Audio Decoder Zero-Input Response for Smooth CELP-MDCT Transitions
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Solution Overview
Problem
Conventional audio decoders face challenges in achieving smooth transitions between CELP and MDCT coding schemes, leading to aliasing and discontinuities due to differences in coding domains, particularly at low to medium bitrates.
Innovation Solution
The solution involves using a zero-input response of a linear predictive filter to modify the second decoded audio information, considering both the first and second decoded audio information, to achieve a smooth transition without introducing additional delay or modifying the first decoded audio information.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If switched audio codecs switch between CELP and MDCT coding schemes, then coding quality and bitrate efficiency are improved, but discontinuities and artifacts are introduced at mode transitions
Solution Approach 1:
The patent applies preliminary action by computing the zero-input response of the linear predictive filter in advance, before the actual mode transition occurs. This pre-computed response is then used to modify the MDCT decoded signal at the transition point, ensuring a smooth transition without discontinuities. The zero-input response captures the tail of the CELP signal, allowing the MDCT decoder to continue smoothly from the CELP frame without abrupt changes.
2Reliability
If conventional approaches modify the first decoded audio information to achieve smooth transition, then transition quality is improved, but additional delay is introduced
Solution Approach 1:
The patent inverts the conventional approach by instead of modifying the first decoded CELP signal, it modifies the second decoded MDCT signal using the zero-input response. This inversion allows the transition to be achieved without delaying the CELP decoding output, as the modification is applied to the MDCT side rather than requiring re-processing of the CELP frame.
3Reliability
If overlap-and-add operation is used to solve discontinuity, then transition quality is improved, but computational complexity increases
Solution Approach 1:
The patent extracts only the essential component needed for the transition - the zero-input response of the linear predictive filter - rather than using the full overlap-and-add operation. By taking out just the zero-input response and using it to modify the MDCT signal, the solution achieves smooth transitions with significantly reduced computational complexity compared to processing the entire overlapping region.
Data Source
AI summary
An audio decoder for providing a decoded audio information on the basis of an encoded audio information includes a linear-prediction-domain decoder configured to provide a first decoded audio information on the basis of an audio frame encoded in a linear prediction domain, a frequency domain decoder configured to provide a second decoded audio information on the basis of an audio frame encoded in a frequency domain, and a transition processor. The transition processor is configured to obtain a zero-input-response of a linear predictive filtering, wherein an initial state of the linear predictive filtering is defined depending on the first decoded audio information and the second decoded audio information, and modify the second decoded audio information depending on the zero-input-response, to obtain a smooth transition between the first and the modified second decoded audio information.