Parametric Stereo Coding for Low-Bitrate Audio Reconstruction
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Solution Overview
Problem
Existing low bitrate audio source coding systems fail to effectively transmit stereo signals at low bitrates, often resulting in artifacts and increased bitrates, especially when dealing with pure mono signals like speech recordings, which can reduce clarity and intelligibility.
Innovation Solution
The system detects signal stereo properties before coding and transmits a stereo width parameter along with a mono sum signal, allowing the decoder to apply appropriate stereo synthesis, and optionally includes a balance parameter for accurate localization, using techniques like cross-correlation and power quotient calculations to efficiently represent stereo information.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Quantity of substance
If pure mono coding is used for low bitrate audio transmission, then bitrate requirements are reduced, but stereo impression and sound quality deteriorate
Solution Approach 1:
The invention extracts only the essential stereo information (stereo width parameter and balance parameter) from the full stereo signal, separating it from the mono content. This extracted parametric data is transmitted alongside the mono-coded audio, enabling stereo reconstruction at the decoder without transmitting complete stereo signals, thus reducing bitrate while preserving stereo impression.
Solution Approach 2:
The invention transforms the stereo signal into parametric form by calculating stereo width and balance parameters that characterize the stereo properties. These parameters are then used to control pseudo-stereo generation at the decoder, allowing flexible adjustment of stereo width to match the original signal's stereo characteristics while maintaining low bitrate transmission.
2Reliability
If pseudo-stereo generation is applied to pure mono signals, then stereo impression is improved, but speech clarity and intelligibility deteriorate due to artifacts
Solution Approach 1:
The invention dynamically adjusts the stereo width parameter based on the detected characteristics of the input signal. For speech signals, the stereo width is reduced or set to mono to avoid artifacts that would degrade speech clarity, while for music or ambient signals, wider stereo effects are applied to enhance the stereo impression. This dynamic adaptation ensures optimal quality for different signal types.
3Loss of information
If traditional sum and difference coding is used for stereo transmission, then stereo redundancy is exploited, but bitrate is still too high for many low bitrate applications
Solution Approach 1:
Instead of transmitting both sum and difference signals with full stereo information, the invention extracts only the critical stereo parameters (stereo width and balance) that define the stereo characteristics. This selective extraction reduces the amount of data to be transmitted compared to traditional S/D coding, achieving lower bitrate while maintaining stereo quality.
Solution Approach 2:
The invention applies partial stereo coding by transmitting only the essential parametric information needed to recreate the stereo effect, rather than transmitting complete stereo signals. This partial approach provides sufficient stereo information for most applications while significantly reducing bitrate requirements compared to full stereo transmission.
4Productivity
If adaptive switching between L/R and S/D coding schemes is implemented, then coding efficiency is improved, but device complexity and processing requirements increase
Solution Approach 1:
The invention simplifies the coding approach by consistently using parametric stereo representation (stereo width and balance parameters) regardless of whether the input signal is stereo or mono. This unified parametric approach eliminates the need for complex adaptive switching between different coding schemes, reducing device complexity while maintaining coding efficiency through appropriate parameter selection.
Data Source
AI summary
The present invention provides improvements to prior art audio codecs that generate a stereo-illusion through post-processing of a received mono signal. These improvements are accomplished by extraction of stereo-image describing parameters at the encoder side, which are transmitted and subsequently used for control of a stereo generator at the decoder side. Furthermore, the invention bridges the gap between simple pseudo-stereo methods, and current methods of true stereo-coding, by using a new form of parametric stereo coding. A stereo-balance parameter is introduced, which enables more advanced stereo modes, and in addition forms the basis of a new method of stereo-coding of spectral envelopes, of particular use in systems where guided HFR (High Frequency Reconstruction) is employed. As a special case, the application of this stereo-coding scheme in scalable HFR-based codecs is described.