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Unformatted text preview: AHMED GAMAL EL-MORSI MOHAMED 4-COMP.3 NO.2 A television transmitter is a device which broadcasts an electromagnetic signal to the television receivers. Television transmitters may be analog or digital. The principals of primarily analog systems are summarized as they are typically more complex than digital transmitters due to the multiplexing of VSB and FM modulation stages. Types of transmitters There are many types of transmitters depending on The system standard Output power Back up facility, usually the Modulator, Multiplexer and Power Amplifier Stereophonic (or dual sound) facility, for analogue TV systems Aural and visual power combining principal, for analogue TV systems Active circuit element in the final amplifier stage Input stage of a transmitter The audio (AF) input (or inputs in case of stereophonic broadcasting) is usually a signal with 15 kHz maximum bandwidth and 0 dBm maximum level. Preemphasis time constant is 50 µs. The signal after passing buffer stages is applied to a modulator, where it modulates an intermediate frequency carrier (IF). The modulation technique is usually frequency modulation (FM) with a typical maximum deviation of 50 kHz (for 1 kHz. input at 0 dBm level). The video (VF) input is a composite video signal (video information with sync) of maximum 1 volt on 75 Ω impedance. (1 V limit is for luminance signal. Some operators may accept superimposed color signals slightly over 1 V.) After buffer and 1 V clipping circuits, the signal is applied to the modulator where it modulates an intermediate frequency signal (which is different from the one used for aural signal.) The modulator is an amplitude modulator which modulates the IF signal in a manner where 1 V VF corresponds to low level IF and 0 volt VF corresponds to high level IF. AM modulator produces two symmetrical side bands in the modulated signals. Thus, IF band width is two times the video band width. (i.e. if the VF bandwidth is 4.2 MHz, the IF bandwidth is 8.4 MHz.) However, the modulator is followed by a special filter known as Vestigal sideband (VSB) filter. This filter is used to suppress a portion of one side band, thus bandwidth is reduced. (Since both side bands contain identical information, this suppression doesn't cause a loss in information.) Although the suppression causes phase delay problems the VSB stage also includes correction circuits to equalise the phase. Output stages The modulated signal is applied to a mixer (also known as frequency converter). Another input to the mixer which is usually produced in a crystal oven oscillator is known as subcarrier. The two outputs of the mixer are the sum and difference of two signals. Unwanted signal (usually the sum) is filtered out and the remaining signal is the radio frequency (RF) signal. Then the signal is applied to the amplifier stages. The number of series amplifiers depends on the required output power. The 1|Page AHMED GAMAL EL-MORSI MOHAMED 4-COMP.3 NO.2 final stage is usually an amplifier consisting of many parallel power transistors. But in older transmitters tetrodes or klystrons are also utilized. In modern solid-state VHF and UHF transmitters, LDMOS power transistors are the device of choice for the output stage, with the latest products employing 50V LDMOS devices for higher efficiency and power density. Even higher energy efficiency is possible using Envelope Tracking, which in the broadcast industry is often referred to as 'drain modulation'. Combining aural and visual signals There are two methods: Split sound system: Actually there are two parallel transmitters one for aural and one for visual signal. The two signals are combined at the output via a high power combiner. In addition to a combiner, this system requires separate mixer and amplifiers for aural and visual signals. This is the system used in most high power applications. Block diagram of a TV transmitter (intercarrier method). Intercarrier system : There are two input stages one for AF and one for VF. But the two signals are combined in low power IF circuits (i.e., after modulators) The mixer and the amplifiers are common to both signals and the system needs no high power combiners. So both the price of the transmitter and the power consumption is considerably lower than that of split sound system of the same power level. But two signals passing through amplifiers produce some intermodulation products. So intercarrier system is not suitable for high power applications and even at lower power transmitters a notch filter to reject the cross modulation products must be used at the output. The output power The output power of the transmitter is defined as the power during sync pulse. (Real output power is variable depending on the content.) But the output power of the transmitting equipment and the output power of the antenna are two different quantities. The output power of the antenna is known as ERP which is actually the transmitter power