On being digital

The defining characteristic of the technology (computers, networks, storage devices, and so on) that supports cyberspace is that it is digital. Traditionally, information has been stored in analogue form, as (say) the shape of ink marks on a page, the strength of a magnetic field on an audio tape, or the depth of a groove on a vinyl record.

Information stored digitally is stored simply as a sequence of numerical values. These numerical values are represented in the binary number system in computers, as binary digits, known as bits. It is initially surprising that virtually any information can be (and is) stored and transmitted digitally:

• Text, such as the contents of this book, or an electronic mail message, is easy to store digitally; just imagine assigning a number to each letter of the alphabet. Adding some extra information to describe the formatting of the text is not much harder.
  
  
• Sounds. We have all become familiar with audio CDs, which store music digitally. Telephone conversations usually work over analogue wires as far as the local telephone exchange, but after that they are converted to digital form and routed over the trunk network.
  
  How can sounds be reduced to a sequence of numerical values? Simply by measuring the height of the sound waveform at regular intervals, and using that as the sequence. To play back the sound, feed the numbers to an electronic gadget (a digital-to-analogue converter in the jargon) that outputs an electronic signal whose size is controlled by the sequence of numbers, and connect the output to a loudspeaker. In the timescales of computers, sound waves are fairly slow: we can only hear sounds that vibrate up to about 20,000 times a second, which gives plenty of time for a computer to do the sampling and recording. (A computer's heart, or 'clock', beats around 400,000,000 times a second in 1998.)
  
  
• Pictures can be expressed in digital form too. First, divide the picture up into very small squares, called pixels. Now, express how much red, how much green, and how much blue colour there is in that pixel, as three numerical values. Give these three numbers for each pixel, and you have described a complete picture.
  
  Reverse the same process to display a digital picture. Computer screens (like televisions) display colours by having lots of tiny red, blue and green dots next to each other; by lighting them up with brightness controlled by those numbers, we can make the original picture appear on the screen.
  
  Digital television has just had its debut in Britain, and digital cameras are becoming more widespread; both work in this way. Pictures are much greedier than sounds. Even one colour picture on a 500 x 500 pixel grid takes 750,000 numbers to express fully, and a full video requires many pictures each second. So, in the digital world, a picture costs much more than a thousand words; a word takes only a handful of numbers! That is why your web browser often goes very slowly when it is receiving pictures over the Internet. Clever compression techniques, which can be applied to any digital data, can, however, reduce the amount of data that must be stored or transmitted. These techniques are particularly effective on pictures which have a lot of redundancy (e.g. large areas of a single colour).
  
  
• Personal information based on an individual's actions ranges from recording purchases made in shops, preferred spending patterns (from bank accounts), movements caught on CCTV or inferred from mobile phone movements or car navigation systems, through to the choices made when viewing web pages.
  
  
• Money has always been somewhat virtual, because banks have always lent much more money than they actually have in their vaults. Nowadays, people, businesses and banks send each other money electronically. Of course, there is more involved than just sending the amount! Somehow it has to be impossible (or at least very difficult) to forge such electronic cash.

In short, anything that can be measured can be represented digitally. For example, computers can record the contours of someone's face or heart, so that surgeons can show it on the screen from any angle and practise complicated surgery on your simulated body. Computers can record details of someone's body movements, so those movements can be used to train someone else, or to tell a computer how to make a computer-animated image move realistically (this was done in some of the long-distance shots in the film Titanic). But there are some things that can't easily be represented in digital form, like love, and terror.

This apparently simple ability to represent virtually anything digitally has huge implications. In the analogue world, the storage and transmission methods for different kinds of information were quite different. Pictures were on paper, sounds were stored on cassette tape or vinyl records, video was stored on VHS cassettes, and so on. The cable television network was separate from the telephone network, which was separate from the (now outdated) telex network. In the digital world, all that changes. The same CD can store a song, a picture or a document, because they are all represented as bits. The same network can transmit any of these things. This is provoking a huge upheaval in the telephone and television industries, as their businesses blur into the computer network industry.