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Transmission of Video Signals

Time:2017-03-31 Source:youtepu


This is not meant to be a text book on transmission but is intended to remove some of the mystery associated with various methods of transmission. Many approximations and simplifications have been used in writing this guide. This is to make the subject more understandable to those people not familiar with the theories. For general application in the design of CCTV systems it should be more than adequate and at least point the way to the main questions that must be addressed. The manufacturers of transmission equipment will usually be only too keen to help in final design.


This first part deals with the transmission of video signals by cables. Part 2 deals with the transmission of video signals by other methods such as microwave, telephone systems, etc.


Diagram 1 illustrates the many methods of getting a picture from a camera to a monitor. The choice will often be dictated by circumstances on the location of cameras and controls. Often there will be more than one option for types of transmission. In these cases there will possibly be trade offs between quality and security of signal against cost.


General Principles


A field of video is created by the CCD being scanned across and down exactly 312 1/2 times and this reproduced on the monitor. A second scan of 312 1/2 lines is exactly 1/2 a line down and interlaced with the first scan to form a picture with 625 lines. This is known as a 2:1 interlaced picture. The combined 625 line is known as a frame of video and made up from two interlaced fields. The total voltage produced is one volt from the bottom of the sync pulse to the top of the white level, hence one volt peak to peak(p/p). The luminance (brightness) element of the signal is from 0.3 volts to one volt, therefore is 0.7 volts maximum. This is known as a composite video signal because the synchronising and video information are combined into a single signal.


In the case of a colour signal, further information has to be provided. The colour information is superimposed onto the video signal by means of a colour sub-carrier. A short reference signal, known as the chroma burst, is added to the back porch after the horizontal sync pulse to detect the difference in position or phase.


The transmission system must be capable of reproducing this signal accurately at the receiving end with no loss of information.


Note that the imaging device is scanned 625 times but the actual resolution is defined by the number of pixels making up the device.


Synchronising

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: Synchronising pulses that tell the monitor when to start a line and a frame; video information that tells the monitor how bright a particular point in the picture should be; chrominance that tells the monitor what colours a particular part of the picture should be (colour cameras only).


Bandwidth

The composite video output from the average CCTV camera covers a bandwidth ranging from 5Hz to many MHz. The upper frequency is primarily determined by the resolution of the camera and whether it is monochrome or colour. For every 100 lines of resolution, a bandwidth of 1MHz approximately is required. Therefore, a camera with 600 lines resolution gives out a video signal with a bandwidth of approximately 6MHz. This principle applies to both colour and monochrome cameras. However colour cameras also have to produce a colour signal (chrominance), as well as a monochrome output (luminance). The chrominance signal is modulated on a 4.43MHz carrier wave in the PAL system therefore a colour signal, regardless of definition, has a bandwidth of at least 5MHz.



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