times the antenna gain. Analog TV Modulation All of the three major analog TV standards are interlaced standards. A TV frame consists of two fields, top and bottom, following sequentially one after another. For NTSC the framerate is 29.97(30) frame per second (fps) with 59.94 (60) fields per sec, PAL and SECAM are 25 fps or 50 field per sec. Despite the fact that the major difference of NTSC, PAL and SECAM is the algorithm for chroma signal encoding, the modulation principles are essentially the same for all of them. 2|Page AHMED GAMAL EL-MORSI MOHAMED 4-COMP.3 NO.2 Fig.2 Real, Imaginary and Amplitude Frequency responses of complex modulation filters and output AM/VSB spectrum The composite signal is modulated to the carrier frequency by amplitude modulation with vestigial sideband (AM/VSB) modulation. It can be described as classical AM modulation SM(t) = SC(t) cos(2π fIF), where SM(t) – is modulated signal, SC(t) – composite signal and fIF – intermediate frequency carrier frequency, and then one side of the modulated spectrum is attenuated by a passband filter. For analog implementation, a passband surface acoustic filter (SAW) may be utilized. Sound signal is an FM modulated signal (except SECAM-L that is AM modulated) on the sound carrier that is located slightly above the boundary of modulated video spectrum. For DSP implementations, the composite signal that is a real signal passes through a complex baseband filter with spatial frequency response and then the complex signal is shifted to the intermediate frequency carrier by the digital rotator (multiplier by , wIF – digital IF, wIF = fIF/fS, fS – sampling frequency). The FM modulation sound signal may also be added to the real composite signal at baseband before complex modulation. 3|Page AHMED GAMAL EL-MORSI MOHAMED 4-COMP.3 NO.2 Analog television or analogue television is the original television technology that uses analog signals to transmit video and audio.[1] In an analog television broadcast, the brightness, colors and sound are represented by rapid variations of either the amplitude, frequency or phase of the signal. Displaying an image A cathode-ray tube (CRT) television displays an image by scanning a beam of electrons across the screen in a pattern of horizontal lines known as a raster. At the end of each line the beam returns to the start of the next line; the end of the last line is a link that returns to the top of the screen. As it passes each point the intensity of the beam is varied, varying the luminance of that point. A color 4|Page AHMED GAMAL EL-MORSI MOHAMED 4-COMP.3 NO.2 television system is identical except that an additional signal known as chrominance controls the color of the spot. Raster scanning is shown in a slightly simplified form below. When analog television was developed, no affordable technology for storing any video signals existed; the luminance signal has to be generated and transmitted at the same time at which it is displayed on the CRT. It is therefore essential to keep the raster scanning in the camera (or other device for producing the signal) in exact synchronization with the scanning in the television. The physics of the CRT require that a finite time interval be allowed for the spot to move back to the start of the next line (horizontal retrace) or the start of the screen (vertical retrace). The timing of the luminance signal must allow for this. Close up image of analog color screen The human eye has a characteristic called Phi phenomenon. Quickly displaying successive scan images will allow the apparent illusion of smooth motion. Flickering of the image can be partially solved using a long persistence phosphor coating on the CRT, so that successive images fade slowly. However, slow phosphor has the negative side-effect of causing image smearing and blurring when there is a large amount of rapid on-screen motion occurring. The maximum frame rate depends on the bandwidth of the electronics and the transmission system, and the number of horizontal scan lines in the image. A frame rate of 25 or 30 hertz is a satisfactory compromise, while the process of interlacing two video fields of the picture per frame is used to build 5|Page AHMED GAMAL EL-MORSI MOHAMED 4-COMP.3 NO.2 the image. This process doubles the apparent number of video frames per second and further reduces flicker and other defects in transmission. Receiving signals The television system for each country will specify a number of television channels within the UHF or VHF frequency ranges. A channel actually consists of two signals: the picture information is transmitted using amplitude modulation on one frequency, and the sound is transmitted with frequency modulation at a frequency at a fixed offset (typically 4.5 to 6 MHz) from the picture signal. The channel frequencies chosen represent a compromise between allowing enough bandwidth for video (and hence satisfactory picture resolution), and allowing enough channels to be packed into the available frequency band. In practice