MDCT Window Sequence for Hybrid Codec Switching
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Solution Overview
Problem
Hybrid coding systems face challenges in achieving a smooth transition between different coding schemes, particularly when switching from a speech coder to a Modified Discrete Cosine Transform (MDCT) based coder, resulting in audible degradation due to discontinuities at frame boundaries.
Innovation Solution
A method is proposed for both encoding and decoding ends of a hybrid coding system, using a sequence of MDCT windows that split the spectrum into nearly uncorrelated components, with the second half of the last window identical to the second half of a single MDCT window, allowing for perfect reconstruction without requiring overlap components from preceding frames.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If a conventional MDCT coder is used for music signals, then high quality audio coding is achieved, but discontinuities occur at frame boundaries when switching from speech coder resulting in audible degradation
Solution Approach 1:
The patent applies preliminary action by setting the second half of the last window in the sequence identical to the second half of a single MDCT window beforehand. This pre-positioning of window parameters ensures that when switching from speech to music coding, the reconstruction process can seamlessly continue without requiring overlap components from the preceding speech frame, thereby preventing audible degradation at frame boundaries.
2Manufacturing precision
If overlap components from preceding frames are used for reconstruction, then perfect reconstruction is achieved, but the complexity of the decoding process increases and extrapolations are required during codec switching
Solution Approach 1:
The patent extracts and eliminates the requirement for overlap components from preceding frames in the reconstruction process. By designing the window sequence such that the second half of the last window matches the second half of a single MDCT window, the invention removes the need to fetch and process overlap components from the previous speech frame, thereby simplifying the decoding process and eliminating the need for extrapolations during codec switching.
3Productivity
If a single MDCT window is used for transformation, then computational efficiency is maintained, but the spectrum is not adequately split into uncorrelated components affecting transition smoothness
Solution Approach 1:
The patent applies segmentation by dividing the MDCT transformation process into a sequence of windows rather than using a single window. This sequence of windows provides better spectral separation into uncorrelated components, improving the quality of transition between coding schemes. The segmentation is designed so that the second half of the last window in the sequence matches the second half of a single MDCT window, maintaining compatibility with standard reconstruction processes.
Data Source
AI summary
Methods and units are shown for supporting a switching from a first coding scheme to a Modified Discrete Cosine Transform (MDCT) based coding scheme calculating a forward or inverse MDCT with a window (h(n)) of a first type for a respective coding frame, which satisfies constraints of perfect reconstruction. To avoid discontinuities during the switching, it is proposed that for a transient frame immediately after a switching, a sequence of windows (h0(n),h1(n),h2(n)) is provided for the forward and the inverse MDCTs. The windows of the window sequence are shorter than windows of the first type. The window sequence splits the spectrum of a respective first coding frame into nearly uncorrelated spectral components when used as basis for forward MDCTs, and the second half of the last window (h2(n)) of the sequence of windows is identical to the second half of a window of the first type.