Mere mention of the word audio in the security industry can provoke fears, questions or even apathy. Can you mix audio and video surveillance without violating the law? Is it technically possible? Is there value in capturing audio recordings? The answers may be surprising: yes, yes and definitely.
The use of covert audio alone--not in combination with covert video--is completely legal. The use of audio with overt (non-hidden) video surveillance cameras is also perfectly legal, so long as the cameras are installed in plain view. Many video cameras and DVRs available to the general public today offer audio capabilities, allowing you to take advantage of this second dimension of security. That said, to be safe it is always a good idea to check with your specific state laws before employing any form of surveillance.
The second dimension of surveillance
Using audio in tandem with video can make a crucial difference in catching a criminal. Case in point, one Sunday morning, I heard a cashier at the local drug store talking about a conversation he overheard while cleaning up some broken glass in one of the aisles. In an adjacent aisle, an older man instructed a 12-year-old girl to fall down on the floor and cut herself with the broken glass. The cashier quickly intervened and escorted the two out of the store. If the cashier had not heard the conversation and intervened quickly, the incident could have been very costly to the store. This type of scenario is unfortunately common, and if the incident had played out differently, audio surveillance could have provided evidence to protect the store from a fraudulent lawsuit and to enable child protective services to take action.
Effective technology is key—especially the need for effective microphones. Simple microphones generate a very small electrical current (“mic level”) and are mostly useless for security applications. “Line level” microphones provide a built-in pre-amplifier that introduces voltage behind the audio vibrations coming from the mic. This stronger signal is required to be used effectively by security monitors and DVRs.
Prices of line level microphones range from $10 to $200, but even inexpensive line level microphones can produce superb voice quality and sensitivity and are easy to integrate into existing video surveillance systems without the need to run additional cabling for audio and power. Existing installations typically involve a single coaxial video cable. Video translators, which connect in a similar fashion to baluns, can be installed to send power, video and the desired audio signal up to 1500 feet over a single coaxial cable. If baluns are already used, they can be switched out to new baluns to support the additional audio signal.
Cabling and recording
For long distances, the preferred method of cabling is to use video translators or baluns. Alternatively, special audio cable is available in the form of shielded twisted pair. However, line level signals are right at home when sent over 75 Ohm coaxial cable such as RG-6 and RG59. For short runs under 100 feet, pre-terminated cable is widely available on the market to accommodate audio, video and power. The same precautions that apply to video cable also apply here, such as not running parallel to AC power lines.
In earlier days, DVRs were typically designed to accommodate a single audio input and most recording was done in time lapse mode so recording audio was not even feasible. Today, progressive manufacturers are catering to demand and making provisions for audio. Most 16-channel DVRs have at least four audio inputs; some have up to 16. Many server-based recorders are capable of providing a one-to-one ratio of video and audio inputs.
Yet, even if a recorder has audio inputs, you cannot just assume a high level of quality. The technology emphasis for most recorders is on video while the audio quality and capabilities can leave much to be desired (i.e. audio synchronization).