Restoration of Smooth Pitch Variations Over Long Timescales - "Wow"
24 June 2004
CEDAR Audio's Managing Director, Gordon Reid, has presented a paper on correcting pitch variations to the Joint Technical Symposium of the world's libraries and sound archives. Co-written with renowned signal processing experts Dr Simon Godsill and Dr Christopher Hicks of Cambridge University, the paper addressed the various causes of these variations, and described a signal processing scheme to correct them in the digital domain. A number of examples of corrected material were played, and these were very warmly received by the 350 attendees.
Mr Reid added, "It's my hope that this research will lead directly to a new method of correction that doesn't rely on things such as hums and bias tones to generate a speed map. These are not always available, rendering existing techniques useless, so we may be on the cusp of a new era that allows us to eliminate wow and other speed deviations from material that was previously thought impossible to restore."
You can encounter smooth pitch variation over long timescales ("wow") on almost any analogue recording medium, and it is one of the most disturbing artifacts encountered when listening to old and/or badly transcribed recordings. There are several mechanisms by which this can occur. One is a variation of the rotation speed of the medium during recording or playback. A second, specific to discs, is eccentricity in the playback process. A third occurs when magnetic tape stretches unevenly during playback or storage.
In some cases it is possible to make mechanical corrections for these defects, but such approaches are generally impractical. Therefore, this paper outlines a signal processing approach for the detection and correction of wow, in which we use the degraded audio to estimate the instantaneous amount of pitch variation, and then recreate the undamaged signal from the existing data. The approach used is as general as possible in order to correct a wide range of related defects.
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