Ghid sisteme de supraveghere - Transmisia semnalului video

Until quite recently, the main interest of constructors, designers and installers was concentrated on image recording - currently the decisive significance in modern surveillance installations have issues related to transmission of video information.

Due to security reasons and tendency towards cost reduction (centralized protection of multiple sites), video surveillance centers are often situated in some distance from the protected buildings and places, thus the leading role of high quality video transmission.

Nowadays, there are two parallel technologies of video transmission: analog and digital, however they are used for realization of different functions - analog transmission in local area systems, digital - for longer distances.

Advantages
Cheap and popular devices, simple installation and operation.

Disadvantages
Limited transmission range, susceptibility to interferences and impact of attenuation of transmission line, limited access for larger group of scattered users.

Use: In local area surveillance installations, at short transmission distances and in low budget installations.

Building analog transmission system.Image processed by a camera to electric continuous signal may be transmitted through various transmission media - the most popular are:

• Copper cables,
• Optical fibers,
• Electromagnetic waves (radio, laser, infrared)

Transmission systems consist of the basic elements:

Basic element of the cable is its copper core.It is surrounded by following layers:

• isolator layer,
• metal shield (mostly copper, in net or coating form) securing the inner wire from outer EM field,
• outer protective layer, separating cable from environment.

Typical coaxial cables used for signal transmission are characterized by wave impedance of 75 ohm.

Advantages of coaxial cables:

• they match for typical inputs/outputs of video equipment,
• metal shield secures from signal "leakage" and protects against interferences,
• possibility of using preemphasis and deemphasis.

Disadvantages of coaxial cables:

• non-zero resistance of metal shields causes voltage drops - due to return currents: when there is difference in mass potential of camera and receiver, it effects in insertion of interfering voltage into transmission line, and as the consequence - horizontal strips in image (fixed or moving, depending on difference of potential - fixed or variable),
• environmental electromagnetic interferences, not always sufficiently attenuated by the shielding, causing deterioration in image quality,
• amplification of signal in transmission line also amplifies interferences (it is not possible differential amplification).

Basic cables parameters:

• signal attenuation expressed in dB/100m, depending on frequency (growing with it)
• line capacitance (caused by relation between wire and shield working as capacitor's plates), measured in F/m, usually pF/m,
• shielding efficiency, showing ability of suppressing outer EM field, shown in dB,
• resistances: of outer (shielding) and inner (core) conductors, in ohms.

Areas of application
Transmission of single signals on short distances, with low level of environmental interferences. Using 75 ohm impedance cable, connections between cameras and monitors may be realized at distances from 100 - 500/600 meters. The maximal distance is strongly related to quality of the cables used - 600m distance can be achieved using the copper cable TRISET-113 E1015.

We don't recommend cheap copper-plated cables - when using them, quality of transmission dramatically decreases.

Depending on the cable type, above 100 - 500 m distances it is recommended to use signal amplifiers - M1840. We should remember that color cameras are more sensitive to so called "cable lengthening" - initial symptom is decrease in color saturation.

It is very popular to use coaxial cables integrated with power supply wires

Pair of copper wires covered with isolation and twisted spirally around themselves. Usually grouped in set of four pairs in common sheath (UTP/FTP cables). Individual wires can be distinguished due to different colors.
UTP/STP cables are used for transmission of one to four video channels. Typical connector - RJ45.

Advantages of twisted-pair cables:

Disadvantages of twisted pair-cables:

Areas of application
Connection of surveillance installation via existing telecommunications cable networks (i.e. telephone, LAN) that usually have not utilized all pairs.

Additional parameters and more detailed description of features you may find here.

- "single signal" transformers/baluns:

The system keeps good quality of color image. It is also possible to transmit several signals when using adequate number of sets (e.g. via one shielded FTP cable) without image quality loss. The system is sensitive to cable length, colors distinctly fade out with length increase.

- "multiple signal" transformers/baluns:

Quality of image is very good no matter the number of transmitted signals is. Some weak reflections may occur when using cameras with large number of frames per second. When the total length of the transmission cable exceeds 300 m, color loss can be observed (up to a complete loss).

