Television

14 Pages 3543 Words

andwidth, it must be transmitted in a very high frequency portion of the spectrum. TV signals are!
assigned to frequencies in the VHF and UHF range. United States TV stations use the frequencies between 54 and 806 MHz. This portion of the spectrum is divided into sixty-eight 6MHz channels that are assigned frequencies. Channels 2 through 7 occupy the frequency range from 54 to 88 MHz. Additional TV channels occupy the space between 470 and 806 MHz. The video signal is most often generated by a TV camera, a very sophisticated electronic device that incorporates lenses and light-sensitive tranducers to convert the scene or object to be viewed into an electrical signal that can be used to modulate a carrier. To do this, the scene to be transmitted is collected and focused by a lens upon a light-sensitive imaging device. Both vacume tube and semiconductor devices are used for converting the light information in the scene into an electrical signal. The scene is divided into smaller segments that can be transmitted serially over a period of time. It is the job of the camera to subdivide the scene in an orderly manner so that an acceptable signal is developed. This process is called scanning. Scanning is a technique that divides a rectangular scene up into individual lines. The standard TV scene dimensions have an aspect ratio of 4:3; that is, the scene width is four units for every 3 units of height. To create a picture, the scene is subdivided into many fine horizontal lines called scan lines. Each line represents a very narrow portion of light variations in the scene. The greater the number of scan lines, the higher the resolution and the greater the detail that can be observed. United States TV standards call for the scene to be divided into a maximum of 525 horizontal lines. The task of the TV camera is to convert the scene into an electrical signal. The camera accomplishes this by transmitting a voltage of 1 volt for black and 0 volts ...