GB2575632A

GB2575632A - Sparse quantization of spatial audio parameters

Info

Publication number: GB2575632A
Application number: GB1811601.2A
Authority: GB
Inventors: Juhani Laaksonen Lasse
Original assignee: Nokia Technologies Oy
Current assignee: Nokia Technologies Oy
Priority date: 2018-07-16
Filing date: 2018-07-16
Publication date: 2020-01-22
Also published as: WO2020016479A1; GB201811601D0

Abstract

A method for spatial audio signal encoding determines a spatial audio direction parameter 108 for two or more audio signals (from eg. a mic array) and uses a quantization grid of quantization points divided into at least two sets of tracks (eg. 701 & 703, fig. 7b) for indexing the spatial audio direction parameter onto a first set of tracks and outputting Direction Index Id. The grid may be spherical with the quantization points at the centres of smaller spheres, and with the spatial audio parameter having an elevation θ and azimuth component φ.

Description

SPARSE QUANTIZATION OF SPATIAL AUDIO PARAMETERS

Field

The present application relates to apparatus and methods for sound-field related parameter encoding, but not exclusively for time-frequency domain direction related parameter encoding for an audio encoder and decoder.

Background

Parametric spatial audio processing is a field of audio signal processing where the spatial aspect of the sound is described using a set of parameters. For example, in parametric spatial audio capture from microphone arrays, it is a typical and an effective choice to estimate from the microphone array signals a set of parameters such as directions of the sound in frequency bands, and the ratios between the directional and non-directional parts of the captured sound in frequency bands. These parameters are known to well describe the perceptual spatial properties of the captured sound at the position of the microphone array. These parameters can be utilized in synthesis of the spatial sound accordingly, for headphones binaurally, for loudspeakers, or to other formats, such as Ambisonics.

The directions and direct-to-total energy ratios in frequency bands are thus a parameterization that is particularly effective for spatial audio capture.

A parameter set consisting of a direction parameter in frequency bands and an energy ratio parameter in frequency bands (indicating the directionality of the sound) can be also utilized as the spatial metadata for an audio codec. For example, these parameters can be estimated from microphone-array captured audio signals, and for example a stereo signal can be generated from the microphone array signals to be conveyed with the spatial metadata. The stereo signal could be encoded, for example, with an AAC encoder or an EVS encoder or any other suitable encoder. A corresponding decoder can decode the audio signals into PCM signals, and process the sound in frequency bands (using the spatial metadata) to obtain the spatial output, for example a binaural output.

The aforementioned solution is particularly suitable for encoding captured spatial sound from microphone arrays (e.g., in mobile phones, VR cameras, stand-alone microphone arrays). However, it may be desirable for such an encoder to have also other input types than microphone-array captured signals, for example, loudspeaker signals, audio object signals, or Ambisonic signals.

Analysing first-order Ambisonics (FOA) inputs for spatial metadata extraction has been thoroughly documented in scientific literature related to Directional Audio Coding (DirAC) and Harmonic planewave expansion (Harpex). This is since there exist microphone arrays directly providing a FOA signal (more accurately: its variant, the B-format signal), and analysing such an input has thus been a point of study in the field.

A further input for the encoder is also multi-channel loudspeaker input, such as 5.1 or 7.1 channel surround inputs.

However with respect to the directional components of the metadata, which may comprise an elevation, azimuth of a resulting direction, for each considered time/frequency subband a quantization and/or encoding which implements uniform granularity along the azimuth and the elevation components separately (when these two parameters are separately added to the metadata) can result in an uneven distribution of quantization and encoding states. For example a uniform approach to both separately results in an encoding scheme with a higher density nearer the ‘poles’ of the direction sphere, in other words directly above or below the locus or reference location.

Furthermore, audio codecs (such as the Immersed Voice Audio Service (IVAS) codec) can operate over a range of different bitrates and in particular at low bitrates. It is incumbent on the audio codec to be able to quantize the directional components of the metadata also over a range of different bitrates.

Summary

According to a first aspect there is a method for spatial audio signal encoding comprising: determining, for two or more audio signals, a spatial audio direction parameter; using a quantization grid of quantization points for indexing the spatial audio direction parameter, wherein the quantization points of quantization grid are divided into at least two sets of tracks; and indexing the spatial audio direction parameter to an index of which a value of the index corresponds to a quantization point on a track from a first set of tracks of the at least two sets of tracks.

Each of the at least two sets of tracks may comprise a plurality of tracks, wherein each track of the at least two sets of tracks may comprise a plurality of quantization points of the quantization grid.

Indexing the spatial audio direction parameter to an index of which a value of the index corresponding to a quantization point on a track from a first set of tracks of the at least two sets of tracks may comprise indexing the spatial audio direction parameter to a quantization point of the plurality of quantization points from the plurality of tracks of the first set of tracks which is closest to the spatial audio direction parameter.

The method may further comprise: indexing a further spatial audio direction parameter to a further index of which a value of the further index corresponds to a quantization point on a track from a second set of tracks of the at least two sets of tracks by indexing the further spatial audio direction parameter to a quantization point of the plurality of quantization points from the plurality of tracks of the second set of tracks which is closest to the further spatial audio direction parameter; and indexing a yet further spatial audio direction parameter to a yet further index of which a value of the yet further index corresponds to a further quantization point on a track from the first set of tracks of the at least two sets of tracks by indexing the yet further spatial audio direction parameter to a quantization point of the plurality or quantization points from the plurality of tracks of the first set of tracks which is closest to the yet further spatial audio direction parameter.

The further index may comprise a reserved value for pointing to a value of an index which corresponds to a quantization point on a track from the first set of tracks of the at least two sets of tracks, and the spatial audio direction parameter may precede the further spatial audio direction parameter.

The method may further comprise: determining a distance measure between the spatial audio direction parameter and the further spatial audio direction parameter; and using the reserved value to index the further spatial audio direction parameter in the case that the determined distance is less than a predetermined threshold value.

The grid may be a spherical grid generated by covering a sphere with smaller spheres, the centres of the smaller spheres may define quantization points of the spherical grid, the spatial audio direction parameter may comprise an elevation and an azimuth component.

The plurality of quantization points may be contiguous and the plurality of quantization points may form a ring or partial ring of contiguous points around the spherical grid.

There is provided according to a second aspect an apparatus for spatial audio signal encoding comprising means for determining, for two or more audio signals, at least one spatial audio direction parameter; means for using a quantization grid of quantization points for indexing the spatial audio direction parameter, wherein the quantization points of quantization grid are divided into at least two sets of tracks; and means for indexing the spatial audio direction parameter to an index of which a value of the index corresponds to a quantization point on a track from a first set of tracks of the at least two sets of tracks.

Each of the at least two sets of tracks may comprise a plurality of tracks, wherein each track of the at least two sets of tracks may comprise a plurality of quantization points of the quantization grid,

The means for indexing the spatial audio direction parameter to an index of which a value of the index corresponding to a quantization point on a track from a first set of tracks of the at least two sets of tracks may comprise: means for indexing the spatial audio direction parameter to a quantization point of the plurality of quantization points from the plurality of tracks of the first set of tracks which is closest to the spatial audio direction parameter.