a technique called vestigial sideband is used to reduce the channel spacing, which would be nearly twice the video bandwidth if pure AM was used. Signal reception is invariably done via a superheterodyne receiver: the first stage is a tuner which selects a television channel and frequency-shifts it to a fixed intermediate frequency (IF). The signal amplifierperforms amplification to the IF stages from the microvolt range to fractions of a volt Structure of a video signal The video carrier is demodulated to give a composite video signal; this contains luminance, chrominance and synchronization signals;[5] this is identical to the video signal format used by analog video devices such as VCRs or CCTV cameras. Note that the RF signal modulation is inverted compared to the conventional AM: the minimum video signal level corresponds to maximum carrier amplitude, and vice versa. To ensure good linearity (fidelity), consistent with affordable manufacturing costs of transmitters and receivers, the video carrier is never shut off altogether. When intercarrier sound was invented later in 1948, not completely shutting off the carrier had the side effect of allowing intercarrier sound to be economically implemented. Each line of the displayed image is transmitted using a signal as shown above. The same basic format (with minor differences mainly related to timing and the encoding of color) is used for PAL, NTSC and SECAM television systems. A monochrome signal is identical to a color one, with the exception that the elements shown in color in the diagram (the color burst, and the chrominance signal) are not present. 6|Page AHMED GAMAL EL-MORSI MOHAMED 4-COMP.3 NO.2 Portion of a PAL video signal. From left to right: end of a video scan line, front porch, horizontal sync pulse, back porch with color burst, and beginning of next line The front porch is a brief (about 1.5 microsecond) period inserted between the end of each transmitted line of picture and the leading edge of the next line sync pulse. Its purpose was to allow voltage levels to stabilise in older televisions, preventing interference between picture lines. The front porch is the first component of the horizontal blanking interval which also contains the horizontal sync pulse and the back porch.[6][7] The back porch is the portion of each scan line between the end (rising edge) of the horizontal sync pulse and the start of active video. It is used to restore the black level (300 mV) reference in analog video. In signal processing terms, it compensates for the fall time and settling time following the sync pulse.[6][7] In color television systems such as PAL and NTSC, this period also includes the colorburst signal. In the SECAM system it contains the reference subcarrier for each consecutive color difference signal in order to set the zero-color reference. In some professional systems, particularly satellite links between locations, the audio is embedded within the back porch of the video signal, to save the cost of renting a second channel. Color b A color signal conveys picture information for each of the red, green, and blue components of an image (see the article on color space for more information). However, these are not simply transmitted as three separate signals, because: such a signal would not be compatible with monochrome receivers (an important consideration when color broadcasting was first introduced). It would also occupy three times the bandwidth of existing television, requiring a decrease in the number of television channels available. Furthermore, typical problems with signal transmission (such as differing received signal levels between different colors) would produce unpleasant side effects. Instead, the RGB signals are converted into YUV form, where the Y signal represents the lightness and darkness (luminance) of the colors in the image. Because the rendering of colors in this way is the goal of both black and white (monochrome) film and black and white (monochrome) television systems, the Y signal is ideal for transmission as the luminance signal. This ensures a monochrome receiver will display a correct picture in black and white, where a given color is reproduced by a shade of gray that correctly reflects how light or dark the original color is. The U and V signals are "color difference" signals. The U signal is the difference between the B signal and the Y signal, also known as B minus Y (B-Y), and the V signal is the difference between the R signal and the Y signal, also known as R minus Y (R-Y). The U signal then represents how "purplish-blue" or its complementary color "yellowish-green" the color is, and the V signal how 7|Page AHMED GAMAL EL-MORSI MOHAMED 4-COMP.3 NO.2 "purplish-red" or its complementary "greenish-cyan" it is. The advantage of this scheme is that the U and V signals are zero when the picture has no color content. Since the human eye is more sensitive to errors in luminance than in color, the U and V signals can be transmitted in a relatively lossy (specifically: bandwidth-limited) way with acceptable results. In the receiver, a single demodulator can extract an additive combination of U plus V. An example is the X demodulator used in the X/Z demodulation system. In that same system, a second demodulator, the Z demodulator, also extracts an additive combination of U plus V, but in a different ratio. The X and Z color difference signals are further matrixed into three color difference signals, (RY), (B-Y), and (G-Y). The combinations of usually two, but sometimes three demodulators were: a. (I) / (Q), (as used in the 1954 RCA CTC-2 and the 1985 RCA "Colortrak" series, and the 1954 Arvin, and some professional color monitors in the 1990s), b. (R-Y) / (Q), as used in the 1955 RCA 21 inch color receiver, c. (R-Y) / (B-Y), used in the first color receiver on the market (Westinghouse, not RCA), d. (R-Y) / (G-Y), (as used in the RCA Victor CTC-4 chassis), e. (R-Y) / (B-Y) / (G-Y), f. (X) / (Z), as used in many receivers of the late '50s and throughout the '60s. In the end, further matrixing of the above color-difference signals c through f yielded the three colordifference signals, (R-Y), (B-Y), and (G-Y). The R,G,B signals in the receiver needed for the display device (CRT, Plasma display or LCD display) are electronically derived by matrixing as follows: R is the additive combination of (R-Y) with Y, G is the additive combination of (G-Y) with Y, and B is the additive combination of (B-Y) with Y. All of this is accomplished electronically. It can be seen that in the combining process, the low resolution portion of the Y signals cancel out, leaving R,G, and B signals able to render a low-resolution image in full color. However, the higher resolution portions of the Y signals do not cancel out, and so are equally present in R, G, and B, producing the higher definition (higher resolution) image detail in monochrome, although it appears to the human eye as a full-color and full resolution picture. Color signals mixed with video signal (two horizontal lines in sequence) In the NTSC and PAL color systems, U and V are transmitted by using quadrature amplitude modulation of a subcarrier. This kind of modulation applies two independent signals to one subcarrier, with the idea that both signals will be recovered independently at the receive end. Before transmission, the subcarrier itself, is removed from the active (visible) portion of the video, and moved, in the form of a burst, to the horizontal blanking portion, which is not directly visible on screen. (More about the burst below.) 8|Page AHMED GAMAL EL-MORSI MOHAMED 4-COMP.3 NO.2 For NTSC, the subcarrier is a 3.58 MHz sine wave. For the PAL system it is a 4.43 MHz sine wave. After the above-mentioned quadrature amplitude modulation of the subcarrier, subcarrier sidebands are produced, and the subcarrier itself is filtered out of the visible portion of the video, since it is the subcarrier sidebands that carry all of the U and V information, and the subcarrier itself carries no information. The resulting subcarrier sidebands is also known as "chroma" or "chrominance". Physically, this chrominance signal is a 3.58 MHz(NTSC) or 4.43 MHz(PAL) sine wave which, in response to changing U and V values, changes phase as compared to the subcarrier, and also changes amplitude. As it turns out, the chroma amplitude (when considered together with the Y signal) represents the approximate saturation of a color, and the chroma phase against the subcarrier as reference, approximately represents the hue of the color. For particular test colors found in the test color bar pattern, exact amplitudes and phases are sometimes defined for test and trouble shooting purposes only. Although, in response to changing U and V values, the chroma sinewave changes phase with respect to the subcarrier, it's not correct to say that the subcarrier is simply "phase modulated". That is because a single sine wave U test signal with QAM produces only one pair of sidebands, whereas real phase modulation under the same test conditions would produce multiple sets of sidebands occupying more frequency spectrum. In NTSC, the chrominance sine wave has the same average frequency as the subcarrier frequency. But a spectrum analyzer instrument shows that, for transmitted chrominance, the frequency component at the subcarrier frequency is actually zero energy, verifying that the subcarrier was indeed removed before transmission. These sideband frequencies are within the luminance signal band, which is why they are called "subcarrier" sidebands instead of simply "carrier" sidebands. Their exact frequencies were chosen such that (for NTSC), they are midway between two harmonics of the frame repetition rate, thus ensuring that the majority of the power of the luminance signal does not overlap with the power of the chrominance ...
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  • Spring '19
  • nb
  • NTSC, PAL, AHMED GAMAL EL-MORSI MOHAMED

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