Fundamentals Of Television Essay, Research Paper
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 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 extremes represented by white and black. The resulting signal is known as the brightness, or luminance and is usually designated by the letter Y.
Resolution in a video system is measured in terms of the number of lines defined within the bounds of the picture. For example, the horizontal resolution is given as the maximum number of alternating black and white vertical lines that can be distinguished. Assume closely spaced vertical black and white lines of the same width, when such lines are scanned they will they will be converted into a square wave. One cycle or period, of this wave is the time for 1 black and 1 white line.
The video signal described so far contains the video or luminance information, which is a black and white version of the scene. To add the color detail, this is done by dividing the light in each scan line into three separate signals, each representing one of the three basic colors, red, green or blue.
In the same way, light in any scene can be divided into its three basic color components by passing the light through red, green and blue filters. This is done in a color TV camera, which is really three cameras in one. The lens focuses the scene on three separate light-sensitive devices such as a videcon tube or a CCD imaging device by way of a series of mirrors and beam splitters. The red light in the scene passes through the red filter, the green passes through the green filter and the blue passes through the blue filter. The result is the generation of three simultaneous signals during the scanning process by the light-sensitive imaging devices.
The R, G and B signals also contain the basic brightness or luminance information. If the color signals are mixed in the correct proportion, the result is the standard B&W video or luminance Y signal. The Y signal is generated by scaling each color signal with a tapped voltage divider and adding the signals together. The Y signal is made up of 30 percent red, 59 percent green and 11 percent blue. The resulting Y signal is what a B&W TV set will see.
The color signals must also be transmitted along with the luminance information in the same bandwidth allotted to the TV signal. This is done by a frequency division multiplexing technique. Instead of all three color signals being transmitted they are combined into color signals referred to as the I and Q signals. I is made up of 60 percent red, 28 percent green and -32 percent blue. Q is made up of 21 percent red, -52 percent green and 31 percent blue. The I and Q signals are referred to as the chrominance signals. To transmit them they are phase-encoded. These I and Q signals are fed to balance modulators along with 3.58 MHz subcarrier signals that are 90 degrees out of phase. The output of each balanced modulator is a double-sideband supressed carrier AM signal. The resulting two signals are added to the Y signal to create the composite video signal. The combined signal modulates the picture carrier. The resulting signal is the NTSC composite video signal. This signal and its sidebands are within the 6MHz TV signal bandwidth.
The I and Q color signals are also called the R – Y and the B – Y signals as the combination of the three color signals produces the effect of subtracting Y from the R or B signals. The phase of these signals with respect to the original 3.58 MHz subcarrier signal determines the color to be seen. In many TV sets an extra phase shift of 57 degrees is inserted to ensure that maximum color detail is seen. There is still 57 degrees between the I and Q signals but their position is moved 57 degrees. The reason for this extra phase shift is that the eye is more sensitive to the color orange. If the I signal is adjusted to the orange phase position better detail will be seen. Because of the frequency of the subcarrier, the sidebands produced during amplitude modulation occur in clusters that are interleaved between the other sidebands produced by the video modulation.
The 3.58 MHz subcarrier is supressed by the balanced modulators and therefore is not transmitted. Only the filtered upper and lower sidebands of the color signals are transmitted. To demodulate these double-sideband AM signals, the carrier must be reinserted at the receiver. A 3.58 MHz oscillator in the receiver generates the subcarrier for the balanced modulator-demodulator circuits.
For the color signals to be accurately recovered, the subcarrier at the receiver must have a phase related to the subcarrier at the transmitter. To ensure the proper conditions at the receiver, a sample of the 3.58 MHz subcarrier signal developed at the transmitter is added to the composite video signal. This is done by gating 8 to 12 cycles of the 3.58 MHz subcarrier and adding it to the horizontal sync and blanking pulse. The receiver uses this signal to phase-synchronize the internally generated subcarrier before it is used in the demodulation process.
In a TV transmitter, the sweep and sync circuits that creates the scanning signals for the vidicons or CCDs as well as generate the sync pulses that are transmitted along with the video and color signals. The sync signals, luminance Y and the color signals are added to form the final video signal that is used to modulate the carrier. Low-level AM is used. The final AM signal is amplified by very high power linear amplifiers and sent to the antenna via a diplexer.
