The dither habit

摘要

A golden rule of digital audio is that before every quantisation or requantisation you must add dither noise to prevent the introduction of distortion or noise modulation. But, asks Keith Howard, is dither sometimes applied unnecessarily, to the detriment of sound quality? Numerous explanations have been offered down the years for the cold reception offered 20-odd years ago by audiophiles to Compact Disc, ranging from rampant jitter to a preponderance of hi-fi systems adapted to the particular foibles of LP. But perhaps the best candidate is poor appreciation of the importance of dither in both the audio and recording industries, as they came to terms with what, at first, seemed an alien technology. Two decades on, largely due to the work of Stanley Lipshitz and John Vanderkooy at the University of Waterloo in Canada, the importance of dither and the correct means to apply it are well understood. There is no longer any excuse for audio signals not being dithered correctly, although that doesn't mean it never happens. I could, for instance, name a prestigious FFT analysis software package whose signal generator plug-in offers up tones that are not correctly dithered and therefore useless for audio measurement purposes.