Video Signal Transmission
Wireless-camera or Cable camera connections are two of the most used means for transmission to the viewer of the images that are captured by the Surveillance Camera. Before selecting anyone of them, one should grasp the main Video Signal.
one area of video is being created by the Convention and sweep exactly 312 1/2 times and this reproduced on the monitor. A second scan of 312 lines 2.1 is accurately half a line down and interlaced with the first cycle to form an image of 625 lines. This is called a 2:1 interlaced image. Line 625 is known as a video frame consists of two interlace fields. Stress is a tension in the bottom of the sync pulse above the target, resulting in a peak to peak (p/p). The luminance (brightness) component of the signal is 0.3 volts to one volt, is 0.7 volts maximum. This is known as a composite video signal, since the timing information and video are combined into a single signal.
Synchronizing
the video signal from a TV camera has to provide a variety of information at the monitor for a correct TV picture to be displayed. This information can be divided into: Synchronizing pulses to tell the monitor when to start a line and a frame; video information that tells the monitor how bright a exacting point in the picture should be; chrominance that tells the monitor what colors a particular part of the picture should be (color cameras only).
Bandwidth
Composite video output media CCTV camera to cover the range of bandwidth from five Hz to several MHz, the higher frequency is mainly determined by the resolution of the camera, and if black and white or color. For every 100 lines, with a bandwidth of about one MHz is required. Therefore, the 600 lines of the resolution camera to trace a video signal with a bandwidth of approximately 6MHz, this principle applies in both color and black and white cameras. However, color cameras are also to produce a color signal (chrominance) and output in black and white (luminance). Chrome signal is modulated carrier wave is a 4. 43MHz PAL because of the color signal, regardless of the definition of bandwidth is less than 5MHz.
Wireless camera Vs Cable camera
The important challenge, for any transmission method, is to be able to pass on the video signal with high fidelity, with minimum disturbance caused by surrounding interferences, from the source, either a Wireless-camera or a Cable camera, to the end user.
The ability to send video signals over a physical distance to the display of the instruments used to view is done in different ways, depending on transmission method selected.
Given the wide range of transmission technologies, we must examine the features and benefits of different kinds to decide in each particular case, which is best suited to perform the function required for successful monitoring and minimum cost.
Several methods exist for transmitting video coaxial cable, telephone line, cable CAT5 (Category 5) cable with adapters, fiber optic, microwave and radio frequency. Due to the needs of specific applications can be found in complex equipment, many video transmission technologies used in the same system of closed circuit television.
Cable Transmission (Cable camera)
All cables, no matter what their length or quality, produce problems when used for the transmission of video signals, the main problem being related to the wide bandwidth requirements of a video signal. All cables produce a loss of signal that is dependent primarily on the frequency, the higher the frequency, the higher the loss. This means that as a video signal travels along a cable it loses its high frequency components faster than its low frequency components. The result of this is a loss of the fine detail (definition) in the picture.
The human eye is very tolerant of errors of this type; a significant loss of detail is generally not objectionable, unless the loss is great. It is fortunate, since the loss of high frequency components is very high on the types of cables used in CCTV systems. For example, using the common coaxial cables URM70 or RG59, 50% of signal lost at 5 MHz to 200 meters of cable. To compensate for these losses, various amplifiers used. These provide the ability to selectively amplify high frequency components of the video signal to compensate for cable losses.
Cable Types
There are two main types of cable used for transmitting video signals, which are: Unbalanced (coaxial) and balanced (twisted pair). The construction of each is shown in diagrams 2 and 3. An unbalanced signal is one in which the signal level is a voltage referenced to ground. For instance a video signal from the camera is between 0.3 and 1.0 volts above zero (ground level). The shield is the ground level.
A balanced signal is a video signal that has been converted for transmission along a medium other than coaxial cable. Here the signal voltage is the difference between the voltages in each conductor.
External interference is taken by all types of cable. The rejection of this interference occurs in different ways. The coaxial cable is based on the central conductor, which, although considered by the outer copper. There are several types of coaxial cable and care must be taken to choose one with a braid of 95%. In the case of a twisted pair cable, interference picked up by the two leaders in the same direction on an equal footing. The video signal moves in the opposite direction to the two leaders. The operation can then be balanced by using the right type of amplifier. This can only react to the signal difference between the two leaders and is known as a differential amplifier.