At the same time the voice or sound signals frequency modulate a carrier that is amplified by class C amplifiers and fed to the same antenna by way of the diplexer. The resulting VHF or UHF signal travels by line-of-sight propagation to the antenna and receiver.
The process involved in receiving a TV signal and recovering it to present the picture and sound outputs in a high-quality manner is complex. Over the course of the past 50 years since its invention, the TV set has evolved from a large vacume tube unit into a smaller and more reliable solid-state unit made with mostly ICs.
The signal from the antenna or the cable is connected to the tuner. The tuner is used to select which TV channel is to be viewed and to convert the picture and sound carriers plus their modulation to an intermediate frequency (IF). As in most superheterodyne receivers, the local oscillator frequency is set higher than the incoming signal by the IF value.
The local oscillators are phase-locked loop frequency synthesizers set to frequencies that will convert the TV signals to the IF. Tuning of the local oscillator is typically done digitally. The PLL synthesizer is tuned by setting the feedback frequency division ratio. In a TV set this is changed by a microprocessor which is part of the master controlled system. The interstage LC-resonant circuits in the tuner are controlled by varactor diodes. By varying the DC bias on the varactors, their capacitance is changed, thereby changing the resonant frequency of the tuned circuits. The bias control signals also come from the control microprocessor. Most TV sets are also tuned by IR remote control.
The standard TV receiver IFs are 41.25 MHz for the sound and 45.75 MHz for the picture. The synthesizer local oscillator is set to 113 MHz. The tuner produces an output that is the difference between the incoming signal and the local oscillator frequencies.
The IF signals are then sent to the video IF amplifiers. Selectivity is usually obtained with a surface acoustic wave filter. This fixed tuned filter is designed to provide the exact selectivity required to pass both of the IF signals with the correct response to match the vestigal sideband signal transmitted. A pattern of interdigital filters on the surface convert the IF signals into acoustic waves that travel across the filter surface. By controlling the shape, sizes and spacing of the interdigital filters, the response can be tailored to any application. Interdigital filters at the output convert the acoustic waves into electrical signals at the IF.
The IF signals are next amplified by IC amplifiers. The video signal is then recovered by an AM demodulator. In older TV sets a simple diode detector was used for video detection. In modern TV sets a synchronous modulator type of synchronous demodulator is used.
The output of the video detector is the Y signal or the composite color signal, which are amplified by the video amplifiers. The Y signal is used to create an AGC voltage output for controlling the gain of the IF amplifiers and mixers.
The composite color signal is taken from the video amplifier output by a filter and fed to color-balanced demodulator circuits. The color burst signal is also picked up by a gating circuit and sent to a phase detector whose output is used to synchronize an oscillator that produces a 3.58 MHz subcarrier signal of the correct frequency and phase. The output of this oscillator is fed to two balanced modulators that recover the I and Q signals. The carriers fed to the two balanced modulators are 90 degrees out of phase. The Q and I signals are combined in matrix with the Y signal, and out comes the R, G and B color signals. These are amplified and sent to the picture tube, which produces the picture.
To recover the sound part of the TV signal a separate sound IF and detector section are used. The sound and picture IF signals are fed to a sound detector circuit. This is a nonlinear circuit that heterodynes the two IFs and generates the sum and the difference of the frequencies. The result is a difference signal that contains both the AM picture and the FM sound modulation. This is the IF sound signal. It is passed to the sound IF amplifiers which also perform a clipping-limiting function which removes the AM, leaving only the FM sound. The audio is recovered with a quadrature detector or differential peak detector. The audio is amplified by one or more audio stages and sent to the speaker. If stereo is used the appropriate demultiplexing is done by an IC, and the left and right channel audio signals are amplified.
A major part of the TV receiver is dedicated to the sweep and synchronizing functions that are unique to TV receivers. To display the picture on a picture tube, special sweep circuits are needed to generate the voltages and currents to operate the picture tube, the sync circuits are needed to keep the sweep in step with the transmitted signal.
