Second MDCT Enhancement Layer for CELP Audio Coding
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Solution Overview
Problem
Conventional CELP codecs are inadequate for providing high-quality audio, especially at high bit rates, as they struggle with music signals due to limitations in pitch lag range and inefficient long-term prediction for irregular harmonics, leading to distorted spectra and insufficient quality in narrowband coding.
Innovation Solution
Introducing an independent second MDCT enhancement layer that adds new spectral envelope coding and fine spectrum coefficient coding for the second coding error, selectively applied based on parameters like pitch lag, spectral sharpness, and energy differences, to enhance audio quality beyond the capabilities of the first MDCT enhancement layer.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a first MDCT enhancement layer is added to CELP core layer, then audio quality is improved, but coding efficiency for music signals at high bit rates is still insufficient
Solution Approach 1:
The enhancement layer is divided into two separate MDCT layers instead of one, where the first layer handles general spectral enhancement and the second layer specifically targets music signal characteristics. This segmentation allows each layer to be optimized for different aspects of audio quality, resolving the contradiction between improved audio quality and coding efficiency for music signals.
Solution Approach 2:
A second enhancement layer is added to the existing single enhancement layer structure, creating a two-dimensional enhancement architecture. This dimensional expansion provides additional coding resources specifically for music signals at high bit rates, allowing the system to achieve both high audio quality and improved coding efficiency simultaneously.
2Adaptability or versatility
If CELP codec is used for narrowband coding, then interoperability with existing standards is maintained, but quality for music signals with irregular harmonics is insufficient
Solution Approach 1:
The audio coding system is segmented into a core CELP layer for narrowband coding that maintains interoperability, and separate MDCT enhancement layers for wideband and super-wideband coding that improve music signal quality. This segmentation allows the core layer to maintain standard compatibility while the enhancement layers provide specialized handling for music signals with irregular harmonics.
Solution Approach 2:
The patent creates a composite coding structure combining CELP (for narrowband/interoperability) with MDCT enhancement layers (for music quality). This composite approach integrates the strengths of different coding methods, maintaining interoperability through the CELP core while achieving superior music signal quality through the MDCT enhancement layers that specifically address irregular harmonics.
Data Source
AI summary
In an embodiment, a method of transmitting an input audio signal is disclosed. A first coding error of the input audio signal with a scalable codec having a first enhancement layer is encoded, and a second coding error is encoded using a second enhancement layer after the first enhancement layer. Encoding the second coding error includes coding fine spectrum coefficients of the second coding error to produce coded fine spectrum coefficients, and coding a spectral envelope of the second coding error to produce a coded spectral envelope. The coded fine spectrum coefficients and the coded spectral envelope are transmitted.