Stereo Audio Encoder TNS Filter Mismatch Control
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Solution Overview
Problem
Multi-channel digital signal processing, particularly in stereo audio coding, faces challenges with artifacts caused by Temporal Noise Shaping (TNS) filtering, which can lead to audible unmasking artifacts when decoding, especially with transient signals and varying prediction gains between channels.
Innovation Solution
A method for processing digital stereo audio signals that involves determining prediction gains for both unmodified and Mid/Side-coded signals, and disabling TNS filtering if the prediction gains differ by more than a predetermined mismatch range, thereby bypassing TNS filtering for current signal frames to prevent unwanted artifacts.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If TNS filtering is applied to improve temporal resolution and masking effect, then coding gain is improved, but quantization noise may be amplified and cause audible unmasking artifacts in the decoder
Solution Approach 1:
The patent changes the parameter of TNS filter application by introducing a conditional switching mechanism based on prediction gain comparison. When the absolute difference between left and right channel prediction gains exceeds a threshold (e.g., 3 dB), TNS filtering is disabled for that channel, thereby preventing noise amplification while maintaining its benefits when conditions are favorable.
Solution Approach 2:
The patent implements dynamic control of TNS filtering by continuously monitoring prediction gain differences between channels and adapting the filtering application in real-time. This dynamic switching allows the system to optimize between coding gain and artifact prevention based on current signal conditions.
2Reliability
If distinct TNS-filters are used for each channel to handle different prediction gains, then channel-specific optimization is achieved, but device complexity and side information increase
Solution Approach 1:
The patent segments the TNS filtering application by channel based on prediction gain comparison. Instead of universally applying complex multi-filter strategies, the system divides the decision-making into simple per-channel evaluations, enabling selective filtering without the full complexity of distinct filters for every scenario.
3Measurement precision
If TNS filtering is applied to transient signals, then temporal masking is improved, but quantization noise amplification and audible artifacts increase
Solution Approach 1:
The patent employs feedback by comparing prediction gain values and using this information to control TNS filter application. The system continuously monitors the prediction gain difference between channels and uses this feedback to dynamically adjust whether TNS filtering should be applied, thereby preventing noise amplification in transient signals while maintaining temporal masking benefits when appropriate.
Data Source
Figure 1~2
AI summary
The invention discloses a method and an encoder for processing a digital audio stereo signal. A digital audio encoder for coding such audio signal comprises a predictive Temporal Noise Shaping (TNS) filter, a Mid-/Side (M/S) coding unit, a control unit for determining a first prediction gain related to the unmodified L/R signal processed by the TNS filter and for determining a second prediction gain related to the M/S-coded L/R signal processed by the TNS filter, wherein the control unit is adapted to disable TNS-filtering - i.e. to bypass the TNS filter - for a current signal frame, if the first and second prediction gains differ by more than a pre-determined mismatch range. Preferably, the first and second prediction gains are determined from signal energy ratios calculated for each channel of the stereo signal including the signal energies of both the TNS-processed (unmodified) L- respectively (unmodified) R-signal and the TNS-processed M/S coded L- respectively M/S coded R-signal divided by the respective signal energies before TNS processing. Furthermore, the control unit is preferably adapted to overrule the disabling of the TNS filter, if the input signal is a near-mono audio signal exhibiting only low energy either in its M- or S-band. In that case, operation of the TNS filter on the stereo audio signal is maintained.