Subband Frequency Shifting for Harmonic Bandwidth Extension
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Solution Overview
Problem
Current audio encoders face limitations in bandwidth extension, particularly at low bit rates, leading to undesired signal corruptions and artefacts such as missing highs, blurred transients, or artificial hissing, due to the inability to maintain the harmonic structure of audio signals.
Innovation Solution
The development of harmonic spectral band extension (HSBE) in the frequency domain, which replicates the harmonic structure of audio signals by copying and modulating spectral values, maintaining the original harmonic structure and reducing artefacts through FIR filters and MDCT/MDST transforms.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If bandwidth extension is applied at low bit rates, then data transmission efficiency is improved, but signal quality deteriorates due to artefacts and loss of harmonic structure
Solution Approach 1:
The patent copies spectral values from lower frequency bands to higher frequency bands to extend the bandwidth. By replicating and modulating the copied spectral values with appropriate phase relationships, the harmonic structure is preserved while achieving bandwidth extension at low bit rates, thus improving signal quality without sacrificing transmission efficiency
Solution Approach 2:
The patent modifies frequency and phase parameters of copied spectral values to maintain harmonic relationships during bandwidth extension. By carefully controlling the phase angle differences and frequency shifts, the method preserves the original signal's harmonic structure while extending the frequency range, resolving the contradiction between compression efficiency and signal fidelity
2Length of moving object
If spectral values are copied and modulated to extend bandwidth, then frequency range is improved, but harmonic structure may be corrupted
Solution Approach 1:
The patent precisely controls frequency and phase parameters when copying and modulating spectral values. By calculating appropriate phase angle differences based on the frequency shift amount and subband bandwidth, the method maintains correct harmonic relationships between original and extended frequency components, thus extending bandwidth while preserving harmonic structure
Solution Approach 2:
The patent uses feedback mechanisms to adjust the phase and frequency parameters of copied spectral values. By monitoring the harmonic relationships and adjusting the modulation parameters accordingly, the system ensures that the extended bandwidth does not corrupt the original harmonic structure, achieving both goals simultaneously
3Reliability
If FIR filters and MDCT/MDST transforms are used to reduce artefacts, then signal quality is improved, but computing complexity increases
Solution Approach 1:
The patent applies FIR filters and MDCT/MDST transforms selectively rather than universally. By applying these computationally intensive operations only where needed to remove specific types of artefacts or in specific frequency bands, the method improves signal quality while minimizing the increase in computing complexity, achieving a practical balance between quality and resource requirements
Data Source
AI summary
A device for producing a frequency-shifted audio signal based on an audio input signal is provided. The device has an interface and a frequency-shifting unit. The interface is configured for receiving the audio input signal. The frequency-shifting unit is configured for producing the frequency-shifted audio signal. The frequency-shifting unit is additionally configured to produce one of the second subband values based on one of the first subband values such that the second phase angle of this second subband value differs from the first phase angle of this first subband value by a phase angle difference, the phase angle difference being dependent on frequency information indicating by which frequency difference the audio input signal is to be shifted in order to obtain the frequency-shifted audio signal, and the phase angle difference being dependent on a frequency bandwidth of one of the first subbands.