On CCTV, no-one can hear you scream.

But technology from a UK company now means cameras can tell if you're being aggressive or calling for help - and will alert security guards straight away.

Cambridge firm Audio Analytic has produced software which it said can analyse the pitch, tone and intonation of noises and work out if they pose a threat.

"A lot of incidents just can't be picked up by video only systems," said Chris Mitchell, Audio Analytic's boss, on BBC World Service's Digital Planet.

"For example in a hospital where somebody, or a nurse, is being threatened early hours in the morning - that's a very difficult thing for CCTV guards who monitor hundreds of channels worth of video signals on 20 screens or so to pick up."

The software goes beyond simply placing microphones onto cameras and listening in. By feeding hundreds of sample sounds into the system, the software can distinguish different threats from various sounds - and not just based on volume.

"We don't work with volume at all in the system because it's so related to how far somebody is from the microphone that it's not a useful metric.

"Our system picks out the most salient characteristics. These are things related to pitch, tone, intonation...."