Multichannel Downmix Phase Compensation for Low-Bitrate Stereo Coding
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Solution Overview
Problem
Current methods for lossy parametric encoding of stereo signals at low bitrates, such as those based on parametric stereo in MPEG-4 Part 3, face challenges with the ILD parameter due to its unrestricted range, leading to complexity issues in calculation and quantization.
Innovation Solution
The proposed solution involves a new mid/side approach using two gain parameters, side gain, and residual gain, which are calculated and transmitted instead of ILD and ICC, allowing for efficient encoding and decoding with absolute phase compensation, limiting the side gain to [-1, 1] and residual gain to [0, 1], and calculating these parameters in the spectral domain for subbands.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If ILD parameter is used for parametric stereo encoding, then spatial image information is captured, but calculation and quantization complexity increases due to unrestricted value range
Solution Approach 1:
The patent transforms the ILD parameter with unrestricted range into a rotated ILD parameter with limited range through phase rotation. Specifically, the downmix signal is rotated by an angle derived from the IPD parameter, which constrains the rotated ILD to a manageable range suitable for efficient quantization while preserving spatial image information.
2Ease of manufacture
If downmixing is performed without phase compensation, then calculation is simpler, but channel energy distribution becomes unbalanced affecting reconstruction quality
Solution Approach 1:
The patent applies preliminary phase rotation to the downmix signal before encoding. By rotating the downmix signal by an angle based on the IPD parameter, the energy distribution between channels is optimized in advance, ensuring better reconstruction quality without adding complex real-time processing at the decoder.
3Loss of information
If ILD values are very small or very big, then spatial information is represented, but efficient calculation and quantization become problematic
Solution Approach 1:
The patent applies a trigonometric transformation to the ILD parameter through phase rotation. The rotated ILD is calculated as arctan2(ILD*sin(IPD), ILD*cos(IPD)), which maps the unrestricted ILD range into a limited range suitable for efficient quantization while preserving the spatial information content.
Data Source
AI summary
An apparatus for downmixing a multi-channel signal having at least two channels, has: a downmixer for calculating a downmix signal from the multi-channel signal, wherein the downmixer is configured to calculate the downmix using an absolute phase compensation, so that a channel having a lower energy among the at least two channels is only rotated or is rotated stronger than a channel having a greater energy in calculating the downmix signal; and an output interface for generating an output signal, the output signal having information on the downmix signal.