ucted by adding the analog levels at each sample time. Figure 4D show the reproduced signal after filtering.As shown in figure4, to convert an analog signal to a digital format, the specific level of the analog signal must be sensed at periodic intervals. These intervals are called sampling times. The analog value at each sample time is converted to a digital code or data bits that is stored in memory.When the information level for a sample time is read from memory, the data bits are converted to the specific analog level represented by that specific sample-time code. The series of sample-time analog levels are then filtered to reproduced the original video level. BASIC A/D ConversionFigure 5 shows the basic circuit involved for A/D conversion. Keep in mind that these circuits are general in nature and do not apply to any specific digital TV. the functions shown are performed within ICs. The first step in A/D conversion is to limit the maximum desired input frequency with low-pas filter (LPF). Although this also limits resolution, the use of a filter prevent high frequency noise and possible beats signals. The limiting effect on resolution are generally not noticeable in the picture.The next step in A/D conversion is to sample the analog signal. The sample clock determine when the samples are taken. The capacitor holds the instantaneous sample until the next sample occurs. This familiar sample-and-hold process generates an equally familiar staircase signal. Note that the staircase signal is still in analog from, since the signal still contains discrete level for each sample.The staircase signal is applied to Quantization circuit. The input of the quantization circuit is a series of voltage comparators. Through a voltage divider, each comparator receives a different reference voltage. The amplitude of the incoming signal determine the number of comparators that produce an output to an encoder.Typically, the output from the comparator s ...
