Audio Companding with Spectral Extension for Pre-Echo Reduction
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Solution Overview
Problem
Current audio codecs using lossy compression techniques introduce noticeable distortion in the form of coding noise, particularly pre-echo artifacts, which are challenging to mitigate without introducing phase distortion or reducing frequency resolution.
Innovation Solution
The method involves using companding techniques to process audio signals by dividing them into time segments, calculating and applying wideband gains in the frequency domain to amplify low-intensity segments and attenuate high-intensity segments during compression, and inversely doing so during expansion to restore the original dynamic range, thereby reducing quantization noise.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If lossy data compression techniques are used to reduce storage or data rate requirements, then productivity is improved, but coding noise or quantization noise is introduced that reduces fidelity
Solution Approach 1:
The audio signal is divided into multiple frames, and each frame is further divided into multiple sub-bands using a filter bank. This segmentation allows different noise shaping strategies to be applied to different time-frequency regions, enabling better control of quantization noise while maintaining compression efficiency.
Solution Approach 2:
The patent applies noise shaping with different characteristics to different sub-bands and frames based on local signal characteristics. By analyzing the signal energy and noise perception in each sub-band, the system optimizes the quantization process locally rather than applying uniform compression, thereby reducing audible artifacts while maintaining overall fidelity.
2Object-affected harmful factors
If filters are used to avoid or minimize pre-echo artifacts, then coding noise is reduced, but phase distortion and temporal smearing are introduced
Solution Approach 1:
The patent extracts the noise shaping function from traditional filtering approaches. Instead of using filters that modify the signal phase, the system shapes the quantization noise in the frequency domain through selective quantization of sub-bands, achieving pre-echo reduction without introducing phase distortion or temporal smearing.
3Object-affected harmful factors
If smaller transform windows are used to reduce pre-echo artifacts, then coding noise is minimized, but frequency resolution is significantly reduced
Solution Approach 1:
The patent transitions from time-domain windowing to frequency-domain noise shaping by introducing a filter bank that decomposes the signal into multiple sub-bands. This dimensional change allows the system to control quantization noise in the frequency domain while maintaining longer transform windows, thereby preserving frequency resolution without suffering from pre-echo artifacts.
Data Source
AI summary
Embodiments are directed to a companding method and system for reducing coding noise in an audio codec. A compression process reduces an original dynamic range of an initial audio signal through a compression process that divides the initial audio signal into a plurality of segments using a defined window shape, calculates a wideband gain in the frequency domain using a non-energy based average of frequency domain samples of the initial audio signal, and applies individual gain values to amplify segments of relatively low intensity and attenuate segments of relatively high intensity. The compressed audio signal is then expanded back to substantially the original dynamic range that applies inverse gain values to amplify segments of relatively high intensity and attenuating segments of relatively low intensity. A QMF filterbank is used to analyze the initial audio signal to obtain a frequency domain representation.