The apparatus may further comprise: means for indexing a further spatial audio direction parameter to a further index of which a value of the further index corresponds to a quantization point on a track from a second set of tracks of the at least two sets of tracks by indexing the further spatial audio direction parameter to a quantization point of the plurality of quantization points from the plurality of tracks of the second set of tracks which is closest to the further spatial audio direction parameter; and means for indexing a yet further spatial audio direction parameter to a yet further index of which a value of the yet further index corresponds to a further quantization point on a track from the first set of tracks of the at least two sets of tracks by indexing the yet further spatial audio direction parameter to a quantization point of the plurality or quantization points from the plurality of tracks of the first set of tracks which is closest to the yet further spatial audio direction parameter.

The apparatus may further comprise: means for determining a distance measure between the spatial audio direction parameter and the further spatial audio direction parameter; and means for using the reserved value to index the further spatial audio direction parameter in the case that the determined distance is less than a predetermined threshold value.

According to a further aspect there is provided an apparatus comprising at least one processor and at least one memory including computer program code, the at least one memory and the computer program code configured to, with the at least one processor, cause the apparatus to: determine, for two or more audio signals, at least one spatial audio direction parameter; use a quantization grid of quantization points for indexing the spatial audio direction parameter, wherein the quantization points of quantization grid are divided into at least two sets of tracks; and index the spatial audio direction parameter to an index of which a value of the index corresponds to a quantization point on a track from a first set of tracks of the at least two sets of tracks.

The apparatus caused to index the spatial audio direction parameter to an index of which a value of the index corresponding to a quantization point on a track from a first set of tracks of the at least two sets of tracks may be caused to: index the spatial audio direction parameter to a quantization point of the plurality of quantization points from the plurality of tracks of the first set of tracks which is closest to the spatial audio direction parameter.

The apparatus may further caused to: index a further spatial audio direction parameter to a further index of which a value of the further index corresponds to a quantization point on a track from a second set of tracks of the at least two sets of tracks by indexing the further spatial audio direction parameter to a quantization point of the plurality of quantization points from the plurality of tracks of the second set of tracks which is closest to the further spatial audio direction parameter; and index a yet further spatial audio direction parameter to a yet further index of which a value of the yet further index corresponds to a further quantization point on a track from the first set of tracks of the at least two sets of tracks by indexing the yet further spatial audio direction parameter to a quantization point of the plurality or quantization points from the plurality of tracks of the first set of tracks which is closest to the yet further spatial audio direction parameter.

The apparatus may be further caused to: determine a distance measure between the spatial audio direction parameter and the further spatial audio direction parameter; and use the reserved value to index the further spatial audio direction parameter in the case that the determined distance is less than a predetermined threshold value.

A computer program comprising program instructions for causing a computer to perform the method as described above.

A computer program product stored on a medium may cause an apparatus to perform the method as described herein.

An electronic device may comprise apparatus as described herein.

A chipset may comprise apparatus as described herein.

Embodiments of the present application aim to address problems associated with the state of the art.

Summary of the Figures

For a better understanding of the present application, reference will now be made by way of example to the accompanying drawings in which:

Figure 1 shows schematically a system of apparatus suitable for implementing some embodiments;

Figure 2 shows schematically the analysis processor as shown in figure 1 according to some embodiments;

Figure 3a shows schematically the metadata encoder/quantizer as shown in figure 1 according to some embodiments;

Figure 3b shows schematically the metadata extractor as shown in figure 1 according to some embodiments;

Figure 3c to 3e shows schematically example sphere location configurations as used in the metadata encoder/quantizer and metadata extractor as shown in figures 3a and 3b according to some embodiments;

Figure 4 shows a flow diagram of the operation of the system as shown in figure 1 according to some embodiments;

Figure 5 shows a flow diagram of the operation of the analysis processor as shown in figure 2 according to some embodiments;

Figure 6 shows a flow diagram of generating a direction index based on an input direction parameter in further detail;

Figure 7(a) shows a depiction of a quantisation points on the surface of a spherical grid;

Figure 7(b) shows how the quantisation points on the surface of a spherical grid can be divided into two sets of tracks of quantisation points;

Figure 7(c) shows a partial section of a spherical grid depicting a division into four sets of tracks of quantisation points;

Figure 8 shows a flow diagram of generating a quantized direction parameter based on an input direction index in further detail;

Figure 9 shows a flow diagram of an example operation of converting a quantized direction parameter from a direction index in further detail; and

Figure 10 shows schematically an example device suitable for implementing the apparatus shown.

Embodiments of the Application

The following describes in further detail suitable apparatus and possible mechanisms for the provision of effective spatial analysis derived metadata parameters for multi-channel input audio signals. In the following discussions multichannel system is discussed with respect to a multi-channel microphone implementation. However as discussed above the input format for the audio codec may be any suitable input format, such as multi-channel loudspeaker, ambisonic (FOA/HOA) etc. It is understood that in some embodiments the channel location is based on a location of the microphone or is a virtual location or direction. Furthermore the output of the example system is a multi-channel loudspeaker arrangement. However it is understood that the output may be rendered to the user via means other than loudspeakers such as a binauralized headphone output. In some embodiments, an audio decoder may output a parametric representation of the encoded and transmitted audio, which can be presented to a user via a loudspeaker arrangement or headphones using an external Tenderer. Furthermore the multi-channel loudspeaker signals may be generalised to be two or more playback audio signals.

As discussed previously spatial metadata parameters such as direction and directto-total energy ratio (or diffuseness-ratio, or any suitable expression indicating the directionality/non-directionality of the sound at the given time-frequency interval) parameters in frequency bands are particularly suitable for expressing the perceptual properties of natural sound fields. Synthetic sound scenes such as 5.1 loudspeaker mixes commonly utilize audio effects and amplitude panning methods that provide spatial sound that differs from sounds occurring in natural sound fields. In particular, a 5.1 or 7.1 mix may be configured such that it contains coherent sounds played back from multiple directions. For example, it is common that some sounds of a 5.1 mix perceived directly at the front are not produced by a centre (channel) loudspeaker, but for example coherently from left and right front (channels) loudspeakers, and potentially also from the centre (channel) loudspeaker. The spatial metadata parameters such as direction(s) and energy ratio(s) do not express such spatially coherent features accurately. As such other metadata parameters such as coherence parameters may be determined from analysis of the audio signals to express the audio signal relationships between the channels.

As expressed above an example of the incorporation of the direction information in the metadata is to use determined azimuth and elevation values. However conventional uniform azimuth and elevation sampling produces a non-uniform direction distribution.

The concept it thus an attempt to determine a direction parameter for spatial metadata and to index the parameter based on points on a sphere covering based distribution of the directions in order to define a more uniform distribution of directions. The points on the sphere can be viewed as forming a spherical grid.

The proposed metadata index may then be used alongside a downmix signal (‘channels’), to define a parametric immersive format that can be utilized, for example the Immersive Voice and Audio Service (IVAS) codec.

The proposed metadata index may then be used alongside a downmix signal (‘channels’), to define a parametric immersive format that can be utilized, e.g., for the Immersive Voice and Audio Service (IVAS) codec.

The concept furthermore addresses the issue of how to index the directional parameters to the spherical grid with different levels of granularity or quantisation resolution, particularly when the distribution of quantisation points on the sphere is fixed. This issue may be particularly pertinent for audio codecs, such as IVAS, which are capable of operating at a number of different coding rates. For example during a low coding rate operating mode it can be beneficial to quantize the directional parameters by indexing the spherical grid at a lower granularity in order to reduce the number of bits to represent each set of directional parameters.

