Dynamic Audio Hum Extraction With Adaptive Notch Depth
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Solution Overview
Problem
Existing audio systems struggle to effectively remove AC hum from audio signals without causing spectral balance degradation or introducing audible side effects, particularly when audio signals have low gain or no masking capability.
Innovation Solution
An adaptive dynamic hum extractor process that dynamically varies the depth of notch filters in response to the audio signal's envelope, using time-delayed and inverted signals to filter out hum components at the fundamental frequency and harmonics, while maintaining transparency by adjusting notch depth based on signal level.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If fixed-notch filters are used to remove hum components, then hum removal effectiveness is improved, but spectral balance of the audio signal is degraded
Solution Approach 1:
The patent applies dynamics by making the notch filter depth variable rather than fixed. The system dynamically adjusts the depth of notches at hum frequencies based on the detected level of the audio signal. When the audio signal level is high, the notch depth is reduced to preserve spectral balance and avoid audible side effects. When the audio signal level is low, the notch depth is increased to effectively remove hum components. This dynamic adaptation resolves the contradiction between hum removal effectiveness and spectral balance preservation.
2Reliability
If fixed time delay is used to create notches at hum frequencies, then hum attenuation is improved, but audible side effects are introduced
Solution Approach 1:
The patent makes the time delay variable rather than fixed. The delay time is dynamically adjusted based on the detected audio signal level. At high signal levels, the delay is reduced to minimize phase distortion and audible side effects. At low signal levels, the delay is increased to maximize hum attenuation. This dynamic adjustment allows the system to achieve effective hum removal without introducing unacceptable audible side effects.
Solution Approach 2:
The patent changes the parameter of delay time based on signal level conditions. By detecting the audio signal level and adjusting the delay time parameter accordingly, the system optimizes the balance between hum attenuation and audible side effects. This parameter change allows the same filter structure to adapt to different operating conditions and resolve the contradiction.
3Reliability
If noise gates and low level expanders are used to attenuate hum, then hum attenuation is improved at low signal levels, but transparency is reduced
Solution Approach 1:
The patent applies dynamics by making the notch depth variable based on signal level detection. Unlike noise gates and expanders that abruptly attenuate low-level signals, this system dynamically adjusts notch depth to match the signal level. At low signal levels where hum is most audible and masking is insufficient, the system increases notch depth to effectively remove hum while maintaining transparency by avoiding the abrupt gain changes characteristic of noise gates and expanders.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
Effectively removes AC hum components without degrading the audio signal's spectral balance, ensuring transparency and minimal alteration of the original audio quality, even at low signal levels.
Implementation Method 1
filtering the input audio signal with one or more notch filters at the fundamental hum frequency
Implementation Method 2
Filtering can be accomplished by delaying the input audio signal with a time delay equal to the inverse of the fundamental power line frequency, varying the level of the delayed input audio signal in relation to the control signal to produce a dynamically varying delayed signal, inverting the dynamically varied delayed signal and summing the inverted dynamically varied delayed signal with the input audio signal to produce dynamic notch filtering
Data Source
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AI summary
An adaptive dynamic audio hum extractor (100) eliminates line frequency hum components (HPLF) and associated higher harmonics from an audio signal (Si). An audio signal containing line frequency hum can be processed by providing dynamically controlled notch (N) filters at the fundamental line frequency (PLF) and additional harmonic multiples of the fundamental frequency. The audio signal (Si) is detected to provide dynamic control of the depth (D) of the notch (N) filters. Alternatively, an audio signal (Si) containing hum can be processed by dividing the spectrum into at least two frequency bands, an unaltered high band combined with a dynamically processed low band. The adaptive dynamically controlled notch (N) filters vary the depth (D) of the notches (N) in relation to the envelope or time averaged level of the bandwidth limited audio signal. This allows masking of the hum components (HPLF) with higher levels of audio, thereby providing transparency devoid of audio path notches (N).