a so-called “last position” can be computed on the basis of the transition lengths informations, for example using an algorithm shown at reference numeral 1036 in FIG. 10b. To summarize, a sample positions and window lengths adjustment, which may be performed by the apparatus 530 or in the step 640 of the method 600 will be performed. From the “warp_contour[ ]” a vector of the sample positions (“sample_pos[ ]”) of the time warped samples on a linear time scale may be computed. For this, first the time contour may be generated using the algorithm shown at reference numerals 1010, 1012. With the helper functions “warp_in_vec( )” and “warp_time_inv( )”, which are shown at reference numerals 1020 and 1022, the sample position vector (“sample_pos[ ]”) and the transition lengths (“warped_trans_len_left” and “warped_trans_len_right”) are computed, for example using the algorithms shown at reference numerals 1030, 1032, 1034 and 1036. Accordingly, the time warp control information 512 is obtained.
Time Warped Signal Reconstruction
In the following, the time warped signal reconstruction, which can be performed on the basis of the time warp control information will be briefly discussed to put the computation of the time warp contour into the proper context.
The reconstruction of an audio signal comprises the execution of an inverse modified discrete cosine transform, which is not described here in detail, because it is well known to anybody skilled in the art. The execution of the inverse modified discrete cosine transform allows to reconstruct warped time domain samples on the basis of a set of frequency domain coefficients. The execution of the IMDCT may, for example, be performed frame-wise, which means, for example, a frame of 2048 warped time domain samples is reconstructed on the basis of a set of 1024 frequency domain coefficients. For the correct reconstruction it is necessitated that no more than two subsequent windows overlap. Due to the nature of the TW-MDCT it might occur that a inversely time warped portion of one frame extends to a non-neighbored frame, thusly violating the prerequisite stated above. Therefore the fading length of the window shape needs to be shortened by calculating the appropriate warped_trans_len_left and warped_trans_len_right values mentioned above.
A windowing and block switching 650b is then applied to the time domain samples obtained from the IMDCT. The windowing and block switching may be applied to the warped time domain samples provided by the IMDCT 650a in dependence on the time warp control information, to obtain windowed warped time domain samples. For example, depending on a “window_shape” information, or element, different oversampled transform window prototypes may be used, wherein the length of the oversampled windows may be given by the equation shown at reference numeral 1040 in FIG. 10c. For example, for a first type of window shape (for example window_shape==1), the window coefficients are given by a “Kaiser-Bessel” derived (KBD) window according to the definition shown at reference numeral 1042 in FIG. 10c, wherein W′, the “Kaiser-Bessel kernel window function”, is defined as shown at reference numeral 1044 in FIG. 10c. 
Otherwise, when using a different window shape is used (for example, if window_shape==0), a sine window may be employed according to the definition a reference numeral 1046. For all kinds of window sequences (“window_sequences”), the used prototype for the left window part is determined by the window shape of the previous block. The formula shown at reference numeral 1048 in FIG. 10c expresses this fact. Likewise, the prototype for the right window shape is determined by the formula shown at reference numeral 1050 in FIG. 10c. 
In the following, the application of the above-described windows to the warped time domain samples provided by the IMDCT will be described. In some embodiments, the information for a frame can be provided by a plurality of short sequences (for example, eight short sequences). In other embodiments, the information for a frame can be provided using blocks of different lengths, wherein a special treatment may be necessitated for start sequences, stop sequences and/or sequences of non-standard lengths. However, since the transitional length may be determined as described above, it may be sufficient to differentiate between frames encoded using eight short sequences (indicated by an appropriate frame type information “eight_short_sequence”) and all other frames.
For example, in a frame described by an eight short sequence, an algorithm shown as reference numeral 1060 in FIG. 10d may be applied for the windowing. In contrast, for frames encoded using other information, an algorithm is shown at reference numeral 1064 in FIG. 10e may be applied. In other words, the C-code like portion shown at reference numeral 1060 in FIG. 10d describes the windowing and internal overlap-add of a so-called “eight-short-sequence”. In contrast, the C-code-like portion shown in reference numeral 1064 in FIG. 10d describes the windowing in other cases.
Resampling
In the following, the inverse time warping 650c of the windowed warped time domain samples in dependence on the time warp control information will be described, whereby regularaly sampled time domain samples, or simply time domain samples, are obtained by time-varying resampling. In the time-varying resampling, the windowed block z[ ] is resampled according to the sampled positions, for example using an impulse response shown at reference numeral 1070 in FIG. 10f. Before resampling, the windowed block may be padded with zeros on both ends, as shown at reference numeral 1072 in FIG. 10f. The resampling itself is described by the pseudo code section shown at reference numeral 1074 in FIG. 10f. 
Post-Resampler Frame Processing
In the following, an optional post-processing 650d of the time domain samples will be described. In some embodiments, the post-resampling frame processing may be performed in dependence on a type of the window sequence. Depending on the parameter “window_sequence”, certain further processing steps may be applied.
For example, if the window sequence is a so-called “EIGHT_SHORT_SEQUENCE”, a so-called “LONG_START_SEQUENCE”, a so-called “STOP_START_SEQUENCE”, a so-called “STOP_START_1152_SEQUENCE” followed by a so-called LPD_SEQUENCE, a post-processing as shown at reference numerals 1080a, 1080b, 1082 may be performed.
For example, if the next window sequence is a so-called “LPD_SEQUENCE”, a correction window Wcorr(n) may be calculated as shown at reference numeral 1080a, taking into account the definitions shown at reference numeral 1080b. Also. The correction window Wcorr(n) may be applied as shown at reference numeral 1082 in FIG. 10g. 
For all other cases, nothing may be done, as can be seen at reference numeral 1084 in FIG. 10g. 
Overlapping and Adding with Previous Window Sequences
Furthermore, an overlap-and-add 650e of the current time domain samples with one or more previous time domain samples may be performed. The overlapping and adding may be the same for all sequences and can be described mathematically as shown at reference numeral 1086 in FIG. 10g. 
