Subband Harmonic Transposition to Reduce Ghost Pitch 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 with reduced computational effort, particularly for signals with prominent periodic structures, which result in artifacts like 'ghost' pitches and increased computational complexity.
Innovation Solution
The implementation of cross-product enhanced subband block-based harmonic transposition, which processes data in blocks of complex subband samples, applies frame-wise phase offsets, and magnitude adjustments to reduce intermodulation products, allowing for the use of filter banks with coarser frequency resolution and lower oversampling while maintaining high output quality.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If complex modulated filter banks with fine frequency resolution and high oversampling are used, then audio quality is improved, but computational complexity becomes very high
Solution Approach 1:
The patent segments the audio signal into subbands using a filter bank, processing each subband separately. This allows the use of coarser frequency resolution within each subband while maintaining overall audio quality, thereby reducing computational complexity compared to processing the entire spectrum with fine resolution.
Solution Approach 2:
The patent applies different processing strategies to different frequency regions through subband decomposition. Each subband can be processed with appropriate resolution and oversampling factors tailored to local requirements, rather than uniformly applying high resolution across all frequencies, thus reducing overall computational burden.
2Speed
If harmonic transposition of order Qφ > 1 is applied to signals with prominent periodic structure, then frequency transposition is achieved, but ghost pitch and metallic character artifacts occur
Solution Approach 1:
The patent introduces cross-product terms as intermediary elements that combine multiple subband signals. These cross-products generate additional spectral components that fill in the missing harmonics caused by transposition, thereby eliminating the ghost pitch artifact while preserving the desired frequency transposition effect.
Solution Approach 2:
The patent combines multiple signal components (direct transposed subbands and cross-product subbands) to create a composite output signal. This composite approach ensures that both the fundamental transposed harmonics and the missing higher-order harmonics are present, eliminating the metallic character and ghost pitch artifacts.
3Manufacturing precision
If cross-product transposition is implemented to address ghost pitch, then missing partials are regenerated, but computational cost increases considerably
Solution Approach 1:
The patent segments the computation of cross-products to occur only within and between specific subbands rather than across the entire frequency spectrum. This selective subband-based cross-product computation significantly reduces the number of operations required compared to full-spectrum cross-product transposition, while still regenerating the necessary missing partials.
Data Source
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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 (101), a subband processing unit (102) configurable by control data (104) and a synthesis filter bank (103).