Audio encoding and decoding using presentation transform parameters

Abstract

A method for encoding an input audio stream including the steps of obtaining a first playback stream presentation of the input audio stream intended for reproduction on a first audio reproduction system, obtaining a second playback stream presentation of the input audio stream intended for reproduction on a second audio reproduction system, determining a set of transform parameters suitable for transforming an intermediate playback stream presentation to an approximation of the second playback stream presentation, wherein the transform parameters are determined by minimization of a measure of a difference between the approximation of the second playback stream presentation and the second playback stream presentation, and encoding the first playback stream presentation and the set of transform parameters for transmission to a decoder.

Claims

1. A method of decoding playback stream presentations from a data stream, the method comprising: receiving and decoding a first rendered playback stream presentation, said first rendered playback stream presentation being a set of M1 signals intended for reproduction on a first audio reproduction system; receiving and decoding a set of transform parameters suitable for transforming an intermediate playback stream presentation into an approximation of a second rendered playback stream presentation, said second rendered playback stream presentation being a set of M2 signals intended for reproduction on a second audio reproduction system, wherein the intermediate playback stream presentation is one of the first rendered playback stream presentation, a down-mix of the first rendered playback stream presentation, and an up-mix of the first rendered playback stream presentation, and wherein the approximation of the second rendered playback stream presentation is an anechoic binaural presentation; receiving and decoding one or more additional sets of transform parameters suitable for transforming the intermediate playback stream presentation into one or more acoustic environment simulation process input signals; applying said transform parameters to said intermediate playback stream presentation to produce said approximation of the second rendered playback stream presentation, applying the one or more additional sets of transform parameters to the intermediate playback stream presentation to generate the one or more acoustic environment simulation process input signals; applying the one or more acoustic environment simulation process input signals to one or more acoustic environment simulation processes to produce one or more simulated acoustic environment signals; and combining the one or more simulated acoustic environment signals with the approximation of the second rendered playback stream presentation.

2. The method of claim 1, wherein the one or more simulated acoustic environment signals comprise one or more of: early reflection signals and late reverberation signals.

3. The method of claim 1, wherein the acoustic environment simulation processes comprises one or more of: an early reflection simulation process and a late reverberation simulation process.

4. The method of claim 3, wherein the early reflection simulation process comprises processing one or more of the acoustic environment simulation process input signals through a delay element.

5. The method of claim 3, wherein the late reverberation simulation process comprises processing one or more of the acoustic environment simulation process input signals through a feedback delay network.

6. A device for decoding playback stream presentations from a data stream, the device having one or more audio components, the device comprising: one or more processors; and a memory storing instructions that, when executed, cause the one or more processors to perform operations comprising: receiving and decoding a first rendered playback stream presentation, said first rendered playback stream presentation being a set of M1 signals intended for reproduction on a first audio reproduction system; receiving and decoding a set of transform parameters suitable for transforming an intermediate playback stream presentation into an approximation of a second rendered playback stream presentation, said second rendered playback stream presentation being a set of M2 signals intended for reproduction on a second audio reproduction system, wherein the intermediate playback stream presentation is one of the first rendered playback stream presentation, a down-mix of the first rendered playback stream presentation, and an up-mix of the first rendered playback stream presentation, and wherein the approximation of the second rendered playback stream presentation is an anechoic binaural presentation; receiving and decoding one or more additional sets of transform parameters suitable for transforming the intermediate playback stream presentation into one or more acoustic environment simulation process input signals; applying said transform parameters to said intermediate playback stream presentation to produce said approximation of the second rendered playback stream presentation, applying the one or more additional sets of transform parameters to the intermediate playback stream presentation to generate the one or more acoustic environment simulation process input signals; applying the one or more acoustic environment simulation process input signals to one or more acoustic environment simulation processes to produce one or more simulated acoustic environment signals; and combining the one or more simulated acoustic environment signals with the approximation of the second rendered playback stream presentation.

7. The device of claim 6, wherein the one or more simulated acoustic environment signals comprise one or more of: early reflection signals and late reverberation signals.

8. The device of claim 6, wherein the acoustic environment simulation processes comprises one or more of: an early reflection simulation process and a late reverberation simulation process.

9. The device of claim 8, wherein the early reflection simulation process comprises processing one or more of the acoustic environment simulation process input signals through a delay element.

10. The device of claim 8, wherein the late reverberation simulation process comprises processing one or more of the acoustic environment simulation process input signals through a feedback delay network.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings in which:

(2) FIG. 1 illustrates a schematic overview of the HRIR convolution process for two sources objects, with each channel or object being processed by a pair of HRIRs/BRIRs.