Legend
Regarding the explanations given, reference is also made to the legend, which is shown in FIGS. 11a and 11d. In particular, the synthesis window length N for the inverse transform is typically a function of the syntax element “window_sequence” and the algorithmic context. It may for example be defined as shown at reference numeral 1190 of FIG. 11b. 
Embodiment According to FIG. 13
FIG. 13 shows a block schematic diagram of a means 1300 for providing a reconstructed time warp contour information which takes over the functionality of the means 520 described with reference to FIG. 5. However, the data path and the buffers are shown in more detail. The means 1300 comprises a warp node value calculator 1344, which takes the function of the warped node value calculator 544. The warp node value calculator 1344 receives a codebook index “tw_ratio[ ]” of the warp ratio as an encoded warp ratio information. The warp node value calculator comprises a warp value table representing, for example, the mapping of a time warp ratio index onto a time warp ratio value represented in FIG. 9c. The warp node value calculator 1344 may further comprise a multiplier for performing the algorithm represented at reference numeral 910 of FIG. 9a. Accordingly, the warp node value calculator provides warp node values “warp_node_values[i]”. Further, the means 1300 comprise a warp contour interpolator 1348, which takes the function of the interpolator 540a, and which may be figured to perform the algorithm shown at reference numeral 920 in FIG. 9a, thereby obtaining values of the new warp contour (“new_warp_contour”). Means 1300 further comprises a new warp contour buffer 1350, which stores the values of the new warp contour (i.e. warp_contour [i], with 2·n_long≦i<3·n_long). The means 1300 further comprises a past warp contour buffer/updater 1360, which stores the “last time warp contour portion” and the “current time warp contour portion” and updates the memory contents in response to a rescaling and in response to a completion of the processing of the current frame. Thus, the past warp contour buffer/updater 1360 may be in cooperation with the past warp contour rescaler 1370, such that the past warp contour buffer/updater and the past warp contour rescaler together fulfill the functionality of the algorithms 930, 932, 934, 936, 950, 960. Optionally, the past warp contour buffer/updater 1360 may also take over the functionality of the algorithms 932, 936, 952, 954, 962, 964.
Thus, the means 1300 provides the warp contour (“warp_contour”) and optimally also provides the warp contour sum values.
Audio Signal Encoder According to FIG. 14
In the following, an audio signal encoder according to an aspect of the invention will be described. The audio signal encoder of FIG. 14 is designated in its entirety with 1400. The audio signal encoder 1400 is configured to receive an audio signal 1410 and, optionally, an externally provided warp contour information 1412 associated with the audio signal 1410. Further, the audio signal encoder 1400 is configured to provide an encoded representation 1440 of the audio signal 1410.
The audio signal encoder 1400 comprises a time warp contour encoder 1420, configured to receive a time warp contour information 1422 associated with the audio signal 1410 and to provide an encoded time warp contour information 1424 on the basis thereof.
The audio signal encoder 1400 further comprises a time warping signal processor (or time warping signal encoder) 1430 which is configured to receive the audio signal 1410 and to provide, on the basis thereof, a time-warp-encoded representation 1432 of the audio signal 1410, taking into account a time warp described by the time warp information 1422. The encoded representation 1414 of the audio signal 1410 comprises the encoded time warp contour information 1424 and the encoded representation 1432 of the spectrum of the audio signal 1410.
Optionally, the audio signal encoder 1400 comprises a warp contour information calculator 1440, which is configured to provide the time warp contour information 1422 on the basis of the audio signal 1410. Alternatively, however, the time warp contour information 1422 can be provided on the basis of the externally provided warp contour information 1412.
The time warp contour encoder 1420 may be configured to compute a ratio between subsequent node values of the time warp contour described by the time warp contour information 1422. For example, the node values may be sample values of the time warp contour represented by the time warp contour information. For example, if the time warp contour information comprises a plurality of values for each frame of the audio signal 1410, the time warp node values may be a true subset of this time warp contour information. For example, the time warp node values may be a periodic true subset of the time warp contour values. A time warp contour node value may be present per N of the audio samples, wherein N may be greater than or equal to 2.
The time contour node value ratio calculator may be configured to compute a ratio between subsequent time warp node values of the time warp contour, thus providing an information describing a ratio between subsequent node values of the time warp contour. A ratio encoder of the time warp contour encoder may be configured to encode the ratio between subsequent node values of the time warp contour. For example, the ratio encoder may map different ratios to different code book indices. For example, a mapping may be chosen such that the ratios provided by the time contour warp value ratio calculator are within a range between 0.9 and 1.1, or even between 0.95 and 1.05. Accordingly, the ratio encoder may be configured to map this range to different codebook indices. For example, correspondences shown in the table of FIG. 9c may act as supporting points in this mapping, such that, for example, a ratio of 1 is mapped onto a codebook index of 3, while a ratio of 1.0057 is mapped to a codebook index of 4, and so on (compare FIG. 9c). Ratio values between those shown in the table of FIG. 9c may be mapped to appropriate codebook indices, for example to the codebook index of the nearest ratio value for which the codebook index is given in the table of FIG. 9c. 
Naturally, different encodings may be used such that, for example, a number of available codebook indices may be chosen larger or smaller than shown here. Also, the association between warp contour node values and codebook values indices may be chosen appropriately. Also, the codebook indices may be encoded, for example, using a binary encoding, optionally using an entropy encoding.
Accordingly, the encoded ratios 1424 are obtained
The time warping signal processor 1430 comprises a time warping time-domain to frequency-domain converter 1434, which is configured to receive the audio signal 1410 and a time warp contour information 1422a associated with the audio signal (or an encoded version thereof), and to provide, on the basis thereof, a spectral domain (frequency-domain) representation 1436.
