Transient-Location Dependent Windowing for Pre-Echo Audio Coding
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Solution Overview
Problem
Conventional audio or image coding systems face challenges in minimizing look-ahead delay while maintaining high coding quality, particularly during signal transients, due to restricted window lengths and inefficiencies in transform switching, leading to pre-echo and post-echo noise.
Innovation Solution
An apparatus and method that utilize a controllable windower to apply specific overlap lengths to audio or image signals based on transient locations, selecting from a set of windows with varying overlap lengths to minimize look-ahead delay and reduce pre-echo noise, allowing for flexible adaptation of transform lengths and overlaps.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If conventional coding systems use fixed window lengths and transform switching, then coding quality is maintained for stationary signals, but look-ahead delay increases and pre-echo noise occurs during transients
Solution Approach 1:
The patent implements dynamic window length adaptation by selecting between long windows (e.g., 2048 samples) for stationary signals and short windows (e.g., 256 samples) for transient signals. The system continuously monitors signal characteristics and adjusts window parameters in real-time, allowing the coding system to transition from static to dynamic operation. This resolves the contradiction by maintaining long windows for quality during stationary periods while switching to short windows to reduce look-ahead delay during transients.
Solution Approach 2:
The patent changes key parameters including window length (from long to short), overlap percentage (from 50% to 25% or 0%), and transform type based on detected transient conditions. When a transient is detected, the system modifies these parameters to optimize performance: reducing window length decreases the look-ahead buffer requirement, and adjusting overlap reduces temporal smearing. This parameter adaptation directly addresses the contradiction between maintaining coding quality and reducing time delay.
2Measurement precision
If long transform windows are used for stationary signals, then coding quality is improved, but pre-echo and post-echo noise increases during transients
Solution Approach 1:
The system dynamically switches window lengths based on transient detection. During stationary signal segments, long windows (e.g., 2048 samples) are used to maintain high coding quality through better frequency resolution. When transients are detected, the system transitions to short windows (e.g., 256 samples) which localize the transform operation in time, preventing pre-echo noise caused by the temporal smearing inherent in long windows. This dynamic adaptation resolves the contradiction between quality and artifact reduction.
Solution Approach 2:
The patent applies different window lengths to different temporal regions of the signal based on local characteristics. Stationary regions receive long-window processing for high quality, while transient regions receive short-window processing to eliminate pre-echo. This localized adaptation of processing parameters ensures that each region is treated optimally, resolving the contradiction between maintaining overall quality and preventing local artifacts.
3Reliability
If transform switching is implemented to handle transients, then transient coding is improved, but device complexity and computational overhead increase
Solution Approach 1:
The patent segments the signal processing into distinct modes: long-window mode for stationary signals and short-window mode for transients. The transient detection unit divides the signal stream into segments that are processed differently based on their characteristics. This segmentation allows the system to implement complex transient handling only where needed, rather than throughout the entire signal, thereby improving transient coding performance while limiting the increase in overall system complexity.
Solution Approach 2:
The system performs preliminary transient detection on incoming signal frames before committing to a processing mode. By detecting transients in advance (using a look-ahead buffer), the system can prepare the appropriate window length and overlap settings beforehand, avoiding complex real-time decisions during coding. This preliminary action simplifies the main coding path while still achieving reliable transient handling.
4Measurement precision
If reduced window overlap is used for transients, then temporal localization is improved, but coding efficiency decreases for stationary signals
Solution Approach 1:
The system dynamically adjusts the overlap percentage between consecutive windows based on transient detection. During stationary signal processing, a 50% overlap is used which provides smooth transitions and maintains coding efficiency through better energy distribution. During transient processing, the overlap is reduced to 25% or 0%, which improves temporal localization by reducing the temporal smearing effect. This dynamic adjustment resolves the contradiction by optimizing overlap for each signal condition.
Solution Approach 2:
The patent applies different overlap percentages to different temporal regions: high overlap (50%) for stationary regions to maintain coding efficiency, and low overlap (25%-0%) for transient regions to improve temporal localization. This localized quality adjustment ensures that each region receives the overlap treatment most appropriate for its characteristics, resolving the contradiction between efficiency and localization.
Data Source
AI summary
An apparatus for encoding an audio or image signal, includes: a controllable windower for windowing the audio or image signal to provide the sequence of blocks of windowed samples; a converter for converting the sequence of blocks of windowed samples into a spectral representation including a sequence of frames of spectral values; a transient location detector for identifying a location of a transient within a transient look-ahead region of a frame; and a controller for controlling the controllable windower to apply a specific window having a specified overlap length to the audio or image signal in response to an identified location of the transient, wherein the controller is configured to select the specific window from a group of at least three windows, wherein the specific window is selected based on the transient location.