13. TV Fundamentals and Signals

1TE4201-Communication Electronics

13 . TV Fundamentals and Signals

Introduction

Interlaced Scanning

Synchronization

Resolution

Television Signal


Introduction

• The concept of television was developed in the 1920s, The concept of television was developed in the 1920s, • feasibility was shown in the 1930s, feasibility was shown in the 1930s, • commercial broadcasting started in the 1940s, commercial broadcasting started in the 1940s, • and the ensuing years have seen the mushrooming growth of an industryand the ensuing years have seen the mushrooming growth of an industry

• Mass-production techniques and specialized ICs utilized in TV assembly Mass-production techniques and specialized ICs utilized in TV assembly enable the aver age consumer to afford this truly sophisticated piece of enable the aver age consumer to afford this truly sophisticated piece of electronics.electronics.


The sound transmitter is FM The sound transmitter is FM system. system. TV uses a ±25-kHz TV uses a ±25-kHz deviationdeviation

The picture signal is The picture signal is amplitude-modulated amplitude-modulated onto a carrier. onto a carrier.

This is done to minimize interference effects between the two at the receiver since an 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 FM receiver is relatively insensitive to amplitude modulation and an AM receiver has rejection capabilities to frequency modulation.rejection capabilities to frequency modulation.

The TV camera The TV camera converts a visual converts a visual scene into an scene into an electrical signal. The electrical signal. The camera is thus a camera is thus a transducer between transducer between light energy and light energy and electrical energy. electrical energy.

the CRT picture the CRT picture tube is the tube is the analogous analogous transducer that transducer that converts the converts the electrical electrical energy back energy back into light into light energy.energy.


A TV Transmitter is actually A TV Transmitter is actually two separate transmitters.two separate transmitters.

the sending and receiving antennas. They convert between electrical energy the sending and receiving antennas. They convert between electrical energy and the electromagnetic energy required for transmission through the and the electromagnetic energy required for transmission through the atmosphere.atmosphere.

The The diplexerdiplexer shown feeding the transmitter antenna feeds both the visual and 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 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 other transmitter. Without the diplexer, the low-output impedance of either transmitter power amplifier would dissipate much of the output power of me transmitter power amplifier would dissipate much of the output power of me other transmitter other transmitter


The TV camera is optically focused so that the scene to be transmitted appears 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 on its light-sensitive area. The most widely used type today is the vidicon.vidicon. It is It is much less costly and more compact in size. Notice the three very thin layers at 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. the tube's front surface. These are the light-active areas.

vidiconvidicon

The scanning electron beam The scanning electron beam (developed by the cathode (developed by the cathode KK and three grids) recharges the and three grids) recharges the mosaic capacitors and mosaic capacitors and produces a video signal produces a video signal voltage drop across voltage drop across RR that is that is proportional to the light proportional to the light intensity at the individual intensity at the individual areas being scanned areas being scanned

The electron beam scans The electron beam scans across (and slowly down) the across (and slowly down) the mosaic areas so as to charge mosaic areas so as to charge up each of the many tiny up each of the many tiny capacitors. Light on the capacitors. Light on the mosaic areas discharges the mosaic areas discharges the capacitors through the load capacitors through the load resistor resistor RR


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


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

the electron beam is made to scan each row sequentiallythe electron beam is made to scan each row sequentially


TRANSMITTER/RECEIVER SYNCHRONIZATIONTRANSMITTER/RECEIVER SYNCHRONIZATION

When the video signal is detected at the receiver, some means of When the video signal is detected at the receiver, some means of synchronizingsynchronizing the transmitter and receiver is necessary: the transmitter and receiver is necessary:

•1. When the TV camera starts scanning line 1, the receiver must also start 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 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.appearing at the center of the TV screen.

•2. The speed that the transmitter scans each line must be exactly duplicated 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.by the receiver scanning process to avoid distortion in the receiver output.

•3. The 3. The horizontal retrace,horizontal retrace, or time when the electron beam is returned back to 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 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 transmitter and receiver. You do not want the horizontal lines starting at the center of the TV screen.center of the TV screen.

