# US 12,223,966 B2 · Selectable linear predictive or transform coding modes with advanced stereo coding

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- Number: US 12,223,966 B2
- Document ID: US12223966B2
- Status: Granted
- Kind: B2
- Application number: 17/728,692
- Filed: 2022-04-25
- Published: 2022-08-04
- Granted: 2025-02-11
- Earliest claimed: 2009-03-17
- Assignees: Dolby International AB
- Applicants: Dolby International AB
- Inventors: Heiko Purnhagen; Pontus Carlsson; Kristofer Kjoerling
- Web page: https://patentgrep.com/patent/US12223966B2

## CPC

- G10L 19/002: Dynamic bit allocation (for perceptual audio coders G10L19/032)
- G10L 19/008: Multichannel audio signal coding or decoding using interchannel correlation to reduce redundancy, e.g. joint-stereo, intensity-coding or matrixing
- H04S 3/02: of the matrix type, i.e. in which input signals are combined algebraically, e.g. after having been phase shifted with respect to each other
- H04S 5/00: Pseudo-stereo systems, e.g. in which additional channel signals are derived from monophonic signals by means of phase shifting, time delay or reverberation
- H04S 5/005: {of the pseudo five- or more-channel type, e.g. virtual surround}
- H04S 5/02: of the pseudo four-channel type, e.g. in which rear channel signals are derived from two-channel stereo signals
- G10L 19/18: Vocoders using multiple modes
- H04S 2400/01: Multi-channel, i.e. more than two input channels, sound reproduction with two speakers wherein the multi-channel information is substantially preserved
- H04S 2400/03: Aspects of down-mixing multi-channel audio to configurations with lower numbers of playback channels, e.g. 7.1 -> 5.1 (H04S2400/01 takes precedence)
- H04S 2420/03: Application of parametric coding 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

Methods and systems for advanced stereo processing of an audio signal are disclosed. The methods and systems include selecting a coding mode of either transform coding or linear predictive coding and performing advanced stereo processing when in the selected coding mode. Both encoding and decoding operations are provided.

## Claims

### Claim 1

A method for encoding a stereo input signal comprising a left channel and a right channel, and having a perceptual stereo image, the method comprising:
selecting either a modified discrete cosine transform (MDCT) based transform coding mode or a linear predictive coding mode as a selected coding mode;
encoding the stereo input signal using only the selected coding mode to produce an encoded output signal; and
generating a bitstream signal including the encoded output signal,
wherein, if the linear predictive coding mode is selected, the encoding comprises:
downmixing the stereo input signal to a mono signal, the mono signal being a sum of the left channel and the right channel,
estimating stereo image parameters, for reconstructing a stereo signal that approximates the perceptual stereo image of the stereo input signal from the mono signal,
generating a residual signal that indicates an error associated with representing the stereo signal by the mono signal and the estimated stereo image parameters,
encoding the mono signal using linear predictive coding to produce an encoded mono signal, and
outputting the encoded mono signal, the residual signal and the stereo image parameters as the encoded output signal,
wherein, if the MDCT based transform coding mode is selected, the encoding comprises:
analyzing the stereo input signal by applying both mid/side stereo coding and left/right stereo coding and selecting either a mid/side stereo coding mode or a left/right stereo coding mode based on an estimated entropy for each stereo coding mode,
encoding the stereo input signal using the selected stereo coding mode in a first frequency band to produce an encoded stereo signal in a first frequency band,
downmixing the stereo input signal to a mono signal in a second frequency band,
encoding the mono signal in the second frequency band using MDCT based transform coding to produce an encoded mono signal in the second frequency band, and
outputting the encoded stereo signal in the first frequency band and the encoded mono signal in the second frequency band as the encoded output signal.

### Claim 2 (depends on 1)

The method of claim 1 wherein the analyzing includes selecting which stereo coding mode would more efficiently code the stereo input signal.

### Claim 3 (depends on 1)

The method of claim 1 wherein the selecting of either the MDCT based transform coding mode or the linear predictive mode is dependent upon characteristics of the stereo input signal.