With respect to figure 1 an example apparatus and system for implementing embodiments of the application are shown. The system 100 is shown with an ‘analysis’ part 121 and a ‘synthesis’ part 131. The ‘analysis’ part 121 is the part from receiving the multi-channel loudspeaker signals up to an encoding of the metadata and downmix signal and the ‘synthesis’ part 131 is the part from a decoding of the encoded metadata and downmix signal to the presentation of the re-generated signal (for example in multi-channel loudspeaker form).

The input to the system 100 and the ‘analysis’ part 121 is the multi-channel signals 102. In the following examples a microphone channel signal input is described, however any suitable input (or synthetic multi-channel) format may be implemented in other embodiments.

The multi-channel signals are passed to a downmixer 103 and to an analysis processor 105.

In some embodiments the downmixer 103 is configured to receive the multi-channel signals and downmix the signals to a determined number of channels and output the downmix signals 104. For example the downmixer 103 may be configured to generate a 2 audio channel downmix of the multi-channel signals. The determined number of channels may be any suitable number of channels. For instance for a two audio channel downmix such as a stereo pair signal can utilize in some embodiments 2 microphone signals corresponding to a left channel and a right channel of the capture device microphone array or the capture scene orientation. The selected pair of microphone signals can furthermore exhibit a distance between the channels that can be at least substantially like the distance between human ears, which can have additional perceptual advantages.

In some embodiments the downmixer 103 is optional and the multi-channel signals are passed unprocessed to an encoder 107 in the same manner as the downmix signal are in this example.

In some embodiments the analysis processor 105 is also configured to receive the multi-channel signals and analyse the signals to produce metadata 106 associated with the multi-channel signals and thus associated with the downmix signals 104. The analysis processor 105 may be configured to generate the metadata which may comprise, for each time-frequency analysis interval, a direction parameter 108, an energy ratio parameter 110, a coherence parameter 112, and a diffuseness parameter 114. The direction, energy ratio and diffuseness parameters may in some embodiments be considered to be spatial audio parameters. In other words the spatial audio parameters comprise parameters which aim to characterize the soundfield created by the multi-channel signals (or two or more playback audio signals in general). The coherence parameters may be considered to be signal relationship audio parameters which aim to characterize the relationship between the multichannel signals.

In some embodiments, more than one direction parameter can be generated. In general, the energy ratio parameter 110, the coherence parameter 112, and the diffuseness parameter 114 each relate to a direction parameter 108. Some types of coherence parameters in particular may relate only to a time-frequency analysis interval and not a particular direction. In addition, the diffuseness parameter 114 may be indicated for each individual analysed direction or over all directions.

In some embodiments the parameters generated may differ from frequency band to frequency band. Thus for example in band X all of the parameters are generated and transmitted, whereas in band Y only one of the parameters is generated and transmitted, and furthermore in band Z any other number of parameters are generated or transmitted. A practical example of this may be that for some frequency bands such as the lowest band or the highest band some of the parameters are not required for perceptual reasons. The downmix signals 104 and the metadata 106 may be passed to an encoder 107.

The encoder 107 may comprise a IVAS stereo core 109 which is configured to receive the downmix (or otherwise) signals 104 and generate a suitable encoding of these audio signals. The encoder 107 can in some embodiments be a computer (running suitable software stored on memory and on at least one processor), or alternatively a specific device utilizing, for example, FPGAs or ASICs. The encoding may be implemented using any suitable scheme. The encoder 107 may furthermore comprise a metadata encoder or quantizer 109 which is configured to receive the metadata and output an encoded or compressed form of the information. In some embodiments the encoder 107 may further interleave, multiplex to a single data stream or embed the metadata within encoded downmix signals before transmission or storage shown in Figure 1 by the dashed line. The multiplexing may be implemented using any suitable scheme.

In the decoder side, the received or retrieved data (stream) may be received by a decoder/demultiplexer 133. The decoder/demultiplexer 133 may demultiplex the encoded streams and pass the audio encoded stream to a downmix extractor 135 which is configured to decode the audio signals to obtain the downmix signals. Similarly the decoder/demultiplexer 133 may comprise a metadata extractor 137 which is configured to receive the encoded metadata and generate metadata. The decoder/demultiplexer 133 can in some embodiments be a computer (running suitable software stored on memory and on at least one processor), or alternatively a specific device utilizing, for example, FPGAs or ASICs.

The decoded metadata and downmix audio signals may be passed to a synthesis processor 139.

The system 100 ‘synthesis’ part 131 further shows a synthesis processor 139 configured to receive the downmix and the metadata and re-creates in any suitable format a synthesized spatial audio in the form of multi-channel signals 110 (these may be multichannel loudspeaker format or in some embodiments any suitable output format such as binaural or Ambisonics signals, depending on the use case) based on the downmix signals and the metadata. In some embodiments, the synthesis processor 139 or Tenderer may be part of the decoder/demultiplexer 133.

With respect to Figure 4 an example flow diagram of the overview shown in Figure 1 is shown.

First the system (analysis part) is configured to receive multi-channel audio signals as shown in Figure 4 by step 401.

Then the system (analysis part) is configured to generate a downmix of the multichannel signals as shown in Figure 4 by step 403.

Also the system (analysis part) is configured to analyse signals to generate metadata such as direction parameters; energy ratio parameters; diffuseness parameters and coherence parameters as shown in Figure 4 by step 405.

The system is then configured to encode for storage/transmission the downmix signal and metadata as shown in Figure 4 by step 407.

After this the system may store/transmit the encoded downmix and metadata as shown in Figure 4 by step 409.

The system may retrieve/receive the encoded downmix and metadata as shown in Figure 4 by step 411.

Then the system is configured to extract the downmix and metadata from encoded downmix and metadata parameters, for example demultiplex and decode the encoded downmix and metadata parameters, as shown in Figure 4 by step 413.

The system (synthesis part) is configured to synthesize an output multi-channel audio signal based on extracted downmix of multi-channel audio signals and metadata with coherence parameters as shown in Figure 4 by step 415.

With respect to Figure 2 an example analysis processor 105 (as shown in Figure 1) according to some embodiments is described in further detail. The analysis processor 105 in some embodiments comprises a time-frequency domain transformer 201.

In some embodiments the time-frequency domain transformer 201 is configured to receive the multi-channel signals 102 and apply a suitable time to frequency domain transform such as a Short Time Fourier Transform (STFT) in order to convert the input time domain signals into a suitable time-frequency signals. These time frequency signals may be passed to a direction analyser 203 and to a signal analyser 205.

Thus for example the time-frequency signals 202 may be represented in the timefrequency domain representation by

Si(b, n), where b is the frequency bin index and n is the frame index and / is the channel index. In another expression, n can be considered as a time index with a lower sampling rate than that of the original time-domain signals. These frequency bins can be grouped into subbands that group one or more of the bins into a band index k = 0,..., K-1. Each subband k has a lowest bin b_klow and a highest bin b_khigll, and the subband contains all bins from b_{k low} to b_khigh. The widths of the subbands can approximate any suitable distribution. For example the Equivalent rectangular bandwidth (ERB) scale or the Bark scale.

