Interleaved Audio Transform Switching for Transient Coding
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Solution Overview
Problem
Modern frequency-domain audio coding systems face challenges in achieving satisfactory quality for audio signals with prominent transients, such as rain or applause, at low bitrates, as they either suffer from time-smearing or inefficiency due to fixed transform lengths.
Innovation Solution
A frequency-domain audio codec that supports transform length switching, allowing for the use of multiple transform lengths within a frame, enabling better adaptation to signal characteristics while maintaining backward compatibility with existing codecs.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If long block coding is used, then temporal resolution is improved, but time-smearing of coding error (pre-echo) occurs
Solution Approach 1:
The audio signal is divided into multiple blocks of different lengths (short and long blocks) instead of using a single fixed block length. The encoder can switch between short blocks (for transient signals to reduce pre-echo) and long blocks (for stationary signals to improve temporal resolution), thereby resolving the contradiction between temporal resolution and pre-echo artifacts.
2Object-generated harmful factors
If short block coding is used, then pre-echo is reduced, but data overhead increases leading to spectral holes
Solution Approach 1:
The block length is made dynamic rather than fixed, allowing the encoder to adaptively select between short and long blocks based on the signal characteristics. This dynamic switching enables the system to use short blocks when needed to reduce pre-echo while using long blocks otherwise to maintain spectral continuity and avoid spectral holes, thus resolving the contradiction.
3Adaptability or versatility
If a new frequency-domain audio codec supporting multiple transform lengths is built, then adaptability to different audio signals is improved, but device complexity and market adoption difficulty increase
Solution Approach 1:
The audio codec is designed to support multiple transform lengths (short and long blocks) within a single unified framework, making it multi-functional. This allows the same codec to handle both transient signals (using short blocks) and stationary signals (using long blocks), improving adaptability without requiring separate specialized codecs for different signal types.
4Manufacturing precision
If a new frequency-domain audio codec is developed to support desired transform length, then coding quality for specific audio signals is improved, but backward compatibility with existing codecs is lost
Solution Approach 1:
The codec implements optional enhanced functionality (support for multiple transform lengths and transform switching) that goes beyond the basic existing codec capabilities. Existing decoders can ignore the additional features and process the bitstream using standard long-block processing, while enhanced decoders can utilize the full capabilities including short blocks and transform switching, thus achieving both improved coding quality and backward compatibility.
Data Source
AI summary
A frequency-domain audio codec is provided with the ability to additionally support a certain transform length in a backward-compatible manner, by the following: the frequency-domain coefficients of a respective frame are transmitted in an interleaved manner irrespective of the signalization signaling for the frames as to which transform length actually applies, and additionally the frequency-domain coefficient extraction and the scale factor extraction operate independent from the signalization. By this measure, old-fashioned frequency-domain audio coders/decoders, insensitive for the signalization, would be able to nevertheless operate without faults and with reproducing a reasonable quality. Concurrently, frequency-domain audio coders/decoders able to support the additional transform length would offer even better quality despite the backward compatibility. As far as coding efficiency penalties due to the coding of the frequency domain coefficients in a manner transparent for older decoders are concerned, same are of comparatively minor nature due to the interleaving.