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AES PARIS 2016

AES PARIS 2016. Engineering Brief EB3-5. Non-intrusive Rumble Filtering by VLF Crossfeed with High Filter Slopes. Douglas Self. The Problem. ● Vinyl disc replay gives rise to large amplitudes of subsonic noise.

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AES PARIS 2016

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  1. AES PARIS 2016 Engineering Brief EB3-5 Non-intrusive Rumble Filtering by VLF Crossfeed with High Filter Slopes Douglas Self

  2. The Problem ● Vinyl disc replay gives rise to large amplitudes of subsonic noise. ● This tends to peak significantly around 8 – 12 Hz due to cartridge/arm resonance. ● This is only an octave away from 20 Hz, the bottom of the RIAA specification. A quite sophisticated conventional filter is required. ● The subsonic noise results mostly from vertical stylus movement so the electrical signals are in anti-phase

  3. Stereo crossfeed ● Stereo crossfeed at low frequencies cancels anti-phase signals, but in-phase signals unaffected. ● Usually low frequency sounds are panned to the centre, for trackability and to make use of both stereo amps. ● Therefore the bass response is unaffected by stereo crossfeed.

  4. Passive crossfeed Fig 1: Anti-phase rolloff is -3 dB at 32 Hz, falling at 6dB/octave

  5. Fig 2: Notional crossfeed circuits using 2nd-order lowpass filters a) Not usable b) Path-2 phase corrected With (b) the 6dB/octave rolloff is on the anti-phase signal, but there is +6 dB LF boost on the in-phase signal .

  6. Fig 3: Langvad/Macaulay crossfeed arrangement with Path 3 added to cancel in-phase +6dB LF boost

  7. Fig 4: Low-impedance implementation of crossfeed with Path 3

  8. Fig 5: Freq response for anti-phase signals of Fig 3 circuit The anti-phase response has a +2dB peak, and a rolloff slope of only 6dB/octave

  9. Fig 6: allpass filter added in Path 1

  10. Fig 7: Frequency response changes as allpass delay is increased, moving to steep slope of 18 dB/octave

  11. Fig 8: As allpass delay further increased, steep slope develops a useful notch at 8.5 Hz. After that notch moves up in frequency and section below it has slope of only 6dB/octave

  12. Note that the steepest slope is 18 dB/octave, not the 12 dB/octave that would be expected from the 2nd-order lowpass filter. • The steepest slope is also 18 dB/octave when 3rd and 4th-order lowpass filters are used. However, these filters give deeper notches at 8.5 Hz.

  13. Fig 9: THD+N for in-phase signals at 2 Vrms(L and R channels) 'Gen' is testgear THD

  14. Fig 10: THD+N for anti-phase signals at 2 Vrms(L and R channels) 'Gen' is testgear THD+N

  15. Awkward resistor values Three possible solutions: • One E96 resistor. (1xE96) Stock problems • Two E24 resistors in parallel. (2xE24) On average 3 times more accurate in setting nominal value than 1xE96. • Three E24 resistors in parallel. (3xE24) On average 30 times more accurate in setting nominal value than 1xE96. More resistors mean lower effective tolerance.

  16. AES PARIS 2016 Engineering Brief EB3-5 Non-intrusive Rumble Filtering by VLF Crossfeed with High Filter Slopes drgself@dsl.pipex.com douglas-self.com

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