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William Thomson [aka Lord Kelvin] 1824-1907

When you can measure what you are speaking about, and express it in numbers, you know something about it; but when you cannot measure it, when you cannot express it in numbers, your knowledge is of a meager and unsatisfactory kind: it may be the beginning of knowledge, but you have scarcely, in your thoughts, advanced to the stage of science.

Popular Lectures and Addresses [1891-1894]


A hundred years later this quote can be expanded to say that when you can measure and express it in numbers, you can put it into a computer. The computer is digital. It can only handle bits of information. Each bit is a binary decision, the answer to one Yes-No question. For convienience, these bits are grouped into groups of eight called bytes. Each byte can represent a number between 0 and 255. The amount of memory needed to store something is measure by the number of bytes of memory it needs. When you type on your keyboard, a number is entered into your computer's memory representing the key you pressed. Every character on your keyboard, every character on your computer screen is represented by a number between 0 and 255.

Just as a keyboard converts your text into numbers, other input devices convert measurements of the analog world into bytes of information. Whatever that is being measured is converted into a voltage that is then quantised by an analog to digital converter into data that the computer can read and then store in its memory. What is important is how often the measurement must be made and the accuracy needed by that measurement. Higher accuracy requires more bytes per measurment.

For example, music is converted to data by sampling the sound pressure 44100 times per second. Microphones are used to convert sound pressure into a voltage. For a CD, two channels are sampled at the rate of 44.1 kHz. Each sample of each channel is quantised to a level between +32767 and -32767 that is represented by two bytes. It takes a total of 700 Megabytes to represent 80 minutes of CD music.

A digital camera uses an array of sensors to convert light intensity into a voltage at a large number of locations. The intensity of light at each location is quantised into a number between 0 and 255 that is represented by one byte. For a color camera, different sensors respond to different colors.

Anything that can be measured can be quantised to a range of numbers and represented by a sequence of bytes that can be stored in a computer.

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