alarm inputs / outputs
Fig.1. Digital webcam.
As a photodetector, in most webcams use CCD (CCD, CCD -
Charge Coupled Device) - a rectangular photosensitive semiconductor
plate with sides ratio of 3: 4, which converts the light falling on it
into an electrical signal. CCD consists of a large number of
photosensitive cells. In order to improve the light sensitivity of CCD,
often form the structure that creates the microlenses in front of each
of the cells. In the technical parameters of Web cameras usually
indicate the format CCD (the length in inches diagonal), the number of
effective pixels, the type of scanning (progressive or interlaced), and
Lens - a lens system designed to project an image of the object of
observation on the light-sensitive element of the webcam. The lens is an
integral part of the web camera, so the correctness of his choice and
installation depends on the quality of video received by a web camera.
Quite often, the webcam comes with a lens. Lenses are characterized by a
number of critical parameters such as focal length, aperture (F), depth
of field, the type of attachment (C, CS), format.
Optical infrared cut off filter installed in a webcam, are optically
accurate plane-parallel plates mounted on top of the CCD. They operate
as optical low pass filter with cutoff frequency of about 700 nm, near
red. They cut off the infrared component of light waves, providing
webcam accurate colors. However, many black and white webcams such
filters are not installed, so that monochrome webcam have a higher
Frame grabber webcam (block digitization) has the transformation of the
analog electrical signal, which formed the CCD, into digital format. The
process of converting the signal consists of three phases:
Sampling - reading the amplitude of the electrical signal at regular
intervals (period). This phase transformation is characterized by a
signal sampling rate.
Quantization - is the process of reporting the results of sampling in
digital form. Changing the level of the electrical signal for the
sampling period is represented as a codeword of 8, 10 or 12 bits, which
give, respectively, 256, 1024 and 4096 levels of quantization. From the
number of quantization levels depends on the accuracy of signal
representation in digital form.
Coding. In addition to information about changes in the level of the
signal received at the previous stage in the process of coding bits are
formed, reporting the end of the sync pulse and the beginning of a new
frame, as well as additional bits of error protection.
Block compression web camera compresses the digitized video signal into
one of the compression formats (JPEG, MJPEG, MPEG-1/2/4, Wavelet).
Thanks to compression, reduces the size of the video frame. It is
necessary for storage and transmission of video over the network. If the
local network to which you connect your webcam, has limited bandwidth,
in order to avoid overflow of network traffic advisable to reduce the
amount of information transmitted by reducing either the frequency of
the frame on the network, or permission of staff. Most compression
formats, which use a Web camera, provides a reasonable compromise
between these two ways of solving the problem of video transmission over
the network. Known today compression formats yield a digital stream with
a bandwidth of 64 KB - 2 MB (with a bandwidth video streams can run
concurrently with other data streams in networks).
Video Compression in the webcam may be represented by both hardware and
software. Software implementation of compression cheaper, but because of
the high computing capacity of compression algorithms, it is inefficient,
especially when it is required to view video from a webcam in online
mode. Therefore, most of the leading manufacturers make a webcam with
the hardware implementation of the compression. For example, each
network camera AXIS Communications Company is equipped with a processor
compression ARTPEC, carrying high-speed video compression format JPEG /
The CPU is the computing core of the webcam. It operates on the
conclusion of the digitized and compressed video, and is responsible for
the built-in Web server and control program for webcams.
Interface for Ethernet is used to connect the webcam to the network
standard Ethernet 10/100 Mbps.
To use the network webcam may have a serial port for the modem and the
mode dial-up in the absence of a local network. A serial port can also
connect to webcam peripherals.
Map of flash memory allows program managers to update the webcam and
store custom HTML-page.
RAM is used for storing temporary data that are generated in the
performance management programs and custom scripts. Many web cameras are
so-called framebuffer. This is part of RAM reserved for temporary
storage of records and filmed a web camera video frames. The information
in the framebuffer is updated cyclically, ie new frame replaces the
oldest. This function is necessary if the webcam does video surveillance
because it allows to restore the events preceding and following the
alarm connected to a webcam security sensors.
Alarm Inputs / outputs are used to connect to a webcam alarm sensors.
When firing one of the sensors generate an alarm, resulting in processor
webcams assembles a set of frames recorded in the framebuffer before,
after and at the time of admission alarm. This recruitment may be sent
to the specified e-mail address or via FTP.