- transmitters:

The system has been designed for specific lengths of the cable, for which transmission quality is optimal, on account of wave adjustment of receiver and transmitter (in the case of other transmission distances blurs and reflections occur in the image). For the transmitter, the acceptable distance is up to 1800 m, but significant image quality decrease appears at distances above 900 m (sharpness and chrominance fade-out). Regulated in the receiver value of signal bias doesn't improve quality, significantly increasing image brightness only, till dark details fade out completely (can be only used in the case of B/W surveillance, in low light conditions).

In none of the above mentioned systems electromagnetic interferences coming form inductive or other electrical devices (e.g. drill, cell phone) influent image quality.

These glass fibers (cores) are surrounded with glass coat. Lower light reflection factor in the core allows to achieve total inner reflection of transferred light rays, generated by laser that is controlled by electrical impulses.

Main types of optical fibers are: single mode and multi-mode. The second one can be additionally divided into step and graded, that is in which the change of light refraction factor between the core and the coat occurs rapidly or fluently. It causes slowdown or acceleration of rays on the medium borders, so difference of ways for particular wavelengths and as the result - chromatic dispersion.

Main parameters of glass fibers are: size, attenuation, number of modes, ability of multiplexing (rays of different colors), reflection attenuation (coming from terminals and conjunction places) and maximum propagation time.

Advantages of optical fibers:

• immunity to electromagnetic interferences
• no generation of electromagnetic interferences
• no stray currents
• no differences of potentials
• low attenuation - presently the lowest achieved value is 0.16 dB/km.
• high durability, around 25 years
• high transmission speed.

Disadvantages of optical fibers:

• high price
• some problems with making terminals and joints
• expensive signal converters

Areas of application:
- systems requiring high bandwidth or long transmission distance,
- CCTV systems built in high interference environment and for long transmission distances.

Example application of L11041 media converters

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Fiber-optic video transmission tests - are described in this article.

Modulation is employed in order to make signal independent of channel interferences and adapt it to long transmission distances. It is realized by changing the original signal, mainly its amplitude and frequency.

In analog video transmission systems there are used amplitude modulations: two-band DSB-AM and pseudo one-band with partially attenuated carrier VSB-AM. With the use of VSB-AM modulation channel spacing is not required - on condition of low differences between them. If we want to allow larger differences of signal level than 3dB (up to 8dB for some receivers) we must use larger channel spacing, equal two or more channels. In the case of DSB-AM modulation, the modulated signal occupies wider band, therefore it is needed to provide channel spacing equal at least two channels. It causes worse use of available transmission band, so the lower total network bandwidth.

Other kind of modulation used in wireless video transmission is FM - frequency modulation. It consists in continuous change of frequency of carrier signal having constant amplitude, so that its power remains unchanged. This kind of modulation is more resistant to interferences and has much higher energetic efficiency. In order to correct S/N ratio, there can be used additional signal processing - preemphasis and deemphasis (additional amplification in transmitter and attenuation in receiver of higher frequencies of the band).

Modulators enable adjustment of electric signals to transmission conditions (i.e. change of transmission frequency into specific channel). Industrial modulators cover UHF band used in television transmissions: 470-862 MHz (channels 21- 69). Reception of the signals is possible on standard TV sets.

Transmission of several video signals with the use of modulators

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The received images have good quality, without interferences and distortions, for all available channels (21 - 69). Measurements were made for 100m coaxial cable. Above this length (e.g. 200 and 300 m), at transmitted frequencies (470 - 862 MHz), attenuation of the cable was to high to receive an usable image.

More about modulators (also catalog pages) - here

Currently popular systems use 5.8 GHz band, less vulnerable to interferences than 2.4 GHz. They are used in situations where it is difficult to install cables, as well as in temporary surveillance systems (events, assemblies, smaller construction sites etc).

We will describe issues related to this way of transmission on the example of CAM5816h M1558 transmitter and receiver of audio/video signals. Other kits can be found here.

CAM5816h 5.8 GHz - transmitter

CAM5816h 5.8 GHz - receiver

The CAM5816h M1558 wireless transmission system has been designed to transmit analog audio and video signals over distances up to 3 km and is primarily dedicated for applications in analog CCTV. The set consists of transmitter (Tx) and receiver (Rx) using unlicensed ISM band (5725 MHz - 5875 MHz). Aside from wireless transmission of video and sound in professional CCTV systems, the set can be used wherever one requires reliable, long-range AV links.

