Figure 20. Analog-to-Digital
To visualize the analog-to-digital conversion process, refer to Figure 20. At the top is one cycle of an analog input signal wave. We've used a simple sine wave to make visualization easier. In this example, the signal has a peak-to-peak amplitude of 20 units, measured by the scale on the left. The sampling frequency is many times higher than the signal being sampled and is shown along the bottom of Figure 20. Once for each cycle of the sam- pling frequency, the sample-and-hold circuit "slices" the input sig- nal, allowing the quantizing circuit to generate a (digital) number equal to the closest (of the 65,536 possible discrete values) quan- tization value of the input signal at the time the sample is taken. This repeats for each successive cycle of the sampling frequency and the quantizer generates a continuous "bit stream" which rep- resents the quantized signal. The continuous stream of digital audio information is converted into a digitally modulated signal using a technique known as linear pulse code modulation.