Variable Frame Length Audio Encoding for Low Bit Rate Quality
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Solution Overview
Problem
Existing audio encoding schemes face challenges in maintaining high audio quality at low bit rates, particularly in mobile communication systems, due to limitations in bit rate and computational resources, leading to artifacts like pre-echoing, ghost-like sounds, and frame discontinuities.
Innovation Solution
The method involves creating a main signal and a side signal from polyphonic signals, with the main signal encoded using various schemes characterized by sub-frames of different lengths, and selecting the best encoding scheme based on signal content to minimize side residual signals, using a balance factor to optimize encoding and reduce computational requirements.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Loss of energy
If long frame lengths are used for encoding, then bit rate is reduced, but temporal resolution deteriorates causing pre-echoing and ghost-like sounds
Solution Approach 1:
The encoding frame is divided into multiple sub-frames of different lengths. The encoder selectively applies different sub-frame lengths to different portions of the signal based on temporal activity. During transient periods, shorter sub-frames are used to maintain temporal resolution, while during steady-state periods, longer sub-frames are used to reduce bit rate.
Solution Approach 2:
The frame length is made dynamic rather than fixed. The encoder adapts the sub-frame length according to the signal characteristics, using short frames when transients are detected and long frames when the signal is stationary. This dynamic adaptation allows the system to optimize between temporal resolution and bit rate based on instantaneous signal conditions.
2Manufacturing precision
If short frame lengths are used for encoding, then temporal resolution is improved, but bit rate increases and computational complexity increases
Solution Approach 1:
Rather than using uniformly short frames, the signal is segmented into regions of different temporal activity. Only the transient portions are encoded with short sub-frames, while the majority of the signal (steady-state portions) is encoded with longer sub-frames, thereby reducing the overall bit rate requirement.
Solution Approach 2:
Different sub-frame lengths are applied locally to different portions of the signal based on their specific requirements. Transient regions receive short sub-frames for high temporal resolution, while steady-state regions receive long sub-frames for efficient compression. This local adaptation optimizes the balance between quality and bit rate.
3Device complexity
If uniform sub-frame lengths are used, then encoding complexity is reduced, but audio quality deteriorates due to frame discontinuities and inability to adapt to signal content
Solution Approach 1:
The sub-frame length structure is made dynamic and adaptive rather than uniform. The encoder detects transient events and adjusts sub-frame lengths accordingly, creating a variable structure that adapts to signal content. This dynamic approach improves audio quality by reducing artifacts while maintaining manageable complexity through algorithmic adaptation.
Solution Approach 2:
The encoding parameters (specifically sub-frame length) are changed based on signal characteristics. The system monitors signal properties and adjusts the sub-frame length parameter to optimize encoding performance for different types of audio content, thereby improving quality without requiring overly complex fixed structures.
4Manufacturing precision
If computational resources are increased to handle short frames, then temporal resolution is improved, but processing power requirements increase
Solution Approach 1:
Processing resources are segmented and allocated dynamically based on signal needs. Rather than continuously using high computational resources, the encoder only applies intensive short-sub-frame processing when transients are detected, thereby reducing average processing power requirements while maintaining temporal resolution when needed.
Solution Approach 2:
Full computational resources are not continuously allocated. Instead, the system applies intensive processing (short sub-frames) only partially, only when and where transients occur. This partial action approach achieves high temporal resolution for critical signal portions without requiring excessive processing power for the entire signal.
Data Source
AI summary
Polyphonic signals are used to create a main signal, typically a mono signal, and a side signal. A number of encoding schemes for the side signal are provided. Each encoding scheme is characterized by a set of sub-frames of different lengths. The total length of the sub-frames corresponds to the length of the encoding frame of the encoding scheme. The encoding scheme to be used on the side signal is selected dependent on the present signal content of the polyphonic signals. In a preferred embodiment, a side residual signal is created as the difference between the side signal and the main signal scaled with a balance factor. The balance factor is selected to minimize the side residual signal. The optimized side residual signal and the balance factor are encoded and provided as encoding parameters representing the side signal.