Phase Coherence Control in Perceptual Audio Codecs

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Solution Overview

Problem

Modern perceptual audio codecs often compromise the vertical phase coherence of audio signals, especially at low bitrates, leading to impaired perceptual quality in signals with rich harmonic overtone content, such as voiced speech and brass instruments, due to the loss of phase coherence over frequency.

Innovation Solution

A system and method that include a phase adjustment unit in both encoders and decoders to measure and restore vertical phase coherence by generating and applying control information indicating the phase coherence of the audio signal, allowing for selective adjustment of phase values to maintain or restore the original phase coherence, particularly in signals where it is crucial for quality.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Productivity

If perceptual audio coding is used to reduce bitrate, then transmission efficiency is improved, but vertical phase coherence is lost

Engineering Contradiction:

Improvetransmission efficiencyVSAvoidvertical phase coherence

Core Design Contradiction:

ProductivityVSReliability

Solution Approach 1:

The encoder performs preliminary analysis of the audio signal to detect vertical phase coherence characteristics before encoding. Control information indicating the degree of VPC is generated and transmitted alongside the coded audio signal, enabling the decoder to restore phase coherence without requiring additional complex processing

Inventive Principle:

Principle #10Preliminary action

Solution Approach 2:

Control information acts as an intermediary carrier that conveys VPC characteristics from encoder to decoder. This side information enables the decoder to adjust phase values and restore vertical phase coherence without directly transmitting the original phase data, thus maintaining low bitrate while improving audio quality

Inventive Principle:

Principle #24Intermediary (Mediator)

2Quantity of substance

If quantization is applied to reduce data rate, then bitrate is reduced, but phase coherence is degraded

Engineering Contradiction:

Improvedata rateVSAvoidphase coherence

Core Design Contradiction:

Quantity of substanceVSManufacturing precision

Solution Approach 1:

The system incorporates feedback by analyzing the coded audio signal at the decoder to determine the degree of vertical phase coherence, then using this information to adjust phase values. This closed-loop approach ensures that phase coherence is restored to match the original signal characteristics despite quantization losses

Inventive Principle:

Principle #23Feedback

3Productivity

If parametric coding is used at low bitrates, then compression efficiency is improved, but vertical phase coherence is weakened

Engineering Contradiction:

Improvecompression efficiencyVSAvoidvertical phase coherence

Core Design Contradiction:

ProductivityVSReliability

Solution Approach 1:

The system applies different processing strategies to different frequency regions and signal components. Phase adjustment is selectively applied to harmonic components where vertical phase coherence is most important, while allowing more compression in non-critical regions, thus maintaining audio quality where it matters most without sacrificing overall compression efficiency

Inventive Principle:

Principle #3Local quality

Data Source

PatentUS10818304B2Phase coherence control for harmonic signals in perceptual audio codecs

Publication Date: 2020.10.27 FRAUNHOFER GESELLSCHAFT ZUR FORDERUNG DER ANGEWANDTEN FORSCHUNG EV

AI summary

A decoder for decoding an encoded audio signal to obtain a phase-adjusted audio signal is provided. The decoder has a decoding unit and a phase adjustment unit. The decoding unit is adapted to decode the encoded audio signal to obtain a decoded audio signal. The phase adjustment unit is adapted to adjust the decoded audio signal to obtain the phase-adjusted audio signal. The phase adjustment unit is configured to receive control information depending on a vertical phase coherence of the encoded audio signal. Moreover, the phase adjustment unit is adapted to adjust the decoded audio signal based on the control information.