Digital Sound

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Quantization

The process of converting a sampled sound into a digital value is termed quantization. The number of distinct sound levels that can be represented is determined by the number of bytes used to store the quantization value.

CD audio, the most common quantization strategy uses 2 bytes (16 bits); capable of representing 65,536 discrete levels. In simple terms, quantization can be viewed as converting real (continuous sound), values into integer (discrete sound) values. This process involves dealing with the error between the sampled discrete values and the actual continuous sound, termed quantization error. In audio theory this is referred to as the signal-to-noise ratio (S/N).

S/N Ratio

The S/N ratio is a ratio between the difference of the highest and lowest frequencies to the average superimposed noise (white noise or static). The higher the S/N ratio the better the sound. CD Audio has a theoretical S/N ratio of about 96 decibels (dB), with actual systems achieving S/N ratios in the low 90 dB. S/N ratios must be greater than 70 dB to prevent backgound noise from becoming audible. Decreasing the quantization to 8-bits, to save 50% of the required storage, decreases the S/N ratio to about 50 dB, approximately the same quality as AM radio.

This two step process just described for sampling and quantizing sound digitially is termed Pulse Code Modulation (PCM). PCM is the standard method employed in the CD Audio format.