Connecting and installing webcams
Typically a web camera connected to the Internet via
10BaseT/100BaseTX/1000BaseTX Ethernet or using a modem through the
serial port. After you physically connect the webcam to the network it
is assigned IP-address. Most webcams have no IP-address by default, it
is set either by using the appropriate functions of a standard web
browser or command DOS, which is used to identify the serial number of
webcam assigned to it by the manufacturer. In addition, manufacturers
are developing special programs for webcams, considerably simplifying
the procedure for assigning webcam IP-address (eg, IP Installer company
AXIS Communications). Thanks to embedded software for the web server,
FTP-server, FTP-client, e-mail client and others, a web camera connected
directly to LAN / WAN / Internet network and works in it as a separate
network device. This distinguishes webcams from other digital cameras
that require mandatory connection to a PC via USB or LPT port. In
addition, the webcam can support the work with custom scripts and
Fig.2. Digital webcam. Connect webcam
To install a webcam on the street or in a
room, many manufacturers include in the design of the casing in a
special fasteners, designed to fit on the walls, ceilings, brackets and
swivel device. Some webcams have built-in rotary devices designed for
mounting on horizontal or vertical surface.
Additional features and functions of the
Motion Detector - is a software module, whose main task is to detect
moving in the field of view webcam on site surveillance. Motion detector
not only detects the movement in the field of view, but also determines
the size of the object and its speed. Depending on the tasks of
surveillance, motion detection web camera adjust to the detection of
moving objects with extreme minimization of false positives (noise
filtering), set a flexible processing logic alarms (an alarming record,
integration with other security equipment).
in most cases carried out by connecting to webcam optional audio module.
For example, the company AXIS
Communications to extend the functionality of web cameras release a
special audio module AXIS 2191 is compatible with most Web cameras AXIS.
Serves to restrict
access to the webcam persons do not have access rights. By default, the
video from a webcam can be viewed from any networked computer that is
equipped with a standard Web browser, for example, Internet Explorer or
Netscape Navigator. However, you can limit the number of persons with
access rights to the webcam, enter your password to the user level. Many
web cameras support multi-level password protection for the separation
of access rights and administration.
Program for webcams
Typically, an image
from a webcam can be viewed via a standard Web browser, for example,
Internet Explorer or Netscape Navigator. However, many manufacturers
develop exclusive programs for webcams.They combine the functions of a
web browser and management software and are used to control, configure,
and view images from a webcam (for example, AXIS Camera Explorer or the
control program of the company JVC Professional to administer network
devices through V. Networks). There is also software for distributed
networked video surveillance systems, supporting equipment from
different manufacturers, for example, Sphinx-DV companies Digicore
Connecting web cameras to the network
Currently, the webcam can be connected to
the network in several ways, which are based on different standards of
This is the most popular way to transfer
data from a webcam to the network. There are two standard networks 10
Mbit Ethernet: 10Base2 (using coaxial cable) and 10BaseT (using twisted
pair). 10Base2 rarely used because of the sensitivity of the coaxial
cable to external interference. 100BaseTX uses standard twisted pair and
provides data transfer rate 100 Mbps.
1000 Mbit Ethrnet, Gigabit Ethernet
Used here are standard 1000BaseTX - an upgraded version 100BaseTX.
This standard is mainly used for
construction of highways LANs.
Standard telephone modems
This is a cheap and fairly common way to
connect the webcam to the global network. The main drawback of this
method of connection - low data rate (maximum speed of data downloads -
56 kbit / s, maximum pumping speed - 33,6 kbit / s). Connect the webcam
to the modem through the serial port.
Standard ISDN (Integrated services Digital
Network) is used for transmission of digitized graphics, audio and video
and other digital data on private or common digital telephone networks.
Standard ISDN provides transmission of data from a web camera at speeds
up to 128 kbit / s on two channels
DSL (Digital Subscriber Line) - technology
that provides bandwidth for simple copper telephone wires. Data transfer
rate from the webcam may vary depending on the company providing this
service. On average it is 1 Mbps for downloading data and 250 kbit / s
for the swap.
Cable modem - a modem that provides Internet
access to cable television networks. Cable modems use an asymmetrical
technology, which is most optimally suited for user access to the
Internet. In this case the maximum possible speed of receiving data from
a webcam so the modem can reach about 40 Mbps (although it usually does
not exceed 1 Mbit / s) and data transfer rate is about 10 Mbps.
With the help of a cellular modem can
connect your webcam to the Internet, using cell lines. Data transfer
rate is usually from 5 to 20 kbps.
More technical information on specific models of web cameras by Axis
Communications and JVC Professional presented on the same site in the
section of your camcorder. Brief specifications and prices on webcams
are in the online store ARMO-Systems.