In some embodiments the analysis processor 105 comprises a direction analyser 203. The direction analyser 203 may be configured to receive the time-frequency signals 202 and based on these signals estimate direction parameters 108. The direction parameters may be determined based on any audio based ‘direction’ determination. Typically such method will utilize knowledge of the microphone array or any other information on the arrangement of the multi-channel signals.

For example in some embodiments the direction analyser 203 is configured to estimate the direction with two or more signal inputs. This represents the simplest configuration to estimate a ‘direction’, more complex processing may be performed with even more signals.

The direction analyser 203 may thus be configured to provide for each frequency band and temporal frame an azimuth denoted as φ(/ςη) and an elevation denoted as Q(k,n). The direction parameter 108, which may then be represented as an index on a spherical grid, may be also be passed to a signal analyser 205

In some embodiments further to the direction parameter the direction analyser 203 is configured to determine an energy ratio parameter 110. The energy ratio may be considered to be a determination of the energy of the audio signal which can be considered to arrive from a direction relative to the total energy for the corresponding time-frequency interval. The direct-to-total energy ratio r(k,n) can be estimated, e.g., using a correlation measure, or any other suitable method to obtain a ratio parameter.

The estimated direction 108 parameters may be output (and passed to an encoder). The estimated energy ratio parameters 110 may be passed to a signal analyser 205.

In some embodiments the analysis processor 105 comprises a signal analyser 205. The signal analyser 205 is configured to receive parameters (such as the azimuth φ(/ςη) and elevation 0(k,ri) or a spherical grid index108, and the direct-to-total energy ratios r(fc,n) 110) from the direction analyser 203. The signal analyser 205 may be further configured to receive the time-frequency signals ($,(£?, n)) 202 from the time-frequency domain transformer 201. All of these are in the time-frequency domain; b is the frequency bin index, k is the frequency band index (each band potentially consists of several bins b), n is the time index, and / is the channel.

Although directions and ratios are here expressed for each time index n, in some embodiments the parameters may be combined over several time indices. Same applies for the frequency axis, as has been expressed, the direction of several frequency bins b could be expressed by one direction parameter in band k consisting of several frequency bins b. The same applies for all of the discussed spatial parameters herein.

The signal analyser 205 is configured to produce a number of signal parameters. In the following disclosure there are the two parameters: coherence and diffuseness, both analysed in time-frequency domain. In addition, in some embodiments the signal analyser 205 is configured to modify the estimated energy ratios (r(fc,n)). The signal analyser 205 is configured to generate the coherence and diffuseness parameters based on any suitable known method.

Summing the energy ratios and the diffuseness values for a given time-frequency interval can produce a value of one or a value smaller than one. In case, the value is smaller than one, the remaining energy can be considered noise.

With respect to Figure 5 a flow diagram summarising the operations of the analysis processor 105 are shown.

The first operation is one of receiving time domain multichannel (loudspeaker) audio signals as shown in Figure 5 by step 501.

Following this is applying a time domain to frequency domain transform (e.g. STFT) to generate suitable time-frequency domain signals for analysis as shown in Figure 5 by step 503.

Then applying direction analysis to determine direction and energy ratio parameters is shown in Figure 5 by step 505.

Then applying analysis to determine coherence parameters (such as surrounding and/or spread coherence parameters) and diffuseness parameters is shown in Figure 5 by step 507. In some embodiments the energy ratio may also be modified based on the determined coherence parameters in this step.

The final operation being one of outputting the determined parameters is shown in Figure 5 by step 509.

With respect to Figure 3a an example metadata encoder and specifically the direction metadata encoder 300 is shown according to some embodiments.

The direction metadata encoder 300 in some embodiments comprises a quantization input 302. The quantization input, which may also be known as an encoding input is configured to define the granularity of spheres arranged around a reference location or position from which the direction parameter is determined. In some embodiments the quantization input is a predefined or fixed value.

The direction metadata encoder 300 in some embodiments comprises a sphere positioner 303. The sphere positioner is configured to configure the arrangement of spheres based on the quantization input value. The proposed spherical grid uses the idea of covering a sphere with smaller spheres and considering the centres of the smaller spheres as points defining a grid of almost equidistant directions, such that the spherical grid comprises a number of points arranged in a form of a sphere.

The concept as shown herein is one in which a sphere is defined relative to the reference location. The sphere can be visualised as a series of circles (or intersections) and for each circle intersection there are located at the circumference of the circle a defined number of (smaller) spheres. This is shown for example with respect to Figure 3c to 3e. For example Figure 3c shows an example ‘equatorial cross-section’ or a first main circle 370 which has a radius defined as the ‘main sphere radius. Also shown in Figure 3c are the smaller spheres (shown as circle cross-sections) 371, 373, 375, 377 and 379 located such that each smaller sphere has a circumference which at one point touches the main sphere circumference and at least one further point which touches at least one further smaller sphere circumference. Thus as shown in Figure 3c the smaller sphere 371 touches main sphere 370 and smaller sphere 373, smaller sphere 373 touches main sphere 370 and smaller spheres 371 and 375, smaller sphere 375 touches main sphere 370 and smaller spheres 373 and 377, smaller sphere 377 touches main sphere 370 and smaller spheres 375 and 379, and smaller sphere 379 touches main sphere

370 and smaller sphere 377.

Figure 3d shows an example ‘tropical cross-section’ or further main circle 380 and the smaller spheres (shown as circle cross-sections) 381, 383, 385 located such that each smaller sphere has a circumference which at one point touches the main sphere (circle) circumference and at least one further point which touches at least one further smaller sphere circumference. Thus as shown in Figure 3d the smaller sphere 381 touches main sphere 380 and smaller sphere 383, smaller sphere 383 touches main sphere 380 and smaller spheres 381 and 385, smaller sphere 385 touches main sphere 380 and smaller sphere 383.

Figure 3e shows an example sphere and the cross sections 370, 380 and smaller spheres (cross-sections) 371 associated with cross-section 370, smaller sphere 381 associated with cross-section 380 and other smaller spheres 392, 393, 394, 395, 397, 398. In this example only the circles with starting azimuth value at 0 are drawn.

The sphere positioner 303 thus in some embodiments be configured to perform the following operations to define the directions corresponding to the covering spheres: Input: Quantization input (number of points on the “Equator”, n(0)=M) Output: number of circles, No, and number of points on each circle, n(i), i =0,Nc-1

1. n(0) = M o 2 π

2. oc=--- n(0)

3. /?(o) = 1 (radius of the circle at the Equator)

4. 0(0) = 0 (elevation)

5. r = 2 /?(0)sin(^) (radius of the smaller spheres)

6. ¢(0) = 0

7. P = arcsin(r—)

8. /?(1) = /?(0) cosp

9. i = 1

10. While n(i - 1) > 1

a. n(i) = (this is valid when r<<R(0))

b. 0(i) = p i

c. Δφ(ί) = (granularity of the azimuth on the circle i)

d. R(i + 1) = 7?(i) cos((i + 1) p)

e. Ifiisodd

i. φ₀(ί) = (first azimuth value on circle i)

f. Else

i. 0_o(i) = 0

g. End if

h. i = i + 1

11. End while

12. Nc=i+1

Step 5 can be also replaced by r = 2 /?(0)sin(-) where the factor k controls the distribution of points along the elevation. For k=4, the elevation resolution is approximately 1 degree. For smaller k, the resolution is correspondingly smaller.