•4. When a complete set of horizontal lines has been scanned, moving the 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 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 (vertical flyback or retrace) must occur simultaneously at both transmitter and receiver.receiver.


Interlaced Scanning

The frame frequencyThe frame frequency is the number of times per second that a complete set of 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 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). 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 Thirty frames per second is not enough to keep the human eye from perceiving flickerflicker as a result of a non-continuous visual presentation. This flicker effect is as a result of a non-continuous visual presentation. This flicker effect is observed when watching old-lime movies.observed when watching old-lime movies. If the frame frequency were increased to 60 per second, the flicker would no 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. longer be apparent, but the video signal bandwidth would have to be doubled. Instead of that solution, the process of Instead of that solution, the process of interlaced scanninginterlaced scanning is used to "trick" the is used to "trick" the human eye into thinking it is seeing 60 pictures per second.human eye into thinking it is seeing 60 pictures per second.


The field frequency is thus 60 Hz with a frame frequency a full scene of 30 Hz. 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 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. when, in fact, there are only 30 full pictures per second.

lines 2, 4, 6, etc., occur during the first field traced in 1/60 slines 2, 4, 6, etc., occur during the first field traced in 1/60 s

lines 1. 3, 5. etc., inter leaved between the even-numbered lines are lines 1. 3, 5. etc., inter leaved between the even-numbered lines are interleaved between the first lines in the next 1/60 s., interleaved between the first lines in the next 1/60 s.,


Synchronization

Horizontal SynchronizationHorizontal SynchronizationTo accommodate the 525 lines (485 visible) every 1/30 s, the To accommodate the 525 lines (485 visible) every 1/30 s, the transmitter must send a synchronization (sync) pulse between transmitter must send a synchronization (sync) pulse between every line of video signal so that perfect transmitter-receiver every line of video signal so that perfect transmitter-receiver synchronization is maintained.synchronization is maintained.Since the horizontal sync pulses occur once for each of the 525 lines Since the horizontal sync pulses occur once for each of the 525 lines every 1/30 s, the frequency of these pulses will be every 1/30 s, the frequency of these pulses will be

525 525 x 30 =x 30 = 15,75 kHz 15,75 kHz

Thus, both transmitter and receiver must contain 15.75-kHz Thus, both transmitter and receiver must contain 15.75-kHz horizontalhorizontal oscillators to control horizontal electron beam movement. oscillators to control horizontal electron beam movement.


• Three horizontal sync pulses are shown along with the video signal for two lines. 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 The actual horizontal sync pulse rides on top of a so-called blanking pulse, as shown. shown. • The blanking pulse is a strong enough signal so that the electron beam retrace at 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 receiver will be "blacked" out and thus invisible to the viewer.

Notice that the back porch Notice that the back porch includes an eight-cycle sine-includes an eight-cycle sine-wave burst at 3,579,545 Hz. It wave burst at 3,579,545 Hz. It is appropriately called the is appropriately called the color burst,color burst, as it is used to as it is used to calibrate the receiver color calibrate the receiver color subcarrier generator. subcarrier generator.

the interval after the end of the the interval after the end of the sync pulse, but before the end sync pulse, but before the end of the blanking pulse, is called of the blanking pulse, is called the the back porch.back porch.

The interval before the horizontal The interval before the horizontal sync pulse appears on the sync pulse appears on the blanking pulse is termed blanking pulse is termed the front the front porch,porch,


Vertical SynchronizationVertical SynchronizationThe vertical retrace and thus vertical sync pulses must occur after each 1/60s 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 since the two interlaced fields that make up one frame (picture) occur 60 times per second. second. Notice that two horizontal sync pulses and the last two lines of video information of Notice that two horizontal sync pulses and the last two lines of video information of a field are initially shown. a field are initially shown.

