Subband Harmonic Transposition to Suppress Ghost Pitches
Find Innovative SolutionsGenerate Solutions
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 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 without compromising audio 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 audio quality, then manufacturing precision of spectral components is improved, but device complexity increases significantly
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, resolving the contradiction between spectral precision and computational complexity.
Solution Approach 2:
The patent changes the processing parameters by using integer-based transposition factors and block-based processing with specific hop sizes, replacing the need for fine frequency resolution and high oversampling. This parameter transformation maintains audio quality while significantly reducing computational complexity compared to traditional methods.
2Speed
If harmonic transposition of order Qφ>1 is applied to signals with prominent periodic structure, then frequency transposition is achieved, but ghost pitch artifacts and metallic character are generated
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 gaps between harmonics, serving as a mediator that prevents ghost pitch artifacts while maintaining the desired frequency transposition effect.
Solution Approach 2:
The patent creates a composite signal structure by combining direct transposition components with cross-product components. This composite approach merges multiple processing paths (direct subband transposition and cross-product generation) to produce an output that maintains periodic structure integrity while avoiding metallic artifacts and ghost pitches.
3Loss of substance
If single sideband modulation based HFR is used for low bandwidth core signal, then bitrate is reduced, but dissonant ringing artifact is produced
Solution Approach 1:
The patent employs dynamic subband processing where the transposition and cross-product operations are adaptively applied based on the signal characteristics in each subband. This dynamic approach allows the system to maintain low bitrate while avoiding ringing artifacts by adjusting processing intensity and selecting appropriate subband combinations based on local signal properties.
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.