The elevation for each point on the circle i is given by the values in 0(i). For each circle above the Equator there is a corresponding circle under the Equator.

Each direction point on one circle can be indexed in increasing order with respect to the azimuth value. The index of the first point in each circle is given by an offset that can be deduced from the number of points on each circle, n(i). In order to obtain the offsets, for a considered order of the circles, the offsets are calculated as the cumulated number of points on the circles for the given order, starting with the value 0 as first offset.

One possible order of the circles could be to start with the Equator followed by the first circle above the Equator, then the first under the Equator, the second one above the Equator, and so on.

Another option is to start with the Equator, then the circle above the Equator that is at an approximate elevation of 45 degrees and then the corresponding circle under the Equator, and then the remaining circles in alternative order. This way for some simpler positioning of loudspeakers, only the first circles are used, reducing the number of bits to send the information.

Other ordering of the circles are also possible in other embodiments.

In some embodiments the spherical grid can also be generated by considering the meridian 0 instead of the Equator, or any other meridian.

The sphere positioner having determined the number of circles and the number of circles, Nc, number of points on each circle, n(i), i =0,Nc-1 and the indexing order can be configured to pass this information to an EA to DI converter 305.

In essence once the sphere positioner 303 has determined the number of circles, Nc the quantisation points of the spherical grid are also determined. Figure 11a depicts a visualisation of quantisation points as a spherical grid, with each black point representing an indexed quantisation point.

The direction metadata encoder 300 in some embodiments comprises a direction parameter input 108. The direction parameter input may define an elevation and azimuth value D = (θ,φ).

The transformation procedures from (elevation/azimuth) (EA) to direction index (DI) and back are presented in the following paragraphs. The alternative ordering of the circles is considered here.

The direction metadata encoder 300 comprises an elevation-azimuth to direction index (EA-DI) converter 305. The elevation-azimuth to direction index converter 305 in some embodiments is configured to receive the direction parameter input 108 and the sphere positioner information and convert the elevation-azimuth value from the direction parameter input 108 to a direction index by quantizing the elevationazimuth value.

With respect to Figure 6 an example method for generating the direction index according to some embodiments is shown.

The receiving of the quantization input is shown in Figure 6 by step 601.

Then the method may determine sphere positioning based on the quantization input as shown in Figure 6 by step 603.

Also the method may comprise receiving the direction parameter as shown in Figure 6 by step 602.

Having receiving the direction parameter and the sphere positioning information the method may comprise converting the direction parameter to a direction index based on the sphere positioning information as shown in Figure 6 by step 605.

The method may then output the direction index as shown in Figure 6 by step 607.

In some embodiments the elevation-azimuth to direction index (EA-DI) converter 305 is configured to perform this conversion according to the following algorithm:

Input: (0, ¢), θ e S_e c [-ϊί],ψ e c [0,2π]

Output: I_d

In some embodiments S_e may take the form of an indexed codebook with N discrete entries, each entry 0, corresponding to a value of elevation for i = 0:N-l. Additionally, the codebook also comprises for each discrete elevation value 0, a set of discrete azimuth values where the number of azimuth values in the set is dependent in the elevation 0,. In other words for each elevation entry 0, there can be differing numbers of discrete azimuth values for j = 0:/(0,), where /(0,) denotes that number of azimuth values in the set of azimuth values associated with the elevation value 0, is a function of the elevation value 0,.

In other embodiments the indexing of the elevation-azimuth value may not use a codebook structure as above for storing discrete elevation and azimuth values. Instead linear quantization can be used where the number of circles N_c and the granularity p as provided by the sphere positioner can be used to uniformly divide the range of elevation from to The position on the elevation range gives a circle index, and the number of azimuth discrete points and corresponding offset, o//(i) are known.

1.

Find the circle index i = <

2- - 1,i/0 > 0 p

0,ife = 0

Q

-2-,if Θ < 0 p

2.

3.

Find the index of the azimuth within the circle i : j = where i' = |^|

The direction index is I_d = + j

The direction index Id 306 may be output.

As stated before the quantisation points of the spherical grid can be determined in accordance with the parameter N_c and that this parameter determines the density of quantisation points within the grid and ultimately the number of bits required to represent the index of the directional parameters.

In some modes of operation the audio codec may be required to index the directional parameters at a lower accuracy than would be required to index a directional parameter using the spherical grid according to the parameter N_c. This may depend, for example, on the total bit rate available to encode the frame of audio (and audio metadata) or the specific bit rate allocation for the metadata parameters. To this end it can be beneficial to use the derived spherical grid, as determined by the parameter

Λ/c, in sparser mode of operation in which the actual quantisation points distributed on the grid do not change, but instead the directional parameters are quantised to quantization points on the spherical grid with a coarser granularity. In other words not every quantization point on the spherical grid is considered as a potential quantization value for a directional parameter. In embodiments, this can be achieved by dividing the spherical grid or a subset of the spherical grid or an approximation of the spherical grid or an approximation of a part of the spherical grid into a number of tracks of quantisation points. Each track can consist of a number of contiguous quantisation points running across the spherical grid in a uniform direction which can resemble a ring or partial ring around the surface of the spherical grid. It is to be appreciated that a track of quantization points can be in its most general form a group of following quantization points which forms a path over the quantization grid. In other words, a first quantization point and a second quantization point in a track can neighbour each other on the grid. Figure 7 (b) shows how a spherical grid may be divided into two sets of tracks with each set having a number (or run) of individual quantization points over the “surface” of the spherical grid. In the case of Figure 7 (b) the spherical grid is divided into two sets of tracks distributed in an alternating fashion over the spherical grid. The first set of tracks are depicted as the tracks formed by the contiguous “runs” of black coloured quantisation points such that track 701 forms a first track of the first set of tracks and track 703 forms a second track of the first set of tracks and so on. Similarly the second set of tracks are depicted in Figure 7(b) as the tracks formed by the contiguous “runs” of grey coloured quantisation points such that track 702 forms the first track of the second set of tracks and track 704 forms a second track of the second set of tracks and so on. Naturally this concept of dividing the spherical grid into a plurality of sets of tracks with a track being a run of contiguous quantisation points over the spherical grid may be extended beyond the example in Figure 7(b) of two sets of tracks. For example Figure 7(c) shows an instance in which the spherical grid is divided into four sets of tracks, a representative sample of tracks of the first set are depicted as 711 and 712, a representative sample of tracks of the second set are depicted as 721 and 722, a representative sample of tracks of the third set are depicted as 731 and 732 and a representative sample of tracks of the fourth set are depicted as 741 and 742.

The concept herein proceeds from the consideration that directional parameters can be indexed at a lower bit rate by confining the indexing/quantisation step for a particular input directional parameter to a particular set of tracks of quantization points. For instance using the above example of Figure 7 (c) in which the spherical grid is divided into four sets of tracks of quantization points the step of quantisation/indexing a series of directional parameters may take the following form. A first directional parameter may be quantised to a quantisation point on a track of the first set of tracks. This can take the form of finding the elevation (or circle) index and azimuth index according to equations 1 to 3 above when searching all quantisation points belonging to the tracks of the first set of tracks. The second directional parameter may by quantised to a quantisation point on a track of the second set of tracks. Again this can take the form of searching all quantisation points belonging to the tracks of the second sub set of tracks and encoding the circle and azimuth indices according to equations 1 to 3. The third directional parameter may be quantised to a quantisation point on a track of the third set of tracks and the fourth directional parameter may be quantised to a quantisation point on a track of the fourth set of tracks. When it comes to quantisation/indexing of the fifth directional parameter, the quantisation process scrolls back to the first set of tracks and the fifth in the series of directional parameters is quantised to a quantisation point on a track of the first set of tracks. Accordingly, the sixth directional parameter can be quantised to a quantisation point on a track of the second set of tracks, seventh directional parameter can be quantised to a quantisation point on a track of the third set of tracks and eighth directional parameters can be quantised to a quantisation point on a track of the fourth set of tracks.

