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Hi-ﬁ audio is included in the standards for MPEG-1/

MPEG-2/MPEG-4. This means you can get high-quality

stereo sound synchronized with the video.

NB. The audio is not synchronized with the images

when using still image standards.

The simplest kind of digital audio is created by samp-

ling and converting the analog audio signal into digital

form. The amount of data can be reduced by approx. 50%

if you use a logarithmic representation. This principle is

used by the audio standard G.711 that is implemented in

the AXIS 2191 Audio Module. Further data reductions

are achievable by simply sending the difference between

the two measurements, subsequently using less band-

width. This principle is used in e.g. G.721.

More advanced compression methods use e.g. mathe-

matical transformations and/or pattern recognition.

These methods are very efﬁ cient, but as they compress

many samples at the same time, the signal gets delayed.

Generally speaking, the more efﬁ cient the compression,

the longer the delays will be.

Audio

Audio can normally be added to the video when using network cameras or

video servers. The audio capability is either built into the network camera/

server, or is supplied as an accessory. It can be either simplex (one way), or

duplex (two-way) and provides a good solution for many reception points,

remote guidance and remote meeting applications.

An AXIS 2120 Network Camera connected to an AXIS 2191 Audio Module.

A UDIO

COMPUTER

SECURITY GUARD

VISITOR

INTERCOM INTERCOM

AUDIO MODULE CAMERA

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Axis 2400+ Manuale Utente Pagina 30