INTRODUCTION TO CONVERSION There are a number of ways in which a video waveform can digitally be represented, but the most useful and therefore common is pulse code modulation, or PCM, which was introduced in Chapter 1. The input is a continuous-time, continuous-voltage video waveform, and this is converted into a discrete-time, discrete-voltage format by a combination of sampling and quantizing. As these two processes are orthogonal (a $64,000 word for at right angles to one another) they are totally independent and can be performed in either order. Figure 4.1a shows an analog sampler preceding a quantizer, whereas Figure 4.1b shows an asynchronous quantizer preceding a digital sampler. Ideally, both will give the same results; in practice each has different advantages and suffers from different deﬁ ciencies. Both approaches will be found in real equipment.