Therefore in embodiments the EA-DI converter 305 indexes a directional parameter using a quantization point from one set of tracks. Furthermore, the order in which each set of tracks is presented to quantize a directional parameter is predetermined, thereby negating the need to transmit set of tracks index to the decoder. Additionally it should be appreciated that the ordering of each set of tracks is not limited to the above uniformly increasing (in terms of the set of tracks index p) mechanism as described above. In fact other patterns may be used such as an alternating pattern in which the sets of tracks may be presented to the incoming directional parameter in a 1, 3, 2, 4 pattern (in terms of the set of tracks index)

It is understood the spherical grid, a subset of the spherical grid, or an approximation of the spherical grid or a section of it can be divided into a varying number sets of tracks each consisting in a varying number of tracks with a varying number of quantization points.

It is to be appreciated that the quantised direction index I_d simply references a point on the spherical grid for an azimuth and elevation value, and that the index of the set of tracks p does not form part of the quantised direction index I_d. By using the above technique of quantising the directional parameters to the quantisation points of a particular set of tracks the number of bits required to index the particular directional parameter (the quantised direction index I_d) is reduced when compared to quantising/indexing to all possible quantisation points of the spherical grid. For instance in the above example of Figure 7(c) the reduction in the number of bits per directional parameter will be of the order 2 bits. Therefore in general the reduction in the number of bits required to index each directional parameter can be given by log₂N_T given that N_T is the total number of sets of tracks and is a number to the power of two. It is understood the bit rate reduction is similarly relative to a subset or approximation of a spherical grid where the at least one set of tracks is used to quantize directions limited to said subset or approximation of the spherical grid instead of quantizing the directions on the full spherical grid.

For example a spherical grid may have a total of 512 quantised directional parameter points, with each direction parameter being a two component vector of azimuth and elevation. In this case indexing a quantised directional component would take a 9 bit direction index I_d. However, should the spherical grid be divided into four sets of tracks as depicted in Figure 7 (c) then a quantised directional component can be represented using a 7 bit direction index I_d. In other words for this particular example a 7 bit direction index I_d is required to index a quantised directional parameter point from one of the sets of four tracks.

Naturally by dividing the spherical grid into a plurality of sets of tracks and then quantizing/indexing an input directional parameter to one of the quantisation points contained by a set of tracks according to the above described scrolling mechanism, or any other suitable selection mechanism, can lead to a reduction in quantisation resolution. However, in some circumstances the effect of the loss of quantisation resolution can be more prevalent particularly if the series of directional parameters entering the EA-DI converter 305 have directional values which are closely correlated to each other. This situation may occur when a series of time frequency tiles (each tile has its own directional parameter) exhibits a strong uniform direction. In such instances it can be seen that forcing each successive input directional parameter to a quantization point from a different set of tracks can result in a perturbation in the quantised value at a time when a smoother quantization profile is desired.

In order to overcome this effect the EA-DI converter 305 can be arranged to compare the current directional parameter to, e.g., the immediate previous directional parameter in order to determine whether the relative change in value is below a predefined threshold. If it is determined that the current directional parameter is sufficiently close to an immediate previous directional parameter it can be advantageous to quantise the current directional parameter to the quantisation point of the quantised immediate previous directional parameter rather than taking a quantisation point from the allotted set of tracks that does not have this same quantization point.

In order to enable the ability to reference a previous directional index value as the directional index value for a current input directional parameter the directional index I_d may be arranged to have at least one reserved value. Whereby the reserved directional index value of a directional index I_d signifies that a previous directional index value from another set of tracks is to be taken as the directional index value for the current directional parameter instead of any of the quantization points/index values from the allocated set of tracks. In other words the reserved directional index value is a pointer to a previous directional index.

Returning to the above example of a 512 point spherical grid, the 7 bit directional index I_d may be arranged to have one index value reserved as a pointer to a previous directional index. For example the index 1111111 may be reserved as the pointer to a previous directional index. In this case, however, the four tracks cannot cover all the 512 points of the example spherical grid. Therefore, the combination of the four tracks will result in a very close approximation of the full grid.

Naturally the directional index Z_dcan be arranged to have more than one reserved value thereby enabling the directional index I_d to point to more than one previous directional index. For example the index 1111110 may be assigned to point to the last but one directional index.

In embodiments, there may be used other rules for the pointer I reserved value than using a previous directional index.

As mentioned above the EA-DI converter 305 can be arranged to compare the current directional parameter to immediate previous directional parameters in order to determine whether the relative change in value is below a predefined threshold. In embodiments EA-DI converter 305 may be arranged to implement this functionality by using a distance measure d between a current direction parameter D = (θ,0) and a previous direction parameter D__± = (θ_₁,0_₁), and checking if the distance measure is less than a predefined threshold d < Thresh. If d is determined to be less than the predefined threshold then the directional parameter index /_dfor the current direction parameter will be set to the reserved value which points to (or refers) to the directional index value associated with the previous direction parameter D__±. In some embodiments, the threshold may be adaptive and based on the bit rate and specifically the highest possible directional resolution allowed for that operating mode.

In embodiments the distance measure d can be considered by taking the L2 norm distance between a previous directional parameter and a current directional parameter points. In the specific case of a quantization grid in the form of a unitary sphere, the distance d is calculated as the L2 norm distance between the current direction parameter D = (Θ, 0) and the previous directional parameter D__± = (θ_₁,0_₁), the L2 norm distance between two points x and y on a unitary sphere may be considered from ||x - y ||² where x and y are spherical coordinates in three dimensional space. In terms the elevation-azimuth pair D = (Θ, 0) the spherical coordinates can be expressed as x = (rcos(0) cos(</>), rcos(0) sin( ¢), rsin(0)) and for the elevation-azimuth pair D__t = (θ_₁,0_₁), the spherical coordinates correspond to y = (rcos(e_₁)cos(0_₁) ,rcos(e_₁)sin(0_₁) ,rsin(e_₁j). By considering a unitary sphere the radius r = 1, and the distance d can be reduced to the calculation d = -(sin(0) + cos(0) cos(^0_₁) cos( φ - 0-J), where it can be seen that the distance d is solely dependent on the values of the angles.

The above method of dividing a quantization grid of quantization points into a plurality of tracks in order to reduce the bits required to quantize the directional parameters has been described in terms of the specific case of a spherical grid. However, it is to be appreciated that the above technique can be applied to other quantization grids such as a two dimensional flat quantization grid whose quantization points are evenly distributed over the flat surface.

With respect to Figure 3b an example metadata extractor 137 and specifically a direction metadata extractor 350 is shown according to some embodiments.