The time warp contour information 1422a may be derived from the encoded information 1424 provided by the time warp contour encoder 1420 using a warp decoder 1425. In this way, it can be achieved that the encoder (in particular the time warping signal processor 1430 thereof) and the decoder (receiving the encoded representation 1414 of the audio signal) operate on the same warp contours, namely the decoded (time) warp contour. However, in a simplified embodiment, the time warp contour information 1422a used by the time warping signal processor 1430 may be identical to the time warp contour information 1422 input to the time warp contour encoder 1420.
The time warping time-domain to frequency-domain converter 1434 may, for example, consider a time warp when forming the spectral domain representation 1436, for example using a time-varying resampling operation of the audio signal 1410. Alternatively, however, time-varying resampling and time-domain to frequency-domain conversion may be integrated in a single processing step. The time warping signal processor also comprises a spectral value encoder 1438, which is configured to encode the spectral domain representation 1346. The spectral value encoder 1438 may, for example, be configured to take into consideration perceptual masking. Also, the spectral value encoder 1438 may be configured to adapt the encoding accuracy to the perceptual relevance of the frequency bands and to apply an entropy encoding. Accordingly, the encoded representation 1432 of the audio signal 1410 is obtained.
Time Warp Contour Calculator According to FIG. 15
FIG. 15 shows the block schematic diagram of a time warp contour calculator, according to another embodiment of the invention. The time warp contour calculator 1500 is configured to receive an encoded warp ratio information 1510 to provide, on the basis thereof, a plurality of warp node values 1512. The time warp contour calculator 1500 comprises, for example, a warp ratio decoder 1520, which is configured to derive a sequence of warp ratio values 1522 from the encoded warp ratio information 1510. The time warp contour calculator 1500 also comprises a warp contour calculator 1530, which is configured to derive the sequence of warp node values 1512 from the sequence of warp ratio values 1522. For example, the warp contour calculator may be configured to obtain the warp contour node values starting from a warp contour start value, wherein ratios between the warp contour start value, associated with a warp contour starting node, and the warp contour node values are determined by the warp ratio values 1522. The warp node value calculator is also configured to compute a warp contour node value 1512 of a given warp contour node which is spaced from the warp contour start node by an intermediate warp contour node, on the basis of a product-formation comprising a ratio between the warp contour starting value (for example 1) and the warp contour node value of the intermediate warp contour node and a ratio between the warp contour node value of the intermediate warp contour node and the warp contour node value of the given warp contour node as factors.
In the following, the operation of the time warp contour calculator 1500 will be briefly discussed taking reference to FIGS. 16a and 16b. 
FIG. 16a shows a graphical representation of a successive calculation of a time warp contour. A first graphical representation 1610 shows a sequence of time warp ratio codebook indices 1510 (index=0, index=1, index=2, index=3, index=7). Further, the graphical representation 1610 shows a sequence of warp ratio values (0.983, 0.988, 0.994, 1.000, 1.023) associated with the codebook indices. Further, it can be seen that a first warped node value 1621 (i=0) is chosen to be 1 (wherein 1 is a starting value). As can be seen, a second warp node value 1622 (i=1) is obtained by multiplying the starting value of 1 with the first ratio value of 0.983 (associated with the first index 0). It can further be seen that the third warp node value 1623 is obtained by multiplying the second warp node value 1622 of 0.983 with the second warp ratio value of 0.988 (associated with the second index of 1). In the same way, the fourth warp node value 1624 is obtained by multiplying the third warp node value 1623 with the third warp ratio value of 0.994 (associated with a third index of 2).
Accordingly, a sequence of warp node values 1621, 1622, 1623, 1624, 1625, 1626 are obtained.
A respective warp node value is effectively obtained such that it is a product of the starting value (for example 1) and all the intermediate warp ratio values lying between the starting warp nodes 1621 and the respective warp node value 1622 to 1626.
A graphical representation 1640 illustrates a linear interpolation between the warp node values. For example, interpolated values 1621a, 1621b, 1621c could be obtained in an audio signal decoder between two adjacent time warp node values 1621, 1622, for example making use of a linear interpolation.
FIG. 16b shows a graphical representation of a time warp contour reconstruction using a periodic restart from a predetermined starting value, which can optionally be implemented in the time warp contour calculator 1500. In other words, the repeated or periodic restart is not an essential feature, provided a numeric overflow can be avoided by any other appropriate measure at the encoder side or at the decoder side. As can be seen, a warp contour portion can start from a starting node 1660 wherein warp contour nodes 1661, 1662, 1663, 1664 can be determined. For this purpose, warp ratio values (0.983, 0.988, 0.965, 1.000) can be considered, such that adjacent warp contour nodes 1661 to 1664 of the first time warp contour portion are separated by ratios determined by these warp ratio values. However, a further, second time warp contour portion may be started after an end node 1664 of the first time warp contour portion (comprising nodes 1660-1664) has been reached. The second time warp contour portion may start from a new starting node 1665, which may take the predetermined starting value, independent from any warp ratio values. Accordingly, warp node values of the second time warp contour portion may be computed starting from the starting node 1665 of the second time warp contour portion on the basis of the warp ratio values of the second time warp contour portion. Later, a third time warp contour portion may start off from a corresponding starting node 1670, which may again take the predetermined staring value independent from any warp ratio values. Accordingly, a periodic restart of the time warp contour portions is obtained. Optionally, a repeated renormalization may be applied, as described in detail above.
The Audio Signal Encoder According to FIG. 17
