Patent Description:
In perceptual audio coding, decorrelators are an important building block for parametric spatial audio coding. Known solutions relate to decorrelators known from parametric spatial audio coding like parametric stereo or MPEG surround. Decorrelators as described in [<NUM>] or [<NUM>] use computationally costly time domain reverberation (reverb) filters with a long impulse response. Decorrelators such as described in [<NUM>] or [<NUM>] require the use of a Quadrature Mirror Filterbank (QMF) with considerable processing delay and computationally expensive Lattice filters.

There is, thus, a need for a decorrelator, a processing system having such a decorrelator and a method for decorrelating portions of an audio signal allowing for a low processing delay and/or low computational complexity decorrelation.

It is an object of the present invention to provide for a decorrelator, a processing system and for a method for decorrelation allowing for a low processing delay and/or decorrelation with a low complexity and high perceptual quality, especially in processing signals containing transients.

This object is achieved by the subject matter as defined in the independent claims.

A finding of the present invention is that dividing a frequency representation in a plurality of parts and for processing, i.e., delaying each of the parts with a separate delay unit, allows for a low processing delay, as the computational the different parts may be performed in parallel. As the same time, such frequency domain operations require a low computational complexity.

According to an embodiment, a decorrelator comprises a plurality of delay units, wherein each delay unit is configured for receiving a part of a frequency representation being based on an audio signal, wherein each delay unit is configured for delaying the received part to provide a delayed part. The decorrelator comprises an envelope shaper configured for receiving an combining signals being based on the delayed parts of the frequency representation, for receiving the frequency representation of the audio signal, for adjusting an energy of the delayed parts in respect of the frequency representation of the audio signal and for providing a combined shape frequency representation.

According to an embodiment, different parts of the frequency representation comprise a same or a different number of frequency bins. Wherein a same number of frequency bins may allow for a same processing time, a different number of frequency bins may allow for an adaptation towards application requirements.

According to an embodiment, the decorrelator comprises a phase shifter configured for phase shifting the frequency representation of the audio signal, or for phase shifting the audio signal in a time domain to obtain a phase shifted audio signal. Phase shifting may allow for a perceived reverberation and therefore for a high audio quality.

According to an embodiment, the phase shifter is configured for a phase shifting the frequency representation of the audio signal and comprises a plurality of Allpass filters, wherein each Allpass filter is configured for phase shifting an associated part of the frequency representation of the audio signal. That is, the Allpass filter may be associated and adapted towards the respective part of the audio signal which may allow for a high overall audio quality.

According to an embodiment, an Allpass filter of the plurality of Allpass filters comprises a set of Allpass filter structures being serially connected to each other, i.e., using Schroeder IIR filters. The Allpass filter structures are adapted for providing different time delays. Alternatively or in addition, the Allpass filter structures comprise a nested Allpass filter structure.

According to an embodiment, a number of Allpass filter structures and/or a circuitry of the Allpass filter structure is equivalent or different between different Allpass filters. This allows for a high flexibility of the decorrelator.

According to an embodiment, the different time delays are based on a prime number multiple of a local sampling rate used for obtaining the frequency representation of the audio signal. This allows for a high perceived audio quality.

According to an embodiment, the set of Allpass filter structures comprises a number of four Allpass filter structures and are adapted for providing a delay of <NUM>, <NUM>, <NUM> and <NUM> time units. Such a time unit may be based on a blocksize of the conversion into the frequency domain. For example, using a blocksize of <NUM> with <NUM>% overlap, a time unit may result in <NUM> samples@<NUM> = <NUM>. Reasonable other time units may be, for example, <NUM> or <NUM> samples or other values. The time units are preferably short enough to allow for sufficient time resolution in the subsequent time/frequency envelope shaping. In an alternative solution, a delay of <NUM>, <NUM>, <NUM> and <NUM> is provided by the four Allpass filter structures. This allows to avoid overlaps in the time domain.

According to an embodiment, a gain factor of the Allpass filter is adapted to a value with a magnitude, i.e., positive or negative values, of <NUM> within a tolerance range. The tolerance range is, for example, <NUM>%, <NUM>% or <NUM>%.

According to an embodiment, the phase shifter is configured for phase shifting the audio signal in a time domain, wherein the phase shifter comprises a set of Allpass filter structures being serially connected to each other, wherein the Allpass filter structures are adapted for providing different time delays. Alternatively or in addition, the Allpass filter structures comprise a nested Allpass filter structure.

According to an embodiment, the different Allpass time delays are based on a prime number multiple of a reciprocal of a sampling rate used for obtaining the frequency representation of the audio signal. Like in the frequency domain, a corresponding advantage may also be obtained in the time domain, In the time domain, different time delays may be based on a prime number being obtained by multiplying each of a set of minimal prime numbers, e.g., <NUM>, <NUM>, <NUM> and <NUM> as one example set or <NUM>, <NUM>, <NUM> and <NUM> as another example set with a downsampling factor used for generating the parts of the frequency representation of the audio signal to obtain an intermediate result and for using a next prime number with respect to the intermediate result. As a next prime number a closest distance may be understood, e.g., to obtain the next larger or next smaller prime-value. In the given example, the values <NUM>, <NUM>, <NUM> and <NUM> may be obtained for the first set and <NUM>, <NUM>, <NUM> and <NUM> may be obtained for the second example set. Here, one time unit may be <NUM> sample. The sample may relate to a sampling frequency being, e.g., <NUM>. In other embodiments, sampling frequency can also be <NUM> or <NUM> or other values.

