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===Analog-to-digital converter===
An [[analog-to-digital converter]] (ADC) can be modeled as two processes: [[Sampling (signal processing)|sampling]] and quantization. Sampling converts a time-varying voltage signal into a [[discrete-time signal]], a sequence of real numbers. Quantization replaces each real number with an approximation from a finite set of discrete values. Most commonly, these discrete values are represented as fixed-point words. Though any number of quantization levels is possible, common word-lengths are [[8-bit]] (256 levels), [[16-bit]] (65,536 levels) and [[24-bit computing|24-bit]] (16.8 million levels). Quantizing a sequence of numbers produces a sequence of quantization errors which is sometimes modeled as an additive random signal called '''quantization noise''' because of its [[stochastic]] behavior. The more levels a quantizer uses, the lower is its quantization noise power.
===Rate–distortion optimization===
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