Television Fundementals

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 signals
derived from each scan line are refereed to as the video signal. They are
transmitted serially one after the other until the entire scene has been sent.

Since the scene contains colors, there are different levels of light along each
scan line. This information is transmitted as different shades of gray between
black and white. Shades of gray are represented by some voltage level between 0-
and 1-V