CSCI 120 Introduction to Computation Bits... and pieces (draft)

You may recall from the previous lecture that the use of electro mechanical relays, and in subsequent years, diodes and transistor, made it possible to construct more advanced computers, e.g. ENIAC. This is accredited to the fact that these devices could function as on/off switches. On one hand, they create the ability to encode logic into the circuits of the computer. This means that the computer can perform different tasks under different conditions, i.e. the notion of a program. For instance, one could encode the logic if A OR B then C. On the other hand, these devices allow the engineers to worry less about the values that could possibly arise in the system: the switch is either on or off. It cannot be anything in between. Therefore, this means that any errors due to fluctuation in voltage levels are greatly reduced. It would be enough to simply distinguish between high voltage and low voltage. This brings us to the question of Analog versus Digital. In simple terms, a digital system encodes information using a number of devices that have discrete states (e.g. on/off switches). An analog system encodes information using a device that have continuous states (e.g. measurement in an electric circuit). To build an intuition for digital versus analog, consider the problem of encoding a number using buckets of water. One possibility is to use two kinds of buckets, full and empty. The buckets will be arranged in a special way to encode the number, a strategy that we may agree upon. Another possibility is to use one bucket only, and fill it up to a level proportional to that number. Although the analog system seems to be more accurate, consider what happens if the buckets moved. Any movement of the bucket in the analog system would alter the level of water and hence cause an error in detecting the encoded number. In the digital system, however, a full bucket remains full, and an empty bucket remains empty (even if a small amount of water moves around). We have encountered an example of digital versus analog before when we examined the abacus and the slide rule. In the following figure, try to identify what is digital and what is analog: