Patent Description:
Although the capacity in telecommunication networks is continuously increasing, it is still of great interest to limit the required bandwidth per communication channel. Less transmission bandwidth for each call allows the mobile network to service a larger number of users in parallel. Additionally, lowering the transmission bandwidth yields lower power consumption in both a mobile device and a base station of the mobile network. Such lower power consumption results in energy and cost saving for a mobile operator, while an end user may experience prolonged battery life and increased talk-time.

One method for reducing transmission bandwidth in speech communication is to utilize the natural pauses in speech. In most conversations, only one talker is active at a time and the natural pauses in speech by the talker in one direction will typically occupy more than half of the signal. A method of utilizing this property of a typical conversation for the purpose of decreasing transmission bandwidth is to employ a Discontinuous Transmission (DTX) scheme where active signal coding is discontinued during speech pauses. DTX schemes are standardized for all 3GPP mobile telephony standards such as <NUM>, <NUM> and VoLTE. DTX schemes are also commonly used in Voice over IP systems.

When implementing a DTX scheme, it is common to transmit a very low bit rate encoding of the background noise to allow a Comfort Noise Generator (CNG) at the receiving end to fill speech pauses with a generated background noise having similar characteristics to the original background noise. The CNG makes the call sound more natural as the generated background noise is not switched on and off with the speech according to the DTX scheme. Complete silence during speech pauses is perceived as annoying to a listener and often leads to the misconception that the call has been disconnected.

The DTX scheme further relies on a Voice Activity Detector (VAD) which indicates to the system when to use active signal encoding methods or low rate background noise encoding methods. The system may be generalized to discriminate between other source types by using a Generic Sound Activity Detector (GSAD also referred to as SAD), which not only discriminates speech from background noise, but also detects music or other relevant signal types.

Communication services may be further enhanced by supporting stereo or multichannel audio transmission. In such instances, a DTX and CNG system may need to consider the spatial characteristics of the audio signal in order to provide a pleasant sounding comfort noise.

<NPL>, discloses an approach for coding of parameters of line spectral pairs using intraframe and interframe prediction.

Telecommunication traditionally utilizes a single channel for voice communication where a single microphone at each communication endpoint is used to capture the sounds uttered by a speaker. Accordingly, there is a need to enhance the communication experience by providing a more precise reconstruction of the spatial environment of the speaker. Such enhancements may increase the intelligibility of the speech as it is easier to separate a voice from the background noise if they are separated in a spatial manner. Further, it is beneficial to have speakers separated in an audio space for a teleconference scenario with more than two participants.

A common comfort noise (CN) generation method used in 3GPP speech codecs is to transmit information to a receiver regarding the energy and spectral shape of the background noise for the speech pauses. Information regarding background noise can be transmitted using a significantly less number of bits compared to regular coding of speech segments.

At the receiver end, the CN is generated by creating a pseudo random signal and shaping the spectrum of the created signal with a filter based on the received information regarding the background noise for the speech pauses. Such signal generation and spectral shaping can be done in the time domain or the frequency domain.

Conventional methods of CN generation for a stereo DTX system use a mono encoder with a DTX system working separately on each channel. For example, a dual mono encoding is used for a dual channel stereo DTX system. Accordingly, the energy and spectral shape of the background noise transmitted to the receiver can be different for the left signal and the right signal. In most cases the difference in energy and spectral shape of the transmitted background noise between the left signal and the right signal may not be large, such differences may result in a significant difference in how "wide" the stereo image of the signal is perceived by a listener. That is, if the pseudo random signals used to generate the CN is synchronized between the left and the right channel the result will be a stereo signal which sounds very "narrow," thereby giving the sensation of a sound originating from within the head of the listener. In contrast, if the pseudo random signals are not synchronized, the very opposite sensation would be given to the listener, i.e. a wide signal.

In most cases, an original background noise will have an energy and spectral shape, also referred to as a stereo image, that is in-between these two extremes, i.e. the narrow signal and the wide signal. This results in a detectable difference in the stereo image of the background noise when the system switches between active (speech) and non-active (noise) coding.

The stereo image of the original background noise may also change during a call. For example, a user may be moving around and/or the environment surrounding the user may be changing. Conventional methods of CN generation, such as a dual mono encoding system, fail to provide any mechanisms to adapt to such changes.

Another disadvantage of using conventional methods of CN generation, such as dual mono encoding, is that the VAD decision will not be synchronized between the channels. This may lead to audible artifacts when, for example, a left channel is encoded with active coding and a right channel is encoded with the low bit rate CN coding. The lack of synchronization of the VAD decision between the channels may cause the pseudo random signals used to generate the CN in the left and the right channel to be synchronized in some time instances and the unsynchronized in others. As a result, the stereo image of the generated CN may toggle between extremely wide and extremely narrow over time.

As shown above, there remains a need for an improved method of CN generation.

