Audio Encoder Transient Processing with High-Resolution Envelope Gain
Find Innovative SolutionsGenerate Solutions
Solution Overview
Problem
Existing audio encoding technologies face challenges in coding transient signals like castanets and applause, which result in pre-echo artifacts due to the temporal spread of quantization noise, leading to reduced perceptual quality, especially at low bit-rates.
Innovation Solution
The implementation of High Resolution Envelope Processing (HREP) which involves pre-processing the audio signal to attenuate high-frequency transient events and post-processing to restore the original dynamics, using time-variable gain information to manage the encoding and decoding of audio signals, thereby reducing the bit-rate demand and improving perceptual quality.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If filterbank based perceptual coding is used to exploit simultaneous masking, then coding efficiency is improved, but quantization noise spreads in time causing pre-echo artifacts
Solution Approach 1:
The encoder performs preliminary identification of transient signal portions and pre-allocates additional bits to these regions before actual coding occurs. This advance preparation ensures that transient signals receive sufficient coding precision without requiring post-processing adjustments, thereby preventing pre-echo artifacts while maintaining overall coding efficiency.
Solution Approach 2:
The coding system applies different coding precision locally: standard precision for stationary signal portions and enhanced precision for identified transient portions. This local differentiation allows the system to concentrate bits where they are most needed (at transients) rather than uniformly distributing them, resolving the contradiction between overall efficiency and local quality requirements.
2Manufacturing precision
If quantization precision is increased for transient signals to avoid pre-echoes, then perceptual quality is improved, but bit-rate demand increases
Solution Approach 1:
The system dynamically changes the coding precision parameter based on signal characteristics. For stationary portions, standard precision is used; for transient portions identified through signal analysis, precision is increased. This parameter adaptation allows the system to achieve high perceptual quality at transients without maintaining high precision throughout the entire signal, thus controlling overall bit-rate demand.
Solution Approach 2:
Instead of applying high coding precision to the entire signal, the system applies excessive precision (additional bits) only partially - specifically to transient portions where it is most needed. This partial application of enhanced precision achieves the necessary perceptual quality improvement without the prohibitive cost of applying it universally.
3Measurement precision
If long filterbank windows are used for stationary signals, then frequency resolution is improved, but transient signal processing deteriorates
Solution Approach 1:
The signal is segmented into stationary portions and transient portions through analysis. Different window lengths are then applied to different segments: long windows for stationary portions to achieve high frequency resolution, and short windows for transient portions to achieve good time resolution. This segmentation allows the system to optimize for frequency resolution where applicable without sacrificing transient processing capability.
Solution Approach 2:
The filterbank window length is made dynamic rather than fixed. The system adapts the window length based on the local signal characteristics - using long windows for stationary regions and short windows for transient regions. This dynamic adaptation resolves the contradiction by allowing the system to have both long and short windows as needed in different parts of the signal.
Data Source
Figure 1
Figure 2
Figure 3a~3c
AI summary
An audio post-processor (100) for post-processing an audio signal (102) having a time-variable high frequency gain information (104) as side information comprises: a band extractor (110) for extracting a high frequency band (112) of the audio signal (102) and a low frequency band (114) of the audio signal (102); a high band processor (120) for performing a time-variable modification of the high frequency band (112) in accordance with the time-variable high frequency gain information (104) to obtain a processed high frequency band (122); and a combiner (130) for combining the processed high frequency band (122) and the low frequency band (114). Furthermore, a pre-processor is illustrated.