(3) FIG. 2 illustrates schematically the binaural pre-rendered content reproduced over loudspeakers (prior art);

(4) FIG. 3 illustrates schematically the binaural pre-rendered content reproduced over loudspeakers;

(5) FIG. 4 illustrates schematically the production of coefficients w to process a loudspeaker presentation for headphone reproduction;

(6) FIG. 5 illustrates schematically the coefficients W (W.sub.E) used to reconstruct the anechoic signal and one early reflection (with an additional bulk delay stage) from the core decoder output;

(7) FIG. 6 illustrates schematically the process of using the coefficients W (W.sub.E) used to reconstruct the anechoic signal and an FDN input signal from the core decoder output.

(8) FIG. 7 illustrates schematically the production and processing of coefficients w to process an anechoic presentation for headphones and loudspeakers.

(9) FIG. 8a-8b are schematic block diagrams of an encoder/decoder according to a further embodiment of the present invention.

(10) FIG. 9a is a schematic block diagram of a decoder according to a further embodiment of the present invention.

(11) FIG. 9b is a schematic block diagram of a simplified version of the decoder in FIG. 9a.

DETAILED DESCRIPTION

(12) The embodiments provide a method for a low bit rate, low complexity representation of channel and/or object based audio that is suitable for loudspeaker and headphone (binaural) playback. This is achieved by (1) creating and encoding a rendering intended for a specific playback reproduction system (for example, but not limited to loudspeakers), and (2) adding additional metadata that allow transformation of that specific rendering into a modified rendering suitable for another reproduction system (for example headphones). The specific rendering may be referred to as a first audio playback stream presentation, while the modified rendering may be referred to as a second audio playback stream presentation. The first presentation may have a set of M1 channels, while the second presentation may have a set of M2 channels. The number of channels may be equal (M1=M2) or different. The metadata may be in the form of a set of parameters, possibly time and frequency varying.

(13) In one implementation, the transformation metadata provides a means for transforming a stereo loudspeaker rendering into a binaural headphone rendering, with the possibility to include early reflections and late reverberation. Furthermore, for object-based audio content, the virtual acoustic attributes, in particular the (relative) level of late reverberation and/or the level, spectral and temporal characteristics of one or more early reflections can be controlled on a per-object basis.

(14) The embodiments are directed to the elimination of artifacts and/or improvement of the reproduction quality and maintaining artistic intent by metadata that guides reproduction on one or more reproduction systems. In particular, the embodiments include metadata with an object, channel or hybrid signal representation that improves the quality of reproduction when the reproduction system layout does not correspond to the intended layout envisioned during content creation. As such, the application and/or effect as a result of the metadata will depend on the intended and actual reproduction systems.

(15) Binaural Pre-Rendered Content Reproduced Over Loudspeakers

(16) As described in the background section, reproduction of binaural pre-rendered content over loudspeakers can result in an unnatural timbre due to the fact that spectral cues inherently present in HRIRs or BRIRs are applied twice; once during pre-rendering, and another time during playback in an acoustic environment. Furthermore, such reproduction of binaural pre-rendered content will inherently have azimuthal localization cues applied twice as well, causing incorrect spatial imaging and localization errors.

(17) FIG. 2 illustrates this form of processing 20. The channel or object 21 is initially convolved 22 with a HRIR 23 before encoding 25. As such, prior to encoding, the channel or object-based content is subjected to loudspeaker reproduction simulation by means of the HRIR or BRIR processing. Subsequently, the processed signal is encoded 25, decoded 26 and reproduced over loudspeakers 27, introducing the aforementioned artifacts.

(18) The spectral artifacts resulting from applying an acoustic pathway from speakers to eardrums twice can, at least in part, be compensated for by applying a frequency-dependent gain or attenuation during decoding or reproduction. These gain or attenuation parameters can subsequently be encoded and included with the content. For headphone reproduction, these parameters can be discarded, while for reproduction on loudspeakers, the encoded gains are applied to the signals prior to reproduction.

(19) One form of suitable consequential processing flow 30 is shown in FIG. 3. In this scheme, when playback is intended for loudspeakers, gain metadata is precomputed 31 when the rendering is created. This metadata is encoded with the binaurally processed signals. During decoding the metadata information is also decoded 32. This is then used to apply gain 33 to the decoded signal to reduce the significance of artifacts. For headphones playback, on the other hand, the stages 31-33 are not required (being discarded) and the decoded information can be directly applied for headphone reproduction.

(20) Implementation Example

(21) In one implementation, to compute the gain metadata 31, the input signals x.sub.i[n] with discrete-time index n and input index i are analyzed in time and frequency tiles. Each of the input signals x.sub.i[n] can be broken up into time frames and each frame can, in turn, be divided into frequency bands to construct time/frequency tiles. The frequency bands can be achieved, for example, by means of a filter bank such as a quadrature mirror filter (QMF) bank, a discrete Fourier transform (DFT), a discrete cosine transform (DCT), or any other means to split input signals into a variety of frequency bands. The result of such transform is that an input signal x.sub.i[n] for input with index i and discrete-time index n is represented by sub-band signals x.sub.i[k, b] for time slot (or frame) k and subband b. The short-term energy in time/frequency tile (K,B) is given by:

(22) ${\sigma }_{x_i}^2\left(K,B\right)=\underset{b,k\in B,K}{.Math.}{x}_i\left[k,b\right]x_i^{*}\left[k,b\right],$
with B, K sets of frequency (b) and time (k) indices corresponding to a desired time/frequency tile.

