The quality and quantity of television service are limited fundamentally by the rate at which it is feasible to transmit the picture” information over the television channel In modem practice the televised image must be capable of being dissected within a few hundredths of a second, into more than I(.)(’.()<)() picture elements.
History of Picture Transmission and Reception
This implies that the electrical impulses corresponding to the pcture elements must ‘pass through the channel a: a rate as high as several million per second.
Moreover, since the picture content may vary, from frame to frame, from simple close-up shots having little fine detail to comprehensive distant scenes in which the limiting detail of the system comes into play, the actual rate of transmitting the picture information .
varies from time to time, from a few impulses per second to several million per second The television channel must be capable, therefore, of handling information over a continuous band of frequencies several million cycles wide wide. In the United States, the television channel occupies a width of six megacycles (6,000,000 hertz, or cycles per second) in the radio spectrum.
Commonly known as community antenna television (CATV) these cable systems use a “community antenna” to “receive broadcast signals (often from communications satellites), which they then retransmit via cables to homes and establishments in the iocal area subscribing to the service.
Subscribers pay a specified monthly service charge in addition to an initial installation fee Since the mid-1970s there has been a proliferation of cable- television systems offering special services Besides bringing high-quality signals to subscribers, the systems provide additional television channels Some of these systems can deliver 50 or more channels because they distribute signals occurring within the normal television broadcast band as well as nonbroadcast frequencies.
A frequency-conversion device is connected to the television set of the subscriber to accommodate these signals of nonbroadcast frequencies. The increased number of channels allows expanded programming, including broadcasts front distant cities, continuous weather and stock-market reports, programs produced by community groups and educational institutions, and access to pay-TV program materials such as recent motion pictures and sports events not telecast by other broadcasters Another feature, offered by more and more cable operators is two-way channel capability.
Great Britain and West Germany. Two-way cable-television systems increasingly allow subscribers with home computers to link up with computer networks, giving the subscribers access to data banks and permitting them to interact with other online users Cable operators have also experimented with video compression, digital transmission, and high-definition television (IIDIA ) In the United States,, government deregulation of the cable-television industry in the 1990s allowed cable companies to experiment with telephony and allowed telephone companies to distribute cabletelevision programming.
This is 600 times as wide as the channel used by each standard sound broadcasting station In fact, each television station uses nearly six times as much spectrum space as all the commercial amplitude-modulation (AVI) sound broadcasting channels combined Since each television station must occupy so much spectrum space, few channels available in a given locality Moreover, the quantity of service is in conflict with the quality of reproduction.
If the detail of the television image is to be increased, other parameters of the transmission being unchanged, th.e channel width must be increased proportionately. and this decreases the number of channels that can be accommodated in the spectrum.
This fundamental conflict between quality of transmission and number of available channels dictates that the quality of reproduction shall just satisfy the typical viewer under normal viewing conditions Anv excess Of performance beyond this ultimately would result in a restriction of program choice.
The first requirement to be met in image analysis is that the reproduced picture shall not FLICKER, since flicker induces severe visual fatigue. Flicker becomes more evident as the brightness of the picture increases If flicker is to be unobjectionable at brightness suitable for home viewing during daylight as well as evening hours (25 to 100 footlamberts), the successive illuminations of the picture screen should occur no fewer than 50 times per second. This is approximately twice the rate of picture repetition needed for smooth reproduction of motion.
To avoid flicker, therefore, twice as much channel space is needed as would suffice to depict motion. The same disparity occurs in motion-picture practice, in which satisfactory performance with respect to flickei requires twice as much film as is necessary for smooth simulation of motion.
A way around this difficult has been found, in motion pictures as well as in television, by projecting each picture twice In motion pictures, the projector interposes a shutter briefly between film and lens while a single frame of the film is being projected In television, each image is analyzed and synthesized in two sets of spaced lines, one of which fits successively within the spaces of the other.
Thus the picture area is illuminated twice during each complete picture transmission, although each line in the image is present only once during.that time.
This technique is feasible because the eye is comparatively insensitive to flicker when the variation of light is confined to a small part of the field of view Hence flicker of the individual lines is not evident If the eye did not have this fortunate property, a television channel would have to occupy about twice as much spectrum space as it now does.
It is thus possible to avoid flicker and simulate rapid motion by a picture rate of about 25 per second, with two screen illuminations per picture. The precise value of the picturerepetition rate used in a given region has been chosen by reference to the electric power frequency that predominates in that region In Europe, where 50-hertz power is the rule,- the television picture rate is 25 per second (50 screen illuminations per second).
In. North America the picture rate is 30 per second (60 screen illuminations per second) to match the 60-hertz power that predominates there The higher picture-transmission rate of North America allows the pictures there to be about five times as bright as those in Europe for the same susceptibility to flicker, but this advantage is offset by a 20 percent reduction in picture detail for equal utilization of the channel.