| In 'History of the Computer - the emergence | | | | system, where the amplitude, or strength, of |
| of Electronics', we saw how the development | | | | the modulation is proportional, or analogous, |
| of Radar during the second world war led to | | | | to the loudness of the original live singing |
| an understanding of pulse technology. At the | | | | or playing. This music had been converted by |
| same time methods were refined for the | | | | a microphone into an variable electrical |
| calculations required for the ballistic | | | | signal, analogous to the sound.With the |
| trajectories. From these beginnings, the | | | | invention in the 1980s of the Compact Disc, |
| digital computer was developed.What is meant | | | | digital techniques were employed to represent |
| by 'Analog' and 'Digital'? A couple of | | | | the changes in sound levels, by using a |
| examples will explain the difference. An | | | | sampling pulse to monitor the loudness of the |
| analog is something which is analogous, | | | | sound. This sampling pulse is used at a high |
| obviously, but you may know that an analogous | | | | frequency, so that it is not audible, and |
| process or function is one that is | | | | traces the progress of the sound. The |
| equivalent, or very similar to another one. | | | | principle is analogous to the movie camera |
| An analogy is often used to explain, or to | | | | where a sequence of still pictures are shown |
| assist the understanding of, some new feature | | | | at a rate of 32 frames per second, so that |
| in terms that are understood.For example a | | | | they appear to show a moving picture. |
| home electric circuit for house wiring to | | | | Similarly a tv uses a frame rate of 50 or 60 |
| operate a lamp. The supply is controlled by | | | | per second (strictly speaking 25 0r 30 |
| a circuit breaker, when it is available at | | | | interlaced).The big advantage of a digital |
| the outlet where the lamp is plugged. This | | | | recording and reproduction system (amplifiers |
| can be compared to plumbing, where the water | | | | etc are also digital) is that, due to the |
| supply is controlled by a valve or tap on | | | | nature of the high frequency pulses, it is |
| entering the home, then pipes carry the water | | | | possible to copy them exactly when |
| to the kitchen, where the supply can be | | | | transferring from one medium to another, for |
| turned on or off by a tap or faucet, and is | | | | example copying a from cd track to |
| immediately available. The rate of flow of | | | | compilation cd. With an analogue system |
| the water can be controlled, which is | | | | there are losses in every transfer, so that a |
| analogous to a dimmer on the lamp.The analogy | | | | recording on a cassette tape copied from a |
| is not absolutely the same, but aids in the | | | | vinyl lp via a home stereo is noticeably |
| understanding of the electricity supply by | | | | poorer quality than the original.A computer |
| someone who knows the plumbing | | | | can also be analog or digital, though the |
| system.Digital, concerning digits or numbers, | | | | digital type have vastly outgrown the analog. |
| refers to the use of numbers to represent all | | | | An analog computer might be used in research |
| things. As an example, a digital watch uses | | | | work, where, for instance a record may be |
| numbers to indicate the time. The numbers | | | | made on a moving chart of the temperature and |
| on the face of traditional watch are | | | | humidity in a room being air conditioned by a |
| distributed around the dial, so that the | | | | new design air conditioner, the chart could |
| hands pointing to them are analogous to the | | | | also be a display on screen. Either way, the |
| passage of time. For example the minute hand | | | | graph is an analog representation of the |
| pointing straight down, half way through the | | | | temperature and pressure.Early electronic |
| circle of the clock, represents the passing | | | | controls in aircraft were run by a form of |
| of half an hour. If the hour hand is | | | | analog computer, the amount the ailerons |
| pointing to the number 3, this indicates 3 | | | | moved was proportional to the movement of the |
| hours of the 12 for a complete circuit. When | | | | joystick, but not directly proportional. A |
| we learnt to tell the time we knew that this | | | | calculation was performed, which depended on |
| was half past three. The digital watch | | | | the speed and height of the aircraft, and |
| however says 3:30.Closer to our topic, we can | | | | other factors. The electronic 'boxes' |
| consider the evolution of the gramophone | | | | involved linear amplifiers and variable |
| record. The vinyl LP was the standard medium | | | | response circuits, all fed by sensors from |
| for music recording and playing from the | | | | the flight controls etc.Tony is an |
| 1950s, tape recordings were also developed in | | | | experienced computer engineer. |
| parallel. Both these media use a modulation | | | | |