# US 2025/0095660 A1 · Spatial coding of higher order ambisonics for a low latency immersive audio codec

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- Number: US 2025/0095660 A1
- Document ID: US20250095660A1
- Status: Published application
- Kind: A1
- Application number: 18/729,248
- Filed: 2023-01-09
- Published: 2025-03-20
- Earliest claimed: 2022-01-20
- Assignees: Dolby Laboratories Licensing Corporation; Dolby International AB
- Applicants: Dolby Laboratories Licensing Corporation; Dolby International AB
- Inventors: Stefanie BROWN; Stefan BRUHN; Rishabh TYAGI
- Web page: https://patentgrep.com/patent/US20250095660A1

## CPC

- G10L 19/008: Multichannel audio signal coding or decoding using interchannel correlation to reduce redundancy, e.g. joint-stereo, intensity-coding or matrixing
- G10L 19/002: Dynamic bit allocation (for perceptual audio coders G10L19/032)
- G10L 19/0204: {using subband decomposition}
- G10L 19/025: Detection of transients or attacks for time/frequency resolution switching
- G10L 19/032: Quantisation or dequantisation of spectral components
- G10L 19/06: Determination or coding of the spectral characteristics, e.g. of the short-term prediction coefficients
- H04S 3/008: {in which the audio signals are in digital form, i.e. employing more than two discrete digital channels (data reduction aspects thereof based on psychoacoustics G10L19/02)}
- H04S 2420/11: Application of ambisonics in stereophonic audio systems
- G10L: SPEECH ANALYSIS TECHNIQUES OR SPEECH SYNTHESIS; SPEECH RECOGNITION; SPEECH OR VOICE PROCESSING TECHNIQUES; SPEECH OR AUDIO CODING OR DECODING
- H04S: STEREOPHONIC SYSTEMS

## Abstract

Described herein is a method of encoding Higher Order Ambisonics, HOA, audio, the method including: receiving an input HOA audio signal having more than four Ambisonics channels; encoding the HOA audio signal using a SPAR coding framework and a core audio encoder; and providing the encoded HOA audio signal to a downstream device, the encoded HOA audio signal including core encoded SPAR downmix channels and encoded SPAR metadata. Further described are a method of decoding Higher Order Ambisonics, HOA, audio, respective apparatuses and computer program products.

## Claims

### Claim 1

A method of encoding Higher Order Ambisonics, HOA, audio, the method including:
receiving an input HOA audio signal having more than four Ambisonics channels;
encoding the HOA audio signal using a SPAR coding framework and a core audio encoder; and
providing the encoded HOA audio signal to a downstream device, the encoded HOA audio signal including core encoded SPAR downmix channels and encoded SPAR metadata.

### Claim 2 (depends on 1)

The method of claim 1, wherein the encoding includes: generating, based on some or all of the Ambisonics channels, a representation of a W channel and a set of ntotal prediction residuals along with computing in SPAR metadata respective prediction coefficients; and selecting, out of the set of ntotal prediction residuals, a subset of nres prediction residuals to be directly coded to obtain a number of ndmx=nres+1 downmix channels to be provided to the downstream device.

### Claim 3 (depends on 2)

The method of claim 2, wherein the selection of the subset of nres prediction residuals is based on a threshold number for directly coded channels indicating a maximum number of directly coded channels.

### Claim 4 (depends on 3)

The method of claim 3, wherein the threshold number for directly coded channels is determined based on one or more of:
information indicative of one or more of a bitrate limitation, a metadata size, a core codec performance, and an audio quality; and
a predetermined set of threshold numbers for directly coded channels.

### Claim 5

(canceled)

### Claim 6 (depends on 2)

The method of claim 2, wherein the subset of nres prediction residuals is selected in accordance with a channel ranking of the Ambisonics channels starting from high-ranked to low-ranked channels.

### Claim 7 (depends on 6)

The method of claim 6, wherein the channel ranking of the Ambisonics channels is based on one or more of:
a perceptual importance of the Ambisonics channels, with Ambisonics channels being higher in the channel ranking having higher perceptual importance;
a channel ranking agreement between encoder and decoder; and
spherical harmonics Ylm(θ, φ) of a given order l forms a subset of the channel ranking of the Ambisonics channels corresponding to spherical harmonics Yl+1m(θ, φ) of an (l+1)-th order, the channel ranking of the Ambisonics channel of the (l+1)-th order staring with the channel ranking of the Ambisonics channels of the lth order.

### Claim 8

(canceled)

### Claim 9 (depends on 7)

The method of claim 7, wherein Ambisonics channels corresponding to one or more of:
a spherical harmonic Ylm(θ, φ) with larger overlap with a left-right-front-rear plane are ranked to be perceptually more important than Ambisonics channels corresponding to a spherical harmonic Ylm(θ, φ) with larger overlap with a height direction, for a given order l;
a spherical harmonic Ylm(θ, φ) with larger overlap with a left-right direction are ranked to have higher perceptual importance than Ambisonics channels corresponding to a spherical harmonic Ylm(θ, φ) with larger overlap with a front-rear direction; and
a spherical harmonic Ylm(θ, φ) with larger overlap in the left-right-front-rear plane of a given order l are ranked to have higher perceptual importance than Ambisonics channels corresponding to a spherical harmonic Yl−1m(θ, φ) of an (l−1)-th order with larger overlap in the height direction.