### Claim 4 (depends on 1)

The method of claim 1 wherein the estimating includes estimating the stereo image parameters in a plurality of frequency bands.

### Claim 5 (depends on 1)

The method of claim 1 wherein a bandwidth of the first frequency band and a bandwidth of the second frequency band is determined based at least in part on a desired target bitrate.

### Claim 6 (depends on 1)

The method of claim 1 wherein the linear predictive coding mode is selected when the stereo input signal is speech.

### Claim 7 (depends on 1)

A non-transitory computer readable medium containing instructions that when executed by a processor perform the method of claim 1.

### Claim 8

A device for encoding a stereo input signal comprising a left channel and a right channel, and having a perceptual stereo image, to produce an encoded output signal, the device comprising:
a mode selector for selecting either a modified discrete cosine transform (MDCT) based transform coding mode or a linear predictive coding mode;
a MDCT based transform encoder for encoding the stereo input signal if the selected coding mode is the MDCT based transform coding mode but not if the selected coding mode is the linear predictive coding mode;
a linear predictive encoder for encoding the stereo input signal if the selected coding mode is the linear predictive coding mode but not if the selected coding mode is the MDCT based transform coding mode; and
a bitstream generator for generating a bitstream signal including the encoded output signal, wherein the linear predictive encoder is configured to:
downmix the stereo input signal to a mono signal, the mono signal being a sum of the left channel and the right channel,
estimate stereo image parameters, for reconstructing a stereo signal that approximates the perceptual stereo image of the stereo input signal, from the mono signal,
generate a residual signal that indicates an error associated with representing the stereo signal by the mono signal and the estimated stereo image parameters,
encode the mono signal using linear predictive coding to produce an encoded first mono signal, and
output the encoded mono signal, the residual signal and the estimated stereo image parameters as the encoded output signal,
wherein the MDCT based transform encoder is configured to:
analyze the stereo input signal by applying both mid/side stereo coding and left/right stereo coding and selecting either a mid/side stereo coding mode or a left/right stereo coding mode based on an estimated entropy for each stereo coding mode,
encode the stereo input signal using the selected stereo coding mode in a first frequency band to produce an encoded stereo signal in the first frequency band,
downmix the stereo input signal to a mono signal in a second frequency band,
encode the mono signal in the second frequency band using MDCT based transform coding to produce an encoded mono signal in the second frequency band, and
output the encoded stereo signal in the first frequency band and the encoded mono signal in the second frequency band as the encoded output signal.

### Claim 9

A method for decoding a bitstream signal to produce a decoded output signal having a left channel and a right channel, the method comprising:
extracting an encoded audio signal from the bitstream signal, the encoded audio signal generated by encoding an input stereo audio signal having a left input channel and a right input channel using a selected coding mode, wherein the selected coding mode is one of a modified discrete cosine transform (MDCT) based transform coding mode or a linear predictive coding mode;
decoding the encoded audio signal using only the selected coding mode to produce a decoded signal; and
outputting the decoded signal as the decoded output signal,
wherein, if the selected coding mode is the linear predictive coding mode, the decoding comprises:
receiving an encoded mono signal, the encoded mono signal being a sum of the left input channel and the right input channel of the input stereo audio signal,
decoding the encoded mono signal using linear predictive decoding to produce a decoded mono signal,
extracting stereo image parameters and a residual signal from the bitstream signal for reconstructing a stereo audio signal that approximates a perceptual stereo image of the input stereo audio signal, wherein the residual signal indicates an error associated with representing the stereo audio signal by the mono signal and the stereo image parameters,
reconstructing the stereo audio signal using the decoded mono signal, the residual signal and the stereo image parameters to produce a reconstructed stereo audio signal that approximates the perceptual stereo image of the input stereo audio signal, and
outputting the reconstructed stereo audio signal as the decoded signal,
wherein, if the selected coding mode is the MDCT based transform coding mode, the decoding comprises:
receiving a stereo signal in a first frequency band, the stereo signal generated using a selected stereo coding mode, the selected stereo coding mode including either mid/side stereo coding or left/right stereo coding,
receiving an encoded mono signal in a second frequency band,
decoding the stereo signal in the first frequency band using the selected stereo coding mode to produce a decoded stereo signal in the first frequency band,
decoding the encoded mono signal in the second frequency band using MDCT based transform decoding to produce a decoded mono signal in the second frequency band, and
generating the decoded signal from the decoded stereo signal in the first frequency band and the decoded mono signal in the second frequency band.