Accordingly, certain embodiments disclosed herein provide a method to encode a channel coherence parameter applied on a frequency band basis, where the coherence parameters of each band form a coherence vector. The coherence vector is encoded using a predictive scheme followed by a variable bit rate entropy coding. The coding scheme further improves the performance through an adaptive inter-frame prediction.

For instance, in one aspect there is provided an audio signal processing method performed by an encoder to encode a vector. The method includes the encoder forming a prediction weighting factor. For each element of the vector, the encoder forms an intra-frame prediction of the vector element and an inter-frame prediction of the vector element. The encoder combines said intra-frame prediction and said inter-frame prediction using the prediction weighting factor into a combined prediction. The encoder forms a prediction residual using said vector element and said combined prediction. The encoder encodes the prediction residual with a variable bit rate scheme. The encoder transmits the encoded prediction residual. In some embodiments, said vector is one of a sequence of vectors. In some embodiments, the encoder reconstructs the vector based on the combined prediction and a reconstructed prediction residual. In some embodiments, the encoder encodes and transmits the prediction weighting factor.

In some embodiments, the first prediction is an intra-frame prediction based on the reconstructed vector elements. In such embodiments, the intra-frame prediction is formed by performing a process which includes selecting a predictor from a set of predictors, applying the selected predictor to the reconstructed vector elements; and encoding an index corresponding to the selected predictor.

In some embodiments, the second prediction is an inter-frame prediction based on one or more vectors previously reconstructed for the sequence of vectors. In such embodiments, the inter-frame prediction is formed by performing a process which may include selecting a predictor from a set of predictors, applying the selected predictor to the one or more previously reconstructed vectors, and encoding an index corresponding to the selected predictor. In some embodiments, a value from the previous reconstructed vector is used for the inter-frame prediction.

In some embodiments, the encoder quantizes the prediction residual to form a first residual quantizer index, wherein the first residual quantizer index is associated with a first code word.

In some embodiments, the step of encoding the prediction residual with the variable bit rate scheme includes encoding the first residual quantizer index as a result of determining that the length of the first code word does not exceed the amount of remaining bits.

In some embodiments, the step of encoding the prediction residual with the variable bit rate scheme includes obtaining a second residual quantizer index as a result of determining that the length of the first code word exceeds the amount of remaining bits, wherein the second residual quantizer index is associated with a second code word, and wherein the length of the second code word is shorter than the length of the first code word. In such embodiments, the encoder determines whether the length of the second code word exceeds the determined amount of remaining bits.

In some embodiments, the encoder is further configured to receive a first signal on a first input channel, receive a second signal on a second input channel, determine spectral characteristics of the first signal and the second signal, determine a spatial coherence based on the determined spectral characteristics of the first signal and the second signal, and determine the vector based on the spatial coherence.

In some embodiments, the method is performed by the encoder in an audio encoder and decoder system comprising at least two input channels. In some embodiments, the encoder is further configured to create a spectrum by performing a process comprising transforming the input channels and analyzing the input channels in frequency bands. In some embodiments, the vector comprises a set of coherence values, and wherein each value corresponds to the coherence between two of the at least two input channels in a frequency band.

In another aspect there is provided an audio signal processing method performed by a decoder to decode a vector. The method includes the decoder obtaining a weighting factor. For each element of the vector the decoder forms an intra-frame prediction of the vector and an inter-frame prediction of the vector. The decoder combines said intra-frame prediction and said inter-frame prediction using the prediction weighting factor into a combined prediction. The decoder decodes a received encoded prediction residual. The decoder reconstructs the vector element based on the combined prediction and the decoded prediction residual. In some embodiments, said vector is one of a sequence of vectors.

In some embodiments, the first prediction is an intra-frame prediction based on the reconstructed vector elements. In such embodiments, the intra-frame prediction is formed by performing a process which includes receiving and decoding a predictor and applying the decoded predictor to the reconstructed vector elements.

In some embodiments, the second prediction is an inter-frame prediction based on one or more vectors previously reconstructed for the sequence of vectors. In such embodiments, the inter-frame prediction is formed by performing a process which may include receiving and decoding a predictor; and applying the decoded predictor to the one or more previously reconstructed vectors. In some embodiments, a value from previous reconstructed vector is used for the inter-frame prediction.

In some embodiments, the step of decoding the encoded prediction residual includes determining an amount of remaining bits available for decoding and determining whether decoding the encoded prediction residual exceeds the amount of remaining bits.

In some embodiments, the step of decoding the encoded prediction residual includes setting the prediction residual as zero as a result of determining that decoding the encoded prediction residual exceeds the amount of remaining bits.

In some embodiments, the step of decoding the encoded prediction residual includes deriving the prediction residual based on a residual quantizer index as a result of determining that decoding the encoded prediction residual does not exceed the amount of remaining bits, wherein the residual quantizer index is a quantization of the prediction residual.

In some embodiments, the step of obtaining the prediction weighting factor comprises (i) deriving the prediction weighting factor or (ii) receiving and decoding the prediction weighting factor.

