The word "television" is made up of two parts-"tele," from the Greek word meaning far, and "vision," meaning to see. Thus, television means seeing from a distance. As used today, the word means the transmission and reception of moving visible images so that at a distant receiver the likeness of the original scene can be viewed.
GENERAL
There is little difference between a television receiver and a common radio receiver. In the radio system, the sound is first converted to an electrical signal, which is transmitted to a distant point. The signal is received and then converted by a speaker into mechanical motion, producing audible sound waves. In the television system, light and sound signals are converted and transmitted to a distant point. Both signals are received and converted back to light and sound by the television set.
Television and radio signals are transmitted through space by means of electromagnetic waves (Fig. 1-1), more commonly known as RF (radio frequency). In radio, we are concerned with the transmission of only one signal, that of music, speech, etc.
Television is far more complicated. To display a picture and reproduce sound, several signals must be transmitted simultaneously. Each of these signals will be discussed later in this Section.
A simplified block diagram of a complete television system is given in Fig. 1-2. The image is picked up by the camera, and an electrical signal corresponding to the image is developed. The signal is then properly amplified, modulated, and transmitted. At the receiver, the signal is demodulated, amplified, and synchronized so that a reproduction of the original image is displayed on the picture tube. For simplicity, the sound circuits are omitted in Fig. 1-2.
Fig. 1-1. Television signals traveling through space as electromagnetic waves
(RF).
Since no method by which the entire picture can be transmit ted simultaneously is feasible with our present knowledge, a different system of transmitting the picture must be employed. This system is known as scanning. In the camera, the scene is focused on a light-sensitive area by an optical lens system. The optical image forms an electric charge corresponding to the light image.
This charge pattern is scanned by an electron beam, which moves across from side to side and from top to bottom at a rate set by the sync generator (Fig. 1-2). This scanning converts the pattern into an electrical current with an instantaneous value corresponding to the amount of light falling on the area being scanned.
Fig. 1-2. Simplified diagram of a television system.
The electron beam is caused to move across the charge pattern in an approximate horizontal line at a uniform speed and then fly back and scan another line until the beam has scanned 525 lines in the desired sequence. This complete scanning is repeated at the rate of 30 frames/sec. When the electron beam falls upon an illuminated portion of the mosaic, current will flow through the output circuit of the camera tube; when the beam falls upon a partially illuminated portion, a smaller current will flow; and when the beam falls upon a dark portion, very little current will flow. This process is depicted in Fig. 1-3. In this manner, current pulses will be generated that will correspond in time sequence to the light and dark areas of the televised image as they are scanned by the electron beam.
(A) Pattern to be scanned
(B) Scanning sequence
(C) Sequential signal voltage.
Fig. 1-3. Development of signal voltages by the scanning process.
PERSISTENCE OF VISION
The human eye is a complex system that converts light energy to electrical nerve impulses. The eye has one shortcoming: It can be deceived. The eye will retain images for a brief period of time. A motion picture uses this phenomenon, known as persistence of vision. Images shown at a rate of greater than 16 images or frames/sec will not allow the eye to detect any change and appear to be a continuous "moving picture." To allow for a safety margin in movies, the scene changes 24 times/sec. At this rate there is no "flicker" common in movies of the early 1900s. For television, the rate is 30 frames/sec.
Figure 1-4 depicts the projection of a moving-picture film. The transparency is enlarged and focused on a screen by a system of lenses. A rotating shutter is used to project each frame twice, thereby reducing the flicker. The shutter also cuts off the light while one frame is quickly pulled down and the next frame placed in the proper position for projection. Another lamp-and-lens system focuses a beam of light through the optical sound track and onto a photocell. The photocell converts the light energy to an electrical signal, which is amplified and fed to a speaker. The method used for television is quite similar.
When the television scanning process takes place at a sufficiently high rate, the eye is deceived into "seeing" the entire picture at once even though a small dot is all that is being produced at any one instant. When the beam strikes the face of the picture tube, the face continues to glow for a short period of time. Also, because of the persistence of vision, the eye will retain this image for a short period of time.
Fig. 1-4. Projection of a moving-picture film.
INTERLACE SCANNING
The method used for scanning the image in television is very similar to that by which you are reading this page. The eye begins at the upper-left-hand corner and travels across the second line of words. This process is repeated until the end of the bottom line of the page is reached, when the eye returns to the top of the next page.
Fig. 1-5. Interlaced scanning pattern on a raster.
In a television system, this scanning is done by electron beam instead of the eye. The resulting voltage pulses, termed video signals, are then amplified and combined with artificially manufactured signals for controlling the timing of the receiver picture tube deflection circuits and for extinguishing (blanking) the electron beam during the return time. The resulting composite signal is then used to modulate a high-frequency transmitter.
In the standard interlaced scanning system, scanning of the horizontal lines is not performed in sequence. Instead, the odd numbered lines are scanned first, that is, 1, 3, 5, etc., and then the beam returns to the top and scans the even-numbered lines. Starting at the upper-left extremity of the picture, as in Fig. 1-5, line 1 is scanned. Instead of proceeding then with line 2, the scanning beam drops two spaces and line 2 is omitted. Line 3 is then scanned, followed by lines 5, 7, 9, and every odd-numbered line of the picture. Upon reaching the bottom of the picture, the scanning spot moves again to the top of the picture and begins another scanning field, which is displaced from the first by the width of one line, so that now lines 2, 4, 6, 8, and all even numbered lines are scanned.