The direction metadata extractor 350 in some embodiments comprises a quantization input 352. This in some embodiments is passed from the metadata encoder or is otherwise agreed with the encoder. For example, the quantization input 352 can be based on a codec operating mode in some embodiments. The quantization input is configured to define the granularity of spheres arranged around a reference location or position.

The direction metadata extractor 350 in some embodiments comprises a direction index input 351. This may be received from the encoder or retrieved by any suitable means.

The direction metadata extractor 350 in some embodiments comprises a sphere positioner 353. The sphere positioner 353 is configured to receive as an input the quantization input and generate the sphere arrangement in the same manner as generated in the encoder. In some embodiments the quantization input and the sphere positioner 353 is optional and the arrangement of spheres information is passed from the encoder rather than being generated in the extractor.

The direction metadata extractor 350 comprises a direction index to elevationazimuth (DI-EA) converter 355. The direction index to elevation-azimuth converter 355 is configured to receive the direction index and furthermore the sphere position information and generate an approximate or quantized elevation-azimuth output. In some embodiments the conversion is performed according to the following algorithm.

Input: I_d

Output: (0,0)

1. Find the circle index i, such that off(t) <I_d< off(i + 1) — if i is odd — if i is even 2 ^J

3. θ =ρ ( '^d //, if i'is odd φ = ) ^Δ</>(ί)

Id~°ff(f) , Δφ(ί') .,.

——-f-r—--if i is even

V Δ</>(ί') 2 ⁷

In other embodiments the direction index I_d can be decoded into an elevation index and azimuth index in order to obtain the quantized elevation value Θ and quantized azimuth value φ from respective codebooks.

With respect to Figure 8 an example method for extracting the direction parameters (or generating quantized direction parameters) according to some embodiments is shown.

The receiving of the quantization input is shown in Figure 8 by step 801.

Then the method may determine sphere positioning based on the quantization input as shown in Figure 8 by step 803.

Also the method may comprise receiving the direction index as shown in Figure 8 by step 802.

Having received the direction index and the sphere positioning information the method may comprise converting the direction index to a direction parameter in the form of a quantized direction parameter based on the sphere positioning information as shown in Figure 8 by step 805.

The method may then output the quantized direction parameter as shown in Figure by step 807.

With respect to Figure 9 an example method for converting the direction index to a quantized elevation-azimuth (DI-EA) parameter, as shown in Figure 8 by step 805, according to some embodiments is shown.

In some embodiments the method comprises finding the circle index value / such that off(i) < I_d < off(i + 1) as shown in Figure 9 by step 901.

Having determined the circle index the next operation is to calculate the circle index in the hemisphere from the sphere positioning information as shown in Figure 9 by step 903.

Then a quantized elevation is determined based on the circle index as shown in Figure 9 by step 905.

Having determined the quantized elevation, the quantized azimuth is determined based on the circle index and elevation information as shown in Figure 9 by step 907.

Although not repeated throughout the document, it is to be understood that spatial audio processing, both typically and in this context, takes place in frequency bands. Those bands could be for example, the frequency bins of the time-frequency transform, or frequency bands combining several bins. The combination could be such that approximates properties of human hearing, such as the Bark frequency resolution. In other words, in some cases, we could measure and process the audio in time-frequency areas combining several of the frequency bins b and/or time indices n. For simplicity, these aspects were not expressed by all of the equations above. In case many time-frequency samples are combined, typically one set of parameters such as one direction is estimated for that time-frequency area, and all time-frequency samples within that area are synthesized according to that set of parameters, such as that one direction parameter.

The usage of a frequency resolution for parameter analysis that is different than the frequency resolution of the applied filter-bank is a typical approach in the spatial audio processing systems.

With respect to Figure 10 an example electronic device which may be used as the analysis or synthesis device is shown. The device may be any suitable electronics device or apparatus. For example in some embodiments the device 1400 is a mobile device, user equipment, tablet computer, computer, audio playback apparatus, etc.

In some embodiments the device 1400 comprises at least one processor or central processing unit 1407. The processor 1407 can be configured to execute various program codes such as the methods such as described herein.

In some embodiments the device 1400 comprises a memory 1411. In some embodiments the at least one processor 1407 is coupled to the memory 1411. The memory 1411 can be any suitable storage means. In some embodiments the memory 1411 comprises a program code section for storing program codes implementable upon the processor 1407. Furthermore in some embodiments the memory 1411 can further comprise a stored data section for storing data, for example data that has been processed or to be processed in accordance with the embodiments as described herein. The implemented program code stored within the program code section and the data stored within the stored data section can be retrieved by the processor 1407 whenever needed via the memory-processor coupling.

In some embodiments the device 1400 comprises a user interface 1405. The user interface 1405 can be coupled in some embodiments to the processor 1407. In some embodiments the processor 1407 can control the operation of the user interface 1405 and receive inputs from the user interface 1405. In some embodiments the user interface 1405 can enable a user to input commands to the device 1400, for example via a keypad. In some embodiments the user interface 1405 can enable the user to obtain information from the device 1400. For example the user interface 1405 may comprise a display configured to display information from the device 1400 to the user. The user interface 1405 can in some embodiments comprise a touch screen or touch interface capable of both enabling information to be entered to the device 1400 and further displaying information to the user of the device 1400. In some embodiments the user interface 1405 may be the user interface for communicating with the position determiner as described herein.

In some embodiments the device 1400 comprises an input/output port 1409. The input/output port 1409 in some embodiments comprises a transceiver. The transceiver in such embodiments can be coupled to the processor 1407 and configured to enable a communication with other apparatus or electronic devices, for example via a wireless communications network. The transceiver or any suitable transceiver or transmitter and/or receiver means can in some embodiments be configured to communicate with other electronic devices or apparatus via a wire or wired coupling.

The transceiver can communicate with further apparatus by any suitable known communications protocol. For example in some embodiments the transceiver or transceiver means can use a suitable universal mobile telecommunications system (UMTS) protocol, a wireless local area network (WLAN) protocol such as for example IEEE 802.X, a suitable short-range radio frequency communication protocol such as Bluetooth, or infrared data communication pathway (IRDA).

The transceiver input/output port 1409 may be configured to receive the signals and in some embodiments determine the parameters as described herein by using the processor 1407 executing suitable code. Furthermore the device may generate a suitable downmix signal and parameter output to be transmitted to the synthesis device.

In some embodiments the device 1400 may be employed as at least part of the synthesis device. As such the input/output port 1409 may be configured to receive the downmix signals and in some embodiments the parameters determined at the capture device or processing device as described herein, and generate a suitable audio signal format output by using the processor 1407 executing suitable code. The input/output port 1409 may be coupled to any suitable audio output for example to a multichannel speaker system and/or headphones or similar.

In general, the various embodiments of the invention may be implemented in hardware or special purpose circuits, software, logic or any combination thereof. For example, some aspects may be implemented in hardware, while other aspects may be implemented in firmware or software which may be executed by a controller, microprocessor or other computing device, although the invention is not limited thereto. While various aspects of the invention may be illustrated and described as block diagrams, flow charts, or using some other pictorial representation, it is well understood that these blocks, apparatus, systems, techniques or methods described herein may be implemented in, as non-limiting examples, hardware, software, firmware, special purpose circuits or logic, general purpose hardware or controller or other computing devices, or some combination thereof.