(23) The discrete-time domain representation of the binaural signals y.sub.1[n], y.sub.r[n], for the left and right ear, respectively, is given by:

(24) $y_1\left[n\right]=\underset{i}{.Math.}{x}_i\left[n\right]*h_{l,i}\left[n\right]$$y_r\left[n\right]=\underset{i}{.Math.}{x}_i\left[n\right]*h_{r,i}\left[n\right]$
with h.sub.l,i, h.sub.r,i, the HRIR or BRIR corresponding to the input index i, for the left and right ears, respectively. In other words, the binaural signal pair y.sub.l[n], y.sub.r[n] can be created by a combination of convolution and summation across inputs i. Subsequently, these binaural signals can be converted into time/frequency tiles using the same process as applied to the signals x.sub.i[k, b]. For these frequency-domain binaural signals, the short-term energy in time/frequency tile (K,B) can thus be calculated as:

(25) ${\sigma }_{y_j}^2\left(K,B\right)=\underset{b,k\in B,K}{.Math.}{y}_j\left[k,b\right]y_j^{*}\left[k,b\right]$

(26) The gain metadata w(K, B) can now be constructed on the basis of energy preservation in each time/frequency tile summed across input objects i in the numerator and across binaural signals j in the denominator:

(27) $w^2\left(K,B\right)=\frac{{.Math.}_i{\sigma }_{x_i}^2\left(K,B\right)}{{.Math.}_j{\sigma }_{y_j}^2\left(K,B\right)}$

(28) The metadata w(K, B) can subsequently be quantized, encoded and included in an audio codec bit stream. The decoder will then apply metadata w(K, B) to frame K and band B of both signals y.sub.l and y.sub.r (the input presentation) to produce an output presentation. Such use of a common w(K, B) applied to both y.sub.l and y.sub.r ensures that the stereo balance of the input presentation is maintained.

(29) Besides the method described above, in which the binaural signals y.sub.l[n], y.sub.r[n] are created by means of time-domain convolution, the binaural rendering process may also be applied in the frequency domain. In other words, instead of first computing the binaural signals y.sub.l[n], y.sub.r[n] in the time domain, one can instead convert the input signals x.sub.i[n] to the frequency-domain representation, and apply the HRIR convolution process in the frequency domain to generate the frequency-domain representation of the binaural signals y.sub.j[k, b], for example by frequency-domain fast convolution methods. In this approach, the frequency-domain representation of the binaural signals y.sub.j[k, b] is obtained without requiring these signals to be generated in the time domain, and does not require a filterbank or transform to be applied on the time-domain binaural signals.

(30) Stereo Content Reproduced Over Headphones, Including an Anechoic Binaural Rendering

(31) In this implementation, a stereo signal intended for loudspeaker playback is encoded, with additional data to enhance the playback of that loudspeaker signal on headphones. Given a set of input objects or channels x.sub.i[n], a set of loudspeaker signals z.sub.s[n] is typically generated by means of amplitude panning gains g.sub.i,s that represents the gain of object i to speaker s:

(32) $z_s\left[n\right]=\underset{i}{.Math.}{g}_{i,s}{x}_i\left[n\right]$

(33) For channel-based content, the amplitude panning gains g.sub.i,s are typically constant, while for object-based content, in which the intended position of an object is provided by time-varying object metadata, the gains will consequently be time variant.

(34) Given the signals z.sub.s[n] to be encoded and decoded, it is desirable to find a set of coefficients w such that if these coefficients are applied to signals z.sub.s [n], the resulting modified signals ŷ.sub.1, ŷ.sub.r constructed as:

(35) ${\hat{y}}_l=\underset{s}{.Math.}{w}_{s,l}{z}_s$

(36) ${\hat{y}}_r=\underset{s}{.Math.}{w}_{s,r}{z}_s$
closely match a binaural presentation of the original input signals x.sub.i[n] according to:

(37) $y_l\left[n\right]=\underset{i}{.Math.}{x}_i\left[n\right]*h_{l,i}\left[n\right]$$y_r\left[n\right]=\underset{i}{.Math.}{x}_i\left[n\right]*h_{r,i}\left[n\right]$

(38) The coefficients w can be found by minimizing the L2 norm E between desired and actual binaural presentation:
E=|y.sub.1−ŷ.sub.1|.sup.2+|y.sub.r−ŷ.sub.r|.sup.2
w=arg min(E)

