The Fundamentals Of Television Essays - Television Technology

The Fundamentals of Television In this report on television I will discuss television signals, the components the make up a television, and how a television produces the picture and sound for the final output. The sound carrier is at the upper end of the spectrum. Frequency modulation is used to impress the sound on the carrier. The maximum frequency deviation is twenty-five kilohertz, considerably less than the deviation permitted by confessional FM stereo. As a result, a TV sound signal occupies less bandwidth in the spectrum than a standard FM broadcast station. Stereo sound is available in TV, and the multiplexing method used to transmit two channels of sound information is virtually identical to that used in stereo transmission for FM broadcasting. The picture information is transmitted on a separate carrier located 4.5 MHz lower in frequency than the sound carrier. The video signal derived from a camera is used to amplitude modulate the picture carrier. Different methods of modulation are used for both sound and picture information so that there is less interference between the picture and sound signals. The full upper sidebands of the picture information are transmitted, but only a portion of the lower sidebands is suppressed to conserve spectrum space. The color information in a picture is transmitted by way of frequency division multiplexing techniques. Two color signals derived from the camera are used to modulate a subcarrier that, in turn, modulates the picture carrier along with the main voice information. The color subcarriers use double-sideband-suppressed carrier AM. The video signal can contain frequency components up to 4.2 MHz. Therefore, if both sidebands were transmitted simultaneously, the picture signal would occupy 8.4 MHz. The vestigal sideband transmission reduces this excessive bandwidth. Because a TV signal occupies so much bandwidth, 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 for white. The scene is divided into 15 scan lines numbered 0 through 14. The scene is focused on the light-sensitive area of a vidicon tube or CCD imaging device that scans the scene one line at time, transmitting the light variations along the lines as voltage levels. Where the white background is being scanned a 0 volt signal occurs. When a black picture element is encountered a 1 volt level is transmitted. The electrical