### Claim 10 (depends on 9)

The method of claim 9 wherein the MDCT based transform decoding further comprises extracting side information from the bitstream signal for reconstruction of one or more subbands not encoded.

### Claim 11 (depends on 10)

The method of claim 10 wherein the side information includes a parameter used to determine a spectral envelope of the one or more subbands not encoded.

### Claim 12 (depends on 9)

The method of claim 9 wherein the MDCT based transform decoding comprises perceptual decoding.

### Claim 13 (depends on 9)

The method of claim 9 wherein the stereo image parameters comprise stereo image parameters for a plurality of frequency bands.

### Claim 14 (depends on 9)

The method of claim 9 wherein a bandwidth of the first frequency band and a bandwidth of the second frequency band is determined based at least in part on a desired target bitrate.

### Claim 15 (depends on 9)

A non-transitory computer readable medium containing instructions that when executed by a processor perform the method of claim 9.

### Claim 16

A device for decoding a bitstream signal to produce a decoded output signal having a left channel and a right channel, the device comprising:
a demultiplexer for extracting an encoded audio signal from the bitstream signal, the encoded audio signal generated by encoding an input stereo audio signal having a left input channel and a right input channel using a selected coding mode, wherein the selected coding mode is one of a modified discrete cosine transform (MDCT) based transform coding mode or a linear predictive coding mode;
a MDCT based transform decoder for decoding the encoded audio signal if the selected coding mode is the MDCT based transform coding mode but not if the selected coding mode is the linear predictive coding mode; and
a linear predictive decoder for decoding the encoded audio signal if the selected coding mode is the linear predictive coding mode but not if the selected coding mode is the MDCT based transform coding mode,
wherein the linear predictive decoder is configured to:
receive an encoded mono signal, the encoded mono signal being a sum of the left input channel and the right input channel of the input stereo audio signal,
decode the encoded mono signal using linear predictive decoding to produce a decoded mono signal,
extract stereo image parameters and a residual signal from the bitstream signal for reconstructing a stereo audio signal that approximates a perceptual stereo image of the input stereo audio signal, wherein the residual signal indicates an error associated with representing the stereo signal by the mono signal and the stereo image parameters,
reconstruct the stereo audio signal using the decoded mono signal, the residual signal and the stereo image parameters to produce a reconstructed stereo audio signal that approximates the perceptual stereo image of the input stereo audio signal, and
output the reconstructed stereo audio signal as the decoded output signal,
wherein MDCT based transform decoder is configured to:
receive a stereo signal in a first frequency band, the stereo signal generated using a selected stereo coding mode, the selected stereo coding mode including either a mid/side stereo coding mode or a left/right stereo coding mode,
receive an encoded mono signal in a second frequency band,
decode the stereo signal in the first frequency band using the selected stereo coding mode to produce a decoded stereo signal in the first frequency band,
decode the encoded mono signal in the second frequency band using MDCT based transform decoding to produce a decoded mono signal in the second frequency band, and
generate the decoded output signal from the decoded stereo signal in the first frequency band and the decoded mono signal in the second frequency band.

### Claim 17 (depends on 16)

The device of claim 16 wherein the MDCT based transform decoding further comprises extracting side information from the bitstream signal for reconstruction of one or more subbands not encoded.

### Claim 18 (depends on 17)

The device of claim 17 wherein the side information includes a parameter used to determine a spectral envelope of the one or more subbands not encoded.

### Claim 19 (depends on 16)

The device of claim 16 wherein the MDCT based transform decoding comprises perceptual decoding.

### Claim 20 (depends on 19)

The device of claim 19 wherein the stereo image parameters comprise parameters for a plurality of frequency bands.