According to an embodiment, the decorrelator comprises a first conversion unit for obtaining the frequency representation of the audio signal from the audio signal for the envelope shaper and comprising a second conversion unit for obtaining a frequency representation from the reverberated audio signal, wherein the parts of the frequency representation form parts of the frequency representation from the reverberated audio signal. This allows to generate the used signal formed directly at the decorrelator.

According to an embodiment, the decorrelator is adapted for additionally implementing a same and predefined delay for a subset or all parts of the frequency representation. That is, a delay that is equal for the respective parts or delay lines may also be applied commonly in a common delay module which allows for simple delay units in the respective delay lines for an associated part.

According to an embodiment, the delay units associated to a spectral part of the plurality of delay units are configured for delaying the associated part of the frequency representation differently when compared to delay units associated to other spectral parts. This allows for a high perceived quality by treating different frequency portions differently.

According to an embodiment, the delay unit is configured for delaying parts of the frequency representation comprising lower frequencies with a higher time delay when compared to parts of the frequency representation comprising higher frequencies.

According to an embodiment, a relationship between different time delays is linear, logarithmic and/or based on a rounding on subband samples. This allows for a high perceived quality.

According to an embodiment, the decorrelator comprises a conversion unit for receiving an converting the audio signal or a reverberated version of the audio signal into the parts by performing a time-block-wise discrete Fourier transform, DFT, or short-time Fourier transform, STFT, wherein the conversion unit is configured for converting blocks having an overlap of <NUM>% within a tolerance range. Such block-wise conversion allows for short delays for a respective part being obtained and for a parallel treatment of the different parts.

According to an embodiment, the envelope shaper is configured for operating in a subband domain and with a temporal resolution of less than <NUM> milliseconds.

According to an embodiment, the decorrelator comprises a signal processing stage configured for receiving a signal based on the combined shaped frequency representation, e.g., as a mono signal, and for processing the mono signal at least to a stereo signal. This allows for an improved perception of a listener.

According to an embodiment, the decorrelator comprises a signal processing stage configured for processing the combined shaped frequency representation at least to a stereo signal and for source extent modelling based on the at least stereo signal, e.g., in the frequency domain.

According to an embodiment, a processing system comprises a decorrelator as described herein and a processing stage for transforming a mid/side decomposed signal to a left/right decomposed signal.

According to embodiments, the processing system may perform transient suppression to suppress echoes, e.g., pre-echoes and/or post-echoes caused by a transient. Such a transient handling may comprise muting the output of a decorrelator and, correspondingly, amplifying an output of a delay compensation unit providing for a portion of the left/right decomposed signal and being in parallel with the decorrelator and connected with the processing stage.

According to an embodiment, a method comprises receiving a plurality of parts of a frequency representation being based on an audio signal, delaying each of the received parts to provide a plurality of delayed parts and receiving and combining signals being based on the delayed parts of the frequency representation. The method comprises receiving the frequency representation of the audio signal and adjusting an energy of the delayed parts in respect of the frequency representation of the audio signal. A combined shaped frequency representation is provided.

According to an embodiment, a computer program or computer program product or a non-transitory storage medium having stored therein instructions to carry out respective instructions is provided for executing such a method, when running on a computer.

Further advantageous embodiments are defined in dependent claims.

Advantageous embodiments are described in more detail whilst making reference to the accompanying drawings, in which:.

In the following description, a plurality of details is set forth to provide a more thorough explanation of embodiments of the present invention. However, it will be apparent to those skilled in the art that embodiments of the present invention may be practiced without these specific details. In other instances, well known structures and devices are shown in block diagram form rather than in detail in order to avoid obscuring embodiments of the present invention. In addition, features of the different embodiments described hereinafter may be combined with each other, unless specifically noted otherwise.

<FIG> shows a schematic block diagram of a decorrelator <NUM> according to an embodiment. Decorrelator <NUM> comprises a number of at least two delay units <NUM><NUM> to <NUM>n with n > <NUM>. Although <FIG> illustrates a number of two delay units <NUM>, the number is preferably higher, e.g., <NUM>, <NUM>, <NUM> or other values to be obtained with a power of <NUM>, wherein embodiments are not limited to such numbers. That is, embodiments may also comprise a number of <NUM>, <NUM>, <NUM> or <NUM> delay units <NUM>. Each delay unit is configured for receiving an associated part <NUM><NUM> to <NUM>n of a frequency representation <NUM> being based on an audio signal. For example, the frequency representation <NUM> may be or may comprise a spectrum being obtained by a Fourier Transform such as a discrete Fourier Transform, DFT, or a short term Fourier transform, STFT. The parts <NUM><NUM> to <NUM>n may be obtained, for example, as a subband of the spectrum, i.e., a part of the frequency domain representation. For example, such a part <NUM><NUM> to <NUM>n may be obtained by using an appropriate window.