The video signal just The video signal just before, during, and after before, during, and after vertical retracevertical retrace

Equalizing pulses at a frequency double the Equalizing pulses at a frequency double the horizontal sweep rate, or 15.75 kHzx2 =31,5 kHzhorizontal sweep rate, or 15.75 kHzx2 =31,5 kHz

They are used to keep the receiver horizontal oscillator in sync during the They are used to keep the receiver horizontal oscillator in sync during the relatively long (830 to 1330 m s) vertical blanking periodrelatively long (830 to 1330 m s) vertical blanking period


To provide adequate resolution, the video signal must include modulating To provide adequate resolution, the video signal must include modulating frequency components from dc up to 4 MHz. This requires a truly frequency components from dc up to 4 MHz. This requires a truly wideband amplifier, and amplifiers that have bandpass characteristics wideband amplifier, and amplifiers that have bandpass characteristics from dc up into the MHz region have come to be known as from dc up into the MHz region have come to be known as video video amplifiersamplifiers

ResolutionResolution is the ability to resolve detailed picture is the ability to resolve detailed picture elements elements

Picture of a blue sky will need only poor resolution (no detail scanning Picture of a blue sky will need only poor resolution (no detail scanning needed as the picture has no change of video levels)needed as the picture has no change of video levels)Picture of a hair will need good resolution (detail scanning needed as the Picture of a hair will need good resolution (detail scanning needed as the picture has small changes of video level)picture has small changes of video level)

Resolution


BW of video signal calculated from the resolution (NTSC American BW of video signal calculated from the resolution (NTSC American system)system)

US TV broadcast has a standard of 525 scanning lines to frame a picture.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 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 line to be 1/(30x525) Hz.=1/15.75kHz.= 63.5 ss

10 10 s is used for flyback (blanking) time. Actual trace time is 63.5-10=53.5 s is used for flyback (blanking) time. Actual trace time is 63.5-10=53.5 ss40 lines are used while vertical retrace (vertical flyback) duration. 525-40=485 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)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=339linesonly 70% of 485 lines are actually used for one frame of picture 0.7x485=339lines

vertical resolutionvertical resolution is therefore consists of 339 horizontal lines is therefore consists of 339 horizontal lines

Aspect ratio ( length : height ratio ) of TV picture frame is 4:3Aspect ratio ( length : height ratio ) of TV picture frame is 4:3To make squares (picture elements) no. of vertical lines needed will be To make squares (picture elements) no. of vertical lines needed will be 4x339/3=452 vertical lines4x339/3=452 vertical lines

It means It means horizontal resolution horizontal resolution will be made of will be made of 452452 picture elements picture elementsIf two adjacent picture elements are alternative black and white to make one 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 53.5cycle, the highest freq. will be 452/2 Hz per horizontal trace of 53.5s.s.

giving 226cycles/53.5giving 226cycles/53.5s or ( 226cyclesx10s or ( 226cyclesx1066s/53.5s/53.5s ) cycles per sec = s ) cycles per sec = 4.22MHz4.22MHz

That is why video BW is found at 4.22MHz. to get highest resolutionThat 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 If BW of video signal = 4MHz, show aspect ratio is 4/3 (NTSC American system)system)

US TV broadcast has a standard of 525 scanning lines to frame a picture.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 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 duration of one line to be 1/(30x525) Hz.=1/15.75kHz.= 63.5 ss

10 10 s is used for flyback (blanking) time. Actual trace time is 63.5-10=53.5 s is used for flyback (blanking) time. Actual trace time is 63.5-10=53.5 ss

If BW of video signal is allowed to 4MHz. or fIf BW of video signal is allowed to 4MHz. or fHH = 4MHz. we can = 4MHz. we can calculate back to find the no. of picture elements that can be used calculate back to find the no. of picture elements that can be used at one horizontal trace time. T = (1/fat one horizontal trace time. T = (1/fHH) = (1/4MHz) = 0.25) = (1/4MHz) = 0.25s (for one s (for one cycle of BW pair)cycle of BW pair)

