By Mark Ward BBC News Online technology correspondent

Mobile callers can be hard to hear

Electronics giant Philips has found a way to dampen the background noise that often intrudes on phone calls made on the move.

Extracting the ambient noise makes it much easier for both people involved with a phone call to hear what is being said.

The technology will first be used in phones that use headsets and could find its way into individual handsets within a couple of years.

Silencing sound

Philips has developed a series of software routines, or algorithms, that analyse the sounds entering the microphone of a handset.

The system needs input from two separate microphones, such as one on the body of a handset and one on an associated headset.

Dieter Therssen, Philips' lead researcher on the active filtering system, said the algorithms compared the input from the two sound sources and worked out what was talk and what was background noise.

"Post-processing these two really makes the intended signal become very prominent over the noise," he said.

The filter passed on what it believed to be speech sounds but dropped the background noise, he said

The Philips technology will appear in headsets first

Mr Therssen added that the system, called LifeVibes, had been tested at busy trade shows and could effectively cut out crowd noise, blaring music played nearby and the shouted conversations of other attendees.

He said that because the system needed input from two microphones, it would be some time before it found its way into handsets which typically only had one microphone on board.

However, he said, it worked well on phones fitted with headsets as input could be taken from the microphone on the body of the phone and its earpiece.

"The first most-heard complaint is the network problems that people encounter," said Mr Therssen. "The second is the lack of clarity and that they have to shout and cannot hear what others are saying on the telephone."

A clearer line meant that people made more calls and talked more, said Mr Therssen. Research had shown that phone calls were 20% longer when people could easily make themselves heard, he said.

Noise cancelling systems have been used for a long time in some passenger aircraft to ensure that the sound track for in-flight films is not drowned by the noise of the jet's engines.

These active noise cancelling systems work by generating sounds that are a mirror image of the engine noise and cancel it out.