CELP Decoder Bandwidth Extension via Up-sampling
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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 beyond the core Code Excited Linear Prediction (CELP) codec's cut-off frequency, due to the need for sequential processing and analysis in the spectral domain using Modified Discrete Cosine Transform (MDCT).
Innovation Solution
A method for decoding audio signals that involves up-sampling the CELP excitation signal, applying non-linear operations, and using bandpass filtering to extend the audio bandwidth beyond the CELP-based decoder element, while synchronizing adaptive codebooks and minimizing alignment errors, and employing complementary all-pass filters to achieve equivalent filtering in both the encoder and decoder.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If MDCT analysis is used to code bandwidth beyond CELP core cut-off frequency, then coding capability for wideband/super-wideband signals is improved, but algorithmic delay increases significantly
Solution Approach 1:
The audio signal is divided into two separate coding paths: CELP coding for the low-frequency band (0-6.4 kHz) and MDCT-based spectral coding for the high-frequency band (6.4-14 kHz). By segmenting the frequency spectrum and applying different coding methods to each segment, the patent achieves wideband/super-wideband coding capability while managing delay through parallel processing of the two bands
Solution Approach 2:
The patent transitions from time-domain sequential processing to frequency-domain parallel processing by applying MDCT to the high-frequency band separately. This dimensional change allows simultaneous encoding of low and high frequency components, reducing the cumulative algorithmic delay that would result from sequential processing
2Adaptability or versatility
If sequential CELP and MDCT processing is used, then bandwidth extension is achieved, but processing complexity and delay increase
Solution Approach 1:
The processing architecture is segmented into independent CELP and MDCT processing streams that operate in parallel rather than sequentially. The input signal is split into low-frequency (for CELP) and high-frequency (for MDCT) components, processed simultaneously through different algorithms, and then combined at the output, reducing overall processing complexity
Solution Approach 2:
The patent applies preliminary frequency-domain decomposition using MDCT to the high-frequency band before encoding, allowing the high-frequency components to be processed independently and in parallel with the CELP low-frequency encoding, rather than requiring sequential time-domain processing
Data Source
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AI summary
A method for decoding an audio signal in a decoder having a CELP-based decoder element including a fixed codebook component, at least one pitch period value, and a first decoder output, wherein a bandwidth of the audio signal extends beyond a bandwidth of the CELP-based decoder element. The method includes obtaining an up-sampled fixed codebook signal by up-sampling the fixed codebook component to a higher sample rate, obtaining an up-sampled excitation signal based on the up-sampled fixed codebook signal and an up-sampled pitch period value, and obtaining a composite output signal based on the up-sampled excitation signal and an output signal of the CELP-based decoder element, wherein the composite output signal includes a bandwidth portion that extends beyond a bandwidth of the CELP-based decoder element.