In the following, an audio signal encoder according to another embodiment of the invention will be briefly described, taking reference to FIG. 17. The audio signal encoder 1700 is configured to receive a multi-channel audio signal 1710 and to provide an encoded representation 1712 of the multi-channel audio signal 1710. The audio signal encoder 1700 comprises an encoded audio representation provider 1720, which is configured to selectively provide an audio representation comprising a common warp contour information, commonly associated with a plurality of audio channels of the multi-channel audio signal, or an encoded audio representation comprising individual warp contour information, individually associated with the different audio channels of the plurality of audio channels, dependent on an information describing a similarity or difference between warp contours associated with the audio channels of the plurality of audio channels.
For example, the audio signal encoder 1700 comprises a warp contour similarity calculator or warp contour difference calculator 1730 configured to provide the information 1732 describing the similarity or difference between warp contours associated with the audio channels. The encoded audio representation provider comprises, for example, a selective time warp contour encoder 1722 configured to receive time warp contour information 1724 (which may be externally provided or which may be provided by an optional time warp contour information calculator 1734) and the information 1732. If the information 1732 indicates that the time warp contours of two or more audio channels are sufficiently similar, the selective time warp contour encoder 1722 may be configured to provide a joint encoded time warp contour information. The joint warp contour information may, for example, be based on an average of the warp contour information of two or more channels. However, alternatively the joint warp contour information may be based on a single warp contour information of a single audio channel, but jointly associated with a plurality of channels.
However, if the information 1732 indicates that the warp contours of multiple audio channels are not sufficiently similar, the selective time warp contour encoder 1722 may provide separate encoded information of the different time warp contours.
The encoded audio representation provider 1720 also comprises a time warping signal processor 1726, which is also configured to receive the time warp contour information 1724 and the multi-channel audio signal 1710. The time warping signal processor 1726 is configured to encode the multiple channels of the audio signal 1710. Time warping signal processor 1726 may comprise different modes of operation. For example, the time warping signal processor 1726 may be configured to selectively encode audio channels individually or jointly encode them, exploiting inter-channel similarities. In some cases, it is advantageous that the time warping signal processor 1726 is capable of commonly encoding multiple audio channels having a common time warp contour information. There are cases in which a left audio channel and a right audio channel exhibit the same relative pitch evolution but have otherwise different signal characteristics, e.g. different absolute fundamental frequencies or different spectral envelopes. In this case, it is not desirable to encode the left audio channel and the right audio channel jointly, because of the significant difference between the left audio channel and the right audio channel. Nevertheless, the relative pitch evolution in the left audio channel and the right audio channel may be parallel, such that the application of a common time warp is a very efficient solution. An example of such an audio signal is a polyphone music, wherein contents of multiple audio channels exhibit a significant difference (for example, are dominated by different singers or music instruments), but exhibit similar pitch variation. Thus, coding efficiency can be significantly improved by providing the possibility to have a joint encoding of the time warp contours for multiple audio channels while maintaining the option to separately encode the frequency spectra of the different audio channels for which a common pitch contour information is provided.
The encoded audio representation provider 1720 optionally comprises a side information encoder 1728, which is configured to receive the information 1732 and to provide a side information indicating whether a common encoded warp contour is provided for multiple audio channels or whether individual encoded warp contours are provided for the multiple audio channels. For example, such a side information may be provided in the form of a 1-bit flag named “common_tw”.
To summarize, the selective time warp contour encoder 1722 selectively provides individual encoded representations of the time warp audio contours associated with multiple audio signals, or a joint encoded time warp contour representation representing a single joint time warp contour associated with the multiple audio channels. The side information encoder 1728 optionally provides a side information indicating whether individual time warp contour representations or a joint time warp contour representation are provided. The time warping signal processor 1726 provides encoded representations of the multiple audio channels. Optionally, a common encoded information may be provided for multiple audio channels. However, typically it is even possible to provide individual encoded representations of multiple audio channels, for which a common time warp contour representation is available, such that different audio channels having different audio content, but identical time warp are appropriately represented. Consequently, the encoded representation 1712 comprises encoded information provided by the selective time warp contour encoder 1722, and the time warping signal processor 1726 and, optionally, the side information encoder 1728.
Audio Signal Decoder According to FIG. 18
FIG. 18 shows a block schematic diagram of an audio signal decoder according to an embodiment of the invention. The audio signal decoder 1800 is configured to receive an encoded audio signal representation 1810 (for example the encoded representation 1712) and to provide, on the basis thereof, a decoded representation 1812 of the multi-channel audio signal. The audio signal decoder 1800 comprises a side information extractor 1820 and a time warp decoder 1830. The side information extractor 1820 is configured to extract a time warp contour application information 1822 and a warp contour information 1824 from the encoded audio signal representation 1810. For example, the side information extractor 1820 may be configured to recognize whether a single, common time warp contour information is available for multiple channels of the encoded audio signal, or whether the separate time warp contour information is available for the multiple channels. Accordingly, the side information extractor may provide both the time warp contour application information 1822 (indicating whether joint or individual time warp contour information is available) and the time warp contour information 1824 (describing a temporal evolution of the common (joint) time warp contour or of the individual time warp contours). The time warp decoder 1830 may be configured to reconstruct the decoded representation of the multi-channel audio signal on the basis of the encoded audio signal representation 1810, taking into consideration the time warp described by the information 1822, 1824. For example, the time warp decoder 1830 may be configured to apply a common time warp contour for decoding different audio channels, for which individual encoded frequency domain information is available. Accordingly, the time warp decoder 1830 may, for example, reconstruct different channels of the multi-channel audio signal, which comprise similar or identical time warp, but different pitch.
