CELP Audio Decoder Bandwidth Extension via MDCT Segmentation
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Solution Overview
Problem
Existing audio signal processing technologies, such as ITU-T G.718 and G.729.1 compliant speech coders, face significant algorithmic delays when encoding and decoding audio signals with bandwidths exceeding the core Code Excited Linear Prediction (CELP) codec's range, due to the need for sequential processing and analysis in the spectral domain using Modified Discrete Cosine Transform (MDCT).
Innovation Solution
The method involves generating a second excitation signal by up-sampling the CELP excitation signal and applying non-linear operations, followed by bandpass filtering to extend the audio bandwidth beyond the CELP-based decoder element, using techniques like up-sampling, fractional pitch period adjustments, and bandpass filtering with complementary all-pass filters to minimize delays and ensure equivalent processing in both encoder and decoder.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Area of stationary object
If MDCT analysis is used to code bandwidth beyond CELP core cut-off frequency, then audio bandwidth extension is achieved, but algorithmic delay increases
Solution Approach 1:
The audio signal is segmented into two parts: the baseband signal processed by CELP codec and the extended bandwidth signal processed separately through MDCT analysis. This segmentation allows independent optimization of each processing path, reducing overall algorithmic delay by avoiding sequential processing of the entire signal through both stages.
Solution Approach 2:
The MDCT analysis is performed on the original input signal before CELP encoding, allowing the extended bandwidth components to be prepared in advance. This preliminary action eliminates the need to wait for CELP decoding before performing spectral analysis, thereby reducing algorithmic delay.
2Area of stationary object
If sequential CELP encoding and MDCT analysis are performed, then bandwidth extension is achieved, but processing time increases
Solution Approach 1:
The system maintains continuous processing by performing MDCT analysis on the original signal in parallel with CELP encoding rather than sequentially. This continuous action ensures that bandwidth extension processing occurs without interruption or waiting, improving overall processing speed while maintaining full bandwidth capability.
3Manufacturing precision
If spectral domain processing is used for bandwidth extension, then audio quality is improved, but computational complexity increases
Solution Approach 1:
Spectral domain processing through MDCT is applied only to the frequency bands beyond the CELP core cut-off frequency, rather than processing the entire audio spectrum. This localized application of spectral processing maintains high audio quality in the extended bands while reducing overall computational complexity by avoiding redundant processing in the baseband range already handled by CELP.
Data Source
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AI summary
A method for decoding an audio signal having a bandwidth that extends beyond a bandwidth of a CELP excitation signal in an audio decoder including a CELP-based decoder element. The method includes obtaining a second excitation signal having an audio bandwidth extending beyond the audio bandwidth of the CELP excitation signal, obtaining a set of signals by filtering the second excitation signal with a set of bandpass filters, scaling the set of signals using a set of energy-based parameters, and obtaining a composite output signal by combining the scaled set of signals with a signal based on the audio signal decoded by the CELP-based decoder element.