Fig.1. Digital webcam. Flowchart

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 sensitivity.
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 sensitivity.
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,
Quantization,
Coding.
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 / MJPEG.
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 JAVA-applets.

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 Web Camera

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).

Transmission audiosignala4

Network 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.

Password Protection

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 Systems.

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 data transmission.

10/100 MbitEthernet

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.

ISDN modems

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

xDSL modems

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 Modems

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.

Cellular modems

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.

Source material: http://www.armosystems.ru