Audio Stream According to FIGS. 19a to 19e 
In the following, an audio stream will be described, which comprises an encoded representation of one or more audio signal channels and one or more time warp contours.
FIG. 19a shows a graphical representation of a so-called “USAC_raw_data_block” data stream element which may comprise a single channel element (SCE), a channel pair element (CPE) or a combination of one or more single channel elements and/or one or more channel pair elements.
The “USAC — raw_data_block” may typically comprise a block of encoded audio data, while additional time warp contour information may be provided in a separate data stream element. Nevertheless, it is usually possible to encode some time warp contour data into the “USAC_raw_data_block”.
As can be seen from FIG. 19b, a single channel element typically comprises a frequency domain channel stream (“fd_channel_stream”), which will be explained in detail with reference to FIG. 9d. 
As can be seen from FIG. 19c, a channel pair element (“channel_pair_element”) typically comprises a plurality of frequency domain channel streams. Also, the channel pair element may comprise time warp information. For example, a time warp activation flag (“tw_MDCT”) which may be transmitted in a configuration data stream element or in the “USAC_saw_data_block” determines whether time warp information is included in the channel pair element. For example, if the “tw_MDCT” flag indicates that the time warp is active, the channel pair element may comprise a flag (“common_tw”) which indicates whether there is a common time warp for the audio channels of the channel pair element. If said flag (common_tw) indicates that there is a common time warp for multiple of the audio channels, then a common time warp information (tw_data) is included in the channel pair element, for example, separate from the frequency domain channel streams.
Taking reference now to FIG. 19d, the frequency domain channel stream is described. As can be seen from FIG. 19d, the frequency domain channel stream, for example, comprises a global gain information. Also, the frequency domain channel stream comprises time warp data, if time warping is active (flag “tw_MDCT” active) and if there is no common time warp information for multiple audio signal channel (flag “common_tw” is inactive).
Further, a frequency domain channel stream also comprises scale factor data (“scale_factor_data”) and encoded spectral data (for example arithmetically encoded spectral data “ac_spectral_data”).
Taking reference now to FIG. 19e, the syntax of the time warp data briefly discussed.
The time warp data may for example, optionally, comprise a flag (e.g. “tw_data_present” or “active Pitch Data”) indicating whether time warp data is present. If the time warp data is present, (i.e. the time warp contour is not flat) the time warp data may comprise a sequence of a plurality of encoded time warp ratio values (e.g. “tw_ratio [i]” or “pitchIdx[i]”), which may, for example, be encoded according to the codebook table of FIG. 9c. 
Thus, the time warp data may comprise a flag indicating that there is no time warp data available, which may be set by an audio signal encoder, if the time warp contour is constant (time warp ratios are approximately equal to 1.000). In contrast, if the time warp contour is varying, ratios between subsequent time warp contour nodes may be encoded using the codebook indices making up the “tw_ratio” information.
CONCLUSION
Summarizing the above, embodiments according to the invention bring along different improvements in the field of time warping.
The invention aspects described herein are in the context of a time warped MDCT transform coder (see, for example, reference [1]). Embodiments according to the invention provide methods for an improved performance of a time warped MDCT transform coder.
According to an aspect of the invention, a particularly efficient bitstream format is provided. The bitstream format description is based on and enhances the MPEG-2 AAC bitstream syntax (see, for example, reference [2]), but is of course applicable to all bitstream formats with a general description header at the start of a stream and an individual frame-wise information syntax.
For example, the following side information may be transmitted in the bitstream:
In general, a one-bit flag (e.g. named “tw_MDCT”) may present in the general audio specific configuration (GASC), indicating if time warping is active or not. Pitch data may be transmitted using the syntax shown in FIG. 19e or the syntax shown in FIG. 19f. In the syntax shown in FIG. 19f, the number of pitches (“numPitches”) may be equal to 16, and the number of pitch bits in (“numPitchBits”) may be equal to 3. In other words, there may be 16 encoded warp ratio values per time warp contour portion (or per audio signal frame), and each warp contour ratio value may be encoded using 3 bits.
Furthermore, in a single channel element (SCE) the pitch data (pitch_data[ ]) may be located before the section data in the individual channel, if warping is active.
In a channel pair element (CPE), a common pitch flag signals if there is a common pitch data for both channels, which follows after that, if not, the individual pitch contours are found in the individual channels.
In the following, an example will be given for a channel pair element. One example might be a signal of a single harmonic sound source, placed within the stereo panorama.
In this case, the relative pitch contours for the first channel and the second channel will be equal or would differ only slightly due to some small errors in the estimation of the variation. In this case, the encoder may decide that instead of sending two separate coded pitch contours for each channel, to send only one pitch contour that is an average of the pitch contours of the first and second channel, and to use the same contour in applying the TW-MDCT on both channels. On the other hand, there might be a signal where the estimation of the pitch contour yields different results for the first and the second channel respectively. In this case, the individually coded pitch contours are sent within the corresponding channel.
In the following, an advantageous decoding of pitch contour data, according to an aspect of the invention, will be described. For example, if the “active PitchData” flag is 0, the pitch contour is set to 1 for all samples in the frame, otherwise the individual pitch contour nodes are computed as follows:

    
    
        there are numPitches+1 nodes,
        node [0] is 1.0;
        node [i]=node[i−1]·relChange[i] (i=1. . . numPitches+1), where the relChange is obtained by inverse quantization of the pitchIdx[i].
    
    


The pitch contour is then generated by the linear interpolation between the nodes, where the node sample positions are 0:frameLen/numPitches:frameLen.
Implementation Alternatives
Depending on certain implementation requirements, embodiments of the invention can be implemented in hardware or in software. The implementation can be performed using a digital storage medium, for example a floppy disk, a DVD, a CD, a ROM, a PROM, an EPROM, an EEPROM or a FLASH memory, having electronically readable control signals stored thereon, which cooperate (or are capable of cooperating) with a programmable computer system such that the respective method is performed.
Some embodiments according to the invention comprise a data carrier having electronically readable control signals, which are capable of cooperating with a programmable computer system, such that one of the methods described herein is performed.
Generally, embodiments of the present invention can be implemented as a computer program product with a program code, the program code being operative for performing one of the methods when the computer program product runs on a computer. The program code may for example be stored on a machine readable carrier.
Other embodiments comprise the computer program for performing one of the methods described herein, stored on a machine readable carrier.
In other words, an embodiment of the inventive method is, therefore, a computer program having a program code for performing one of the methods described herein, when the computer program runs on a computer.
A further embodiment of the inventive methods is, therefore, a data carrier (or a digital storage medium, or a computer-readable medium) comprising, recorded thereon, the computer program for performing one of the methods described herein.
A further embodiment of the inventive method is, therefore, a data stream or a sequence of signals representing the computer program for performing one of the methods described herein. The data stream or the sequence of signals may for example be configured to be transferred via a data communication connection, for example via the Internet.
A further embodiment comprises a processing means, for example a computer, or a programmable logic device, configured to or adapted to perform one of the methods described herein. A1
A further embodiment comprises a computer having installed thereon the computer program for performing one of the methods described herein.
In some embodiments, a programmable logic device (for example a field programmable gate array) may be used to perform some or all of the functionalities of the methods described herein. In some embodiments, a field programmable gate array may cooperate with a microprocessor in order to perform one of the methods described herein.
While this invention has been described in terms of several embodiments, there are alterations, permutations, and equivalents which fall within the scope of this invention. It should also be noted that there are many alternative ways of implementing the methods and compositions of the present invention. It is therefore intended that the following appended claims be interpreted as including all such alterations, permutations, and equivalents as fall within the true spirit and scope of the present invention.
REFERENCES
[1] L. Villemoes, “Time Warped Transform Coding of Audio Signals”, PCT/EP2006/010246, Int. patent application, November 2005
[2] Generic Coding of Moving Pictures and Associated Audio: Advanced Audio Coding. International Standard 13818-7, ISO/IECJTC1/SC29/WG11 Moving Pictures Expert Group, 1997

