ONCE upon a time telephone companies made telephones, television companies made televisions and computer companies made computers. Today this is changing and a lot of interesting products and a lot of money will be made by marrying the right technologies together.

These new multimedia hybrids are emerging fast in a process known as convergence. It is not new; convergence was much in vogue in the 1980s when the computer and the telephone were supposed to join seamlessly together and provide enhanced profits for all concerned. It did not happen, but it could have done.

It is easy to imagine the world today with every PC having a modem, a device that allows computer data to be transmitted down telephone lines, built in. If modems were built in as standard, they would add no more than a few pounds to the price of a PC. This would have given birth to information services and 'user-friendly' communications software would have developed, allowing us to ask our computer 'how do you make a Christmas cake?' or 'how many dollars are there to the pound?' and have the computer automatically dial into the relevant database.

This level of integration may still come about but by a slightly different route - via the convergence of the consumer electronics, entertainment and communications industries with the computing industry. Already the coming together of these sectors is bearing fruit. Philips' CD-I essentially uses sophisticated computer technology to manipulate information stored on a disc and show the results on a TV screen.

It is not the storage of information on a CD-type disc that is new; it is the control and the type of information that is different. But CD-I will be genuinely innovative when it gets its video ability at the end of this year.

Because of the limitations of the hardware, taking moving images off a CD type disc requires some very clever tricks - which are turning up in other areas.

For example, delivering movies 'on demand' is the holy grail of the cable television industry and it is just becoming technically and economically possible. Here the user dials up a central facility and orders a movie. A matter of minutes later he or she will be able to watch it on TV.

The key technology to allow this to happen is video compression - the same technology used to get images off a CD-I disc. What is essentially happening is that your film and dozens of others are being sent over a single TV channel but the equipment in your home is de-scrambling the signal and picking out the signal you want.

Video compression technology was developed by the computer industry to solve its problems of sending one picture over a narrow information channel. The cable TV industry created a business opportunity by using compression to send dozens of pictures over a much wider channel.

But the convergence of computing and the other sectors will go much wider. Apple plans a range of products that will take computer power out of the office and make it easy for everyone to use. Some of these products will be able to communicate using mobile telephone techniques wherever they are. Since the system knows where you are, finding a local Chinese restaurant will be as easy as asking a local policeman.

Film companies are looking at ways of creating interactive movies, in which you make choices during the film that change the way the plot develops.

The possibilities are endless; the problem is sorting out the potentially lucrative marriages and making them work. Keeping the different industries moving in the same direction will be essential because one thing has been shown time and again in convergent markets: if the various players all decide to go their own way it will be very difficult for anyone to bring interesting and profitable products to the market. Today's modem-less PC is a lesson to all.