Vinyl discs generate large amounts of subsonic disturbances, someimes only 20 dB or so below the level of the music. This can be filtered out by conventional highpass filters, but it is very hard to this without also removing the lower audio frequencies. An alternative approach is to make use of the fact that the subsonic disturbances essentially cause vertical stylus movements, which cause anti-phase signals in the Left and Right channels. L-R crossfeed at low frequencies cancels the anti-phase signals, converting bass information to mono. The popularity of mono sub-woofers shows that this is subjectively completely acceptable, and unlike conventional subsonic filtering, there is no loss of deep bass.
Low-frequency crossfeed is not a new idea but has never caught on, probably because in all previous attempts the anti-phase filtering slope always comes out as -6dB/octave, no matter what order of lowpass filter is used to control the crossfeed. To solve this we have used some quite sophisticated allpass filtering to time-correct the lowpass filter group delay, which gives a much steeper slope of -18dB/octave from a 2nd-order Butterworth filter. The intrusion of crossfeed into the audio band is minimised.
The Devinyliser was presented to the 140th AES Convention at Paris in June 2016 by Douglas Self. The Powerpoint presentation explains the operation in detail, and can be downloaded free of charge from The Douglas Self Site here.
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