Since each field is completed in 1/60 sec, both fields consume 1/30 sec. A raster consists of multiple frames, with each frame made up of two fields. Thirty complete frames are scanned in 1 sec, each having been broken up into two projections as a means of flicker reduction.
Given a line frequency of 15,750 and a picture repetition rate of 30 Hz, the number of lines per picture is 15,750/30, or 525 lines, the number of lines per field being 262 1/2 , as shown in Fig. 1-5.
Notice that the odd field ends on a half-line, and that the even field starts on a half line.
Approximately 490 of these 525 lines in each frame are active, the remainder occurring during the vertical retrace period (the beam is moving from the bottom to the top of the screen) when the viewing tube is blanked out. It is necessary, of course, that complete synchronism be maintained between scanning at the transmitter and at the receiver. To achieve this, synchronizing pulses are transmitted along with the video signal to lock the receiver oscillators, both vertical and horizontal, into step with those at the transmitter.
Thus, in addition to the video signal, which contains the actual picture, four other signals must be transmitted before the picture can be properly displayed on the screen. The horizontal sync pulse is transmitted to keep the horizontal deflection circuit in the receiver in step with the left-to-right movement of the scanning beam in the camera. Since the return of the beam from the right to the left of the screen (retrace) would produce objectionable interference if displayed on the screen, a horizontal blanking pulse is transmitted to black out this portion of the scan. Similarly, a vertical sync pulse and a vertical blanking pulse are also transmitted to keep the vertical deflection in step and to blank the beam when it is returning from bottom to top. Figure 1-6 shows the plan of the sync and sound sections in a TV receiver.
FREQUENCY BANDS
Fig. 1-6. TV picture and sound section.
The entire spectrum of radio frequencies is broken down into several portions called bands. These bands and the designations used to identify them are given in Table 1-1.
The assignment of the various types of services (radio, television, radar, amateur, etc.) within the various bands has been partially standardized by international agreements. Thus, world wide communication is possible, and interference caused by one country operating one type of service and another country operating an entirely different service on the same frequency is eliminated. The actual establishment of operating standards, assignment of portions of the frequency bands listed previously for certain specific purposed, and assignment of individual frequencies in the United States are controlled by the FCC (Federal Communications Commission).
Table 1-1. Frequency Bands and Designations Band No. Frequency Classification
Abbreviation
Fig. 1-7. Television-channel- frequency assignments.
The standard amplitude-modulated (AM) broadcast stations in the United States are located between 535 and 1605 kHz (kilo hertz) in a portion of the medium-frequency band. Frequency modulated (FM) radio broadcasting is assigned the space between 88 and 108 MHz (megahertz) in the VHF (very-high frequency) band. The frequencies assigned for television broad casting are located in a portion of the VHF band and a portion of the UHF (ultra-high-frequency) band. Originally 13 VHF channels, each 6 MHz wide, were allocated for television. Later channel 1 was deleted. In 1952, 70 additional channels were allocated between 470 and 890 MHz in the UHF band. The frequency assignments for all 82 channels are given in Fig. 1-7.
The VHF channels are divided in two bands. Channels 2 to 6, called the low-band VHF channels, are located between 54 and 88 MHz. (There is a 4-MHz break between channels 4 and 5 for other services.) Channels 7 to 13, called the high-band VHF channels, are located between 174 and 216 MHz. (The frequencies between 88 and 174 MHz are reserved for other services.) Special letter designations have been given to frequencies below channel 2 and between channels 6 and 7, and between channels 13 and 14. These letter channels are used by CATV companies to increase their channel offerings to the public.
From the foregoing, it will be observed that a single television channel is 6 MHz wide, in contrast to the entire AM broadcast band, which is only 1 MHz wide. Also, each AM broadcast channel is only 10 to 20 kHz wide. The wide television channel is necessary to transmit both audio and video information with clarity and sharpness. It also illustrates that video information must be transmitted on very high frequencies to obtain a satisfactory ratio of carrier frequency to bandwidth.
STANDARD TELEVISION CHANNEL
A standard television channel is shown in Fig. 1-8. As previously noted, the bandwidth of each television channel is 6 MHz, and both the video and sound signal must be transmitted within this limit. The AM picture signal is always at the low frequency end of each channel allocation and occupies approximately 5 MHz of the total 6-MHz bandwidth. The FM sound signal is always at the high-frequency end of the 6-MHz channel and has a maximum deviation of 25 kHz (a total carrier swing of 50 kHz). The actual frequencies of the picture and sound carriers are included in Fig. 1-7.
Fig. 1-8. Signal distribution is a standard television channel.
An unusual feature of the AM picture signal is that the high frequency sideband is approximately 4 MHz wide whereas the low-frequency sideband is only 1.25 MHz wide. This un-symmetrical distribution permits transmission of a better definition picture within the 6-MHz bandwidth. This transmission of one side band and a portion of the other sideband of the picture signal, as effected in television practice, is termed vestigial sideband transmission.
