Audio High-Frequency Reconstruction with Spectral Envelope Correction

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Solution Overview

Problem

Conventional High Frequency Reconstruction (HFR) technologies introduce artificial spectral discontinuities and level variations in the highband signal, particularly for audio signals with large variations in the lowband range, leading to perceived loss of high frequency energy and audible discontinuities.

Innovation Solution

An additional correction step is introduced in the HFR signal generation process, involving the use of spectral gain coefficients derived from a frequency-dependent curve to adjust the energy of low frequency subband signals, ensuring the highband signal resembles the original spectral envelope without introducing undesirable artifacts.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Loss of energy

If conventional HFR technologies are used to reconstruct high frequency signals from low frequency signals, then bit rate efficiency is improved, but spectral discontinuities and level variations are introduced in the highband signal

Engineering Contradiction:

Improvebit rate efficiencyVSAvoidspectral accuracy

Core Design Contradiction:

Loss of energyVSManufacturing precision

Solution Approach 1:

The patent applies preliminary action by pre-processing the lowband signal through spectral envelope modification before transposition. The spectral envelope is adjusted to compensate for expected discontinuities that will occur during the HFR process, thereby preventing rather than correcting the spectral inaccuracies in the highband signal

Inventive Principle:

Principle #10Preliminary action

Solution Approach 2:

The patent introduces an intermediary spectral envelope adjustment stage between the lowband signal and the transposition process. This intermediary modifies the spectral characteristics of the lowband signal to ensure that when transposed to highband, the resulting signal has reduced spectral discontinuities and better matches the original highband spectral envelope

Inventive Principle:

Principle #24Intermediary (Mediator)

2Manufacturing precision

If spectral envelope adjustment is applied to correct highband discontinuities, then spectral accuracy is improved, but the complexity of the envelope adjuster increases

Engineering Contradiction:

Improvespectral accuracyVSAvoidenvelope adjuster complexity

Core Design Contradiction:

Manufacturing precisionVSDevice complexity

Solution Approach 1:

By performing spectral envelope modification on the lowband signal before transposition, the patent reduces the burden on the subsequent envelope adjuster. The preliminary action pre-compensates for spectral issues, allowing the envelope adjuster to operate with simpler logic and fewer computational resources

Inventive Principle:

Principle #10Preliminary action

3Productivity

If high frequency reconstruction is performed using transposition methods, then coding efficiency is improved, but artificial spectral characteristics are introduced

Engineering Contradiction:

Improvecoding efficiencyVSAvoidartificial spectral artifacts

Core Design Contradiction:

ProductivityVSObject-generated harmful factors

Solution Approach 1:

The patent converts the harmful artificial spectral characteristics into beneficial corrections by analyzing the spectral envelope of the lowband signal and using it to pre-adjust the spectral characteristics before transposition. The artificial characteristics generated by transposition are anticipated and compensated for in advance

Inventive Principle:

Principle #22Blessing in disguise (Convert harm into benefit)

Solution Approach 2:

The patent applies parameter changes by modifying the spectral envelope parameters of the lowband signal before transposition. The spectral envelope is adjusted to have reduced slope and modified energy distribution, which translates to reduced spectral discontinuities in the highband signal after transposition

Inventive Principle:

Principle #35Parameter changes

Data Source

PatentUS9117459B2Processing of audio signals during high frequency reconstruction

Publication Date: 2015.08.25 DOLBY INTERNATIONAL AB

AI summary

The application relates to HFR (High Frequency Reconstruction/Regeneration) of audio signals. In particular, the application relates to a method and system for performing HFR of audio signals having large variations in energy level across the low frequency range which is used to reconstruct the high frequencies of the audio signal. A system configured to generate a plurality of high frequency subband signals covering a high frequency interval from a plurality of low frequency subband signals is described. The system comprises means for receiving the plurality of low frequency subband signals; means for receiving a set of target energies, each target energy covering a different target interval within the high frequency interval and being indicative of the desired energy of one or more high frequency subband signals lying within the target interval; means for generating the plurality of high frequency subband signals from the plurality of low frequency subband signals and from a plurality of spectral gain coefficients associated with the plurality of low frequency subband signals, respectively; and means for adjusting the energy of the plurality of high frequency subband signals using the set of target energies.