Audio Coder Critical Sampling MDCT Overlap
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Solution Overview
Problem
Existing audio coding technologies, such as AMR-WB+, face inefficiencies due to non-critical sampling and increased data overhead, leading to suboptimal sound quality and frequency response when switching between ACELP and TCX modes, particularly due to the steep overlap regions and discarded samples during frame transitions.
Innovation Solution
Implementing a Modified Discrete Cosine Transform (MDCT) for transforming overlapping time domain frames to the frequency domain, allowing for critical sampling and smoother cross-fading between frames, thereby reducing overhead and improving sound quality.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If non-critical sampling is used in existing audio coding technologies, then implementation is simpler, but data overhead increases and sound quality deteriorates
Solution Approach 1:
The patent changes the sampling parameter from non-critical to critical sampling, where the transform block size equals the frame size. This parameter change eliminates the need for additional padding or overlap data, thereby reducing data overhead while maintaining implementation feasibility through the standardized MDCT process
2Ease of manufacture
If non-critical sampling is used, then implementation is simpler, but frequency response quality deteriorates
Solution Approach 1:
By changing the sampling parameter to critical sampling where transform block size equals frame size, the patent achieves continuous frequency response without the artifacts introduced by non-critical sampling. This parameter change improves frequency response quality while the standardized MDCT implementation keeps complexity manageable
3Adaptability or versatility
If steep overlap regions are used during mode switching, then transition between ACELP and TCX modes is achieved, but sound quality deteriorates
Solution Approach 1:
The patent applies preliminary windowing functions to the time-domain signal before MDCT transformation. This preliminary action smooths the transitions between overlapping frames and during mode switching, preventing the steep overlap regions from causing audible artifacts and thereby maintaining sound quality during mode transitions
4Device complexity
If samples are discarded during frame transitions, then processing is simplified, but sound quality deteriorates
Solution Approach 1:
The patent converts the potentially harmful effect of frame boundaries and overlaps into a benefit by using windowing functions that deliberately create smooth transitions. The overlap region, which could cause discontinuities, is transformed into a beneficial cross-fade region that improves sound quality during mode transitions and frame processing
Data Source
AI summary
An audio encoder adapted for encoding frames of a sampled audio signal to obtain encoded frames, wherein a frame includes a number of time domain audio samples. The audio encoder includes a predictive coding analysis stage for determining information on coefficients of a synthesis filter and a prediction domain frame based on a frame of audio samples. The audio encoder further includes a time-aliasing introducing transformer for transforming overlapping prediction domain frames to the frequency domain to obtain prediction domain frame spectra, wherein the time-aliasing introducing transformer is adapted for transforming the overlapping prediction domain frames in a critically-sampled way. Moreover, the audio encoder includes a redundancy reducing encoder for encoding the prediction domain frame spectra to obtain the encoded frames based on the coefficients and the encoded prediction domain frame spectra.