In some embodiments, the decoder generates signals for at least two output channels based on the reconstructed vector.

In yet another aspect there is provided an encoder according to claim <NUM>. prediction, encode the prediction residual with a variable bit rate scheme and transmit the encoded prediction residual.

In yet another aspect there is provided a decoder according to claim <NUM>.

The embodiments disclosed herein provide prediction and residual coding which offers rate scalability suitable for the variable bit budget. The residual coding may be truncated in relation to the predictive scheme. The adaptive inter-frame prediction finds a balance between the advantages of inter-frame redundancy while minimizing the risk of error propagation in case of frame loss.

A method of achieving a spatial representation of a signal is to use multiple microphones and to encode a stereo or multichannel signal. <FIG> shows an illustration of a parametric stereo encoder <NUM> and decoder <NUM>. The encoder <NUM> performs an analysis of the input channel pair 106A-106B and obtains a parametric representation of a stereo image through parametric analysis <NUM> and reduces the channels a single channel through down-mix <NUM> thereby obtaining a down-mixed signal. The down-mixed signal is encoded with a mono encoding algorithm by a mono encoder <NUM> and the parametric representation of the stereo image is encoded by a parameter encoder <NUM>. The encoded down-mixed signal and parametric representation of the stereo image is transmitted through a bitstream <NUM>. The decoder <NUM> employs a mono decoder <NUM> to apply a mono decoding algorithm and obtains a synthesized down-mixed signal. A parameter decoder <NUM> decodes the received parametric representation of the stereo image. The decoder <NUM> transforms the synthesized down-mix signal into a synthesized channel pair through parametric synthesis <NUM> using the decoded parametric representation of the stereo image.

<FIG> illustrates a parametric stereo encoding and decoding system <NUM> according to some embodiments. As shown in <FIG>, the parametric stereo encoding and decoding system <NUM> comprises a mono encoder <NUM> including a CNG encoder <NUM> and a mono decoder <NUM> including a CNG decoder <NUM>. In some embodiments, the input signals 106A-106B comprise a channel pair denoted as [l(m, n) r(m, n)], where l(m, n) and r(m, n) denote the input signals for the left and right channel, respectively, for sample index n of frame m. The signals are processed in frames of length N samples at a sampling frequency Fs, where the length of the frame may include an overlap such as look-ahead and memory of past samples.

The parametric stereo encoding and decoding system <NUM> further comprises a coherence analysis <NUM> in the parametric analysis <NUM> and a coherence synthesis <NUM> in the parametric synthesis <NUM>. The parametric analysis <NUM> includes the capability to analyze the coherence of the input signals 106A-106B. The parametric analysis <NUM> may analyze the input signals 106A-106B when the mono encoder <NUM> is configured to operate as the CNG encoder <NUM>. In some embodiments, the input signals 106A-106B may be transformed to the frequency domain by means of, for example, a DFT or any other suitable filter-bank or transform such as QMF, hybrid QMF, and MDCT. In some embodiments, a DFT or MDCT transform may be used to transform the input signals 106A-106B to the frequency domain. In such embodiments, the input signals 106A-106B are typically windowed before the transformation. The choice of window depends on various parameters, such as time and frequency resolution characteristics, algorithmic delay (overlap length), reconstruction properties, etc. As an example, the DFT transformed channel pair denoted as [l(m, n) r(m, n)] is given by <MAT> A general definition of the channel coherence Cgen(f) for frequency f is given by <MAT> where Sxx(f) and Syy (f) represent the power spectra of the two channels 106A-106B and Sxy(f) is the cross power spectrum. In the exemplary DFT based solution, the channel coherence spectra may be represented by the DFT spectra given by <MAT> where * denotes the complex conjugate. To reduce the number of bits required to encode the coherence values, the spectrum is divided into sub frequency bands (also referred to as coherence bands). In some embodiments, the bandwidth of the sub frequency bands is configured to match the perceived frequency resolution with narrow bandwidth for the low frequencies and increasing bandwidth for higher frequencies. It is to be noted that terms channel coherence and spatial coherence are used interchangeably throughout the description.

Accordingly, the analysis of the coherence provides a value per sub frequency band, thereby forming a vector of coherence values, Cm = [C<NUM>,m C<NUM>,m. CNbnd,m], where Nbnd is the number of coherence bands, b is the band index, and m is the frame index. The coherence values Cb,m are then encoded to be stored or transmitted to a decoder. In some embodiments, the power spectra may be averaged over time or low-pass filtered to form more stable estimates of the power spectrum. Further details regarding the coherence analysis is described in International Application Publication No. <CIT>.

