Spectrum Coding Apparatus for Wideband Audio Harmonic Structure Preservation
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Solution Overview
Problem
Conventional voice and audio coding techniques fail to maintain the harmonic structure of signals when compressing wideband signals, leading to quality degradation and annoying sounds due to discontinuity in spectral energy, especially when substituting low-frequency bands for high-frequency bands without considering the harmonic structure.
Innovation Solution
A method that estimates the shape of the high-frequency band spectrum using a filter with the low-frequency band as its internal state, codes the filter coefficients to adjust the spectral outline, and divides the band into subbands to prevent discontinuity, ensuring the harmonic structure is maintained during compression.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If the frequency band is widened to improve sound quality and realism, then the sense of realism improves, but the bit rate increases
Solution Approach 1:
The patent divides the frequency band into low-frequency band (0 to FL) and high-frequency band (FL to FH), processing each band separately. The low-frequency band is coded directly while the high-frequency band is estimated by filtering and scaling the low-frequency band, reducing the overall coding complexity and bit rate requirement while maintaining quality across the extended frequency range.
2Quantity of substance
If the low-frequency band spectrum is substituted for the high-frequency band spectrum to reduce bit rate, then the bit rate decreases, but the harmonic structure collapses and quality deteriorates
Solution Approach 1:
The patent introduces a filter as an intermediary between the low-frequency band spectrum and the high-frequency band estimation. This filter, with its internal state set to the low-frequency band spectrum, processes the signal to generate an estimated high-frequency band that preserves harmonic structure. Additionally, a scaling factor is applied as another intermediary to adjust the amplitude, preventing collapse of the harmonic structure while maintaining bit rate reduction.
3Quantity of substance
If the low-frequency band spectrum is substituted for the high-frequency band spectrum to reduce bit rate, then the bit rate decreases, but spectral energy discontinuity occurs and annoying sounds appear
Solution Approach 1:
The patent introduces a filter as an intermediary between the low-frequency band spectrum and the high-frequency band estimation. This filter, with its internal state set to the low-frequency band spectrum, processes the signal to generate an estimated high-frequency band that preserves harmonic structure. Additionally, a scaling factor is applied as another intermediary to adjust the amplitude, preventing collapse of the harmonic structure while maintaining bit rate reduction.
Solution Approach 2:
The patent applies a scaling factor to adjust the amplitude of the estimated high-frequency band spectrum. This parameter change ensures that the spectral energy is continuous across the boundary between low-frequency and high-frequency bands, preventing discontinuities that would cause annoying sounds. The scaling factor is determined based on the relationship between the two bands to maintain energy continuity.
Data Source
AI summary
A spectrum coding apparatus capable of performing coding at a low bit rate and with high quality is disclosed. This apparatus is provided with a section that performs the frequency transformation of a first signal and calculates a first spectrum, a section that converts the frequency of a second signal and calculates a second spectrum, a section that estimates the shape of the second spectrum in a band of FL≦k<FH using a filter having the first spectrum in a band of 0≦k<FL as an internal state and a section that codes an outline of the second spectrum determined based on a coefficient indicating the characteristic of the filter at this time.