Each delay unit <NUM><NUM> to <NUM>n is configured for delaying the received part <NUM><NUM> to <NUM>n so as to provide a delayed part <NUM>'<NUM> to <NUM>'n, i.e., for having a delay in the time domain.

The decorrelator <NUM> further comprises an envelope shaper <NUM> configured for receiving signals being based on the delay parts <NUM>'<NUM> to <NUM>'n. Such signals may be the delayed parts <NUM>'<NUM> to <NUM>'n themselves or processed variants thereof. The envelope shaper <NUM> is configured for combining the received signals. In addition, the envelope shaper is configured for receiving the frequency representation <NUM> of the audio signal. The envelope shaper <NUM> is configured for adjusting an energy of the delayed parts <NUM>'<NUM> to <NUM>'n in respect of the frequency representation <NUM> of the audio signal. The envelope shaper <NUM> is configured for providing a combined shaped frequency representation <NUM>. In the combined shaped frequency representation <NUM>, the respective parts <NUM><NUM> to <NUM>n, signals resulting thereof respectively, may be decorrelated with regard to one another and/or with regard to the frequency representation <NUM>.

Although the envelope shaper <NUM> is illustrated so as to receive the combined frequency representation <NUM>, as an alternative, the envelope shaper <NUM> may receive the respective information by receiving the possibly non-delayed or commonly treated parts <NUM><NUM> to <NUM>n.

<FIG> shows a schematic block diagram of a decorrelator <NUM> according to an embodiment. The decorrelator <NUM> is configured for receiving an audio signal <NUM>. The decorrelator <NUM> may comprise a conversion unit <NUM> configured for generating the frequency representation <NUM> shown in <FIG>. The conversion unit <NUM> may provide for parts <NUM><NUM> to <NUM><NUM> being obtained by an example STFT. For example, the frequency representation may comprise a number of <NUM> frequency bins in total. Alternatively, <NUM> bins may be used. For example, two types of Digital Fourier Transforms (DFT) may be used, a so-called "evenly stacked" and an "oddly stacked". For example, as "standard" DFT the evenly stacked version may be considered having, in the example provided, <NUM> bands (<NUM> complex, one real and one imaginary). The oddly stacked may comprise <NUM> (complex) bands. Both transforms can be used in embodiments described herein. The parts <NUM><NUM> to <NUM><NUM> may comprise, partly or completely, a same or different number of bins. For example, part <NUM><NUM> may comprise the first to the ninth bin, e.g., <NUM> bins. Part <NUM><NUM> comprises, for example, bins <NUM> to <NUM> and, thus, a number of ten bins. The adaptation or selection with regard to the number of bins may be based on the sampling frequency being in the illustrated example <NUM>, the overlap that is, for example, <NUM>% and/or a number of parts <NUM>, to <NUM><NUM> to be generated. The parts <NUM><NUM> to <NUM><NUM> may comprise an equal or different number of frequency bins such that some or all parts <NUM><NUM> to <NUM><NUM> may also be generated so as to comprise a same number of frequency bins.

The decorrelator <NUM> further comprises a delay section <NUM> having delay lines <NUM><NUM> to <NUM><NUM>, each delay line <NUM><NUM> to <NUM><NUM> being associated with one specific part <NUM><NUM> to <NUM><NUM> and configured for receiving said part, a processed version thereof respectively. The delay units <NUM><NUM> to <NUM><NUM> may be associated to a respective spectral part <NUM><NUM> to <NUM><NUM>. Such a delay unit <NUM><NUM> to <NUM><NUM> may be configured for delaying the associated part of the frequency representation <NUM> differently when compared to delay units associated to other spectral parts. Alternatively or in addition, a relationship between different time delays may be one of linear, logarithmic and/or based on a rounding on super band samples.

The decorrelator <NUM> further comprises a phase shifter <NUM> being coupled to the delay section <NUM>, the phase shifter <NUM> configured for receiving the delayed parts <NUM>'<NUM> to <NUM>'<NUM>. Phase shifting using the phase shifter <NUM> may allow for a reverberation in the signal parts. However, according to embodiments, a sequence of the delay section <NUM> and the reverberation section <NUM> may also be changed such that a respective part <NUM><NUM> to <NUM><NUM> may first be subject of a reverberating filter and afterwards being delayed.

The phase shifter <NUM> may be configured for phase shifting the frequency representation <NUM> of the audio signal, a processed, e.g., delayed, version thereof. The phase shifting may also be performed prior to converting the audio signal <NUM> into the frequency domain, a corresponding phase shifter may be configured for phase shifting the audio signal <NUM> in the time domain to obtain a phase shifted audio signal. In the short configuration where the phase shifter <NUM> is configured for phase shifting the frequency representation of the audio signal <NUM>, the delayed version thereof respectively, the phase shifter may comprise a plurality of Allpass filters <NUM><NUM> to <NUM><NUM>. In the shown example, the Allpass filters <NUM><NUM> to <NUM><NUM> are configured to receive the delayed parts <NUM>'<NUM> to <NUM>'<NUM>. The term Allpass filter is to be understood that the frequency range to be passed corresponds to the frequency range of the respective part <NUM><NUM> to <NUM><NUM>. Wherein this may include examples where each of the Allpass filters <NUM><NUM> to <NUM><NUM> passes the complete frequency range provided in the frequency representation, the passband of different Allpass filters <NUM><NUM> to <NUM><NUM> may also differ from one another based on the different frequency bins contained in the respective parts <NUM><NUM> to <NUM><NUM>.

