Video Surveillance: The Sound of Security Success

How to integrate audio into a video surveillance system


Dynamic microphones are rarely used in surveillance because of poor audio sensitivity and ability to reproduce important low frequencies. If a dynamic microphone is required, typically it will use an XLR connector or, if that is not supported, an adapter.

The condenser microphone is known for the highest audio sensitivity and quality. While some back-electret microphones can provide similar quality, the condenser is often used in professional recording studios. Surveillance manufacturers and third-party accessory providers have recently launched new condenser microphones designed specifically for the security world with discreet and flexible installation options.

The third option, electret condenser microphones, are typically ones found in headsets and computer microphones — so it is not surprising that these are the types of microphones often built-in to network cameras. This type of microphone normally needs 1-10 Volts to work, which can be powered by the camera itself.

 Lastly, another sometimes overlooked audio device for network video applications is the audio module. These devices enable audio support and I/O ports that can be located far away from a network camera. Audio modules, for example, are great for a city surveillance application where a PTZ is located high up on a pole, but the sound should be captured at street level.

 

How to Select Audio Equipment

When selecting a camera  or encoder for video and audio capabilities, you must determine the type of communication that is important for the application. If the end-user requires two-way communication, then obviously a microphone and speaker (either internal or external) are required. From there, determine which of the three communication options will satisfy the needs: simplex, half-duplex or full-duplex.

In simplex mode, audio is sent in one direction only. In half-duplex mode, audio is sent in both directions, but only one party at a time can be heard (think walkie-talkie). In full-duplex mode, audio is sent to and from the operator simultaneously.

When audio is used merely to listen in on a scene or for video analytics, simplex will do the job. If you are working with an operator team and communication with people in the video scene is intermittent, then half-duplex will be acceptable; however, if the use of audio is for visitor management, emergency response or video conferencing, full-duplex is the choice. Remember, though, that while full duplex has the simultaneous audio advantage, it also increases the bandwidth required.

 

Five Best Practices for Installation

Once the appropriate IP camera, encoder and/or audio accessories are selected, there are a few other crucial installation and configuration best practices:

1. Audio equipment placement: Although an audio signal can be amplified later, appropriate placement of equipment will reduce noise. In full-duplex mode, the microphone should face away from the speaker at a reasonable distance to reduce feedback.

2. Amplify the signal as early as possible: This minimizes noise in the signal chain. In addition, make sure the signal levels are close to, but not over, the clipping level where audio becomes distorted. Proper gain selection is especially important to surveillance installations. If possible, the gain should be applied as early in the signal path as possible, preferably in the microphone itself.

3. Apply appropriate signal processing technologies to improve audio quality: Audio quality can be improved by adjusting the input gain and using different features such as echo cancellation and speech filter.

4. Select the right codec and bit rate: The type of codec and bit rate selected will affect audio quality. In general, the higher bit rate, the better the audio quality.

5. Understand legal implications: States and countries have different restrictions on the use of audio and video surveillance. Check with the local authorities about the legality of the system – or what’s required to make it legal – before product procurement.

 

The Future?

Most articles end with a look toward the future of technology and what it’ll bring. No need here. The technological marriage of audio, video and access control technology is here and it’s a strong one. It’s up to the expertise of the integrator to determine if audio is needed and then how to apply it.