Unbalanced (Coaxial) Cables
This type of cable is made in many different types of impedance. In this case impedance is measured between the inner conductor and the outer sheath. 75 Ohm impedance cable is the standard used in CCTV systems. Most video equipment is designed to operate at this impedance. Coaxial cables with an impedance of 75 Ohms are available in many different mechanical formats; including single wire armoured and irradiated PVC sheathed cable for direct burial, the cables available range in performance from relatively poor to excellent. Performance is normally measured in high frequency loss per 100 meters, the lower this loss figure, the less the distortion to the video signal. Therefore, higher quality cables should be used when transmitting the signal over long distances.
Another factor that should be considered carefully when selecting coaxial cables is the quality of the cable screen. This, as its name suggests, provides protection from interference for the centre core, as once interference enters the cable it is almost impossible to remove.
Balanced (Twisted Pair) Cables
In each pair of twisted pair is twisted with a twist slowly around one to two turns per meter. These cables are manufactured in different types of impedance from 100 to 150 ohms being the most common. Balanced cables have been used for many years in the largest cable networks in the world. When circumstances require, these benefits have on the coaxial cables of similar size. Twisted pair cables are often used where there would be an unacceptable loss due to a long series of coaxial cable.
The main advantages are:
1) The ability to reject unwanted interference.
2) Lower losses at high frequencies per unit length.
3) Smaller size.
4) Availability of multi-pair cables.
5) Lower cost.
2) Lower losses at high frequencies per unit length.
3) Smaller size.
4) Availability of multi-pair cables.
5) Lower cost.
Interests must be taken into account in relation to the price of the equipment of this type of transmission. Launch amplifier to convert the video signal is required for the camera and to reconstruct the control signal amplifier at the end of equalization.
Wireless Transmission (Wireless-camera)
To prevent the coaxial cables, if it is not practical for a monitoring solution of some houses, we can use two linked devices: a video transmitter and a receiver both established in the same preselected frequency and use of directional antennas a better connection. A video transmitter is included in the corresponding camera video wireless receiver purchased separately or part of the orchestra. The video signal from the wireless camera is sent by radio to the common frequency for the receiver that is connected by means of other elements of a surveillance system.
Radio Frequency (RF) is a reliable though short distance video transmission technology. It is regulated by national authority. For limited power dedicated installations, no special authorizations are required. For longer range and best reception unobstructed line of sight should be preferred, although normal walls are not an obstacle.
The main advantage of a wireless camera is that none of the coaxial cables should be installed so that less work is involved. It's easy and convenient way to move items to a new location, where appropriate. It is similar in principle to the mobile phone (with the exception of the directional antenna). Another advantage may be important when tracking objects of great value, is that the recording device is out of sight, hidden somewhere difficult for intruders to locate and destroy.
But wireless camera to be operated, continuous stream, usually by wire from a supply low voltage adapter (transformer) plugged into a standard wall outlet located somewhere near the camera location. (The solution of the battery is not recommended because of the short time between battery changes). Using a wireless camera is becoming increasingly popular where wiring methods are either impossible or difficult, and has been used successfully to reduce cabling costs even within large buildings. Environmental conditions in the region may affect its performance.
If more than one wireless camera is required, the amount of spectrum available limits the number of wireless cameras that can be deployed. If there are only four different frequencies can be used so that only four cameras can be installed on a given site.
The three different radio frequencies most used are:
• 2.4 GHz (Giga Hertz or 109 cycles per second) or
• 1.2 GHz or
• 900 MHz (Mega Hertz or 106 cycles per second)
• 1.2 GHz or
• 900 MHz (Mega Hertz or 106 cycles per second)
Within each one of them a few different channels may be provided to avoid interference with other devices present nearby using the same frequency.
Radio signals can be disrupted by interference, nor inevitable (from competing devices at the same frequency) or it is generated in order to disrupt or interfere with the continuous monitoring of activities. It should be noted that the transmission path may be offensive: if this is a real risk, appropriate measures be taken to protect the integrity of the transmission. Although it could not cover the use of home monitoring, it is noted that the communication wireless camera can be subject to interception. To sum up, the Wireless-camera can be used almost anywhere, but the most common are discreet surveillance or baby sitter hidden cameras, door control and so on.
The choice of means of transmission depends on factors like distance, obstacles, environment in-house design, and cost, besides the option of mobility in the future must be considered. In addition, nearly all modes of transmission suffer from various forms of interference or loss to be minimized by good design.
However, for simple home surveillance purposes, the practical choice is probably limited to either coaxial Cable or Wireless-camera (radio) transmission. Finally, we recommendations wireless solution for a simple surveillance case, and to wired solutions for all the other cases.
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