Each of the Allpass filters <NUM><NUM> to <NUM><NUM> is configured for phase shifting an associated part of the frequency representation of the audio signal.

That is, a number of Allpass filter structures and/or a circuitry of the Allpass filter structure may be the same, i.e., equal or comparable, or may, alternatively, be different between different Allpass filters <NUM><NUM> to <NUM><NUM>.

A time delay provided by the delay lines <NUM><NUM> to <NUM><NUM> may be same or may be different for different parts <NUM><NUM> to <NUM><NUM>. As indicated in <FIG>, parts of the frequency representation comprising lower frequencies may be delayed with a higher time delay when compared to parts of the frequency representation comprising higher frequencies. From bin <NUM> to higher bins, a represented frequency may increase. As represented in the z-domain, the time delay may decrease with an increase of frequencies.

Signals <NUM><NUM> to <NUM><NUM> may comprise a result of the delaying and the phase shifting, e.g., as an output of the Allpass filters <NUM><NUM> to <NUM><NUM>.

The envelope shaper <NUM> may be configured for receiving signals <NUM><NUM> to <NUM><NUM> and an unfiltered and undelayed version thereof, i.e., the parts <NUM><NUM> to <NUM><NUM>, i.e., the frequency representation of the audio signal <NUM>. The parts <NUM>, to <NUM><NUM> may be understood as subbands. The envelope shaper <NUM> may be configured for operating in a subband domain. For example, a temporal resolution of the envelope shaper <NUM> may be at most or less than <NUM> milliseconds, e.g., <NUM> milliseconds, <NUM> milliseconds, <NUM> milliseconds or less.

The decorrelator <NUM> may comprise another conversion unit <NUM> that may provide for an inverse operation when compared to the conversion unit <NUM>. For example, the conversion rate <NUM> may perform an inverse short term Fourier transform iSTFT. The combined shape frequency representation <NUM> may comprise information with regard to the frequency domain that is present in each of the bins such that the combined shaped frequency representation <NUM> may be treated correspondingly to the output of the conversion unit <NUM>. That is, the conversion unit <NUM> may receive the processed versions of the parts <NUM><NUM> to <NUM><NUM> of the frequency representation <NUM> and for synthesizing a synthesized signal <NUM> from the processed versions <NUM>'<NUM> to <NUM>'<NUM> based on, e.g., an overlap-add procedure. The signal <NUM> may be provided, for example, at an interface <NUM> of the decorrelator <NUM>.

The envelope shaper <NUM> may be configured for shaping spectral bins in time and/or frequency. Shaping may be performed by the envelope shaper <NUM> for individual bins and/or for groups of bins, e.g., by implementing an interdependent or an at least groupwise common shaping processing.

When referring again to conversion unit <NUM>, same may be configured for receiving and converting the audio signal <NUM> or a reverberated version thereof into the parts <NUM>, to <NUM><NUM>, wherein the number of <NUM> is an example only. The reverberated version of the audio signal <NUM> may be an input in case the phase shifter <NUM> operates in the time domain and may thus be arranged upstream of the conversion unit <NUM>. The conversion unit <NUM> may perform a time-block-wise discrete Fourier transform, DFT, or a short-time Fourier transform, STFT. The conversion unit may be configured for converting blocks having an overlap of, e.g., <NUM>% within a tolerance range. For example, the tolerance range may be <NUM>% as far as possible, at most <NUM>%, at most <NUM>%, at most <NUM>% or more.

The blocks may comprise a block length of, for example, <NUM> samples, <NUM> samples or <NUM> samples, wherein a value of <NUM> may be preferred.

<FIG> shows a schematic block diagram of a decorrelation <NUM>. When compared to the decorrelator <NUM>, the decorrelator <NUM> may additionally comprise a pre-delay <NUM>, wherein the term pre-delay does not limit the delay to be implemented directly prior or subsequent to any specific block. The pre-delay <NUM> may be located at any stage prior to the envelope shaper <NUM>, preferably and when operating in the frequency domain, after the conversion unit <NUM>. That is, for example, a sequence between the Allpass filters of the reverberation or phase shifter <NUM> and the pre-delay <NUM> may be swapped when compared to the illustration in <FIG>. The pre-delay <NUM> or the delay block <NUM> may be configured to additionally implement a same and predefined delay for a subset or all of the parts <NUM><NUM> to <NUM><NUM> of the frequency representation. This may allow for implementing the same delay to each part <NUM><NUM> to <NUM><NUM> or a group thereof for combining the processing at this stage and to use delay lines <NUM><NUM> to <NUM><NUM> for adding a probably individual delay to differ from the common delay implemented in block <NUM>. In one example, the pre-delay <NUM> is configured to allow for a constant pre-delay for all spectral bands.