(39) The solution to minimize the error E can be obtained by closed-form solutions, gradient descent methods, or any other suitable iterative method to minimize an error function. As one example of such solution, one can write the various rendering steps in matrix notation:
Y=XH
Z=XG
Ŷ=XGW=ZW
This matrix notation is based on single-channel frame containing N samples being represented as one column:

(40) ${\stackrel{.fwdarw.}{x}}_i=\left(\begin{array}{c}{x}_i\left[0\right]\\ .Math.\\ x_i\left[N-1\right]\end{array}\right)$
and matrices as combination of multiple channels i={1, . . . , I}, each being represented by one column vector in the matrix:
X=[{right arrow over (x)}.sub.1 . . . {right arrow over (x)}.sub.I]

(41) The solution for W that minimizes E is then given by:
W=(G*X*XG+ϵI).sup.−1G*X*XH
with (*) the complex conjugate transpose operator, I the identity matrix, and c a regularization constant. This solution differs from the gain-based method in that the signal Ŷ is generated by a matrix rather than a scalar W applied to signal Z including the option of having cross-terms (e.g. for example the second signal of Ŷ being (partly) reconstructed from the first signal in Z).

(42) Ideally, the coefficients w are determined for each time/frequency tile to minimize the error E in each time/frequency tile.

(43) In the sections above, a minimum mean-square error criterion (L2 norm) is employed to determine the matrix coefficients. Without loss of generality, other well-known criteria or methods to compute the matrix coefficients can be used similarly to replace or augment the minimum mean-square error principle. For example, the matrix coefficients can be computed using higher-order error terms, or by minimization of an L1 norm (e.g., least absolute deviation criterion). Furthermore various methods can be employed including non-negative factorization or optimization techniques, non-parametric estimators, maximum-likelihood estimators, and alike. Additionally, the matrix coefficients may be computed using iterative or gradient-descent processes, interpolation methods, heuristic methods, dynamic programming, machine learning, fuzzy optimization, simulated annealing, or closed-form solutions, and analysis-by-synthesis techniques may be used. Last but not least, the matrix coefficient estimation may be constrained in various ways, for example by limiting the range of values, regularization terms, superposition of energy-preservation requirements and alike.

(44) In practical situations, the HRIR or BRIR h.sub.l,i, h.sub.r,i will involve frequency-dependent delays and/or phase shifts. Accordingly, the coefficients w may be complex-valued with an imaginary component substantially different from zero.

(45) One form of implementation of the processing of this embodiment is shown 40 in FIG. 4. Audio content 41 is processed by a hybrid complex quadrature mirror filter (HCQMF) analysis bank 42 into sub-band signals. Subsequently, HRIRs 44 are applied 43 to the filter bank outputs to generate binaural signals Y. In parallel, the inputs are rendered 45 for loudspeaker playback resulting in loudspeaker signals Z. Additionally, the coefficients (or weights) w are calculated 46 from the loudspeaker and binaural signals Y and Z and included in the core coder bitstream 48. Different core coders can be used, such as MPEG-1 Layer 1, 2, and 3, e.g. as disclosed in Brandenburg, K., & Bosi, M. (1997). “Overview of MPEG audio: Current and future standards for low bit-rate audio coding”. Journal of the Audio Engineering Society, 45(1/2), 4-21 or Riedmiller, J., Mehta, S., Tsingos, N., & Boon, P. (2015) “Immersive and Personalized Audio: A Practical System for Enabling Interchange, Distribution, and Delivery of Next-Generation Audio Experiences”. Motion Imaging Journal, SMPTE, 124(5), 1-23, both hereby incorporated by reference. If the core coder is not able to use sub-band signals as input, the sub-band signals may first be converted to the time domain using a hybrid complex quadrature mirror filter (HCQMF) synthesis filter bank 47.

(46) On the decoding side, if the decoder is configured for headphone playback, the coefficients are extracted 49 and applied 50 to the core decoder signals prior to HCQMF synthesis 51 and reproduction 52. An optional HCQMF analysis filter bank 54 may be required as indicated in FIG. 4 if the core coder does not produce signals in the HCQMF domain. In summary, the signals encoded by the core coder are intended for loudspeaker playback, while loudspeaker-to-binaural coefficients are determined in the encoder, and applied in the decoder. The decoder may further be equipped with a user override functionality, so that in headphone playback mode, the user may select to playback over headphones the conventional loudspeaker signals rather than the binaurally processed signals. In this case, the weights are ignored by the decoder. Finally, when the decoder is configured for loudspeaker playback, the weights may be ignored, and the core decoder signals may be played back over a loudspeaker reproduction system, either directly, or after upmixing or downmixing to match the layout of loudspeaker reproduction system.

(47) It will be evident that the methods described in the previous paragraphs are not limited to using a quadrature mirror filter banks; as other filter bank structures or transforms can be used equally well, such as a short-term windowed discrete Fourier transforms.

