13. TV Fundamentals and Signals Introduction Interlaced Scanning Synchronization Resolution Television Signal

Introduction The concept of television was developed in the 1920s, feasibility was shown in the 1930s, commercial broadcasting started in the 1940s, and the ensuing years have seen the mushrooming growth of an industry Mass-production techniques and specialized ICs utilized in TV assembly enable the aver­age consumer to afford this truly sophisticated piece of electronics.

The picture signal is amplitude-modulated onto a carrier. the CRT picture tube is the analogous transducer that converts the electrical energy back into light energy. The TV camera converts a visual scene into an electrical signal. The camera is thus a transducer between light energy and electrical energy. The sound transmitter is FM system. TV uses a ±25-kHz deviation This is done to minimize interference effects between the two at the receiver since an FM receiver is relatively insensitive to amplitude modulation and an AM receiver has rejection capabilities to frequency modulation.

the sending and receiving antennas the sending and receiving antennas. They convert between electrical energy and the electromagnetic energy required for transmission through the atmosphere. A TV Transmitter is actually two separate transmitters. The diplexer shown feeding the transmitter antenna feeds both the visual and aural signals to the antenna while not allowing either to be fed back into the other transmitter. Without the diplexer, the low-output impedance of either transmitter power amplifier would dissipate much of the output power of me other transmitter

The electron beam scans across (and slowly down) the mosaic areas so as to charge up each of the many tiny capacitors. Light on the mosaic areas discharges the capacitors through the load resistor R vidicon The scanning electron beam (developed by the cathode K and three grids) recharges the mosaic capacitors and produces a video signal voltage drop across R that is proportional to the light intensity at the individual areas being scanned The TV camera is optically focused so that the scene to be transmitted appears on its light-sensitive area. The most widely used type today is the vidicon. It is much less costly and more compact in size. Notice the three very thin layers at the tube's front surface. These are the light-active areas.

Charge-Coupled Devices (CCD) is designed for camcorder cameras , security monitors, video phones, imaging, and video conferencing. Its color imaging capabilities include 410.000 pixels in a 1/4-in area. Pixel pitch is 4,9 by 5.6 mm. Pixel den­sity is 811 pixels on the horizontal axis and 507 on the vertical axis. Despite its high resolution, this CCD fits into a standard 14-pin DIP.

Scanning To understand how these tiny individual outputs can serve to represent an entire scene, let the camera focuses the letter "T" onto the capacitors of the vidicon. but instead of a million capacitors this system has just 30. arranged in 6 rows with 5 capacitors per row. Each separate area is called a pixel, which is short for picture element- The greater the number of pixe!s, the greater can be the quality (or resolution) of the transmitted picture. The letter “T" is focused on the light-sensitive area such that all of rows 1 and 6 are illuminated while all of row 2 is dark and the centers of rows 3.4. and 5 are dark. the electron beam is made to scan each row sequentially

TRANSMITTER/RECEIVER SYNCHRONIZATION When the video signal is detected at the receiver, some means of synchronizing the transmitter and receiver is necessary: 1. When the TV camera starts scanning line 1, the receiver must also start scanning line I on the CRT output display. You do not want the top of a scene appearing at the center of the TV screen. 2. The speed that the transmitter scans each line must be exactly duplicated by the receiver scanning process to avoid distortion in the receiver output. 3. The horizontal retrace, or time when the electron beam is returned back to the left-hand side to start tracing a new line, must occur coincidentally at both transmitter and receiver. You do not want the horizontal lines starting at the center of the TV screen. 4. When a complete set of horizontal lines has been scanned, moving the electron beam from the end of the bottom line to the start of the top line (vertical flyback or retrace) must occur simultaneously at both transmitter and receiver.

Interlaced Scanning The frame frequency is the number of times per second that a complete set of 485 lines (complete picture) is traced. That rate for broadcast TV is 30 times per second. or a complete scene (frame) is traced every 1/30 (second). Thirty frames per second is not enough to keep the human eye from perceiving flicker as a result of a non-continuous visual presentation. This flicker effect is observed when watching old-lime movies. If the frame frequency were increased to 60 per second, the flicker would no longer be apparent, but the video signal bandwidth would have to be doubled. Instead of that solution, the process of interlaced scanning is used to "trick" the human eye into thinking it is seeing 60 pictures per second.

lines 2, 4, 6, etc., occur during the first field traced in 1/60 s lines 1. 3, 5. etc., inter­leaved between the even-numbered lines are interleaved between the first lines in the next 1/60 s., The field frequency is thus 60 Hz with a frame frequency a full scene of 30 Hz. This illusion is enough to convince the eye that 60 pictures per second occur when, in fact, there are only 30 full pictures per second.

Synchronization Horizontal Synchronization To accommodate the 525 lines (485 visible) every 1/30 s, the transmitter must send a synchronization (sync) pulse between every line of video signal so that perfect transmitter-receiver synchronization is maintained. Since the horizontal sync pulses occur once for each of the 525 lines every 1/30 s, the frequency of these pulses will be 525 x 30 = 15,75 kHz Thus, both transmitter and receiver must contain 15.75-kHz horizontal oscillators to control horizontal electron beam movement.

Three horizontal sync pulses are shown along with the video signal for two lines. The actual horizontal sync pulse rides on top of a so-called blanking pulse, as shown. The blanking pulse is a strong enough signal so that the electron beam retrace at the receiver will be "blacked" out and thus invisible to the viewer. the interval after the end of the sync pulse, but before the end of the blanking pulse, is called the back porch. The interval before the horizontal sync pulse appears on the blanking pulse is termed the front porch, Notice that the back porch includes an eight-cycle sine-wave burst at 3,579,545 Hz. It is appropriately called the color burst, as it is used to calibrate the receiver color subcarrier generator.

