High-Frequency Envelope Processing for Transient Audio Coding
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Solution Overview
Problem
Existing audio processing technologies face challenges in encoding transient signals, such as applause, due to pre-echo artifacts and high bit-rate demands, particularly in coding schemes like MP3 and AAC, which struggle with temporal masking and quantization noise spreading.
Innovation Solution
The implementation of High Resolution Envelope Processing (HREP) involves a pre-processor that temporally flattens high-frequency signals and generates side information, and a post-processor that temporally shapes these signals, using time-variable high-frequency gain information to reduce short-time dynamics and improve coding efficiency.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If classic filterbank based perceptual coders (MP3, AAC) are used to encode transient signals, then spectral signal decomposition and quantization can be achieved, but quantization noise spreads out in time causing pre-echo artifacts that exceed the original signal components
Solution Approach 1:
The encoder performs preliminary detection of transient signal components before quantization and coding. By identifying transient regions in advance, the system can apply specialized processing (such as time-domain noise shaping or adjusted quantization strategies) to prevent quantization noise from spreading before the transient onset, thereby eliminating pre-echo artifacts while maintaining spectral decomposition precision
Solution Approach 2:
The patent applies different processing strategies to different parts of the signal. For transient regions, specialized noise shaping and quantization methods are applied locally, while non-transient regions use standard perceptual coding. This localized approach ensures that quantization noise does not spread before transient onsets where it would be audible, while maintaining efficient coding elsewhere
2Productivity
If quantization is applied in the spectral domain, then coding efficiency is improved, but quantization noise spreads over more than 40 milliseconds causing pre-echoes before signal onsets
Solution Approach 1:
The patent introduces an intermediary processing stage between spectral decomposition and quantization. This intermediary performs time-domain analysis to identify transient regions and generates control information that guides the quantization process. By using this intermediary, the system maintains the coding efficiency of spectral domain quantization while preventing temporal spread of quantization noise before transient onsets through adaptive noise shaping
3Measurement precision
If high coding precision is used for transient signal portions, then perceptual quality is improved, but the amount of bits required increases significantly making constant bit rate coding difficult
Solution Approach 1:
The patent employs dynamic bit rate allocation that adapts to the temporal characteristics of the signal. For transient regions, higher precision coding is applied only where necessary (at and near the transient onset), while using fewer bits for regions where quantization noise would be masked or less perceptible. This dynamic approach maintains high perceptual quality for transients while controlling overall bit rate demand for constant bit rate coding
4Duration of action of moving object
If short window sizes are used for transient coding, then pre-echo duration is reduced, but coding efficiency decreases and more bits are required
Solution Approach 1:
The patent segments the signal into transient and non-transient regions, then applies different window sizes and coding strategies to each segment. For transient portions, short window sizes are used locally to limit pre-echo duration, while longer window sizes are used for surrounding non-transient regions to maintain coding efficiency. This segmentation allows the system to achieve short pre-echo duration where critical while maintaining overall coding efficiency
Data Source
AI summary
An audio post-processor for post-processing an audio signal having a time-variable high frequency gain information as side information includes: a band extractor for extracting a high frequency band of the audio signal and a low frequency band of the audio signal; a high band processor for performing a time-variable modification of the high frequency band in accordance with the time-variable high frequency gain information to obtain a processed high frequency band; and a combiner for combining the processed high frequency band and the low frequency band. Furthermore, a pre-processor is illustrated.