-9); (b) diagram of a noisy system with a bit error rate of 1.79 X 10-4. of the signals on top of each other. The resulting pattern is known as the eye diagram. Respec- tively a wide-open eye diagram represents a system with high performance while noise, jitter, and other disturbances have the effect of closing the eye and increasing the probability of bit errors. Figure 1.21(a) shows an example of a wide-open eye that corresponds to a bit error rate of 2.45 X 10-9. Similarly, Figure 1.21(b) represents the eye diagram of a noisy transmission system with a bit error rate of 1.79 X 10-4, Notice that the optimum bit error rate performance is achieved if both the horizontal and vertical decision thresholds are placed at their optimal place, which is, in fact, in the middle of the eye. The vertical threshold can change because of temperature effects or com- ponent inaccuracy. Inaccuracy in timing is also possible and is known as jitter. Jitter can considerably affect the bit error rate performance of a receiver. As an example, if the sam- Resources pling takes place at the point shown by marker M2 in Figure 1.21(a), the bit error rate will be equal to 2.27 X 10-5. Marker M1, however, shows the optimum sampling time. Jitter and Time Extraction As mentioned before, jitter is defined as the short-term variation of digital signals from their ideal position. Though this is mainly due to the mistuning of the digital regenerators and repeaters (e.g., random thermal noise from a crystal), it can also be affected by the pattern of the transmitted data as well. There are basically two main impairments associated with jitter. The first problem is the impact on the bit error rate performance of the receiver as previously discussed. Second, jitter can cause irregular spacing between the coded samples, which can result in low-frequency distortion component. Fortunately, this latter effect is not a serious problem in primary PCM voice systems. The problem of jitter can become more significant with an increase in the number of repeat