Then there will be 0.25/2=0.125Then there will be 0.25/2=0.125s for each picture elements for each picture elementTherefore there will be (53.5Therefore there will be (53.5s/0.125s/0.125s)=428 picture elements along s)=428 picture elements along horizontal line. horizontal resolution is therefore 428horizontal line. horizontal resolution is therefore 428

Aspect ratio is (hor-reso/vert-reso)=(428/0.7x(525-40))=(428/339)=1.26Aspect ratio is (hor-reso/vert-reso)=(428/0.7x(525-40))=(428/339)=1.261.33=4/31.33=4/3


Calculate the increase in horizontal resolution if video BW is increased to Calculate the increase in horizontal resolution if video BW is increased to 5MHz and vertical resolution is kept as before at 525 lines (53.55MHz and vertical resolution is kept as before at 525 lines (53.5s for hor s for hor trace time)trace time)

If BW of video signal is allowed to 5MHz. or fIf BW of video signal is allowed to 5MHz. or fHH = 5MHz. we can = 5MHz. we can calculate back to find the no. of picture elements that can be used at calculate back to find the no. of picture elements that can be used at one horizontal trace time. one horizontal trace time.

T = (1/fT = (1/fHH) = (1/5MHz) = 0.2) = (1/5MHz) = 0.2s (for one cycle of BW pair)s (for one cycle of BW pair)Then there will be 0.2/2=0.1Then there will be 0.2/2=0.1s for each picture elements for each picture element

Therefore there will be (53.5us/0.1Therefore there will be (53.5us/0.1s)=535 picture elements s)=535 picture elements along horizontal line along horizontal line

horizontal resolution is therefore 535 horizontal resolution is therefore 535


Calculate the increase in vertical resolution if video BW is increased to Calculate the increase in vertical resolution if video BW is increased to 5MHz and horizontal resolution is kept as before at 428 picture 5MHz and horizontal resolution is kept as before at 428 picture elements per hor traceelements per hor trace

T = (1/fT = (1/fHH) = (1/5MHz) = 0.2) = (1/5MHz) = 0.2s (for one cycle of BW pair)s (for one cycle of BW pair)Then there will be 0.2/2=0.1Then there will be 0.2/2=0.1s for each picture elements for each picture element

For 428 pic elements, hor trace time is 428x0.1=42.8For 428 pic elements, hor trace time is 428x0.1=42.8ss

If 10If 10s is used for blanking, hor time will be s is used for blanking, hor time will be 42.8+10=52.842.8+10=52.8s Time allowed for one picture frame is s Time allowed for one picture frame is 1/30 sec1/30 sec

So there will be (1/30)/52.8So there will be (1/30)/52.8s=632 lines per pictures=632 lines per picture

Taking vertical retrace of 32 lines and 70% actual vertical resolution,Taking vertical retrace of 32 lines and 70% actual vertical resolution,

Vertical resolution will be (632-32)x0.7=420 line=vertical resolutionVertical resolution will be (632-32)x0.7=420 line=vertical resolution


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

Television Signal


The lower visual sideband The lower visual sideband extends only 1.25 MHz below its extends only 1.25 MHz below its carrier with the remainder carrier with the remainder filtered out, but the upper filtered out, but the upper sideband is transmitted in full. sideband is transmitted in full.

The audio carrier is 4.5 MHz above The audio carrier is 4.5 MHz above the picture carrier with FM the picture carrier with FM sidebands as created by its ±25-sidebands as created by its ±25-kHz deviation kHz deviation

54- to 60-MHz limit shown is the allocation for 54- to 60-MHz limit shown is the allocation for channel 2 channel 2

part of the 6-part of the 6-MHz bandwidth MHz bandwidth

is occupied by a is occupied by a "vestige" of the "vestige" of the lower sideband lower sideband

(about 0,75 MHz (about 0,75 MHz out of 4 MHz). It out of 4 MHz). It

is therefore is therefore commonly commonly

referred to as referred to as vestigial-vestigial-sidebandsideband

operation. operation.


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

13. TV Fundamentals and Signals

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