## Claims

1. A time warp contour calculator apparatus for use in an audio signal decoder for providing a decoded audio signal representation on the basis of an encoded audio signal representation, comprising:
a warp ratio decoder that includes a mechanism for receiving an input signal including encoded warp ratio information; and
a warp node value calculator in communication with the warp ratio decoder;
wherein the warp ratio decoder is configured to process the encoded warp ratio information to derive a sequence of warp ratio values,
wherein the warp node value calculator is configured to process the sequence of warp ratio values to acquire, starting with a time warp start value, warp contour node values,
wherein ratios between the time warp contour node values and the time warp contour starting value associated with a time warp contour start node are determined by the warp ratio values; and
wherein the warp node value calculator is configured to compute a time warp contour node value of a given time warp contour node,
wherein the given time warp contour node is spaced from the time warp contour starting node with an intermediate time warp contour node in between on the basis of a product-formation,
wherein the product-formation comprises as factors a ratio between the time warp contour node value of the intermediate time warp contour node and the time warp contour starting value and a ratio between the time warp contour node value of the given time warp contour node and the time-warp contour node value of the intermediate time warp contour node;
wherein the warp node value calculator includes a mechanism for outputting an output signal that includes the warp contour node values.

2. The time warp contour calculator according to claim 1, wherein the time warp contour calculator is configured to periodically restart from the time warp contour starting value.

3. The time warp contour calculator according to claim 1, wherein the warp ratio decoder is configured to map the encoded warp ratio information onto a sequence of warp ratio values using a mapping rule,
wherein the mapping rule describes a mapping of a plurality of warp ratio Codebook indices onto corresponding warp ratio values,
wherein the mapping rule is chosen such that the mapping rule comprises a plurality of pairs of reciprocal warp ratio values, such that product of two warp ratio values of a pair of at least approximately reciprocal warp ratio values lies between 0.9997 and 1.0003.

4. The time warp contour calculator according to claim 1, wherein the warp ratio decoder is configured to map the encoded warp ratio information onto a sequence of warp ratio values using a mapping rule,
wherein the mapping rule describes a mapping of a plurality of warp ratio codebook indices onto corresponding warp ratio values,
wherein the mapping rule is chosen such that the warp ratio values, onto which the warp ratio codebook indices are mapped, are within a range between 0.97 and 1.03.

5. The time warp contour calculator according to claim 1, wherein the warp ratio is configured to map the encoded warp ratio information onto a sequence of warp ratio values using a mapping rule,
wherein the mapping rule describes a mapping of a plurality of warp ratio Codebook indices onto corresponding warp ratio values,
wherein the mapping rule is chosen asymmetrically such that a range of ascending warp ratio values is larger than a range of descending warp ratio values.

6. The time warp contour calculator according to claim 1, wherein the time warp contour calculator is configured to receive a side information indicating a non-varying time warp contour or a varying time warp contour for a given frame of the encoded audio signal representation, and, in dependence on the side information indicating a non-varying time warp contour or a varying time warp contour, to acquire the time warp contour node values for the given frame on the basis of the encoded warp ratio information, or to set the time warp contour node values for the given frame to the warp contour start value.

7. The time warp contour calculator according to claim 1, wherein the time warp contour calculator is configured to linearly interpolate between the time warp contour node values, to acquire time warp contour values of a new time warp contour portion.

8. The time warp contour calculator according to claim 1, wherein the warp node value calculator is configured to iteratively acquire a sequence of time warp contour node values, wherein the time warp contour calculator is configured to acquire a subsequent time warp contour node value from a present time warp contour node value by multiplying the present time warp contour node value with a corresponding time warp ratio value.

9. An audio signal encoder apparatus for providing an encoded representation of an audio signal, the audio signal encoder comprising:
an input mechanism for receiving the audio signal;
a mechanism for obtaining time warp contour information;
a time warp contour encoder in communication with the mechanism for obtaining the time warp contour information, wherein the time warp contour encoder is configured to
receive the time warp contour information associated with the received audio signal,
compute ratios between pairs of subsequent node values of a time warp contour, and
encode the ratios between subsequent node values of the time warp contour; and

a time warping signal encoder in communication with the input mechanism, wherein the time warping signal encoder is configured to
acquire an encoded representation of a spectrum of the received audio signal, taking into account a time warp described by the time warp contour information;

wherein the encoded representation of the received audio signal comprises the encoded ratios and the encoded representation of the spectrum;
wherein the node values are sample values of the time warp contour described by the time warp contour information; and
an output mechanism in communication with the time warp contour encoder and with the time warping signal encoder, for outputting an output signal that includes the encoded representation of the audio signal.