<FIG> shows a schematic block diagram of an Allpass filter <NUM> according to an embodiment that may be operated at least as a part of one of filters <NUM><NUM> to <NUM><NUM> of decorrelator <NUM> and/or <NUM>. Allpass <NUM> may comprise a structure of a Schroeder IIR filter, for example, and may comprise a forward branch <NUM> in combination with a backward branch <NUM> in combination with a delay block <NUM> to provide for a respective output signal <NUM> being based on an input signal <NUM> of the Allpass filter <NUM>. An Allpass filter <NUM> of decorrelator <NUM> and/or <NUM> may comprise one or more of such Allpass filters <NUM> being connected serially to one another. To provide for different time delays in different Allpass filters <NUM><NUM> to <NUM><NUM>, a different number of Allpass filter structures <NUM> may be serially connected.

In other words, <FIG> shows an Allpass filter stage.

<FIG> shows a schematic block diagram of an Allpass filter structure <NUM> being a nested Allpass filter structure. Alternatively or in addition to Allpass filter structure <NUM>, one or more Allpass filter structures <NUM> may form at least a part of an Allpass filter <NUM><NUM> to <NUM><NUM> of the decorrelator <NUM> and/or <NUM>. Although showing two delay blocks <NUM>, and <NUM><NUM>, a different and especially higher number of delay blocks <NUM> may be present resulting possibly in an increased number of forward branches <NUM> and/or backward branches <NUM>. Further, gains g<NUM>/- g<NUM> and/or g<NUM>/-g<NUM> may be adopted.

When considering, for example, to serially connect delay blocks <NUM> in one or more Allpass filter structures <NUM> and/or one or more Allpass filter structures <NUM>, different Allpass filters <NUM><NUM> to <NUM><NUM> may be implemented so as to comprise a different time delay when compared to other Allpass filters. For example, the different delays of different Allpass filter structures and/or circuitries of Allpass filter structures may be based on a prime number multiple of a local sampling rate, e.g., <NUM>, used for obtaining the frequency representation <NUM> of the audio signal <NUM>. For example, a set of Allpass filter structures forming at least a part of an Allpass filter may comprise a number of four Allpass filter structures, e.g., Allpass filter structures <NUM>. The different delay blocks therein may be adapted for providing a delay of <NUM>, <NUM>, <NUM> and <NUM>. According to a different example, the number of four Allpass filter structures may provide a delay of <NUM>, <NUM>, <NUM> and <NUM> units in the z-domain. Those values may form a set of prime values, i.e., a number of <NUM>, <NUM>, <NUM>, <NUM> or more prime values may be grouped.

When transferring this embodiment, the sets of prime values respectively, to the possible operations of the Allpass filters in the time domain, the time delays are based on a prime number multiple of a reciprocal of a sampling rate used for obtaining the frequency representation of the audio signal in an embodiment. For example, the different time delays may be based on a prime number being obtained by multiplying each of a set of prime numbers as mentioned, for example, <NUM>, <NUM>, <NUM> and <NUM> or <NUM>, <NUM>, <NUM> and <NUM> with a down sampling factor used for generating the parts of the frequency representation of the audio signal to obtain an intermediate result. Instead of the intermediate result, a next prime number with respect to the intermediate result may be used. For example, when referring to the downsampling factor of <NUM> and considering the sets of prime numbers above, such a result may be the delay of <NUM>, <NUM>, <NUM> and <NUM> on the one hand and <NUM>, <NUM>, <NUM> and <NUM> on the other hand, wherein each delay may relate to a multiplication with <NUM> sample at the sampling rate which is, for a sampling rate of <NUM> approximately <NUM>. Other sets of prime numbers are possible without limitation.

When referring, for example, to <FIG>, the gain factor g of the Allpass filter may be adapted to a value of <NUM> within a tolerance range of, for example, ± <NUM>%, ± <NUM>% or t <NUM>%. However, the gain value may also have a negative value of, e.g., -<NUM> within the mentioned tolerance range. That is, the gain factor may be adapted to a value with a magnitude of <NUM> within the tolerance range.

In other words, additionally to the serial out pass configuration of <FIG>, also a nested configuration in which the delay element of an outer Schroeder Allpass is replaced by another inner Allpass configuration or a combination of both configurations may be implemented. <FIG> shows a simple nested Allpass filter stage.

<FIG> shows a schematic block diagram of a decorrelator <NUM> according to an embodiment. The decorrelator <NUM> comprises the phase shifter <NUM> configured to operate in the time domain. An Allpass filter structure <NUM>' may be configured for using the respective next prime numbers when compared to the sets of prime numbers as described in connection with decorrelator <NUM> and/or <NUM>. For ensuring a precise operation of decorrelator <NUM> same may comprise conversion units <NUM><NUM> and <NUM><NUM>. Whilst conversion unit <NUM>, may provide for the frequency representation of the audio signal, conversion unit <NUM><NUM> may receive the reverberated or phase shifted audio signal <NUM>' provided by the phase shifter <NUM>'. The obtained parts <NUM>"<NUM> to <NUM>"<NUM> may be delayed by delay units <NUM><NUM> to <NUM><NUM> arriving at a comparable input for the envelope shaper <NUM> when compared to the decorrelator <NUM> and/or <NUM> whilst allowing for a time-domain based reverberation. That is, the parts of the frequency representation may form parts of the frequency representation from the reverberated audio signal <NUM>'.

According to embodiments, a decorrelator as described herein may be combined with further functionality, i.e., the output signal can be further processed.

