A
television system involves equipment located at the source of production,
equipment located in the home of the viewer, and equipment used to convey the
television signal from the producer to the viewer. The purpose of all of this
equipment, as stated in the introduction to this article, is to extend the
human senses of vision and hearing beyond their natural limits of physical
distance. A television system must be designed, therefore, to embrace the
essential capabilities of these senses, particularly the sense of vision. The aspects of vision that must be
considered include the ability of the human eye to distinguish the brightness,
colours, details, sizes, shapes, and positions of objects in a scene before it.
Aspects of hearing include the ability of the ear to
distinguish the pitch, loudness, and distribution of sounds. In working to
satisfy these capabilities, television systems must strike appropriate compromises between the
quality of the desired image and the costs of reproducing it. They must also be
designed to override, within reasonable limits, the effects of interference and
to minimize visual and audial distortions in the transmission and reproduction
processes. The particular compromises chosen for a given television
service—e.g., broadcast or cable service—are embodied in the television
standards adopted and enforced by the responsible government agencies in each
country. Television technology must deal with the fact that human vision
employs hundreds of thousands of separate electrical circuits, located in the optic nerve running from the retina to the brain, in order to convey simultaneously in two
dimensions the whole content of a scene on which the eye is focused. In
electrical communication, however, it is feasible to employ only one circuit (i.e.,
the broadcast channel) to connect a transmitter with a receiver. This
fundamental disparity is overcome in television practice by a process known as
image analysis, whereby the scene to be televised is broken up by the camera’s
image sensors into an orderly sequence of electrical waves and these waves are
sent over the single channel, one after the other. At the receiver the waves
are translated back into a corresponding sequence of lights and shadows, and
these are reassembled in their correct positions on the viewing screen. This
sequential reproduction of visual images is feasible only because the visual
sense displays persistence; that is, the brain retains the
impression of illumination for about one-tenth of a second after the source of
light is removed from the eye. If, therefore, the process of image synthesis
takes less than one-tenth of a second, the eye will be unaware that the picture
is being reassembled piecemeal, and it will appear as if the whole surface of
the viewing screen is continuously illuminated. By the same token, it will then be
possible to re-create more than 10 pictures per second and to simulate thereby
the motion of the scene so that it appears to be continuous. In practice, to
depict rapid motion smoothly it is customary to transmit from 25 to 30 complete
pictures per second. To provide detail sufficient to accommodate a wide range
of subject matter, each picture is analyzed into 200,000 or more picture elements, or pixels. This analysis
implies that the rate at which these details are transmitted over the
television system exceeds 2,000,000 per second. To provide a system suitable
for public use and also capable of such speed has required the full resources
of modern electronic technology.
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, 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 flicker requires twice as much film as is necessary for smooth simulation of motion. A way
around this difficulty 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 picture-repetition 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 alternating current 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 alternating current 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.
Resolution
The
second aspect of performance to be met in a television system is the detailed
structure of the image. A printed engraving may possess several million
halftone dots per square foot of area. However, engraving reproductions are
intended for minute inspection, and so the dot structure must not be apparent
to the unaided eye even at close range. Such fine detail would be a costly
waste in television, since the television picture is viewed at comparatively
long range. Standard-definition television (SDTV) is
designed on the assumption that viewers in the typical home setting are located
at a distance equal to six or seven times the height of the picture screen—on
average some 3 metres (10 feet) away. Even high-definition
television (HDTV) assumes a viewer who is seated no closer than three
times the picture height away. Under these conditions, a picture structure of
about 200,000 picture elements for SDTV (approximately 800,000 for HDTV) is a
suitable compromise. The physiological basis of this compromise lies in the
fact that the normal eye, under conditions typical of television viewing, can
resolve pictorial details if the angle that these details subtend at the eye is
not less than two minutes of arc. This implies that the SDTV structure of
200,000 elements in a picture 16 cm (0.5 foot) high can just be resolved at a
distance of about 3 metres (10 feet), and the HDTV structure can be resolved at
about 1 metre (3 feet). The structure of both pictures may be objectionably
evident at short range—e.g., while tuning the receiver—but it would be
inappropriate to require a system to assume the heavy costs of transmitting
detail that would be used by only a small part of the audience for a small part
of the viewing time.
Picture shape
The
third item to be selected in image analysis is the shape of the picture. For
SDTV, as is shown in the figure, the universal picture is a rectangle that is
one-third wider than it is high. This 4:3 ratio (or aspect ratio) was
originally chosen in the 1950s to match the dimensions of standard 35-mm
motion-picture film (prior to the advent of wide-screen cinema) in the interest
of televising film without waste of frame area. HDTV sets, introduced in the
1980s, accommodate wide-screen pictures by offering an aspect ratio of 16:9.
Regardless of the aspect ratio, in both SDTV and HDTV the width of the screen
rectangle is greater than its height in order to incorporate the horizontal
motion that predominates in virtually all televised events.
The fourth determination in image analysis is the path over
which the image structure is explored at the camera and reconstituted on the
receiver screen. In standard television, the pattern is a series of parallel
straight lines, each progressing from left to right, the lines following in
sequence from top to bottom of the picture frame. The exploration of the image structure proceeds at a
constant speed along each line, since this provides uniform loading of the transmission channel under
the demands of a given structural detail, no matter where in the frame the
detail lies. The line-by-line, left-to-right, top-to-bottom dissection and
reconstitution of television images is known as scanning, from its similarity
to the progression of the line of vision in reading a page of printed matter.
The agent that disassembles the light values along each line is called the scanning spot, in reference to the focused beam of electrons
that scans the image in a camera tube and recreates the image in a picture
tube. Tubes are no longer employed in most video cameras (see the
section Television cameras and
displays), but
even in modern transistorized cameras the image is dissected into a series of
“spots,” and the path of dissection is called the scanning pattern, or raster.
Deflection
signals
The
scanning spot is made to follow the interlaced paths described above by being
subjected to two repetitive motions simultaneously (see the figure). One
is a horizontally directed back-and-forth motion in which the spot is moved at
constant speed from left to right and then returned as rapidly as possible,
while extinguished and inactive, from right to left. At the same time a
vertical motion is imparted to the spot, moving it at a comparatively slow rate
from the top to the bottom of the frame. This motion spreads out the more rapid
left-to-right scans, forming the first field of alternate lines and empty
spaces. When the bottom of the frame is reached, the spot moves vertically
upward as rapidly as possible, while extinguished and inactive. The next
top-to-bottom motion then spreads out the horizontal line scans so that they
fall in the empty spaces of the previously scanned field. Precise interlacing
of the successive field scans is facilitated if the total number of lines in the
frame is an odd number. All the numbers of lines used in standard television
were chosen for this reason.
The picture signal
The
translation of the televised scene into its electrical counterpart results in a
sequence of electrical waves known as the television picture signal. This is
represented graphically in the diagram as a wave form, in which the range of
electrical values (voltage or current) is plotted vertically and time is
plotted horizontally. The electrical values correspond to the brightness of the
image at each point on the scanning line, and time is essentially the position
on the line of the point in question. The television signal wave form is
actually a composite made up of three individual signals, as is shown in the
figure. The first is a continuous sequence of electrical values corresponding
to the brightness along each line. This signal contains what is known as the
luminance information. The luminance signal is interspersed with blanking
pulses, which correspond to the times during which the scanning spot is
inactivated and retraced from the end of one line to the beginning of the next,
as described above. Superimposed on the blanking pulses are additional short
pulses corresponding to the synchronization signals (also described above),
whose purpose is to cause the scanning spots at the transmitter and receiver to
retrace to the next line at precisely the same