(48) This scheme has various benefits compared to conventional approaches. These can include: 1) The decoder complexity is only marginally higher than the complexity for plain stereo playback, as the addition in the decoder consists of a simple (time and frequency-dependent) matrix only, controlled by bit stream information. 2) The approach is suitable for channel-based and object-based content, and does not depend on the number of objects or channels present in the content. 3) The HRTFs become encoder tuning parameters, i.e. they can be modified, improved, altered or adapted at any time without regard for decoder compatibility. With decoders present in the field, HRTFs can still be optimized or customized without needing to modify decoder-side processing stages. 4) The bit rate is very low compared to bit rates required for multi-channel or object-based content, because only a few loudspeaker signals (typically one or two) need to be conveyed from encoder to decoder with additional (low-rate) data for the coefficients w. 5) The same bit stream can be faithfully reproduced on loudspeakers and headphones. 6) A bit stream may be constructed in a scalable manner; if, in a specific service context, the end point is guaranteed to use loudspeakers only, the transformation coefficients w may be stripped from the bit stream without consequences for the conventional loudspeaker presentation. 7) Advanced codec features operating on loudspeaker presentations, such as loudness management, dialog enhancement, etcetera, will continue to work as intended (when playback is over loudspeakers). 8) Loudness for the binaural presentation can be handled independently from the loudness of loudspeaker playback by scaling of the coefficients w. 9) Listeners using headphones can choose to listen to a binaural or conventional stereo presentation, instead of being forced to listen to one or the other.

(49) Extension with Early Reflections

(50) It is often desirable to include one or more early reflections in a binaural rendering that are the result of the presence of a floor, walls, or ceiling to increase the realism of a binaural presentation. If a reflection is of a specular nature, it can be interpreted as a binaural presentation in itself, in which the corresponding HRIRs include the effect of surface absorption, an increase in the delay, and a lower overall level due to the increased acoustical path length from sound source to the ear drums.

(51) These properties can be captured with a modified arrangement such as that illustrated 60 in FIG. 5, which is a modification on the arrangement of FIG. 4. In the encoder 64, coefficients W are determined for (1) reconstruction of the anechoic binaural presentation from a loudspeaker presentation (coefficients W.sub.Y), and (2) reconstruction of a binaural presentation of a reflection from a loudspeaker presentation (coefficients W.sub.E). In this case, the anechoic binaural presentation is determined by binaural rendering HRIRs H.sub.a resulting in anechoic binaural signal pair Y, while the early reflection is determined by HRIRs H.sub.e resulting in early reflection signal pair E. To allow the parametric reconstruction of the early reflection from the stereo mix, it is important that the delay due to the longer path length of the early reflection is removed from the HRIRs H.sub.e in the encoder, and that this particular delay is applied in the decoder.

(52) The decoder will generate the anechoic signal pair and the early reflection signal pair by applying coefficients W (W.sub.Y; W.sub.E) to the loudspeaker signals. The early reflection is subsequently processed by a delay stage 68 to simulate the longer path length for the early reflection. The delay parameter of the block 68 can be included in the coder bit stream, or can be a user-defined parameter, or can be made dependent on the simulated acoustic environment, or can be made dependent on the actual acoustic environment the listener is in.

(53) Extension with Late Reverberation

(54) To include the simulation of late reverberation in the binaural presentation, a late-reverberation algorithm can be employed, such as a feedback-delay network (FDN). An FDN takes as input one or more objects and or channels, and produces (in case of a binaural reverberator) two late reverberation signals. In a conventional algorithm, the decoder output (or a downmix thereof) can be used as input to the FDN. This approach has a significant disadvantage. In many use cases, it can be desirable to adjust the amount of late reverberation on a per-object basis. For example, dialog clarity is improved if the amount of late reverberation is reduced.

(55) In an alternative embodiment per-object or per-channel control of the amount of reverberation can be provided in the same way as anechoic or early-reflection binaural presentations are constructed from a stereo mix.

(56) As illustrated in FIG. 6, various modifications to the previous arrangements can be made to accommodate further late reverberation. In the encoder 81, an FDN input signal F is computed 82 that can be a weighted combination of inputs. These weights can be dependent on the content, for example as a result of manual labelling during content creation or automatic classification through media intelligence algorithms. The FDN input signal itself is discarded by weight estimation unit 83, but coefficient data W.sub.F that allow estimation, reconstruction or approximation of the FDN input signal from the loudspeaker presentation are included 85 in the bit stream. In the decoder 86, the FDN input signal is reconstructed 87, processed by the FDN 88, and included 89 in the binaural output signal for listener 91.

(57) Additionally, an FDN may be constructed such that, multiple (two or more) inputs are allowed so that spatial qualities of the input signals are preserved at the FDN output. In such cases, coefficient data that allow estimation of each FDN input signal from the loudspeaker presentation are included in the bitstream.