Vertical Synchronization The vertical retrace and thus vertical sync pulses must occur after each 1/60s since the two interlaced fields that make up one frame (picture) occur 60 times per second. Notice that two horizontal sync pulses and the last two lines of video information of a field are initially shown. Equalizing pulses at a frequency double the horizontal sweep rate, or 15.75 kHzx2 =31,5 kHz The video signal just before, during, and after vertical retrace They are used to keep the receiver horizontal oscillator in sync during the relatively long (830 to 1330 m s) vertical blanking period

Resolution Resolution is the ability to resolve detailed picture elements To provide adequate resolution, the video signal must include modulating frequency components from dc up to 4 MHz. This requires a truly wideband amplifier, and amplifiers that have bandpass characteristics from dc up into the MHz region have come to be known as video amplifiers Picture of a blue sky will need only poor resolution (no detail scanning needed as the picture has no change of video levels) Picture of a hair will need good resolution (detail scanning needed as the picture has small changes of video level)

BW of video signal calculated from the resolution (NTSC American system) US TV broadcast has a standard of 525 scanning lines to frame a picture. 525 lines should be traced during one frame at 1/30 sec, making duration of one line to be 1/(30x525) Hz.=1/15.75kHz.= 63.5 ms 10 ms is used for flyback (blanking) time. Actual trace time is 63.5-10=53.5 ms 40 lines are used while vertical retrace (vertical flyback) duration. 525-40=485 lines is used for a visible picture (485 visible lines on the TV screen) only 70% of 485 lines are actually used for one frame of picture 0.7x485=339lines vertical resolution is therefore consists of 339 horizontal lines Aspect ratio ( length : height ratio ) of TV picture frame is 4:3 To make squares (picture elements) no. of vertical lines needed will be 4x339/3=452 vertical lines It means horizontal resolution will be made of 452 picture elements If two adjacent picture elements are alternative black and white to make one cycle, the highest freq. will be 452/2 Hz per horizontal trace of 53.5ms. giving 226cycles/53.5ms or ( 226cyclesx106ms/53.5ms ) cycles per sec = 4.22MHz That is why video BW is found at 4.22MHz. to get highest resolution

If BW of video signal = 4MHz, show aspect ratio is 4/3 (NTSC American system) US TV broadcast has a standard of 525 scanning lines to frame a picture. 525 lines should be traced during one frame at 1/30 sec, making duration of one line to be 1/(30x525) Hz.=1/15.75kHz.= 63.5 ms 10 ms is used for flyback (blanking) time. Actual trace time is 63.5-10=53.5 ms If BW of video signal is allowed to 4MHz. or fH = 4MHz. we can calculate back to find the no. of picture elements that can be used at one horizontal trace time. T = (1/fH) = (1/4MHz) = 0.25ms (for one cycle of BW pair) Then there will be 0.25/2=0.125ms for each picture element Therefore there will be (53.5ms/0.125ms)=428 picture elements along horizontal line. horizontal resolution is therefore 428 Aspect ratio is (hor-reso/vert-reso)=(428/0.7x(525-40))=(428/339)=1.261.33=4/3

Calculate the increase in horizontal resolution if video BW is increased to 5MHz and vertical resolution is kept as before at 525 lines (53.5ms for hor trace time) If BW of video signal is allowed to 5MHz. or fH = 5MHz. we can calculate back to find the no. of picture elements that can be used at one horizontal trace time. T = (1/fH) = (1/5MHz) = 0.2ms (for one cycle of BW pair) Then there will be 0.2/2=0.1ms for each picture element Therefore there will be (53.5us/0.1ms)=535 picture elements along horizontal line horizontal resolution is therefore 535

Calculate the increase in vertical resolution if video BW is increased to 5MHz and horizontal resolution is kept as before at 428 picture elements per hor trace T = (1/fH) = (1/5MHz) = 0.2ms (for one cycle of BW pair) Then there will be 0.2/2=0.1ms for each picture element For 428 pic elements, hor trace time is 428x0.1=42.8ms If 10ms is used for blanking, hor time will be 42.8+10=52.8ms Time allowed for one picture frame is 1/30 sec So there will be (1/30)/52.8ms=632 lines per picture Taking vertical retrace of 32 lines and 70% actual vertical resolution, Vertical resolution will be (632-32)x0.7=420 line=vertical resolution

Television Signal The maximum modulating rate for the video signal is 4 MHz. Since it is amplitude-modulated onto a carrier, a bandwidth of 8 MHz is implied. However, The FCC allows only a 6-MHz (only is a relative term here since 6 MHz is enough to contain 600 AM radio broadcast stations of 10 kHz each) bandwidth per TV station, and that must also include the FM audio signal. Once the entire TV signal is generated, it is amplified and driven into an antenna that converts the electrical energy into radio (electromagnetic) waves. These waves travel through the atmosphere to be intercepted by a TV receiving antenna and fed into the receiver once again as an electrical signal. That signal consists of the video, audio, and synchronizing signals. The synchronizing signals are contained in the video signal.

54- to 60-MHz limit shown is the allocation for channel 2 part of the 6-MHz bandwidth is occupied by a "vestige" of the lower sideband (about 0,75 MHz out of 4 MHz). It is therefore commonly referred to as vestigial-sideband operation. The lower visual sideband extends only 1.25 MHz below its carrier with the remainder filtered out, but the upper sideband is transmitted in full. The audio carrier is 4.5 MHz above the picture carrier with FM sidebands as created by its ±25-kHz deviation

Notice the VHF channels are broken up into two bands:- 54 to 88 MHz and 174 to 216 MHz. The UHF band (channels 14 to 69) is continuous and eats up a tremendous chunk of the usable frequency spectrum,