Eye on Video: Adding audio intelligence

Adding a listening post to your surveillance system


Audio can be analyzed by a network camera in much the same way as video. Audio detection nicely complements video motion detection since it can react to events in areas outside the camera's view or too dark for video motion detection to function properly. When intelligent audio detects a suspicious sound - such as a pane of glass breaking or voices in a room that should be unoccupied - it can trigger a response in much the same way intelligent motion detection or door contact systems can. The system can instruct the network camera to record and send audio and video, send e-mail or other alert, or activate alarms or other external devices. In systems with PTZ or network dome cameras, audio alarm detection can direct a camera to automatically turn to a preset location, such as a specific window or doorway.

If you use directional microphones, the audio system can even ascertain which direction the sound is coming from and point a PTZ camera in that direction. This feature is particularly useful in city center surveillance projects, where operators often monitor a large array of fixed and PTZ cameras.

Audio detection offers a number of deployment options. You can enable audio detection all the time, during specific times or disable it during certain events, such as closed-door meetings. You can set it to trigger a sequence of responses if the incoming sound level rises above, falls below or passes a certain level of sound intensity.

Choosing an audio compression algorithm

For efficient transmission and storage, analog audio signals must be converted into digital audio through a sampling process and then compressed to reduce the file size.

Sampling refers to number of times per second a sample of an audio signal is taken. Generally, the sample rate must be twice the maximum required frequency. For example, if you want to capture human speech which is normally below 4 kHz, you need a sample rate of at least 8 kHz. In general. The higher the sampling frequency, the better the audio quality and the greater the bandwidth and storage required.

Compression is defined by bit rate. The higher the compression level, the lower the bit rate and the lower the audio quality, especially for more complex sounds. Higher compression levels may also introduced more transmission delay, but save on bandwidth and storage.

There are a number of coding and decoding (codec) algorithms for audio data, each with different sampling frequencies, bit rates and levels of compression. All of these factors affect audio quality and file size.

 

Audio compression
algorithm

License

Sampling
Frequency

Bit Rate

Compression Standard

AAC-LC

Licensed

8-96 kHz

2-300+ kbit/s

MPEG, MPEG-2, MPEG-4

G.711 PCM

Unlicensed

8 kHZ

64 kbit/s

ITU-T

G.726 ADPCM

Unlicensed

8 kHz

16, 24, 32 and 40 kbit/s

ITU-T

G.722.2 or AMR-WB

Licensed

16 kHz

6.60-23.85 kbit/s

ITU-T

 

Tips for proper deployment

There are a number of factors to consider when deploying audio to ensure that you achieve the best quality from the installation.

Audio equipment and placement. Place the microphone as close as possible to the source of the sound. For two-way communication, face the microphone away from and some distance from the speaker to reduce feedback.

Signal amplification. Amplify the signal as early as possible to minimize noise in the signal chain. Set the signal level as close to, but not over, the clipping level which is the level at which audio becomes distorted.

Acoustical adjustments. Adjust the input gain and use different features such as echo cancellation and speech filter to reduce distortion, eliminate feedback and screen background noise.

Codec and bit rate selection. Choosing a variable bit rates that adjusts to the complexity of the audio will help you achieve a higher quality stream than a constant bit rate file of the same size.

Shielded cabling. Use shielded audio cable to minimize disturbance and noise. Avoid running the cable near power cables and cables carrying high frequency switching signals. Keep the audio cables as short as possible. If you need to run a long audio cable, be sure that the cable, amplifier and microphone are balanced to reduce noise.

Legal restrictions. Some countries place restrictions on audio and video surveillance. Check with local authorities to determine what is allowable before you start.

Where audio intelligence is going