The set ensures high performance video/audio transmission, even in high-interference environments. This allows to use it in many various installations, such as video monitoring systems in cities, factories, at parking lots, in parks, businesses and many others.

The user can choose one of eight non-overlapping channels, avoiding some congested ones. With high quality video and easy mounting (no problem with changing the location of the camera and transmitter, if needed), the equipment an irreplaceable in many situations where wired solutions are not economical or feasible. The CAM5816h set can also be a great complement to any existing wired CCTV system, allowing to seamlessly extend its range without interference in the previously installed wiring.

Digital video transmission is the base and future of modern telecommunications installations. Its application area is much larger than just surveillance systems and it concerns also such issues as: telephony, education, e-commerce, advertisement and many more aspects of modern economy.

Basic knowledge of the features of digital transmission is a necessary condition to work in any of telecommunications branches, including the field of surveillance systems.

The main issues of digital transmission are:

• signal processing, i.e. conversion from analog to digital form - digitalization and compression,
• transmission issues.

One of main problems digital transmission meets is the fact that converted signal would cover much wider bandwidth after the conversion from analog to digital form than it does being transmitted in original analog shape. Therefore special compression is used, which partially eliminates from the digitalized form the information that is not perceptible by humans or inessential.

Out of the MJPEG, MPEG-4 and H.264 compression methods, the latter is the most efficient and popular now.

MPEG-2/4 compression, for the reason of more expensive codecs and weak adaptation to existing data transmission networks, is used for professional transmission rather e.g. in broadcast television systems.

Distortions.
For every kind of compression there is a limit, below which even small increase of compression factor causes significant quality decrease. Distortions are becoming visible then. The reason is rejection of too much data concerning edges, shapes, small details, colors.

Practical expansion of video signal transmission began together with the use of networks based on TCP/IP protocol (especially Internet). Digital video transmission in computer networks has big advantage - it allows to transmit coherently all signals related to remote control of objects or places: alarm signals, control signals, video data, voice data.

An extension of transmission abilities in computer networks is streaming transmission.

Streaming can be established between two points or between single source and multiple receivers. Stream transmission consists in simultaneous sending of multiple digital data (video, voice and other digital information e.g. control data), then received in continuous stream form. The transmitted data can used even before it is downloaded completely. In the case of Internet streaming, the data coming from the network to a computer is directed to specific devices - for example graphic or sound cards - responsible for processing, and immediately removed after displaying or playing. For playing back a file, the elements have to be transmitted consecutively as well as adequately buffered. To ensure this, the "player" and the server have to communicate with each other, to settle to which moment data has been already played, in order to enable the player to gather adequate portion of data, allowing fluent playback in moments when network bandwidth decreases.

Common TCP protocol does not fit this kind of communication (although it is used for that), but the future of streaming is clear thanks to new protocols, specially prepared for this purpose:

• UDP (User Datagram Protocol)
• RTSP (Real Time Streaming Protocol)
• RTP (Real Time Protocol)

IP address
Address of a computer in TCP/IP network is called IP address. Usually when we enter IP address we use dot-decimal notation (e.g. 195.13.38.254).

Computer connected to the network may have fixed IP address (so called fixed IP) or variable address being assigned e.g. at each connection via modem. There is also possible assignment of IP address for some time period (e.g. once/24h). Dynamic IP address assignment is performed by DHCP protocol (Dynamic Host Configuration Protocol). This protocol is commonly used for the reason that it allows automatic IP configuration for work in specific environment.

Besides public addresses (being "seen" in the Internet), an Internet provider may use private addresses (so called non-routing) from pools 192.168.x.x, 172.16.x.x and 10.x.x.x. These addresses are used in local area network, usually connected to the Internet by the means of a router, masking the inner network with one external routing address.

MAC address.
MAC address (Media Access Control) consists of 48 bits. We can divide it into two basic parts: LAN card manufacturer code assigned by IEEE, and unique factory number. MAC address is used for identification of individual LAN card in the local network and may be used e.g. for limitation of Internet access for the specific device.

