Subband Harmonic Transposition to Suppress HFR Audio Artifacts
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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 reduced 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 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 fine frequency resolution and high oversampling are used for 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, processing each subband independently with lower computational requirements. This segmentation allows coarse frequency resolution per subband while maintaining overall high audio quality through the combination of multiple subbands.
Solution Approach 2:
The patent changes the processing parameters by using block-based processing with frame-wise phase offsets and magnitude adjustments instead of continuous fine-resolution processing. This parameter change reduces computational complexity while preserving perceptual audio quality through controlled phase and magnitude modifications.
2Device complexity
If traditional harmonic transposition is used for signals with prominent periodic structures, then processing is simpler, but ghost pitch artifacts and metallic character occur
Solution Approach 1:
The patent introduces cross-product terms as intermediary elements that mediate between the original signal components and the transposed output. These cross-products generate additional spectral components that fill in the gaps caused by traditional transposition, eliminating ghost pitch artifacts while maintaining processing efficiency.
Solution Approach 2:
The patent combines multiple signal components (original subband signals and their cross-products) to create a composite transposed signal. This composite approach integrates both the direct transposition components and the cross-product components, resulting in a more complete and artifact-free reconstruction.
3Productivity
If coarse frequency resolution is used to reduce computational complexity, then processing efficiency is improved, but intermodulation products and artifacts increase
Solution Approach 1:
The patent extracts and separately processes the phase and magnitude components of each subband signal. By taking out these individual components and applying frame-wise phase offsets and magnitude adjustments, the method reduces intermodulation products that would otherwise be generated by coarse frequency resolution processing.
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.