Subband Harmonic Transposition for Low-Artifact High-Frequency Reconstruction
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Solution Overview
Problem
Existing high-frequency reconstruction (HFR) methods in audio source coding systems face challenges in achieving high-fidelity audio reproduction at low computational complexity, particularly for signals with prominent periodic structures, leading to artifacts like 'ghost' pitches and increased computational complexity due to the need for fine frequency resolution and high oversampling.
Innovation Solution
The implementation of a cross-product enhanced HFR system that processes complex subband samples in blocks, applying frame-wise phase offsets and magnitude adjustments to reduce intermodulation products, allowing for coarser frequency resolution and lower oversampling while maintaining high output quality, using a subband block-based harmonic transposition method.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If complex modulated filter banks with very fine frequency resolution and high degree of oversampling are employed to achieve high quality HFR, then audio quality is improved, but computational complexity becomes very high
Solution Approach 1:
The patent segments the audio signal into multiple subbands using a filter bank with coarser frequency resolution. By processing each subband separately and combining them, the system achieves high-quality HFR without requiring very fine frequency resolution across the entire spectrum, thus reducing computational complexity while maintaining audio quality.
Solution Approach 2:
The patent changes the oversampling parameter from high degree to lower degree by employing a different transposition approach that works effectively with coarser filter banks. This parameter change allows the system to achieve the required audio quality with significantly reduced computational complexity by avoiding the need for high oversampling factors.
2Speed
If harmonic transposition is applied to signals with prominent periodic structure, then frequency transposition is achieved, but ghost pitch artifacts and metallic sounding character occur
Solution Approach 1:
The patent introduces cross-product terms as an intermediary mechanism to generate the missing harmonic partials that are lost during transposition. By computing cross-products of transposed subband signals, the system reconstructs the full harmonic series and eliminates ghost pitch artifacts, thereby removing the harmful effects while preserving the desired frequency transposition.
Solution Approach 2:
The patent converts the harmful ghost pitch artifacts into beneficial full harmonic series by using cross-product terms. The spurious spectral components that would normally cause metallic sounding character are transformed into desirable harmonic partials through the cross-product enhancement, turning a harmful effect into a beneficial one.
3Productivity
If coarser frequency resolution and lower oversampling are used to reduce computational complexity, then processing efficiency is improved, but audio quality and suppression of intermodulation products deteriorate
Solution Approach 1:
The patent applies dynamic phase offsets to the transposed subband signals, allowing the system to adaptively compensate for the coarser frequency resolution and lower oversampling. This dynamic adjustment maintains the suppression of intermodulation products and preserves audio quality while enabling the use of computationally efficient coarser filter banks.
Solution Approach 2:
The patent combines multiple transposed subband signals with cross-product terms to create a composite signal that achieves high audio quality. By synthesizing the transposed subbands together with the cross-product enhancement, the system obtains the benefits of both coarse frequency resolution (for efficiency) and fine effective resolution (for quality), achieving a composite solution that satisfies both requirements.
Data Source
AI summary
The invention provides an efficient implementation of cross-product enhanced high-frequency reconstruction (HFR), wherein a new component at frequency QΩ+rΩ0 is generated on the basis of existing components at Ω and Ω+Ω0. The invention provides a block-based harmonic transposition, wherein a time block of complex subband samples is processed with a common phase modification. Superposition of several modified samples has the net effect of limiting undesirable intermodulation products, thereby enabling a coarser frequency resolution and/or lower degree of oversampling to be used. In one embodiment, the invention further includes a window function suitable for use with block-based cross-product enhanced HFR. A hardware embodiment of the invention may include an analysis filter bank, a subband processing unit configurable by control data and a synthesis filter bank.