Passive equipment (the components of structured cabling i.e. cables, connectors, distribution frames/boxes) connects active network devices:

• Regenerator (repeater) is used for conjunction of particular network segments, it also restores received (distorted for the reason of attenuation or cable crosstalk). It can join network segments with different transmission media used.
• Switch -- Switches also restore the transmitted signals. The packet transmission is performed only to the output where the receiver is connected. Switch learns it from MAC addresses of LAN cards, and transfers packets designated for the specified receiver; this behavior causes decrease of network traffic. Switches work in full-duplex system (simultaneous transmission both ways).
• VLAN switch - Kind of switch allowing virtual LAN network creation. This network allows optimizing of packet traffic intensity in particular network segments.
• Bridge - It is used for transmitting and possibly filtering of data between two networks, however these networks don't have to use the same transmission medium. A bridge follows MAC addresses placed in packets being sent to it. Both a bridge and a switch act the same way, and the difference (beside the fact that bridges have two ports, switches up to dozens) between them is quite subtle - e.g. the way of packet transmission.
• Router - It is used for connecting a few networks together, managing and filtering traffic between them. The two most popular areas of application are connection of LAN network to WAN network (usually to the Internet) and connection of a few LANs together. Routers usually have several ports working in different technologies (i.e. Ethernet, Frame Relay and ATM) and can make conversions between them. Routers can recognize destination addresses of packets and decide where and how they should be sent. Additionally, routers may also perform many other functions, such as firewall. They are made as standalone, specialized devices or implemented on computers running suitable software.

VLAN (Virtual Local Area Network) - a network of computers that behave as if they were connected to the same LAN even though they may actually be physically located in different segments of the Web.

To separate such group of devices in traditional Ethernet network, adequate topology would have to be used. VLAN allows avoiding these limits, and guarantees that devices that belong to different VLAN sub-networks can't communicate with each other, so they are secured from access of unauthorized persons.

Virtual Private Network. Another way of limiting unauthorized access to a network. Advanced techniques of encryption and protocol tunneling are used to establish secure connection. The use of existing network i.e. the Internet for transmission of private data allows to significantly reduce costs - and taking special measures - to keep the same security level as in private networks.

VPN enables secure access to resources of the protected network for authorized remote users. They may be company workers, also when traveling, staff of surveillance center etc.

Risks that occur in a computer network can be divided into following classes:

• access of unauthorized people to data transmitted in the network or saved on computers connected to it,
• access of unauthorized people to other resources (e.g. computing power),
• data loss caused by malicious actions from outside
• data fake (it mostly concerns e-mail correspondence).

All bugs and failures of systems and protocols can be used to threaten security of a network. The invaders usually make use of:

• eeaknesses of TCP/IP protocol, allied protocols and also net services (DNS, SMTP),
• software bugs,
• mistakes of the administrator or the user of the network.

The use of digital transmission broke through series of barriers within surveillance installations, enabling high flexibility and effectiveness through:

• long distance image transmission (ability of remote viewing of current events as well as recorded material and control over whole monitoring equipment and performed operations)
• working in whole network,
• extended ability of control and automation,
• easy cooperation with alarm and telecommunications systems (ability of joining surveillance systems with other systems, e.g. alarm system, fire alarm, e-mail or SMS notification).

Depending on users' requirements, digitalization and compression can be performed:

• during recording (in digital recording systems)
• in dedicated devices (video servers)
• in cameras with built-in web servers (mostly using HTTP protocol)
• via web cameras (connected to USB ports of computers).

Block diagram of a distributed monitoring system of a chain of grocery stores,
with local recording on DVRs and remote access to the surveillance data via the Internet

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Bandwidth of a link is the main factor that limits image transmission. Each compressed frame of an image from B/W camera occupies about 7.5KB (Kbytes) so singular camera transmits 7.5kB/s or 60Kbps at 1 frame/s (1 fps) recording speed.

The figures below show rough calculations of possible number of employed cameras depending on available transmission bandwidth:

Web camera is a camera integrated with system that changes analog signal to digital form, and thanks to built in Ethernet interface, allows to transmit signal further - in the form of IP packets. It enables remote viewing trough the network. It is commonly used in simple image sharing systems e.g. from ski slopes etc.