In other words, <FIG> shows an alternative implementation of a decorrelator with regard to <FIG>.

Further, the inventive decorrelators may be combined with transient handling processing. Transients may cause artifacts in the decorrelated stereo signal such as post-echoes or unwanted panning effects. To mitigate this, a transient handling can be combined with the decorrelator described herein. Transient handling may mute the decorrelator output to preserve the direct onset waveform and suppress the post-echo caused by the pre-delay.

<FIG> shows a schematic block diagram of a decorrelator <NUM> according to an embodiment. Decorrelator <NUM> comprises at least a part of decorrelator <NUM>, wherein alternatively or in addition at least parts of decorrelator <NUM>, <NUM> and/or <NUM> may be arranged. Decorrelator <NUM> may comprise a signal processing stage <NUM> configured for processing the combined shaped frequency representation <NUM> or a signal based thereon. The combined shaped frequency representation <NUM> may be considered as a mono signal, i.e., it may represent a single channel. From the received mono signal the processing stage may provide at least signals <NUM>, and <NUM><NUM> representing a stereo signal.

A source extender <NUM> that models the perceptual effect of a spatially extended sound source from a mono signal of a point source and a decorrelated version thereof may be coupled to the decorrelator <NUM>. The source extender <NUM> may comprise filters <NUM><NUM> to <NUM><NUM> allowing for a source extend modelling based on the stereo signal having signals <NUM>, and <NUM><NUM>. The source extend modeling may be performed, for example, in the frequency domain and may result in stereo output signals <NUM><NUM>, e.g., a left channel and <NUM><NUM>, e.g., a right channel. It should be noted that the source extender <NUM> may also form a part of the decorrelator <NUM>.

In other words, <FIG> shows a schematic block diagram of source extent processing.

<FIG> shows a schematic block diagram of a processing system <NUM> according to an embodiment. Processing system <NUM> may comprise decorrelator <NUM>. Alternatively or in addition, decorrelator <NUM>, <NUM>, <NUM> and/or <NUM> may be arranged. The processing system <NUM> comprises a processing stage <NUM> configured for transforming a mid/side decomposed signal <NUM> to a left/right decomposed signal <NUM>. That is, the mid/side decomposed signal <NUM> may comprise at least a first signal <NUM><NUM>, e.g., representing one of the mid/middle or side portion and a second signal <NUM><NUM> representing the other portion. The processing stage <NUM> may be configured for transforming the signals <NUM><NUM> to <NUM><NUM> and possibly additional signals into at least signals <NUM><NUM> to <NUM><NUM> representing a left channel and a right channel. One channel, e.g., the left channel L, may be obtained, for example, by adding the mid component M and the side component M+S; whilst the other, e.g., right channel may be obtained by subtracting one component from the other e.g., M-S. According to a different approach both channels may be obtained by using <NUM> % or a factor of <NUM> thereof, i.e., <NUM>(M+S) and <NUM>(M-S). Other factors and/or determination rules are possible.

According to an embodiment, signal <NUM>, is provided by the decorrelator of the processing system <NUM>. The other signal <NUM><NUM> may be provided by a delay compensation unit <NUM> that is connected in parallel to the decorrelator <NUM> and is configured for also receiving the audio signal <NUM>. The delay compensation unit <NUM> is, thus, connected with the processing stage <NUM>. The delay compensation unit <NUM> may be configured for providing a time delay that is comparable to the decorrelator. Preferably, for frequency domain embodiments, the delay equals the processing delay introduced by the STFT analysis/synthesis of the decorrelator. However, the decorrelator <NUM> may provide for additional signal processing leading to a decorrelation such that the signal <NUM><NUM> may comprise a similar delay when compared to signal <NUM><NUM>. According to an embodiment, the signal <NUM><NUM> may be unprocessed with exception of the time delay.

The decorrelator <NUM> in the processing system <NUM> may provide the combined shaped frequency representation as at least one part of the mid/side decomposed signal to the processing stage <NUM>. The processing stage <NUM> may transform the combined shaped frequency representation together with delay signal <NUM><NUM> to the left/right decomposed signal in the frequency domain. The output of the processing stage <NUM> may be a L/R signal <NUM>. The decorrelator <NUM> itself may produce a mono signal S (Side, component <NUM>), in that respect it is only part of it. With the transient handling, the direct part M (<NUM><NUM>; <NUM>'<NUM>) and the decorrelator output S (Signal <NUM>) may become closely coupled, since the signal S will be muted and be "replaced" by an amplified M signal (Signal <NUM>'<NUM>). As a consequence, both units, decorrelator and "upmixing unit" <NUM> are closely coupled and so processing stage <NUM> finally provides the decorrelated stereo signal. If the decorrelator would be operated standalone with mono output, e.g., without processing stage <NUM>, then delay compensated direct signal, without any scaling, should be added directly to the mono output to fill the muted gap and provide a "complete" signal.

In other words, <FIG> shows a decorrelator in M/S to L/R setup with delay compensation of mono (mid-signal) input.

