Multi-Band Speaker Limiter With Phase Alignment for Clipping Control
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Solution Overview
Problem
Conventional limiter systems in active speakers cause audible clipping distortion when input signals exceed the maximum voltage limit, particularly affecting bass performance and leading to uneven volume adjustments across frequencies.
Innovation Solution
A dual-band limiter system with lowpass and highpass filters, along with allpass filters, is implemented to separate signals into low-frequency and high-frequency bands, allowing for distinct limiting thresholds and phase adjustments, thereby preventing clipping distortion by mixing the limited signals from each band.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a conventional limiter limits signals at all frequencies below a limiting threshold, then the maximum output voltage is limited, but signals at all frequencies are equally compressed causing sudden volume drops in human voices
Solution Approach 1:
The patent divides the audio signal into multiple frequency bands (low-frequency band and high-frequency band) using separate filter paths. Each band is processed by its own limiter with independent threshold settings, allowing differential compression rather than uniform compression across all frequencies. This segmentation resolves the contradiction by enabling frequency-selective limiting that avoids sudden volume drops in vocal ranges while still preventing clipping distortion.
Solution Approach 2:
The patent applies different limiting characteristics to different frequency regions. The low-frequency path uses a first limiting threshold while the high-frequency path uses a second limiting threshold. This local quality approach allows the system to maintain appropriate volume levels in the human voice frequency range while still limiting low-frequency bass signals that are more prone to exceeding maximum voltage limits.
2Reliability
If different limiting thresholds are imposed on different bands, then clipping distortion is avoided, but signals near crossover frequency are filtered out by both lowpass and highpass filters
Solution Approach 1:
The patent introduces an allpass filter as an intermediary component in each frequency path. The allpass filter adjusts the phase of signals without affecting their amplitude or frequency content. By placing the allpass filter between the frequency separation stage and the limiter, the system compensates for phase shifts introduced by the lowpass and highpass filters, ensuring that signals near the crossover frequency are properly aligned and summed without cancellation or distortion.
3Manufacturing precision
If low-frequency signals are amplified to achieve good bass performance, then bass quality improves, but the low-frequency signal is more likely to reach the maximum voltage limit of the amplifier
Solution Approach 1:
The patent applies limiting action before the final power amplification stage. By placing the limiters in the signal path before the power amplifier, the system preemptively prevents low-frequency bass signals from exceeding the maximum voltage limit. This preliminary limiting action allows the bass signals to be amplified to high levels for good bass performance while ensuring they never exceed the amplifier's voltage capabilities, thus avoiding clipping distortion.
Data Source
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Figure 3
Figure 4A~5B
AI summary
A limiter system for an active speaker may include at least one lowpass filter configured to receive an input signal and output a signal lower than a crossover frequency, at least one highpass filter, configured to receive an input signal and output a signal higher than the crossover frequency, a first allpass filter configured to adjust the phase of the signal lower than the crossover frequency, a second allpass filter configured to adjust the phase of the signal higher than the crossover frequency, a first limiter, configured to receive and limit the signal from the first allpass filter, a second limiter, configured to receive and limit the signal from the second allpass filter, and a mixer, configured to mix the signal lower from the first limiter and the signal from the second limiter.