View of a Wi-Fi camera with bidirectional audio transmission and possibility of WPA-PSK data encryption

Video web server is a device changing analog signal from cameras to digital form, and thanks to built-in Ethernet interface allows for further data transmission in the form of IP packets. The device can be connected to LAN, to cable modem or to xDSL modem. Traffic generated by the device to the Internet depends on the viewing size and the quality chosen by the user. Managing system automatically chooses number of frames per second, depending on available transmission capability. It is usually within 512 - 3000 Kbps range.

An example of such a device can be one-channel video web serverP1401 K2131. To "video in" we can connect camera or other video signal source, and then transmit the converted and compressed signal through the network. The included software enables remote registration on a computer. The web server also allows audio transmission.

CPD-560 digital recorder with built-in network module

Web servers also support larger number of cameras, e.g. SED-2300QK2111 - 4-channel video server. Access to real time images is possible for any authorized network user (privilege limited by password) - with Internet Explorer (ver.6.0 or later).

Back view of 4-channel web server

The video server has 4 alarm connectors (inputs/outputs). Alarm events, in fact connected with them screenshots, may also be sent to e-mail address/es or by FTP protocol. There is also possibility to control PTZ cameras.

Recording may be performed on any computer within the network, having installed the included software.

There are on the market two main groups of devices used for digital recording:

• digital recording cards mounted in PC computers (GeoVison, AD),
• independent digital recorders - stand-alone devices with built-in dedicated operating system and data carrier (HDD or/and DVD/CD-RW) on which video data is recorded).

DVR cards.
We will present the basic features of DVR cards on the example of M8205 card. The card uses TCP/IP protocol and client-server architecture, and ensures remote access - via LAN, Dialup, ISDN, Internet.

Maximal number of simultaneous remote connections - 5.
Connection with ADACS server (the same applies to other card servers) can be established in two ways: through client software or with Internet browser (with Java support).

When connecting to the network we should be sure that server/client allows traffic to/from adequate ports (specific firewall configuration on surveillance computer and router).

During remote work, the user can change setup parameters of images from individual cameras and zones of motion detection.

Internet browser limits control features in comparison to client software (the firmware for the product), but allows to share images with users that only have typical software.

Video transmission with dynamic IP.
Many DVR cards allow video transmission via the Internet. There is no problem if we have fixed IP - we have access through standard Internet browser, e.g. IE or through additional client software.

Many ISPs do not ensure fixed IP, but there are available two simple solutions. The hardware solution is to run router with DDNS support (the devices which have this feature are e.g. Access Point with xDSL router and 4p switch TP-Link TL-WR542G N2950) and using Internet service, e.g. http://www.no-ip.org.

The software solution is used when we already have a router and we can't change or don't want to change it (e.g. provided by ISP ADSL modem/router, usually with USB interface). It is possible to install client software on one of the computers connected to LAN. The software will update the entry in DDNS service every time when IP address changes. Such a client application is available e.g. on http://www.no-ip.org.

Control of speed dome cameras
The connection is realized by RS485/RS232 converter, and the control process is performed using mouse pointer directly on the screen with the image from the camera. ADACS has implemented protocols allowing control of cameras of the following manufacturers: Videotec, Dynacolor, Samsung, Panasonic, Pelco, Vortex, Sensormatic, Kalatel, GANG SZ, Lilin, YAAN, YOKO.

Sound transmission.
Sound playback is also available in the moment of intruding a zone. Many other cards also enable direct sound transmission.

Caution: If the computer, with which we want to connect from outside (via Internet), is a part of LAN network and is behind a router (usually computers in LAN are seen as having common single IP address), in the router/server acting as firewall we have to redirect traffic of the following ports: 20900, 20910 and 21060.

Important feature of these cards is that they can be combined, so we can extend our system with additional inputs. We can install in one computer 1, 2, 3 or 4 cards.

New technical features, also related to network work, should rather be found in DVR cards than in standalone DVRs. It is mainly for the reason of longer designing and implementation process.

However, in the case of fulfilling all requirements, the standalone DVRs should be preferred, for their stability of work and easy and intuitive operation.

There is a large group of recorders designed for network operation. During selection of a recorder we should draw our attention to features allowing to backup the video onto external device/carrier and possibility of playback on regular PC.

Description of digital recorders available in DIPOL offer.