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Audio can easily be integrated into network video as the network can carry any type of data, which reduces the need for extra cabling -- as opposed to analog systems where an audio cable must be installed from endpoint to endpoint. A network camera captures audio at the camera, integrating it into the video stream, and then sending it back for monitoring and/or recording over the network.

This makes it possible to use audio from remote locations. For instance, monitoring personnel at a company's headquarters can interact with "surveillance scenes" at remote branch offices. They can inform possible perpetrators that they are under surveillance and listen in on situations using the audio as an additional confirmation method. With both visual and audio communication, you can also conduct remote education, troubleshooting, or broadcast both sights and sounds of a location or event on Web sites.


Components involved in a network video system with audio

Audio functionality is normally delivered as an integrated component of the network camera (e.g. AXIS 211A) or video server, but the same functionality can also be achieved using an audio module accessory together with a compatible network camera/video server with no built-in audio support.

Audio level alarm
Audio can also be used in network cameras or video servers as an independent detection method, which triggers video recordings and alarms when audio levels above a certain threshold are detected. See AXIS 230 MPEG-2 Network Camera.

Audio transmission
Audio can be compressed and transmitted as an integrated part of the video stream, if MPEG-1/MPEG-2/MPEG-4 or any of the H.x video conferencing standards are used. It can also be transmitted in parallel if using a still image standard, such as JPEG. However if synchronized audio and video is prioritized, MPEG is the preferred choice. Nonetheless, there are many situations where synchronized audio is less important or even undesirable (for example if audio is to be monitored but not recorded).

Audio compression
Digital audio compression allows for efficient transmission and storage of audio data. As with video, there are many audio compression techniques, which offer different levels of compressed audio quality. In general, higher compression levels introduce more latency. Audio in digital form offers many advantages, for example high noise immunity, stability, and reproducibility. It also allows for efficient implementation of many audio post-processing functions, such as noise filtering and equalization.

Popular audio compression formats include:

- G.711 PCM providing high quality audio at 64kbit/s bit rate

- G.726 ADPCM providing audio at 32 or 24kbit/s bit rate

- MP3 (which stands for ISO-MPEG Audio Layer-3), a popular format geared towards music, with bit rates around 100 kbit/s

Audio modes
When using Axis network cameras, there are several audio modes to choose from: simplex (one way: only from client or from camera), half duplex (audio can be sent from both the client and the camera but only one direction at a time), and duplex mode (simultaneous two-way audio).

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