When decoding a CNG frame, the decoder <NUM> produces two CNG frames corresponding to the two synthesis channels 210A-210B. In some embodiments, the two CNG frames are generated to have a minimum coherence/correlation. Such CNG frames with minimum coherence/correlation may be generated by operating the CNG decoder <NUM> two separate times with the same parameters, but using two different pseudo-random number generators according to some embodiments. In some embodiments, the two CNG frames with minimum coherence/correlation may be generated by applying a decorrelator function which modifies the fine structure of the CNG frame while maintaining a minimum impact on the magnitude spectrum. The target coherence is then obtained by combining the two generated CNG signals using a method described in International Application Publication No. <CIT>.

The proposed solution disclosed herein applies to a stereo encoder and decoder architecture or a multi-channel encoder and decoder where the channel coherence is considered in channel pairs. Referring back to <FIG>, the mono encoder <NUM> may comprise a stereo encoder VAD according to some embodiments. The stereo encoder VAD may indicate to the CNG encoder <NUM> that a signal contains background noise, thereby activating the CNG encoder <NUM>. Accordingly, a CNG analysis comprising the coherence analysis <NUM> is activated in the parametric analysis <NUM> and the mono encoder <NUM> initiates the CNG encoder <NUM>. As a result, an encoded representation of the coherence and the mono CNG is bundled together in the bitstream <NUM> for transmission and/or storing. The decoder <NUM> identifies the stereo CNG frame in the bitstream <NUM>, decodes the mono CNG and the coherence values, and synthesizes the target coherence as described, for instance, in International Application Publication No. <CIT>.

The disclosed embodiments described herein relate to the encoding and decoding of the coherence values for the CNG frames.

The encoding of the coherence vector described herein considers the following properties: (<NUM>) adaptable encoding to a varying per-frame bit budget Bm, (<NUM>) the coherence vector shows strong frame-to-frame similarity, and (<NUM>) error propagation should be kept low for lost frames.

To address the varying per-frame bit budget, a coarse-fine encoding strategy is implemented. More specifically, the coarse encoding is first achieved at a low bit rate and the subsequent fine encoding may be truncated when the bit limit is reached.

In some embodiments, the coarse encoding is performed utilizing a predictive scheme. In such embodiments, a predictor works along the coherence vector for increasing bands b and estimates each coherence value based on the previous values of the vector. That is, an intra-frame prediction of the coherence vector is performed and is given by: <MAT> Each predictor set P(q) consists of (Nbnd - <NUM>) predictors, each predictor comprising (b - <NUM>) predictor coefficients for each band b where q = <NUM>,<NUM>,. Nq and Nq indicates a total number of predictor sets. As shown above, there are no previous values when b = <NUM> and the intra-frame prediction of the coherence is zero. As an example, a predictor set number q when there are six coherence bands, Nbnd = <NUM>, is given by <MAT> As another example, the total number of predictor sets may be four, i.e. Nq = <NUM>, which indicates that the selected predictor set may be signaled using <NUM> bits. In some embodiments, predictor coefficients for a predictor set q may be addressed sequentially and stored in a single vector of length <MAT>.

<FIG> is a flow chart illustrating an encoding process <NUM> according to some embodiments. The encoding process <NUM> may be performed by the encoder <NUM> according to the following steps:.

In step <NUM>, for each frame m, a bit variable (also referred to as a bit counter) to keep track of the bits spent for the encoding is initialized to zero (Bcurr,m = <NUM>). The encoding algorithm receives a coherence vector (Cb,m) to encode, a copy of the previous reconstructed coherence vector (Ĉb,m-<NUM>), and a bit budget Bm. In some embodiments, the bits spent in preceding encoding steps may be included in Bm and Bcurr,m. In such embodiments, the bit budget in the algorithm below can be given by Bm - Bcurr.

In step <NUM>, a predictor set P(q*) which gives the smallest prediction error out of the available predictors P(q), q = <NUM>,<NUM>,. , Nq is selected. The selected predictor set is given by <MAT> In some embodiments, b = <NUM> is omitted from the predictor set because the prediction is zero and contribution to the error will be the same for all predictor sets. The selected predictor set index is stored and the bit counter (Bcurr,m) is increased with the required number of bits, e.g. Bcurr,m := Bcurr,m + <NUM> if two bits are required to encode the predictor set.

In step <NUM>, a prediction weighting factor α is computed. The prediction weighting factor is used to create a weighted prediction as described in step <NUM> below. The prediction weighting factor needs to be available in the decoder <NUM>. In some embodiments, the prediction weighting factor α is encoded and transmitted to the decoder <NUM>. In such embodiments, the bit counter (Bcurr,m) is increased by the amount of bits required for encoding the prediction weighting factor. In other embodiments, the decoder may derive the prediction weight factor based on other parameters already available in the decoder <NUM>.

For each of the bands b = <NUM>,<NUM>,. Nbnd in step <NUM>, the following steps are performed:.

In step <NUM>, an intra-frame prediction value, <MAT>, is obtained. There are no preceding encoded coherence values for the first band (b = <NUM>). In some embodiments, the intra-frame prediction for the first band may be set to zero, <MAT>. In some embodiments, the intra-frame prediction for the first band may be set to an average value C, <MAT>.