TELEVISION-SIGNAL COMPONENTS
The combination of video, blanking, and sync signals is called the composite video signal.
Video Signal The video (picture) signal is arbitrarily represented as a jagged line in Fig. 1-9. (Note the horizontal scale of this portion of the signal is greatly compressed in Fig. 1-9; if drawn to scale, it would be many times wider than the blanking signal.) You will recall that the amplitude of the video signal varies in accordance with the degree of brightness at any particular instant. Figure 1-10 shows a typical example. Here, the variations in the tones along line X-X in the picture of Fig. 1-10 will produce the video modulating voltage of Fig. 1-11.
Fig. 1-9. Composite video signal.
Fig. 1-10. Television subject.
Fig. 1-11. Variations in video voltage of a scanning line.
Notice in Fig. 1-11 that any darker portion of the picture produces an increase in the amplitude of the signal. This type of modulation is called negative polarity of transmission. Any black area on the screen is represented by an absence of light. In other words, the beam is cut off. This type of transmission is standard in the United States.
Blanking Signal
Two blanking signals are transmitted, one at the end of each line, and the other at the end of each field. Notice in Fig. 1-9 that the amplitude of the blanking signals is greater than any portion of the video signal. Hence, the beam is cut off during the blanking pulses so that the retrace of the scanning beams cannot be viewed on the screen.
Sync Signal
Sitting atop the horizontal and vertical blanking signals is a rectangular pulse, called the sync pulse, which is employed to keep the deflection circuits in the receiver in step with those at the transmitter. Since the amplitude of the blanking signal is sufficient to cut the beam of the picture tube off, the sync pulse is said to be in the "blacker than black" region.
If one long unbroken pulse were transmitted for vertical synchronization, the horizontal sync pulsed during this time would be absent. During that time the horizontal deflection circuits in the receivers would lack synchronization and drop out of step. In order that horizontal synchronization be maintained during their vertical retrace period, the vertical sync pulse is broken by serrations. These serrations then maintain horizontal synchronization during the vertical retrace period.
Equalizing Pulse
A fourth group of signals, termed equalizing pulses, is also included in Fig. 1-9. These pulses are transmitted to ensure uniform spacing of the interlaced scanning lines and prevent loss of synchronism of the horizontal circuits during the retrace intervals between fields. In Fig. 1-9 all pulse shapes, their relative amplitudes, and their durations are standardized. The only variable is the picture signal, which varies from line to line as the subject is scanned.
SUMMARY
The television picture is broken up into individual elements instead of the entire picture being transmitted simultaneously. A process called interlaced scanning is employed to break up the picture for transmission. In this system, 525 horizontal lines are scanned across the picture. All odd-numbered lines are scanned first; then the beam returns to the top of the picture, and the even numbered lines are scanned. Thus, two fields of 262 1/2 lines each are interlaced to form the complete picture or frame (525 lines). The horizontal lines are scanned at a rate of 15,750/sec. Each field of 262 1/2 lines is repeated 60 times/sec, and 30 complete pictures are transmitted per second. We see the complete picture without flicker for two reasons: (1) The picture tube continues to glow for a period of time after being struck by the beam; and (2) the persistence of vision of the human eye makes it sensitive only to changes that occur at a rate of 1/46 sec or slower.
In addition to the picture or video signals, blanking signals to black out the beam during retrace when the beam returns from the right to the left side of the screen and from the bottom to the top of the screen, are also transmitted. Sync signals, which keep the receiver deflection circuits in step with the transmitter, are situated on top of the blanking signal.
Negative modulation is employed for the picture signal; that is, an increase in the light intensity of the televised scene causes a decrease in the radiated power. Conversely, a decrease in the light intensity causes an increase in the radiated power. The primary reason for adopting negative transmission is that any noise pulse present with the signal will usually cause an increase in signal strength. With negative transmission, noise will be produced as a black spot, whereas with positive transmission, it would appear as a bright flash of light. The black spots are far less annoying. Also, blanking and sync pulses must be transmitted as black; therefore, with negative transmission, they will appear at the maximum amplitude, ensuring proper synchronization even at low signal strength.
The entire composite video signal, including the picture, sync, and blanking signals, plus the sound signal, is transmitted in a channel 6 MHz wide. To obtain maximum bandwidth, a system termed vestigial sideband transmission is employed. In the Uni ted States, 82 channels (12 VHF and 70 UHF) are allocated for television broadcasting.
QUIZ
1. Radio and television signals are transmitted through space by what means?
2. In radio, how many signals are being transmitted? In television?
3. How many horizontal scanning lines are produced per second?
4. Explain the process used in scanning horizontal lines.
5. How many channels are assigned to television broadcasting?
6. What is the bandwidth of each television channel? Why?
7. How many fields exist in one complete picture or frame?
8. How many lines are in each field?
9. What channels are in the low-band VHF, high-band VHF, and UHF?
10. What range of frequencies is used in the low-band VHF, high-band VHF, and UHF?
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