Fundamentals of Video

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Introduction

Video signals are the signals used to send closed circuit television pictures from one place to another. Television (TV) is literally, tele-vision, a means of viewing one place from somewhere else. The word video comes from the Latin verb Videre, to see.

A television picture is made up from a number of horizontal lines on the television screen, which are laid down, or scanned, from the top to the bottom of the television screen. There are now only two standards for TV pictures in general use, 525 lines in the USA (EIA) and Japan and 625 lines elsewhere (CCIR). The descriptions that follow are based on the 625-line system. The number of lines describes how each still picture is created, but a television picture is made up from a number of still pictures displayed every second.

There is a characteristic of the human eye known as ‘persistence of vision.’ The eye retains an impression of an image for a fraction of a second after it has disappeared. If a series of still images is presented at a rate of about 14 per second an impression of continuous movement will be perceived. This, however, would give rise to a very distracting flicker. If the rate were increased to 24 images per second, the flicker would be almost unnoticeable. Increasing this to 50 images per second would eliminate noticeable flicker.

To transmit 50 complete images per second would be needlessly complex and expensive to produce. The solution is to adopt what is known as interlaced scanning. Instead of scanning the full 625 lines 50 times a second, the scanning speed is effectively doubled and so is the vertical spacing of the lines. Therefore, one scan produces 312 1/2 lines from the top to the bottom of the picture. This is known as one field. The next scan is arranged to start at a precise position exactly between the lines of the first scan, so that the lines of the second field interlace, like fingers, between the lines of the first field. In this way, a complete frame of video is created made up from two fields.

On a TV screen, the phosphor on the screen continues to glow from the first scan while the second scan is being displayed. In this way, although only 25 complete pictures (frames) are presented per second the screen is scanned 50 times (fields) per second. The result is to achieve a flicker rate of 50 Hz (cycles per second) while only using a bandwidth for 25 frames per second. Some broadcast televisions now use a technique called “100Hz technology” to further reduce the flicker on the TV screen. However, this technique is not generally used in CCTV monitors due to the extra cost involved.

Interlaced.png
Diagram 2.1 Interlaced Fields

The relationship between the length of the horizontal lines and the height of the picture is always the same and is known as the aspect ratio. It is given by the following ratio.

Aspectra.gif

Monochrome Video Signal Components

The signal used to carry the scanning pictures from one place to another is called the video signal. A voltage is generated proportional to the brightness of the image at any point on a horizontal line. For the brightest parts, corresponding to a white area, a level of one volt is produced; this is the white level.

For the darkest parts corresponding to a black image, a voltage of approximately 0.3 volts is produced; this is the black level. Between these levels, the camera will produce a voltage proportional to the shade of grey of the image.

However, the brightness signal is not the only part of the video signal normally produced by a camera. Some method is required of synchronising the monitor on which the camera picture is being displayed to the field and line scanning process.

This is to enable it to re-create the picture that the camera is viewing. The method used to achieve this is to add pulses for the start of each field and the start of each line. The synchronising, or sync, pulses for the start of each field are called Vertical Sync Pulses. These vertical sync pulses reduce the voltage from the black level down to zero voltsand take up a time space equivalent to 25 horizontal lins, i.e. 1.6 milliseconds. The sync pulses for the start of each line are called Horizontal Sync Pulses. The horizontal sync pulses are also from the black level down to zero volts and are 4.7 microseconds in long.

The type of video signal that contains both video and synchronising information is known as composite video.

Composite.png
Diagram 2.2 composite video signal

The relationship in level between the video signal and sync pulses is normally given by the following formula:

Vidsync.gif

The complete horizontal line lasts 64 microseconds. There is a short period between the end of the video signal for a line and the leading edge of the next horizontal sync pulse. This is known as the front porch.

There is also a short period between the trailing edge of the horizontal sync pulse and the start of the video signal of the next line. This is known as the back porch. Considering the times for the horizontal sync and the front and back porches, the actual length of the video signal in a horizontal line is 52 microseconds. In practice only 47 to 50 microseconds is visible due to over-scanning at the monitor.

There is not just one sync pulse. The nominally 625 line system uses 25 lines for field blanking, therefore 50 lines in one frame. This leaves 575 lines for picture information. The 25 lines are used as follows:


This article is an extract from chapter 2 of The 'The Principles & Practice of CCTV' which is generally accepted as the benchmark for CCTV installation in the UK.