Loudspeaker Crossover Group Delay Equalization With All-Pass Filters
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Solution Overview
Problem
Multi-way loudspeakers experience uneven group delay due to crossover filters, leading to smearing of sound, and existing methods either require pre-processing, introduce significant latency, or necessitate a large number of filters, which are impractical for real-time applications.
Innovation Solution
The method employs a series of all-pass filters to normalize group delay in the low-frequency range and pure delay in the high-frequency range, allowing for real-time equalization with low latency across the entire frequency spectrum, using either all-pass filter correction or reverse block processing techniques.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If crossover filters are used in multi-way loudspeakers to ensure well-formed magnitude response, then frequency distribution is improved, but group delay becomes uneven causing sound smearing
Solution Approach 1:
An all-pass filter is introduced as an intermediary component in the low-frequency signal path. This filter has a flat magnitude response (does not affect frequency distribution) but provides a frequency-dependent phase shift that compensates for the group delay introduced by the crossover filter, thereby equalizing the overall group delay across frequencies
Solution Approach 2:
The phase response parameter of the low-frequency path is modified by introducing the all-pass filter. The filter's phase characteristic is specifically designed to counteract the phase distortion from the crossover, changing the temporal parameters (group delay) without altering the magnitude response
2Loss of time
If pre-processing audio in reverse time through all-pass filter is used to achieve uniform group delay, then group delay equalization is improved, but latency and complexity increase making it inconvenient for real-time applications
Solution Approach 1:
Instead of applying complex reverse-time pre-processing to the entire audio signal, the invention applies a forward-time all-pass filter with a specifically designed phase characteristic that inverts or compensates for the crossover's phase distortion. This approach achieves the same group delay equalization effect but in a simpler, causal, real-time manner
Solution Approach 2:
The correction is applied locally only to the low-frequency signal path where the group delay problem exists, rather than processing the entire audio spectrum. The all-pass filter is designed to operate specifically in the low-frequency range, leaving high-frequency paths unchanged and minimizing overall system complexity
3Loss of time
If a large number of all-pass filters are used to cover the full frequency range from below 100Hz to above 20kHz, then group delay equalization is improved, but noise accumulation and computational load increase
Solution Approach 1:
The frequency spectrum is segmented into low-frequency and high-frequency ranges, with the all-pass filter correction applied only to the low-frequency segment. This segmentation allows the use of a minimal number of filter elements (just enough to correct the low-frequency group delay) rather than attempting to correct the entire frequency spectrum, thereby reducing noise accumulation and computational requirements
Data Source
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AI summary
Methods are provided for equalising the group delay of a sound reproduction system, in particular a system comprising acoustic transducers with at least one crossover between a lower-frequency and a higher-frequency range. A correction is applied to a signal in the lower-frequency range, including the crossover region, to substantially equalise the group delay for the lower- frequency range, and a signal delay is applied to a signal in the higher-frequency range to bring it into closer alignment with the equalised lower-frequency range signal. The methods may be implemented in the design of an acoustic transducer system and also via a computer program product, which can be implemented as an update or enhancement to an existing digital signal processor loudspeaker system.