<FIG> shows a schematic block diagram of a processing system <NUM> according to an embodiment. When compared to the processing system <NUM>, the processing system <NUM> comprises a transient suppressor <NUM> configured for detecting a transient in the audio signal <NUM> or the frequency representation <NUM> thereof at an input of the decorrelator. The transient suppressor may comprise a transient detection unit <NUM> configured for receiving the audio signal <NUM> or the frequency representation thereof. The transient detection unit <NUM> may detect a transient in the audio signal, e.g., by processing the audio signal <NUM>. The transient suppressor <NUM> may further comprise a mute unit <NUM> configured for receiving the combined shaped frequency representation <NUM> and for muting the same based on a control signal. However, it is to be noted that a same or comparable effect may also be obtained when controlling the decorrelator <NUM> or the decorrelator contained in the processing system <NUM> so as to mute the output of the decorrelator. That is, the mute unit <NUM> may also form a part of the decorrelator. However, signal <NUM>, forming the input of the processing stage <NUM> may be muted based on a detected transient in the audio signal <NUM>. The transient suppressor <NUM> may be configured for temporarily muting the portion provided by the decorrelator to suppress echoes at the processing stage <NUM>, wherein the echoes may relate to pre-echoes and/or post-echoes. When operating in the time domain, a window may be used for a soft muting to avoid additional transients to be caused by the muting. If done in the frequency domain, the STFT windowing being described in connection with decorrelators <NUM>, <NUM> and <NUM> may provide for such an effect automatically, i.e., in a synergetic manner.

With regard to the processing stage <NUM>, muting the output of the decorrelator <NUM> might lead to an unwanted shift in the input energy of the signal processing stage <NUM>. To avoid negative effects an amplifier <NUM> may be connected between the delay compensation unit <NUM> and the signal processing stage <NUM> to temporarily amplify the signal <NUM><NUM> to obtain amplified signal <NUM>'<NUM>. Amplification of signal <NUM><NUM> may be conditional to muting the output of the decorrelator <NUM>. That is, the transient suppressor <NUM> may be configured for amplifying the portion of the delay compensation unit <NUM> corresponding to muting the portion of the decorrelator.

A level of amplification may be fixed or may be controlled. According to one example, if applied, the amplification factor of amplifier <NUM> may be a factor of <MAT> when compared to an unmuted portion of the decorrelator. That is, when muting the output of the decorrelator, the amplifier <NUM> may amplify signal <NUM><NUM> by <MAT> whilst not amplifying signal <NUM><NUM> during times where the mute is off, i.e., g=<NUM>.

Optionally and to avoid unwanted effects during the transient suppression, the transient suppressor <NUM> may be configured for suppressing a detected transient in the audio signal and for suppressing a following transient not earlier than a predefined inhibition time. For example, the transient suppressor <NUM> may comprise a control unit <NUM> configured for controlling and/or applying a hold time, a hysteresis and/or an inhibition time. For example, the hold time may be shorter when compared to the inhibition time. The hold time may relate to a time during which the output of the decorrelator <NUM> is muted responsive to a detected transient, i.e., a property determined by the transient detection unit <NUM>. The inhibition time may be longer when compared to the hold time, to avoid unwanted effects. For example, the hold counter, i.e., the time for muting, may be <NUM>, <NUM>, <NUM>, <NUM>, <NUM> or <NUM> blocks, whilst the inhibition time may be at least twice the time, e.g., at least <NUM>, at least <NUM>, at least <NUM> or <NUM> blocks or any other time duration.

According to an example, the control unit <NUM> may also provide for a hysteresis to mitigate on/off toggling of transient suppression for audio signals like low rate pulse trains. That is, the inhibition time provided by the control unit <NUM> may be a first inhibition time. The transient suppressor <NUM> may be configured for restarting the inhibition time as a second inhibition time being longer than the first inhibition time in case a transient occurs during the first inhibition time. That is, even if the hold time has lapsed but the inhibition time has not yet lapsed and in case a new transient is determined (regardless if the hold time has lapsed or not) the inhibition timer may be restarted. Optionally, the restarted inhibition timer may be longer when compared to the cancelled inhibition timer. In other words, when a very first transient is detected, then a hold counter and an inhibit counter are both started. The transient may be muted until the hold counter has reached its stop count, e.g., <NUM> blocks. Then, the hold counter may be reset and muting may stop. The inhibit counter may reach its stop count/reset much later in time, e.g., <NUM> blocks. If during said ongoing inhibit counting process a new transient is detected, then just the inhibit counter is restarted, but with a higher stop count value, e.g., <NUM> blocks. In this way, hysteresis is implemented by conditional switching and stop count modifications. That is, during the inhibit counter running, a new triggering of transient suppression or muting may be deactivated.

The transient suppressor <NUM> may be configured for operating in the frequency domain. Alternatively or in addition, the transient suppressor <NUM> may be configured for muting the portion of the decorrelator for a longer time when compared to a pre-delay of the decorrelator. That is, in case a transient is detected in the audio signal <NUM>, then the mute should still be in effect when the transient arrives at the output of the decorrelator.