In some alternative embodiments, the coherence value of the first band may be encoded separately. In such embodiments, the first value is encoded using a scalar quantizer to produce reconstructed value ĈSQ,<NUM>,m. Accordingly, the intra-frame prediction for the first band may be set to the reconstructed value, <MAT>. The bit counter, Bcurr,m, is increased by the amount of bits required to encode the coherence value of the first band. For example, if <NUM> bits are used to encode the coherence value of the first band, <NUM> bits are added to the current amount of bits spent for the encoding, for example, Bcurr,m := Bcurr,m + <NUM>.

For the remaining bands b = <NUM>,<NUM>,. , Nbnd, the intra-frame prediction <MAT> is based on previously encoded coherence values, i.e. <MAT>.

In step <NUM>, an inter-frame prediction value, Ĉinter,b,m, is obtained based on previously reconstructed coherence vector elements from one or more preceding frames. In cases where the background noise is stable or changing slowly, the frame-to-frame variation in the coherence band values Cb,m will be small. Hence, an inter-frame prediction using the values from previous frame will often be a good approximation which yields a small prediction residual and a small residual coding bit rate. As an example, a last reconstructed value for band b may be used for an inter-frame prediction value, i.e. Ĉinter,b,m = Ĉb,m-<NUM>. An inter-frame linear predictor considering two or more preceding frames can be formulated as <MAT>, where Ĉinter,m denotes the column vector of inter-frame predicted coherence values for all bands b of frame m, Ĉm-n represents the reconstructed coherence values for all bands b of frame m - n and gn is the linear predictor coefficients which span Ninter preceding frames. gn may be selected out of a predefined set of predictors, in which case the used predictor needs to be represented with an index that may be communicated to a decoder.

In step <NUM>, a weighted prediction, <MAT>, is formed based on the intra-frame prediction, <MAT>, the inter-frame prediction, Ĉinter,b,m, and the prediction weighting factor α. In some embodiments, the weighted prediction is given by <MAT> <MAT>.

In step <NUM>, a prediction residual is computed and encoded. In some embodiments, the prediction residual is computed based on the coherence vector and the weighted prediction, i.e. <MAT>. In some embodiments, a scalar quantizer is used to quantize the prediction residual to an index Ib,m. In such embodiments, the index is given by Ib,m = SQ(rb,m) where SQ(x) is a scalar quantizer function with a suitable range. An example of a scalar quantizer is shown in Table <NUM> below. Table <NUM> shows an example of reconstruction levels and quantizer indices for a prediction residual.

In some embodiments, the index Ib,m is encoded with a variable length codeword scheme that consumes fewer bits for smaller values. Some examples for encoding the prediction residual are Huffman coding, Golomb-Rice coding, and unary coding (the unary coding is the same as the Golomb-Rice coding with divisor <NUM>). In the step of encoding the prediction residual, the remaining bit budget (Bm - Bcurr,m) needs to be considered. If the length of the codeword Lcode(Ib,m) corresponding to index Ib,m fits within the remaining bit budget, i.e. Lcode(Ib,m) ≤ Bm - Bcurr,m, the index Ib,m is selected as the final index <MAT>. If the remaining bits are not sufficient to encode the index Ib,m, a bit rate truncation strategy is applied. In some embodiments, the bit rate truncation strategy includes encoding the largest possible residual value, assuming that smaller residual values cost fewer bits. Such a rate truncation strategy can be achieved by reordering a codebook as illustrated by table <NUM> in <FIG> shows an exemplary quantizer table <NUM> with unary codeword mapping for the scalar quantizer example shown in Table <NUM>. In some embodiments, a bit rate truncation may be achieved by advancing upwards in the table <NUM> in steps of two until codeword <NUM> is reached. That is, <FIG> illustrates a truncation scheme of moving upwards from a long code word to a shorter code word. To maintain the correct sign of the reconstructed value, each truncation steps takes two steps up the table <NUM>, as indicated by the dashed and solid arrows for negative and positive values respectively. By moving upward in the table <NUM> in steps of two, a new truncated codebook index <MAT> can be found. The upward search continues until <MAT> is satisfied or the top of the table <NUM> has been reached.

If the length of the codeword determined by the upward search fits does not exceed bit budget, the final index is selected <MAT> and <MAT> is output to the bitstream and the reconstructed residual is formed based on the final index, i.e. r̂b,m = <MAT>.

If after the upward search, the length of the codeword still exceeds the bit budget, <MAT>, this means that the bit limit has been reached Bm = Bcurr,m. In such instances, the reconstructed residual is set to zero r̂b,m = <NUM> and an index is not added to the bitstream. Since the decoder keeps a synchronized bit counter, Bcurr,m, the decoder may detect this situation and use r̂b,m = <NUM> without explicit signaling.

In an alternative embodiment, if the length of the codeword associated with the initial index exceeds the bit budget, the residual value is immediately set to zero, thereby foregoing the upward search describe above. This could be beneficial if computational complexity is critical.

