Vinyl discs create subsonic anti-phase signals because they are never perfectly flat and cause vertical stylus movement. This is often made worse by cartridge-arm resonance, giving amplitudes peaking around 10 Hz and requiring 40 dB of attenuation to reduce them to the vinyl noise floor. A conventional rumble filter needs very steep slopes to do this without unduly affecting the bottom of the audio band at 20 Hz. L-R crossfeed at low frequencies cancels the anti-phase signals, converting bass information to mono. This is not a new idea but has never caught on, probably because in published implementations the anti-phase filtering slope always comes out as –6dB/octave, no matter what order of lowpass filter is used to control the crossfeed. It is demonstrated that time-correction of the lowpass filter group delay with simple allpass filtering gives a much steeper slope of –18dB/octave for 2nd, 3rd, and 4th-order Butterworth filters, and intrusion into the audio band is minimized; this is believed novel. A practical design using 2nd-order filters was built and measured and gave the desired results.
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