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Телевизионный сигнал - Television signal

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Телевизионный сигнал
Television signal

 

Television signal

There is  a need to be familiar with the television signal, so that routing procedures and monitoring of the signal can be handled with ease. The best way to approach an understanding of the television signal is first to look at the needs of a monochrome system and then develop this to look at the colour system, i.e. in the way in which the systems have evolved.

Television is the process of transmitting moving pictures. In practice a series of still pictures is transmitted, but at a rapid enough rate to see the change from one picture to the next picture as 'continuous' movement. The persistence of vision (a form of image retention) of the eye/brain means that this is possible.

This has a parallel in the film world with film shot at 24 pictures/second. Unfortunately, although a picture rate of 24 Hz is sufficient for conveying movement without 'jerky' transitions, the perceived picture suffers from flicker. To overcome this the picture rate needs to be above about 42 Hz. In the film world it would be expensive to shoot at 48 Hz - one would need twice as much film! The solution is a simple one: in the cinema the film projector simply displays each picture or frame twice, resulting in an effective picture rate of 48 Hz. In television the expense of a high picture rate is also prohibitive (see later note on bandwidth) - basically the more information to be transmitted in a second, the more expensive it becomes!

Aspect ratio is the ratio of picture width to picture height, the current standard of 4:3 dating back to the 1930s when early high-definition systems were developed. 4:3 was chosen as a compromise on film formats and the availability of suitable display tubes. The trend is towards 16:9 widescreen, with 14:9 as an interim measure.

The number of lines/picture is an important parameter: too few and the line structure becomes visible; too many and information is transmitted which the viewer will not see! The limit of the acuity of the eye (ability to see fine detail) occurs when the structure of one television line subtends an angle of 1 second.

To overcome 'flicker' effects in television an 'effective' picture rate linked to mains frequency was considered desirable, i.e. 60 Hz in the USA, 50 Hz in Europe. To transmit fine detail at these rates is too expensive, so a system of interlace scanning was adopted to halve the picture rate while still maintaining an effective flicker rate of 60 Hz and 50 Hz respectively.

The basic principle of interlace scanning is each picture is divided into a number of lines, but having initially numbered the lines in sequence they are scanned as two separate fields, one field scanning the odd-numbered lines and the second field made up of the even-numbered lines.

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