In step <NUM>, a reconstructed coherence value Ĉb,m is formed based on the reconstructed prediction residual and the weighted prediction, i.e. <MAT>.

In step <NUM>, the bit counter is incremented accordingly. As described above, the bit counter is increased throughout the encoding process <NUM>.

In some embodiments, the frame-to-frame variations in the coherence vector are small. Hence, the inter-frame prediction using the previous frame value is often a good approximation which yields a small prediction residual and a small residual coding bit rate. Additionally, the prediction weighting factor α serves the purpose of balancing the bit rate versus the frame loss resilience.

<FIG> is a flow chart illustrating a decoding process <NUM> according to some embodiments. The decoding process <NUM> corresponding to the encoding process <NUM> may be performed by the decoder <NUM> according to the following steps:
In step <NUM>, a bit counter, Bcurr,m, configured to keep track of the bits spent during the decoding process <NUM> is initialized to zero, i.e. Bcurr,m = <NUM>. For each frame m, the decoder <NUM> obtains a copy of the last reconstructed coherence vector Ĉb,m-<NUM> and a bit budget Bm.

In step <NUM>, a selected predictor set P(q*) is decoded from the bitstream <NUM>. The bit counter is increased by the amount of bits required to decode the selected predictor set. For example, if two bits are required to decode the selected predictor set, the bit counter, Bcurr,m, is increased by two, i.e. Bcurr,m := Bcurr,m + <NUM>.

In step <NUM>, the prediction weighting factor α corresponding to the weighting factor used in the encoder <NUM> is derived.

In step <NUM>, an intra-prediction value, <MAT>, is obtained. The intra-frame prediction for the first band is obtained similarly to step <NUM> of the encoding process <NUM>. Accordingly, the intra-frame prediction for the first frame may be set to zero ( <MAT> <NUM>), an average value C ( <MAT>) or a coherence value of the first band may be decoded from the bitstream <NUM> and the intra-frame prediction for the first frame may be set to reconstructed value ĈSQ,<NUM>,m ( <MAT>). If the coherence value of the first band is decoded, the bit counter, Bcurr,m, is increased by the amount of bits required for the decoding. For example, if three bits are required for decoding the coherence value of the first band, the bit counter, Bcurr,m, is increased by three, i.e. Bcurr,m := Bcurr,m + <NUM>.

For the remaining bands b = <NUM>,<NUM>,. , Nbnd, the intra-frame prediction <MAT> is based on the previously decoded coherence values, i.e. <MAT> <MAT>.

In step <NUM>, an inter-frame prediction value, Ĉinter,b,m, is obtained similarly to step <NUM> of the encoding process <NUM>. As an example, a last reconstructed value for band b may be used for an inter-frame prediction value, i.e. Ĉinter,b,m = Ĉb,m-<NUM>.

In step <NUM>, a reconstructed prediction residual, r̂b,m, is decoded. If the bit counter, Bcurr,m, is below the bit limit, i.e. Bcurr,m < Bm, the reconstructed prediction residual is derived from an available quantizer index <MAT>. If the bit counter equals or exceeds the bit limit, the reconstructed prediction residual is set to zero, i.e. r̂b,m = <NUM>.

In step <NUM>, a coherence value Ĉb,m is reconstructed based on the reconstructed prediction residual and the weighted prediction, i.e. <MAT>. In step <NUM>, the bit counter is incremented.

In some embodiments, further enhancements of the CNG may be required in the encoder. In such embodiments, a local decoder will be run in the encoder where the reconstructed coherence values Ĉb,m are used.

<FIG> is a flow chart illustrating a process <NUM>, according to some embodiments, that is performed by an encoder <NUM> to encode a vector. Process <NUM> may begin with step <NUM> in which the encoder forms a prediction weighting factor. The following steps <NUM> through <NUM> may be repeated for each element of the vector. In step <NUM>, the encoder forms a first prediction of the vector element. In step <NUM>, the encoder forms a second prediction of the vector element. In step <NUM>, the encoder combines said first prediction and said second prediction using the prediction weighting factor into a combined prediction. In step <NUM>, the encoder forms a prediction residual using said vector element and said combined prediction. In step <NUM>, the encoder encodes the prediction residual with a variable bit rate scheme. In step <NUM>, the encoder reconstructs the vector element based on the combined prediction and a reconstructed prediction residual. In step <NUM>, the encoder transmits the encoded prediction residual. In some embodiments, the encoder encodes also the prediction weighting factor and transmits the encoded prediction weighting factor.

In some embodiments, the second prediction is an inter-frame prediction based on one or more vectors previously reconstructed for the sequence of vectors. In such embodiments, the inter-frame prediction is formed by performing a process which may include selecting a predictor from a set of predictors, applying the selected predictor to the one or more previously reconstructed vectors, and encoding an index corresponding to the selected predictor. In embodiments, where the inter-frame prediction is based on only one previously reconstructed vector, a value from the previous reconstructed vector may be used for the inter-frame prediction, i.e., for frequency band b, a last reconstructed value (i.e. vector element) for band b may be used for an inter-frame prediction value.