The embodiments of this invention may be implemented by computer software executable by a data processor of the mobile device, such as in the processor entity, or by hardware, or by a combination of software and hardware. Further in this regard it should be noted that any blocks of the logic flow as in the Figures may represent program steps, or interconnected logic circuits, blocks and functions, or a combination of program steps and logic circuits, blocks and functions. The software may be stored on such physical media as memory chips, or memory blocks implemented within the processor, magnetic media such as hard disk or floppy disks, and optical media such as for example DVD and the data variants thereof, CD.

The memory may be of any type suitable to the local technical environment and may be implemented using any suitable data storage technology, such as semiconductor-based memory devices, magnetic memory devices and systems, optical memory devices and systems, fixed memory and removable memory. The data processors may be of any type suitable to the local technical environment, and may include one or more of general purpose computers, special purpose computers, microprocessors, digital signal processors (DSPs), application specific integrated circuits (ASIC), gate level circuits and processors based on multi-core processor architecture, as non-limiting examples.

Embodiments of the inventions may be practiced in various components such as integrated circuit modules. The design of integrated circuits is by and large a highly automated process. Complex and powerful software tools are available for converting a logic level design into a semiconductor circuit design ready to be etched and formed on a semiconductor substrate.

Programs can automatically route conductors and locate components on a semiconductor chip using well established rules of design as well as libraries of pre-stored design modules. Once the design for a semiconductor circuit has been completed, the resultant design, in a standardized electronic format (e.g., Opus, GDSII, or the like) may be transmitted to a semiconductor fabrication facility or fab for fabrication.

The foregoing description has provided by way of exemplary and non-limiting examples a full and informative description of the exemplary embodiment of this invention. However, various modifications and adaptations may become apparent to those skilled in the relevant arts in view of the foregoing description, when read in conjunction with the accompanying drawings and the appended claims. However, all such and similar modifications of the teachings of this invention will still fall within the scope of this invention as defined in the appended claims.

Claims

1. A method for spatial audio signal encoding comprising determining, for two or more audio signals, a spatial audio direction parameter;

using a quantization grid of quantization points for indexing the spatial audio direction parameter, wherein the quantization points of quantization grid are divided into at least two sets of tracks; and indexing the spatial audio direction parameter to an index of which a value of the index corresponds to a quantization point on a track from a first set of tracks of the at least two sets of tracks.

2. The method as claimed in Claim 1, wherein each of the at least two sets of tracks comprises a plurality of tracks, wherein each track of the at least two sets of tracks comprises a plurality of quantization points of the quantization grid.

3. The method as claimed in Claim 2, wherein indexing the spatial audio direction parameter to an index of which a value of the index corresponding to a quantization point on a track from a first set of tracks of the at least two sets of tracks comprises:

indexing the spatial audio direction parameter to a quantization point of the plurality of quantization points from the plurality of tracks of the first set of tracks which is closest to the spatial audio direction parameter.

4. The method as claimed in Claims 2 and 3, wherein the method further comprises:

indexing a further spatial audio direction parameter to a further index of which a value of the further index corresponds to a quantization point on a track from a second set of tracks of the at least two sets of tracks by indexing the further spatial audio direction parameter to a quantization point of the plurality of quantization points from the plurality of tracks of the second set of tracks which is closest to the further spatial audio direction parameter; and indexing a yet further spatial audio direction parameter to a yet further index of which a value of the yet further index corresponds to a further quantization point on a track from the first set of tracks of the at least two sets of tracks by indexing the yet further spatial audio direction parameter to a quantization point of the plurality or quantization points from the plurality of tracks of the first set of tracks which is closest to the yet further spatial audio direction parameter.

5. The method as claimed in Claims 2 to 4, wherein the further index comprises a reserved value for pointing to a value of an index which corresponds to a quantization point on a track from the first set of tracks of the at least two sets of tracks, and wherein the spatial audio direction parameter precedes the further spatial audio direction parameter.

6. The method as Claimed in Claim 5, wherein the method further comprises:

determining a distance measure between the spatial audio direction parameter and the further spatial audio direction parameter; and using the reserved value to index the further spatial audio direction parameter in the case that the determined distance is less than a predetermined threshold value.

7. The method as Claimed in Claims 1 to 6, wherein the grid is a spherical grid generated by covering a sphere with smaller spheres, wherein the centres of the smaller spheres define quantization points of the spherical grid, wherein the spatial audio direction parameter comprises an elevation and an azimuth component.

8. The method as Claimed in Claim 7, wherein the plurality of quantization points are contiguous and the plurality of quantization points form a ring or partial ring of contiguous points around the spherical grid.

9. An apparatus for spatial audio signal encoding comprising:

means for determining, for two or more audio signals, at least one spatial audio direction parameter;

means for using a quantization grid of quantization points for indexing the spatial audio direction parameter, wherein the quantization points of quantization grid are divided into at least two sets of tracks; and means for indexing the spatial audio direction parameter to an index of which a value of the index corresponds to a quantization point on a track from a first set of tracks of the at least two sets of tracks.

10. The apparatus as claimed in Claim 9, wherein each of the at least two sets of tracks comprises a plurality of tracks, wherein each track of the at least two sets of tracks comprises a plurality of quantization points of the quantization grid.

11. The apparatus as claimed in Claim 10, wherein means for indexing the spatial audio direction parameter to an index of which a value of the index corresponding to a quantization point on a track from a first set of tracks of the at least two sets of tracks comprises:

means for indexing the spatial audio direction parameter to a quantization point of the plurality of quantization points from the plurality of tracks of the first set of tracks which is closest to the spatial audio direction parameter.

12. The apparatus as claimed in Claims 10 and 11, wherein the apparatus further comprises:

means for indexing a further spatial audio direction parameter to a further index of which a value of the further index corresponds to a quantization point on a track from a second set of tracks of the at least two sets of tracks by indexing the further spatial audio direction parameter to a quantization point of the plurality of quantization points from the plurality of tracks of the second set of tracks which is closest to the further spatial audio direction parameter; and means for indexing a yet further spatial audio direction parameter to a yet further index of which a value of the yet further index corresponds to a further quantization point on a track from the first set of tracks of the at least two sets of tracks by indexing the yet further spatial audio direction parameter to a quantization point of the plurality or quantization points from the plurality of tracks of the first set of tracks which is closest to the yet further spatial audio direction parameter.

13. The apparatus as claimed in Claims 10 to 12, wherein the further index comprises a reserved value for pointing to a value of an index which corresponds to a quantization point on a track from the first set of tracks of the at least two sets of tracks, and wherein the spatial audio direction parameter precedes the further spatial audio direction parameter.

14. The apparatus as Claimed in Claim 13, wherein the apparatus further comprises:

means for determining a distance measure between the spatial audio direction parameter and the further spatial audio direction parameter; and means for using the reserved value to index the further spatial audio direction parameter in the case that the determined distance is less than a predetermined threshold value.

15. The apparatus as Claimed in Claims 9 to 14, wherein the grid is aspherical grid generated by covering a sphere with smaller spheres, wherein the centres of the smaller spheres define quantization points of the spherical grid, wherein the spatial audio direction parameter comprises an elevation and an azimuth component.

16. The apparatus as Claimed in Claim 15, wherein the plurality of quantization points are contiguous and the plurality of quantization points form a ring or partial ring of contiguous points around the spherical grid.

Intellectual Property Office