### Claim 10

(canceled)

### Claim 11 (depends on 7)

The method of claim 7, wherein pairs formed by Ambisonics channels corresponding to spherical harmonics Ylm(θ, φ) for a given order l with |m|=l are ranked to be perceptually more important than HOA channels for the given order l with |m|<l.

### Claim 12

(canceled)

### Claim 13

(canceled)

### Claim 14 (depends on 7)

The method of claim 7, wherein one or more prediction residuals to be subsequently added to the subset of nres prediction residuals are selected based on a ranking promoting Ambisonics channels corresponding to a spherical harmonic; Yl±l(θ, φ) over Ambisonics channels corresponding to a spherical harmonic Ylθ(θ, φ) ahead of Ambisonics channels corresponding to a spherical harmonic Ylm (θ, φ), where 0<|m|<l.

### Claim 15 (depends on 2)

The method of claim 2, wherein the encoding further includes representing parametric channels based on computing in SPAR metadata respective coefficients from the remaining ndec=ntotal−nres prediction residuals.

### Claim 16 (depends on 15)

The method of claim 15, wherein the computing in SPAR metadata includes one or more of:
computing a plurality of cross-prediction coefficients for use by a decoder to reconstruct at least part of the ndec parametric channels from the nres directly coded prediction residuals;
computing a plurality of decorrelator coefficients for use by the decoder to account, during reconstruction, for remaining energy not accounted for by the prediction coefficients and the cross-prediction coefficients; and
computing at least one of the prediction coefficients, the cross-prediction coefficients and the decorrelator coefficients with a first time resolution of t1 milliseconds which is larger than a second time resolution of t2 milliseconds of an encoder filterbank.

### Claim 17

(canceled)

### Claim 18

(canceled)

### Claim 19 (depends on 16)

The method of claim 16, wherein the computing with the second time resolution of t2 milliseconds is only performed for high frequency bands; and optionally performed upon detection of a transient.

### Claim 20

(canceled)

### Claim 21 (depends on 15)

The method of claim 15, wherein the computing in SPAR metadata further includes computing a normalization term for channels corresponding to a given Ambisonics order l, by using only covariance estimates of channels corresponding to the order l.

### Claim 22 (depends on 15)

The method of claim 15, wherein the encoding further includes obtaining a bitrate limitation value, selecting, out of a set of SPAR quantization modes, a SPAR quantization mode to meet the bitrate limitation value and applying the selected SPAR quantization mode to the SPAR metadata.

### Claim 23 (depends on 22)

The method of claim 22, wherein some or all of the modes in the set of SPAR quantization modes include re-allocating bits to coefficients relating to Ambisonics channels being ranked higher in the channel ranking from coefficients relating to Ambisonics channels being ranked lower in the channel ranking.

### Claim 24 (depends on 22)

The method of claim 22, wherein the computing in SPAR metadata includes one or more of:
computing a plurality of cross-prediction coefficients for use by a decoder to reconstruct at least part of the ndec parametric channels from the nres directly coded prediction residuals;
computing a plurality of decorrelator coefficients for use by the decoder to account, during reconstruction, for remaining energy not accounted for by the prediction coefficients and the cross-prediction coefficients; and
computing at least one of the prediction coefficients, the cross-prediction coefficients and the decorrelator coefficients with a first time resolution of t1 milliseconds which is larger than a second time resolution of t2 milliseconds of an encoder filterbank; and
wherein some or all of the modes in the set of SPAR quantization modes include one or more of:
selecting a subset of cross-prediction coefficients to be omitted from the plurality of cross-prediction coefficients;
selecting a subset of decorrelator coefficients to be omitted from the plurality of decorrelator coefficients; and
wherein selectin, the subset of coefficients is based on the channel ranking of the Ambisonics channels.

### Claim 25

(canceled)

### Claim 26

(canceled)

### Claim 27 (depends on 7)

The method of claim 7, wherein the received input HOA audio signal consists of Ambisonics channels that are ranked to have a relatively high perceptual importance.

### Claim 28

(canceled)

### Claim 29

(canceled)

### Claim 30

(canceled)

### Claim 31

(canceled)

### Claim 32

(canceled)

### Claim 33

(canceled)

### Claim 34

(canceled)

### Claim 35 (depends on 1)

An apparatus including memory and one or more processor configured to perform the method according to claim 1.

### Claim 36

(canceled)

### Claim 37 (depends on 1)

A program comprising instructions that, when executed by one or more processors, cause the one or more processor, to carry out the method according to claim 1.

### Claim 38 (depends on 1)

A non-transitory computer-readable storage medium storing instructions that, when executed by one or more processors, cause the one or more processors to perform the operations of claim 1.