In many electronic circuits, such as those in radios, the electric current can be of any strength. These circuits are called analogue circuits. In digital circuits, the current can have only two strengths: on and off. Digital circuits are used in a range of devices, including computers, cameras, CD and DVD players and smartphones, in which the flow of electricity is controlled to represent information.

Binary number system

Numbers are represented in digital circuits using the binary number system. This uses only the digits 0 and 1 and so can easily be represented in electronic circuits by turning currents on or off. In the decimal system (numbers we use in everyday life: 1, 2, 3, etc.), the digits of a number represent ones, tens, hundreds and so on. In the binary system, the digits 0 and 1 represent ones, twos, fours, eights and so on. In digital circuits, each 0 or 1 is called a bit. A four-bit binary “word” can represent decimal numbers up to 15 (one 8, one 4, one 2 and one 1). The binary “word” 1101, for example, represents the number 13 (one 8, one 4, no 2s and one 1). So 13 is represented by the four-bit “word” 1101, its digital code. A sequence of eight bits, the number needed to represent a number or letter on a computer, is called a byte.

Almost any sort of information (from simple letters to complex moving images) can be converted into a series of numbers, represented by the digits 0 and 1. This means that this information can be represented in digital electronic circuits by controlling the flow of electricity (0 is for “off” and 1 is for “on”) through it. Computers rely on this fact to store numbers, words, pictures and sounds. They use circuits called logic circuits to process and manipulate the information.

Pixels

Anything that appears on a computer’s monitor is called computer graphics. These can be as simple as plain white text on a black screen, or as complicated as animated three-dimensional images. Whatever the graphics are, they are made up of small coloured squares called pixels (short for picture elements) in a grid pattern. The concentration of pixels in a picture is called resolution. High-resolution graphics can be viewed on a large screen without the pixels being visible. Graphics can have a different range of colours, too. In eight-bit graphics, each pixel is represented by eight bits, and so can be any one of 256 colours.

Storing pixels
on a computer

Each pixel on a computer monitor screen has a unique position, colour and brightness. This information exists as binary codes in the computer. This pixel is 4 units across, 6 down, so its position code is 4, 6. Its colour, green, is coded 5 and its brightness is 3 on a scale 1 to 10. So its code is 4653.

The pixel code is stored as a binary number, which, inside the computer, exists as electrical signals. 1 means an electrical pulse, 0 means no electrical pulse.

​​​​​​​Every pixel that makes up a computer graphic—one of many hundreds of thousands—has its own code. The tiny portion of the image of Tower Bridge, London, shown here has 100 alone. The higher the definition of the picture, the more pixels it has and the more binary digits required to represent them.

Storage on disc

Information can be recorded in digital form on a compact disc (CD). Sounds, images, video or text are turned into a series of digital codes. These are stored on the disc as a series of pits and flats.

The disc is “read” by a laser beam. A series of mirrors and lenses focus a laser beam on to the underside of a CD. The reflected beam “reads” the various pits and flats that represent the coded information. The machine turns these into electrical signals.