The sweep and sync operations begin in the video amplifier. The demodulated video includes the vertical and horizontal blanking and sync pulses. The sync pulses are stipped off the video signal with a sync seperator circuit and fed to the sweep circuits. The horizontal sync pulses are used to synchronize a horizontal oscillator to 15,734 Hz. This oscillator drives a horizontal output stage that developes a sawtooth of current that drives magnetic deflection coils in the picture tube yoke that sweep the electron beams in the picture tube.
The horizontal output stage is also part of a switching power supply. The horizontal output transistor drives a step-up-step-down transformer called the flyback. The 15.734 KHz pulses developed are stepped up, rectified, and filtered to develope the 30 to 35 KV high direct current required to operate the picture tube. Step-down windings on the flyback produce the lower voltage pulses that are rectified and filtered into low voltages that are used as power supplies for most of the circuits in the receiver.
The sync pulses are also fed to an IC that takes the horizontal sync pulses during the vertical blanking interval and integrates them into a 60 Hz sync pulse that is used to synchronixe a vertical sweep oscillator. The output from this oscillator is a sawtooth sweep voltage at the field rate of 60 Hz. This output is amplified and converted into a linear sweep current that drives the magnetic coils in the picture tube yoke. These coils produce vertical deflection of the electron beams in the picture tube.
In most modern TV sets, the horizontal and vertical oscillators are replaced by digital sync circuits. The horizontal sync pulses from the sync seperator are normally used to phase -lock a 31.468 KHz voltage controlled oscillator that runs at two times the normal horizontal rate of 15.734 KHz. Dividing this by two in a flip-flop gives the horizontal pulses that are amplified and shaped in the horizontal output stage to drive the deflection coils on the picture tube. A digital frequency divider divides the 31.468 KHz signal by 525 to get a 59.94 Hz signal for vertical sync. This signal is shaped into a current sawtooth and amplified by the vertical output stage which drives the deflection coils on the picture tube.
A picture tube is a vactume tube called a cathode-ray tube (CRT).Both monochrome and color picture tubes are available. The tube is housed in a bell shaped glass enclosure. A filament heats a cathode that emits electrons. The negatively charged electrons are attracted and accelerated by positive-bias voltages on the elements in an electron gun assembly. The electron gun also focuses the electrons into a very narrow beam. A control grid that is made negative with respect to the cathode controls the intensity of the electron beam and the brightness of the spot it makes.
The beam is accelerated forward by a very high voltage applied to an internal metallic coating called aquadag. The face or the front of the picture tube is coated internally with a phosphor that glows and produces white light when it is struck by the electron beam.
Around the neck of the picture tube is a structure of magnetic coils called the deflection yoke. The horizontal and vertical current linear sawtooth waves generated by the sweep and synchronizing circuits are applied to the yoke coils, which produce magnetic fields inside the tube that influence the position of the electron beam. When electrons flow, a magnetic field is produced around the conductor through which current flows. The magnetic field that occurs around the electron beam is moved or deflected by the magnetic field produced by the deflection coils in the yoke. Thus the electron beam is sweep across the face of the picture n tube in the interlaced manner.
As the beam is being sweep across the face of the tube to trace out the scene, the intensity of the electron beam is varied by the luminance, or Y, signal, which is applied to the cathode or in some cases to the control grid. By varying the grid voltage, the beam can be made stronger or weaker, thereby varying the intensity of the light spot produced by the beam when it strikes the phosphor. Any shade of gray, from white to black can be reproduced this way.
To produce color, the inside of the picture tube is coated with many tiny red, green and blue phosphor dots arranged in groups of three called triads. Some tubes use a pattern of red, green and blue stripes. These dots or stripes are energived by three seperate cathodes and electron guns driven by the red, green and blue color signals. A metallic plate with holes for each dot triad called a shadow mask is between the guns and the phosphor dots to ensure that the correct beam strikes the correct color dot. By varying the intensity of the color beams the dot triads can be made to produce any color. The dots are small enough so that the eye cannot see them individually at a distance. What the eye sees is a color picture swept out on the face of the tube.
This report was intended to cover the fundamentals of television. I covered TV signals, signal bandwidth, the process of generating a video signal, TV receiver fundamentals, TV tuner fundamentals, voice IF and demodulation, sound IF and demodulation, synchronizing circuits and the picture tube. I hope that you have found this report on television to be informative and enjoyable.