(58) In this case it may be desirable to control the spatial positioning of the object and or channel in respect to the FDN inputs.

(59) In some cases, it may be possible to generate late reverberation simulation (e.g., FDN) input signals in response to parameters present in a data stream for a separate purpose (e.g, parameters not specifically intended to be applied to base signals to generate FDN input signals). For instance, in one exemplary dialog enhancement system, a dialog signal is reconstructed from a set of base signals by applying dialog enhancement parameters to the base signals. The dialog signal is then enhanced (e.g., amplified) and mixed back into the base signals (thus, amplifying the dialog components relative to the remaining components of the base signals). As described above, it is often desirable to construct the FDN input signal such that it does not contain dialog components. Thus, in systems for which dialog enhancement parameters are already available, it is possible to reconstruct the desired dialog free (or, at least, dialog reduced) FDN input signal by first reconstructing the dialog signal from the base signal and the dialog enhancement parameters, and then subtracting (e.g., cancelling) the dialog signal from the base signals. In such a system, dedicated parameters for reconstructing the FDN input signal from the base signals may not be necessary (as the dialog enhancement parameters may be used instead), and thus may be excluded, resulting in a reduction in the required parameter data rate without loss of functionality.

(60) Combining Early Reflections and Late Reverberation

(61) Although extensions of anechoic presentation with early reflection(s) and late reverberation are denoted independently in the previous sections, combinations are possible as well. For example, a system may include: 1) Coefficients W.sub.Y to determine an anechoic presentation from a loudspeaker presentation; 2) Additional coefficients W.sub.E to determine a certain number of early reflections from a loudspeaker presentation; 3) Additional coefficients W.sub.F to determine one or more late-reverberation input signals from a loudspeaker presentation, allowing to control the amount of late reverberation on a per-object basis.

(62) Anechoic Rendering as First Presentation

(63) Although the use of a loudspeaker presentation as a first presentation to be encoded by a core coder has the advantage of providing backward compatibility with decoders that cannot interpret or process the transformation data w, the first presentation is not limited to a presentation for loudspeaker playback. FIG. 7 shows a schematic overview of a method 100 for encoding and decoding audio content 105 for reproduction on headphones 130 or loudspeakers 140. The encoder 101 takes the input audio content 105 and processes these signals by HCQMF filterbank 106. Subsequently, an anechoic presentation Y is generated by HRIR convolution element 109 based on an HRIR/HRTF database 104. Additionally, a loudspeaker presentation Z is produced by element 108 which computes and applies a loudspeaker panning matrix G. Furthermore, element 107 produces an FDN input mix F.

(64) The anechoic signal Y is optionally converted to the time domain using HCQMF synthesis filterbank 110, and encoded by core encoder 111. The transformation estimation block 114 computes parameters W.sub.F (112) that allow reconstruction of the FDN input signal F from the anechoic presentation Y, as well as parameters W.sub.Z (113) to reconstruct the loudspeaker presentation Z from the anechoic presentation Y. Parameters 112 and 113 are both included in the core coder bit stream. Alternatively, or in addition, although not shown in FIG. 7, transformation estimation block may compute parameters W.sub.E that allow reconstruction of an early reflection signal E from the anechoic presentation Y.

(65) The decoder has two operation modes, visualized by decoder mode 102 intended for headphone listening 130, and decoder mode 103 intended for loudspeaker playback 140. In the case of headphone playback, core decoder 115 decodes the anechoic presentation Y and decodes transformation parameters W.sub.F. Subsequently, the transformation parameters W.sub.F are applied to the anechoic presentation Y by matrixing block 116 to produce an estimated FDN input signal, which is subsequently processed by FDN 117 to produce a late reverberation signal. This late reverberation signal is mixed with the anechoic presentation Y by adder 150, followed by HCQMF synthesis filterbank 118 to produce the headphone presentation 130. If parameters W.sub.E are also present, the decoder may apply these parameters to the anechoic presentation Y to produce an estimated early reflection signal, which is subsequently processed through a delay and mixed with the anechoic presentation Y.

(66) In the case of loudspeaker playback, the decoder operates in mode 103, in which core decoder 115 decodes the anechoic presentation Y, as well as parameters W.sub.Z. Subsequently, matrixing stage 116 applies the parameters W.sub.Z onto the anechoic presentation Y to produce an estimate or approximation of the loudspeaker presentation Z. Lastly, the signal is converted to the time domain by HCQMF synthesis filterbank 118 and produced by loudspeakers 140.

(67) Finally, it should be noted that the system of FIG. 7 may optionally be operated without determining and transmitting parameters W.sub.Z. In this mode of operation, it is not possible to generate the loudspeaker presentation Z from the anechoic presentation Y. However, because parameters W.sub.E and/or W.sub.F are determined and transmitted, it is possible to generate a headphone presentation including early reflection and/or late reverberation components from the anechoic presentation.

