SBR Frame Structure for Transient Audio Encoding at Low Bit Rates
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Solution Overview
Problem
Existing audio encoding methods using SBR technique for low bit rates result in audible artifacts, particularly during transients, due to coarse temporal grids leading to pre-echo issues, while fine grids increase bit consumption.
Innovation Solution
Introduce a new SBR frame class (LD_TRAN) where grid boundaries align with frame boundaries, using a transient position indication to define a short grid area around transients, reducing the need for variable frame classes and minimizing bit transmission.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Loss of substance
If a coarse temporal grid is used for encoding the spectral envelope, then bit consumption is reduced, but audible artifacts and pre-echo occur during transients
Solution Approach 1:
The temporal grid is segmented into different regions: coarse grid areas for stationary portions and fine grid areas for transient portions. This segmentation allows the encoder to use different temporal resolutions in different time regions, reducing bit consumption in stationary areas while maintaining quality during transients.
Solution Approach 2:
The temporal grid resolution is made dynamic rather than static. The encoder adapts the grid fineness based on the detected signal characteristics - using coarse grids during stationary periods and fine grids during transients. This dynamic adaptation resolves the contradiction by adjusting resolution according to actual signal needs.
2Object-affected harmful factors
If a fine temporal grid is used for encoding the spectral envelope, then audible artifacts and pre-echo are reduced, but bit consumption increases
Solution Approach 1:
Different temporal resolutions are applied locally to different portions of the signal. Fine temporal resolution is applied only to transient portions where it is needed, while coarse resolution is used for stationary portions. This local differentiation eliminates the need to use fine resolution globally, thereby reducing overall bit consumption while maintaining quality where required.
Solution Approach 2:
The temporal resolution parameter of the grid is changed dynamically based on signal characteristics. The encoder detects transients and adjusts the grid fineness parameter accordingly - increasing resolution during transients and decreasing it during stationary periods. This parameter adaptation resolves the contradiction by matching resolution to actual signal requirements.
3Object-affected harmful factors
If variable frame classes are used to handle transients, then audio quality is improved, but device complexity and decoding delay increase
Solution Approach 1:
A single unified frame structure is used that can handle both stationary and transient portions through internal grid adaptation rather than requiring multiple specialized frame classes. The transient position indication and adaptive grid mechanism allow one frame type to serve multiple purposes, reducing complexity while maintaining quality.
Solution Approach 2:
The encoder preliminarily identifies transient positions and incorporates this information into the frame structure before decoding. By pre-marking transient locations with position indications, the decoder can directly use this information to adjust grid boundaries without complex real-time analysis, reducing decoding complexity and delay.
Data Source
AI summary
The transient problem may be sufficiently addressed, and for this purpose, a further delay on the side of the decoding may be reduced if a new SBR frame class is used wherein the frame boundaries are not shifted, i.e. the grid boundaries are still synchronized with the frame boundaries, but wherein a transient position indication is additionally used as a syntax element so as to be used, on the encoder and/or decoder sides, within the frames of these new frame class for determining the grid boundaries within these frames.