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Digital Dither: Processing with Resolution Far Below the Least Significant Bit

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With appropriate analog dither and accurate analog-to-digital and digital-to-analog converters, a simple digital storage and replay system can be completely free of distortion while maintaining a signal-to-noise ratio far in excess of analog techniques. We show that such excellent performance can easily be seriously degraded by not paying attention to the details of the digital signal processing that is increasingly common in present-day digital audio systems. Digital equalization, gain changing, oversampling interpolation, editing, or sampling rate conversion can all reintroduce quantization distortion and/or other deleterious audible effects such as noise modulation and limit cycle oscillations. The use of appropriate digital dithering before final rounding can keep the digital signal free of such degradation. In this paper we point out some of the pathology of undithered digital signal processing, indicate how proper digital dithering can maintain the signal quality, introduce and analyze a new form of high-pass dither which results in a very small increase in audible noise level, and introduce some aspects of dithered noise-shaping quantizers which reduce the audible effect of dithering still further.

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