In some embodiments, the process <NUM> includes a further step in which the prediction residual is quantized to form a first residual quantizer index, wherein the first residual quantizer index is associated with a first code word.

In some embodiments, the step of encoding the prediction residual with the variable bit rate scheme includes obtaining a second residual quantizer index as a result of determining that the length of the first code word exceeds the amount of remaining bits, wherein the second residual quantizer index is associated with a second code word, and wherein the length of the second code word is shorter than the length of the first code word. In such embodiments, the process <NUM> includes a further step in which the encoder determines whether the length of the second code word exceeds the determined amount of remaining bits.

In some embodiments, the process <NUM> includes a further step in which the encoder receives a first signal on a first input channel, receives a second signal on a second input channel, determines spectral characteristics of the first signal and the second signal, determines a spatial coherence based on the determined spectral characteristics of the first signal and the second signal, and determines the vector based on the spatial coherence.

In some embodiments, the process <NUM> is performed by the encoder in an audio encoder and decoder system comprising at least two input channels. In some embodiments, the process <NUM> includes a further step in which the encoder creates a spectrum by performing a process comprising transforming the input channels and analyzing the input channels in frequency bands. In some embodiments, the vector comprises a set of coherence values, and wherein each value corresponds to the coherence between two of the at least two input channels in a frequency band.

<FIG> is a flow chart illustrating a process <NUM>, according to some embodiments, that is performed by a decoder <NUM> to decode a vector. Process <NUM> may begin with step <NUM> in which the decoder obtains a prediction weighting factor. The following steps <NUM> through <NUM> may be repeated for each element of the vector. In step <NUM>, the decoder forms a first prediction of the vector element. In step <NUM>, the decoder forms a second prediction of the vector element. In step <NUM>, the decoder combines said first prediction and said second prediction using the prediction weighting factor into a combined prediction. In step <NUM>, the decoder decodes a received encoded prediction residual. In step <NUM>, the decoder reconstructs the vector element based on the combined prediction and the prediction residual. In some embodiments, said vector is one of a sequence of vectors.

In some embodiments, the second prediction is an inter-frame prediction based on one or more vectors previously reconstructed for the sequence of vectors. In such embodiments, the inter-frame prediction is formed by performing a process which may include receiving and decoding a predictor; and applying the decoded predictor to the one or more previously reconstructed vectors. In embodiments, where the inter-frame prediction is based on only one previously reconstructed vector, a value from the previous reconstructed vector may be used for the inter-frame prediction, i.e., for frequency band b, a last reconstructed value (i.e. vector element) for band b may be used for an inter-frame prediction value.

In some embodiments, the process <NUM> further includes a step in which the decoder generates signals for at least two output channels based on the reconstructed vector.

<FIG> is a block diagram of encoder <NUM> according to some embodiments. As shown in <FIG>, encoder <NUM> may comprise: a processing circuit (PC) <NUM>, which may include one or more processors (P) <NUM> (e.g., a general purpose microprocessor and/or one or more other processors, such as an application specific integrated circuit (ASIC), field-programmable gate arrays (FPGAs), and the like); a network interface <NUM> comprising a transmitter (Tx) <NUM> and a receiver (Rx) <NUM> for enabling encoder <NUM> to transmit data to and receive data from other nodes connected to a network <NUM> (e.g., an Internet Protocol (IP) network) to which network interface <NUM> is connected; circuitry <NUM> (e.g., radio transceiver circuitry comprising an Rx <NUM> and a Tx <NUM>) coupled to an antenna system <NUM> for wireless communication with UEs); and local storage unit (a. , "data storage system") <NUM>, which may include one or more non-volatile storage devices and/or one or more volatile storage devices (e.g., random access memory (RAM)). In embodiments where PC <NUM> includes a programmable processor, a computer program product (CPP) <NUM> may be provided. CPP <NUM> includes a computer readable medium (CRM) <NUM> storing a computer program (CP) <NUM> comprising computer readable instructions (CRI) <NUM>. CRM <NUM> may be a non-transitory computer readable medium, such as, but not limited, to magnetic media (e.g., a hard disk), optical media, memory devices (e.g., random access memory, flash memory), and the like. In some embodiments, the CRI <NUM> of computer program <NUM> is configured such that when executed by data processing apparatus <NUM>, the CRI causes encoder <NUM> to perform steps described herein (e.g., steps described herein with reference to the flow charts and/or message flow diagrams). In other embodiments, encoder <NUM> may be configured to perform steps described herein without the need for code. That is, for example, PC <NUM> may consist merely of one or more ASICs. Hence, the features of the embodiments described herein may be implemented in hardware and/or software.