(68) Multi-Channel Loudspeaker Presentation

(69) It will be appreciated by the person skilled in the art that the first playback stream presentation encoded in the encoder may be a multichannel presentation, e.g. a surround or immersive loudspeaker presentation such as a 5.1, 7.1, 7.1.4, etc. presentation. Embodiments of the invention discussed above where the second playback stream presentation is a stereo presentation, e.g. with reference to FIG. 4, will operate in a similar manner, although the size of the matrices will be adjusted. For example, while a 2×2 parameter matrix is sufficient for a stereo-to-stereo transformation, a 5×2 matrix is required for a transformation from a five channel surround presentation to a stereo presentation, and a 6×2 matrix for a transformation from a 5.1 surround presentation (five full bandwidth channels and a low-frequency effects (LFE) channel) to a stereo presentation. As a consequence, the amount of side information needed for presentation transform parameters would increase with the number of channels in the loudspeaker presentation, and also the computational complexity of the decoding process would increase correspondingly.

(70) In order to avoid or minimize such increase in computational complexity when a first presentation with M1 channels is transformed to a second presentation with M2 channels, where M1>M2, e.g. when a surround or immersive loudspeaker presentation is transformed to a binaural stereo presentation, it may be advantageous to downmix the first presentation to an intermediate presentation before determining the transform parameters. For example, a 5.1 surround presentation may be downmixed to a 2.0 stereo loudspeaker presentation.

(71) FIG. 8a shows an encoder 200 where the audio content 201 is rendered by renderer 202 to a 5.1 surround loudspeaker presentation S, which is encoded by the core encoder 203. The 5.1 presentation S is also converted by a downmix module 204 into an intermediate 2-channel (stereo) downmix presentation Z. For example, the left channel of Z (Z.sub.L), may be expressed as a weighted sum of the left channel (S.sub.L), the left side channel (S.sub.LS), the center channel (S.sub.C) and the low frequency effect channel (S.sub.LFE) of the surround presentation S, according to the following equation:
Z.sub.L=(S.sub.L+a*S.sub.C+b*S.sub.LS+c*S.sub.LFE)
where a, b and c are suitable constants, e.g. a=b=sqrt(0.5)=0.71, c=0.5.

(72) The audio content is also input to a binaural renderer 205 configured to render an anechoic binaural signal Y. A parameter computation block 206 receives the anechoic signal Y and the stereo downmix signal Z and computes stereo-to-anechoic parameters W.sub.Y. Compared to FIG. 4 above, the renderer 202 is a multi-channel variant of the renderer 45, as the output in both cases is provided to the core encoder 203/48. Blocks 205 and 206 are in principle identical to blocks 43 and 46.

(73) Further, the encoder may also in include a block 207 (corresponding to block 82 in FIG. 6) for rendering an FDN input signal, and the computation block 206 may then be configured to also compute a set of FDN parameters W.sub.F (corresponding to block 83 in FIG. 6).

(74) FIG. 8b shows a decoder 210, where a core decoder 211 receives and decodes a 5.1 surround presentation S as well as the parameter sets W.sub.Y and W.sub.F. The surround presentation S is converted into a 2-channel (stereo) downmix signal Z by means of a downmix module 212 that operates in the same way as its counterpart 204 in the encoder. A first matrixing block 213 applies the parameters W.sub.Y to the stereo presentation Z to provide a reconstructed anechoic signal Ŷ. A second matrixing block 214 applies the parameters W.sub.F to the stereo presentation Z to provide a reconstructed FDN input signal. The FDN input signal is used in FDN 215 to provide a late reverberation signal, which is added 216 to the reconstructed anechoic signal to provide the binaural output. It is noted that the processing in blocks 213-216 is similar to that in the decoder 86 in FIG. 6.

(75) For low target bit-rates it is known to use parametric methods to convey a 5.1 presentation with help of a 2.1 downmix and a set of coupling parameters, see e.g. ETSI TS 103 190-1 V1.2.1 (2015-06). In such a system, the core decoder effectively performs an up-mix in order to provide the decoded 5.1 presentation. If the embodiment in FIG. 8b is implemented in such a decoder, the result will be a decoder as depicted in FIG. 9a. It is noted that the core decoder 311 in FIG. 9a includes an up-mix module 312 for up-mixing a 2.1 presentation into a 5.1 presentation. The 5.1 presentation is then down-mixed to a 2.0 presentation by the downmix module 212, just as in FIG. 8b.