10. The audio signal encoder according to claim 9, wherein the time warp contour encoder is configured to check, whether a non-flat time warp contour is available for a given frame of the audio signal, and to set a flag within the encoded representation of the audio signal to indicate the absence of a varying time warp contour if a varying time warp contour is not available for the given frame of the audio signal, and
to omit an inclusion of encoded ratio values into the encoded representation of the audio signal if a varying time warp contour is not available for the given frame of the audio signal.

11. A method for providing a decoded audio signal representation on the basis of an encoded audio signal representation, the method comprising:
receiving an input signal that includes encoded warp ratio information;
processing the encoded warp ratio information to derive a sequence of warp ratio values; and
processing the sequence of warp ratio values, to acquire, starting from a time warp contour start value, a plurality of time warp contour node values,
wherein ratios between the time warp contour node values and the time warp contour starting value associated with the time warp contour starting node are determined by the warp ratio values;
wherein a time warp contour node value of a given time warp contour node, which given time warp contour node is spaced from the time warp contour starting node with an intermediate time warp contour node in between, is computed on the basis of a product- formation, comprising as factors
a ratio between the time warp contour node value of the intermediate time warp contour node and the time warp contour starting value, and
a ratio between the time warp contour node value of the given time warp contour node and the time warp contour node value of the intermediate time warp contour node; and

outputting an output signal that includes the resultant decoded audio signal representation.

12. A method for providing an encoded representation of an audio signal, the method comprising:
with an input mechanism, receiving the audio signal;
receiving a time warp contour information associated with the audio signal;
computing ratios between pairs of subsequent node values of a time warp contour;
encoding the ratios between subsequent node values of the time warp contour; and
acquiring an encoded representation of a spectrum of the audio signal, taking into account a time warp described by the time warp contour information;
wherein the encoded representation of the audio signal comprises the encoded ratios and the encoded representation of the spectrum;
wherein the node values are sample values of the time warp contour described by the time warp contour information; and
with an output mechanism, outputting an output signal that includes the encoded representation of the audio signal.

13. A non-transitory computer readable medium comprising a computer program for performing, when executed by a computer, a method for providing a decoded audio signal representation on the basis of an encoded audio signal representation, the method comprising:
receiving an input signal, wherein the input signal includes encoded warp ratio information;
processing the encoded warp ratio information to derive a sequence of warp ratio values;
processing the sequence of warp ratio values, to acquire, starting with a time warp contour start value, a plurality of time warp contour node values, and
outputting an output signal that includes the resultant decoded audio signal representation;
wherein ratios between the time warp contour node values and the time warp contour starting value associated with the time warp contour starting node are determined by the warp ratio values;
wherein a time warp contour node value of a given time warp contour node, which given time warp contour node is spaced from the time warp contour starting node with an intermediate time warp contour node in between, is computed on the basis of a product- formation, comprising as factors
a ratio between the time warp contour node value of the intermediate time warp contour node and the time warp contour starting value, and
a ratio between the time warp contour node value of the given time warp contour node and the time warp contour node value of the intermediate time warp contour node.

14. A non-transitory computer readable medium comprising a computer program for performing, when executed by a computer, a method for providing an encoded representation of an audio signal, the method comprising:
receiving the audio signal;
receiving a time warp contour information associated with the audio signal;
determining, on the basis of the received time warp contour information, ratios between pairs of subsequent node values of a time warp contour;
encoding the determined ratios between subsequent node values of the time warp contour;
acquiring an encoded representation of a spectrum of the received audio signal, taking into account a time warp described by the received time warp contour information; and
outputting an output signal that includes the encoded representation of the received audio signal;
wherein the encoded representation of the received audio signal comprises the encoded ratios and the encoded representation of the spectrum;
wherein the node values are sample values of the time warp contour described by the time warp contour information.

15. An audio signal decoder, wherein the audio signal decoder is configured to provide a decoded audio signal representation on the basis of an encoded audio signal representation, the audio signal decoder comprising:
a mechanism for receiving an input signal that includes encoded warp ratio information;
a time warp contour calculator that includes a warp ratio decoder and a warp node value calculator;
the warp ratio decoder for processing the received encoded warp ratio information to derive a sequence of warp ratio values, and
the warp node value calculator for processing the derived sequence of warp ratio values, to acquire, starting from a time warp contour start value, warp contour node values,
wherein ratios between the time warp contour node values and the time warp contour starting value associated with a time warp contour start node are determined by the warp ratio values; and
wherein the warp node value calculator is configured to compute a time warp contour node value of a given time warp contour node, which given time warp contour node is spaced from the time warp contour starting node with an intermediate time warp contour node in between, on the basis of a product-formation comprising as factors
a ratio between the time warp contour node value of the intermediate time warp contour node and the time warp contour starting value, and
a ratio between the time warp contour node value of the given time warp contour node and the time-warp contour node value of the intermediate time warp contour node;

wherein the audio signal decoder comprises a warp decoder configured to perform a resampling in dependence on the warp contour node values; and
a mechanism for outputting an output signal that includes the decoded audio signal representation.