Eye on Video: Adding audio intelligence

Adding a listening post to your surveillance system


The movie industry introduced its first "talkie" back in 1927. Yet video surveillance, for the most part, has remained oddly silent. Given that what we hear adds as much to our understanding of events as the images we see, the lack of an audio component can seriously impact the ability of security personnel to effectively protect property and people.

Consider a video surveillance system sans audio. A cry for help, the sound of breaking glass, a gunshot, or an explosion in the vicinity of a camera - but outside the field of view - would escape notice. Even in a parking under visual surveillance, without audio support security staff might never know that a vehicle's alarm had gone off.

Audio covers a 360-degree area, enabling a video surveillance system to extend its coverage beyond a camera's field of view. Intelligent audio can instruct a pan/tilt/zoom (PTZ) or dome camera or an operator of the camera to visually verify an audio alarm, giving remote security personnel additional information about the environment on which to base their response.

In addition to being a listening post, security personnel can use audio to communicate with visitors or intruders. If a person in a camera's field of view exhibits suspicious behavior - such as loitering near a bank machine or entering a restricted area - a remote security guard can send a verbal warning to the individual. In cases where the camera reveals a person who is injured, the guard can remotely assure the victim that help is on the way. Audio naturally dovetails with a variety of security applications. In access control, intercom technology is a strong fit. A doorman can remotely greet visitors before buzzing them into a building. In an unmanned garage, patrons can request assistance from a remote security attendant.

Deployment obstacles: analog vs. network video system

The industry expects audio adoption to increase as network video systems become more commonplace. This is primarily due to audio being easier to implement in network video systems than analog CCTV systems.

Analog systems require users to install separate audio and video cables from the camera and microphone location to the recording and viewing station. If the distance is too long, you need to add balanced audio equipment, which increases installation difficulty and cost. A simpler way would be to tie the analog cameras into a network video system, using video encoders with built-in audio support.

Network video systems equipped with audio support process the audio and send both the audio and video over the same network cable used for monitoring and/or recording. This eliminates the need for extra cabling and makes synchronizing the audio and video much easier. (For more information, see athe previous article in this series, Eye on Video: Intelligent Video Architecture, What to consider when deciding upon centralized or distributed surveillance analytics)

Selecting audio equipment

Network cameras or encoders that support audio generally include a built-in microphone, but very rarely a built-in speaker. While a built-in microphone may be adequate for some applications, other may require a more sensitive external microphone. External microphones fall into four main categories: condenser, electret condenser, dynamic and directional.

Condenser microphones offer the highest audio sensitivity and quality. These are the same microphones used in professional sound studios.

Electret condenser microphones offer a high level of sensitivity and are less expensive than condenser microphones.

Dynamic microphones are rarely used in security or video surveillance because they typically do not possess sufficient audio sensitivity.

Directional microphones pick up sound based on a particular pattern. An omni-directional microphone picks up audio equally well in all directions. Unidirectional microphones have audio sensitivity in one specific direction.

Adding audio detection alarms

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