In an embodiment an encoder <NUM> comprises a processing circuitry <NUM>, the processing circuitry being configured to cause the encoder to form a prediction weighting factor, and for each element of the vector: form a first prediction of a vector element, form a second prediction of the vector element, form a prediction weighting factor, and to combine said first prediction and said second prediction using the prediction weighting factor into a combined prediction. The processing circuitry is further configured to cause the encoder to form a prediction residual using said vector element and said combined prediction, encode the prediction residual with a variable bit rate scheme and transmit the encoded prediction residual.

<FIG> is a block diagram of decoder <NUM> according to some embodiments. As shown in <FIG>, decoder <NUM> may comprise: a processing circuit (PC) <NUM>, which may include one or more processors (P) <NUM> (e.g., a general purpose microprocessor and/or one or more other processors, such as an application specific integrated circuit (ASIC), field-programmable gate arrays (FPGAs), and the like); a network interface <NUM> comprising a transmitter (Tx) <NUM> and a receiver (Rx) <NUM> for enabling decoder <NUM> to transmit data to and receive data from other nodes connected to a network <NUM> (e.g., an Internet Protocol (IP) network) to which network interface <NUM> is connected; circuitry <NUM> (e.g., radio transceiver circuitry comprising an Rx <NUM> and a Tx <NUM>) coupled to an antenna system <NUM> for wireless communication with UEs); and local storage unit (a. , "data storage system") <NUM>, which may include one or more non-volatile storage devices and/or one or more volatile storage devices (e.g., random access memory (RAM)). In embodiments where PC <NUM> includes a programmable processor, a computer program product (CPP) <NUM> may be provided. CPP <NUM> includes a computer readable medium (CRM) <NUM> storing a computer program (CP) <NUM> comprising computer readable instructions (CRI) <NUM>. CRM <NUM> may be a non-transitory computer readable medium, such as, but not limited, to magnetic media (e.g., a hard disk), optical media, memory devices (e.g., random access memory, flash memory), and the like. In some embodiments, the CRI <NUM> of computer program <NUM> is configured such that when executed by data processing apparatus <NUM>, the CRI causes decoder <NUM> to perform steps described herein (e.g., steps described herein with reference to the flow charts and/or message flow diagrams). In other embodiments, decoder <NUM> may be configured to perform steps described herein without the need for code. That is, for example, PC <NUM> may consist merely of one or more ASICs. Hence, the features of the embodiments described herein may be implemented in hardware and/or software.

In an embodiment a decoder <NUM> comprises a processing circuitry <NUM>, the processing circuitry being configured to cause the decoder to obtain a weighting factor, and for each element of the vector: form a first prediction of a vector element, form a second prediction of the vector element, obtain a prediction weighting factor and to combine said first prediction and said second prediction using the prediction weighting factor into a combined prediction. The processing circuitry is further configured to cause the decoder to decode a received encoded prediction residual and reconstruct the vector element based on the combined prediction and the decoded prediction residual.

<FIG> is a diagram showing functional units of encoder <NUM> according to some embodiments. As shown in <FIG>, encoder <NUM> includes a first forming unit <NUM> for forming a first prediction of the vector element; a second forming unit <NUM> for forming a second prediction of the vector element; a third forming unit <NUM> and an encoding unit <NUM> for forming and encoding a prediction weighting factor; a combining unit <NUM> for combining said first prediction and said second prediction using the prediction weighting factor into a combined prediction; a fourth forming unit <NUM> for forming a prediction residual using said vector element and said combined prediction; an encoding unit <NUM> for encoding the prediction residual with a variable bit rate scheme; and a transmitting unit <NUM> for transmitting the encoded prediction weighting factor and the encoded prediction residual.

<FIG> is a diagram showing functional units of decoder <NUM> according to some embodiments. As shown in <FIG>, decoder <NUM> includes a first forming unit <NUM> for forming a first prediction of the vector element; a second forming unit <NUM> for forming a second prediction of the vector element; an obtaining unit <NUM> for obtaining a prediction weighting factor; a combining unit <NUM> for combining said first prediction and said second prediction using the prediction weighting factor into a combined prediction; a receiving unit <NUM> and a decoding unit <NUM> for receiving and decoding an encoded prediction residual; and a reconstructing unit <NUM> for reconstructing the vector element based on the combined prediction and the prediction residual.

Also, while various embodiments of the present disclosure are described herein, it should be understood that they have been presented by way of example only, and not limitation.

Claim 1:
An audio signal processing method (<NUM>) for encoding a vector, the method comprising:
forming (s602) a weighting factor;
for each element of the vector:
- forming (s604) an intra-frame prediction of the vector element;
- forming (s606) an inter-frame prediction of the vector element;
- combining (s608) said intra-frame prediction and said inter-frame prediction using the weighting factor into a combined prediction;
- forming (s610) a prediction residual using said vector element and said combined prediction;
- encoding (s612) the prediction residual with a variable bit rate scheme; and transmitting (s616) the encoded prediction residual.