On call

People could receive emergency messages on their mobile phones via an audio system — even when networks are down or out of reach, such as when underground.

Intrasonics Sound Link and Sync (SLS), developed by Cambridge's

, works by embedding data in an audio signal which can be transmitted over a radio, TV or PA system and sent using an encoded link via SLS to mobiles in the vicinity.

Rob Morland, director of Intrasonics data communications explained: 'It is essentially a way of hiding data in an audio stream. That audio can then be broadcast, stored, transmitted over TV or radio networks and played over any standard loudspeaker. It can then be picked up with the standard microphone in your mobile.'

In the event of an incident, such as on the Underground, SLS would send a further code using an audio signal from the driver's cab. this second code will activate the first, bringing up an icon on your mobile, alerting you to a message. By clicking on the link through to a remote server, the relevant message is then displayed on the screen.

'We envisage hiding a link in an emergency message, such as "please keep your bags with you", already being transmitted. In the event of an incident, wherever you were, on any transport system, you would be in reach of an audio announcement. The link would be sent over that PA system — you don't need a mobile network — and it will instruct the phone to show a message.'

This could be any number of messages detailing, for example, evacuation procedures, location details or evasive action to take.

To bury the data within a normal message, it is encoded using a spread spectrum technique, similar to that used by the 3 network. Morland said that with SLS it is encoded in the audio domain using complex techniques derived from physchoacoustics — essentially a compression technique that describes which part of a given digital audio signal can be removed without significant losses in the quality of sound. Such compression is a feature of nearly all modern audio compression formats such as MP3 and MPEG.

For the consumer to use the technology he or she would have to download specific software to interpret the coded message. It would also require a powerful mobile to handle the sophisticated decoding software which runs at 10 million instructions per second (mips).

Morland admitted one reason they are bringing the technology out now is that there has been a significant growth in the penetration of Symbian phones in the

market with six million registered handsets in 2005. This figure is projected to increase to 11 million by the end of the year.

Following trials in the far east, the company is in discussion with the government and relevant transport authorities to establish whether there is sufficient interest in the idea for cities such as

. But more immediately, interactive advertising and gaming will provide greater revenue streams. 'We are in discussion with major broadcasters and mobile operators,' said Morland.