In other words, decorrelators according to embodiments operate in the short time Fourier transform (STFT) domain on overlapping transform blocks with short duration. This enables a small processing delay of a few milliseconds, e.g., <NUM> milliseconds assuming a transform size of <NUM> and <NUM> sample rate, as opposed to the high delay of the PS/MDS decorrelator as described in [<NUM>] or [<NUM>] that may arrive at a delay time of <NUM> milliseconds at <NUM> sample rate. Moreover, the described decorrelators can be implemented using very low computational Allpass filters and may therefore be computationally much more efficient than time domain decorrelation as described in [<NUM>] or [<NUM>]. If further downstream spectral processing is required or wanted, e.g., a source extent modelling, the described decorrelators may be interfaced directly to this processing stage in the STFT domain to achieve low computational complexity.

Decorrelators as described herein may thus provide for a short processing delay and a moderate computational complexity. Decorrelators can be combined with additional downstream processing to model audio objects having a spatial dimension, the so-called Spatially Extended Sound Sources (SESS) with a perceptual property of "Source Extend".

In other words, <FIG> and <FIG> show preferred embodiments of the present invention. First, the input signal or audio signal (sound of a point source, for example) may be fed into the decorrelator <NUM> comprising a time-block-wise DFT with, e.g., <NUM> sample block length and, e.g., <NUM>% overlap. Next, the spectral bins of the DFT are time-delayed for a frequency dependent duration, where low frequencies may have a higher delay and high frequencies may have a lower delay. For example, delay may be <NUM> subband samples (<NUM> milliseconds at <NUM>) for low frequencies and may decrease down to <NUM> subband sample for the highest bins, i.e., z-<NUM>. The decrease in delay over time may be linear, logarithmic or otherwise with rounding to integer numbers of subband samples. Next, each bin is sent through an Allpass filter, preferably comprising a chain of simple Allpass filters or a nested Allpass filter structure. An example Allpass filter is shown in <FIG>. A different structure is shown in <FIG>. With regard to <FIG>, one possible chain may comprise or consist of four such Allpass filters. The parameter g may be chosen to be, for example, <NUM> and the delays Mi may be prime numbers. Note that <FIG> shows the very first part of the chain, i.e., M<NUM>. As these filters may operate on downsampled spectral bands, e.g., downsampling factor <NUM>, the delays may be very low, e.g., prime numbers <NUM>, <NUM>, <NUM> and <NUM> or, as another example, <NUM>, <NUM>, <NUM> and <NUM>. Following, a time/frequency envelope shaping may be applied. Input signals to the envelope shaping may be the DFT bins directly and their delayed and filtered versions. Finally, an IDFT with overlap add may synthesize the output signal. The output signal may be further processed in time domain to obtain a left/right stereo signal from a mono input signal in a configuration as shown in <FIG>. Alternatively, the left/right stereo signal can be assembled in DFT frequency domain and further processed in frequency domain, e.g., for a source extent/SESS modelling by fast convolution, if beneficial for overall computational efficiency.

A configuration for source extent modelling is shown in <FIG>. In contrast to other embodiments, the alternative embodiment having delays Mi may be chosen as prime numbers being approximately <NUM> times (corresponding the aforementioned downsampling factor) larger than the ones chosen in subband domain, e.g., <NUM>, <NUM>, <NUM> and <NUM> (for the set of prime values <NUM>, <NUM>, <NUM> and <NUM>) or <NUM>, <NUM>, <NUM> and <NUM> (for the set of prime values <NUM>, <NUM>, <NUM> and <NUM>). For different sets of prime values with a different number of prime numbers and/or different prime numbers, corresponding values may be chosen. Further, the alternative embodiment may require an additional STFT to obtain the direct signal input to the time/frequency envelope shaper.

<FIG> shows an example decorrelator in M/S to L/R setup with transient handling processing. Aspects of these embodiments are:.

Embodiments of the present invention relate to
An/a apparatus/method for decorrelation of an audio signal.

<FIG> shows a schematic block diagram of a method <NUM> according to an embodiment that may be implemented, for example, by a decorrelator described herein. Method <NUM> comprises a step <NUM> in which a plurality of parts that are based on an audio signal are received. In <NUM> each of the received parts is delayed to provide for a plurality of delayed parts. <NUM> comprises receiving and combining signals being based on the delayed parts of the frequency representation. <NUM> comprises receiving the frequency representation of the audio signal. <NUM> comprises adjusting an energy of the delayed parts in respect of the frequency representation of the audio signal. <NUM> comprises providing a combined shaped frequency representation, e.g., using the envelope shaper <NUM>.

The inventive encoded audio signal can be stored on a digital storage medium or can be transmitted on a transmission medium such as a wireless transmission medium or a wired transmission medium such as the Internet.

Claim 1:
A decorrelator comprising:
a plurality of delay units (<NUM>) , wherein each delay unit (<NUM>) is configured for receiving a part (<NUM><NUM>-<NUM>n) of a frequency representation being based on an audio signal (<NUM>); wherein each delay unit (<NUM>) is configured for delaying the received part (<NUM><NUM>-<NUM>n) to provide a delayed part (<NUM>'<NUM>-<NUM>'n); and
an envelope shaper (<NUM>) configured for receiving and combining signals being based on the delayed parts (<NUM>'<NUM>-<NUM>'n) of the frequency representation; for receiving the frequency representation of the audio signal (<NUM>); for adjusting an energy of the delayed parts (<NUM>'<NUM>-<NUM>'n) in respect of the frequency representation of the audio signal (<NUM>); and for providing a combined shaped frequency representation.