(76) However, in this context, when a 2.1 presentation is already included in the bit stream, the up-mix to 5.1 is not necessary and can be omitted in order to simplify the decoder. Such a simplified decoder is depicted in FIG. 9b. Here, the core decoder 411 only decodes the 2.1 presentation. This presentation is received by a simplified down-mix module 412, which is configured to convert the 2.1 presentation to a 2.0 presentation, according to:
Lo=a*L+b*LFE
Ro=a*R+b*LFE
where L, R and LFE are the left and right full bandwidth channels and the low-frequency effects channel of the decoded 2.1 presentation, a and b are suitable constants, taking the effect of the up-mix and down-mix performed by modules 312 and 212 in FIG. 9a into account.

(77) The process described in FIGS. 9a and 9b assumes a 2.1 downmix and corresponding coupling parameters. A similar approach can be employed in a system using for example a 3.1 downmix and corresponding coupling parameters. Alternatively, the system in FIGS. 8a and 8b could also carry additional side information that allows to upmix the 5.1 presentation to an object-based representation, as discussed in ETSI TS 103 190-1 V1.2.1 (2015-06).

(78) Interpretation

(79) Reference throughout this specification to “one embodiment”, “some embodiments” or “an embodiment” means that a particular feature, structure or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, appearances of the phrases “in one embodiment”, “in some embodiments” or “in an embodiment” in various places throughout this specification are not necessarily all referring to the same embodiment, but may. Furthermore, the particular features, structures or characteristics may be combined in any suitable manner, as would be apparent to one of ordinary skill in the art from this disclosure, in one or more embodiments.

(80) As used herein, unless otherwise specified the use of the ordinal adjectives “first”, “second”, “third”, etc., to describe a common object, merely indicate that different instances of like objects are being referred to, and are not intended to imply that the objects so described must be in a given sequence, either temporally, spatially, in ranking, or in any other manner.

(81) In the claims below and the description herein, any one of the terms comprising, comprised of or which comprises is an open term that means including at least the elements/features that follow, but not excluding others. Thus, the term comprising, when used in the claims, should not be interpreted as being limitative to the means or elements or steps listed thereafter. For example, the scope of the expression a device comprising A and B should not be limited to devices consisting only of elements A and B. Any one of the terms including or which includes or that includes as used herein is also an open term that also means including at least the elements/features that follow the term, but not excluding others. Thus, including is synonymous with and means comprising.

(82) As used herein, the term “exemplary” is used in the sense of providing examples, as opposed to indicating quality. That is, an “exemplary embodiment” is an embodiment provided as an example, as opposed to necessarily being an embodiment of exemplary quality.

(83) It should be appreciated that in the above description of exemplary embodiments of the invention, various features of the invention are sometimes grouped together in a single embodiment, FIG., or description thereof for the purpose of streamlining the disclosure and aiding in the understanding of one or more of the various inventive aspects. This method of disclosure, however, is not to be interpreted as reflecting an intention that the claimed invention requires more features than are expressly recited in each claim. Rather, as the following claims reflect, inventive aspects lie in less than all features of a single foregoing disclosed embodiment. Thus, the claims following the Detailed Description are hereby expressly incorporated into this Detailed Description, with each claim standing on its own as a separate embodiment of this invention.

(84) Furthermore, while some embodiments described herein include some but not other features included in other embodiments, combinations of features of different embodiments are meant to be within the scope of the invention, and form different embodiments, as would be understood by those skilled in the art. For example, in the following claims, any of the claimed embodiments can be used in any combination.

(85) Furthermore, some of the embodiments are described herein as a method or combination of elements of a method that can be implemented by a processor of a computer system or by other means of carrying out the function. Thus, a processor with the necessary instructions for carrying out such a method or element of a method forms a means for carrying out the method or element of a method. Furthermore, an element described herein of an apparatus embodiment is an example of a means for carrying out the function performed by the element for the purpose of carrying out the invention.

(86) In the description provided herein, numerous specific details are set forth. However, it is understood that embodiments of the invention may be practiced without these specific details. In other instances, well-known methods, structures and techniques have not been shown in detail in order not to obscure an understanding of this description.

(87) Similarly, it is to be noticed that the term coupled, when used in the claims, should not be interpreted as being limited to direct connections only. The terms “coupled” and “connected,” along with their derivatives, may be used. It should be understood that these terms are not intended as synonyms for each other. Thus, the scope of the expression a device A coupled to a device B should not be limited to devices or systems wherein an output of device A is directly connected to an input of device B. It means that there exists a path between an output of A and an input of B which may be a path including other devices or means. “Coupled” may mean that two or more elements are either in direct physical or electrical contact, or that two or more elements are not in direct contact with each other but yet still co-operate or interact with each other.

(88) Thus, while there has been described what are believed to be the preferred embodiments of the invention, those skilled in the art will recognize that other and further modifications may be made thereto without departing from the spirit of the invention, and it is intended to claim all such changes and modifications as falling within the scope of the invention. For example, any formulas given above are merely representative of procedures that may be used. Functionality may be added or deleted from the block diagrams and operations may be interchanged among functional blocks. Steps may be added or deleted to methods described within the scope of the present invention.