Movie-making by modem

British film and special effects production houses will soon be able to work on Hollywood movies while Los Angeles sleeps, if a trial of the first public high-speed, fibre-optic network is a success.

British Telecom, Metropolitan Fibre Systems, and Videotron are bidding to provide networking technology to film effects companies in Soho, London, which will enable them to move high-resolution video files around the district, and ultimately between the UK and the US. BT looks to have the lead, as its offer of Asynchronous Transfer Mode (ATM) networking technology would use fibre-optic cabling that is already laid.

The winner of the contract is due to be announced in September. The new network would allow digitised video footage to be transferred from the US when Hollywood goes to bed, edited in the UK during the London morning and sent back across the Atlantic in the afternoon, ready for the LA morning. It could also form the basis for a "virtual company", in which up to 30 production houses with 60 machines could work on one film at the same time.

Soho's production houses excel at the computer graphics required in many modern blockbusters. But until now, they have only been able to move video on tape. Different tape formats mean many production houses need up to eight separate machines to be able to read them all. Footage from Hollywood then has to be brought over by courier, rendered in the UK and sent back for approval. If changes have to be made, the process begins again.

"The bottom line is that we are trying to get rid of tape because it is too expensive to try to second guess which new tape format will become standard, and too difficult to move pictures around, because each tape has to be downloaded to a machine, treated, and uploaded to a tape again," says Chris Lyon, director of technology at VTR.

It is technically possible, but impractical, to send video clips by modem over the telephone line: each frame of digitised video takes 1.4 megabytes, roughly the size of a floppy disk, and each second of film consists of 25 frames, or 35Mb. Sending that at high speed by synchronous transmission, where both ends of the line operate at the same speed and confirm the reception of each packet of data, is very difficult. But asynchronous transmission, in which the sender "squirts" data down the line without waiting for confirmation, can be much faster. The key task then is to make sure that the receiver assembles the data correctly.

ATM splits chunks of data into cells, each composed of 53 bytes, five for addressing information, and 48 for data. It was invented by France Telecom's CNET laboratories in the Eighties for use in public telephone networks, but was not taken seriously by the telephone companies until 1992. It was then also adopted by the data communications industry as a rapid means of moving data. Many European telephone operators already use ATM in their own networks but none, except Deutsche Telekom, offers the service to customers.

Many effects companies are installing ATM local area networks to speed up the process of moving digitised video footage around the building. Soho's Cinesite, which worked on the special effects for the film First Knight, finds its new ATM network is about 100 times faster than its Ethernet predecessors.

Fitted to the back of each of the computers on the network is an ATM interface card that converts frames of film into ATM format. The interface card sets up a permanent virtual circuit (virtual because the cells may travel by any number of routes) between source and destination. The cells are converted into beams which travel at the speed of light down the fibre- optic cabling. When the data reaches its destination, it is reorganised into its correct sequence by the information contained in the header.

Although the data travels at the speed of light down the cable, organising and reorganising the information slows the process down. ATM currently runs at 155 megabits per second, which will transfer full video at half speed. But products able to transmit data at 622 megabits per second are currently on trial.