video signal

7/27/2019 Video Signal

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Vision

Let's think about human vision for a moment. The eye receives an image, and hundreds of thousands of fibres in the optic nerve simultaneously send to the brain signals that, takentogether, represent the whole scene. Human vision uses an abundance of "channels," all

at once.

In television however, the entire scene must be sent through a single channel. Think of

it as a serial process, sent down a series circuit. Within the camera an electrical signal isformed to represent the changing brightness and colour of each part. This signal is sent to themonitor. At the monitor the signal is transformed back into light, and the image is assembledon the viewing screen in its proper relative position.

The perception of motion comes to us by a series of still images - Muybridge�s famous�galloping horse�experiment of thelate 1800s

In the television system, the picture we want to see is "scanned" sequentially, top tobottom, left to right. This repetition occurs at a rate of approximately 30 times everysecond, so we say that television runs at a rate of 30 frames per second.

Even though the picture elements are laid down on the screen one after the other, they allmust be perceived at once. This requirement is met by persistence of vision - a property of the eye. When light entering the eye is shut off, the impression of light persists for about a tenth of a second. So, if all the picture elements in the image are presentedsuccessively to the eye in a tenth of a second or less, the whole area of the screen appearsilluminated, although in fact only one spot of light is present at any instant. Activity in thescene is represented, as in motion pictures, by a series of still pictures, each differing slightlyfrom those preceding and following it

Black and White Television

Electrons

Before we go on further in our discussion of video, let's take a moment to have a look atelectrons, since they are the basis of this whole television business anyway, and a quickoverview will be invaluable in our discussion of monitors and TV sets later on.

The electron is often described as a "particle of electricity." The characteristic that we care

about for now is that an electron has an electric charge. By the way, an electric current downa wire is a flow of electrons, too.



Electrons were discovered in 1895 by Joseph J. Thomson, a British physicist, in the form of cathode rays - actually a stream of electron particles. What's really interesting aboutcathode rays is that they can be deflected by magnetic and electric fields . An electron isessentially weightless - it has a mass of about 9.1083 X 10

-28grams.

Let's now try to do something useful with this stream of electrons. If we move them in a

particular pattern across our picture tube, we get:

Scanning

The process of breaking down the scene into picture elements and reassembling them on thescreen is known as scanning. It's like your eye's motion when you read this page. Inscanning, the scene is broken into a series of horizontal lines.

The principle of scanning (note how each line breaks down the scene into discrete elements of picture intensity)

At The Monitor

At the television monitor this signal is recovered and controls the picture tube. The picturetube creates an image that is composed of horizontal lines just like those produced in thecamera. As the camera examines the topmost line, a spot of light produced by the picturetube moves across the screen and produces the topmost line of light on the screen. Thevideo signal causes the spot of light to become brighter or darker as it moves, and so thepicture elements scanned by the camera are reproduced line by line at the monitor, until thewhole area of the screen is covered, completing the image. Then the process is repeated.

Try This At Home!

Go to your friend's place. The one who didn't get the loan of the colour TV when they movedto Toronto, so they have the clunker black and white set instead. Take a magnifying glasswith you.

Turn on the set, tune in a channel, and hold the magnifying glass up to the screen. Notice thescanning lines that make up the picture. If you're friend asks what you're doing, tell 'em you'repractising your Sherlock Holmes impression.

By the way, this doesn't really work on a colour TV, but there's a nifty thing we can look for,

that we'll mention later on.



Interlace

Field One

Field Two

Interlaced Together... Because the phosphor coating on the picture tube can only keep the picture information for acertain amount of time, flicker results if scanning from top to bottom only occurs at 30 framesper second. To avoid flicker, each still picture is presented twice by a process known as



interlaced scanning. After the topmost line is scanned, space for another line is left

immediately below it, and the next scanned line appears just below the empty space. As thescanning proceeds, alternate lines are scanned, with empty spaces between them. Thisrepresents the first showing of the still picture. The next image also consists of spaced lines,and its lines fall precisely in the empty spaces of the preceding image, so the whole screen isfilled by the two sets of interlaced scanning lines. These two sets of scanning lines are called

fields. There are two fields to each frame of television scanning.

The Sawtooth Wave

Consider for a moment the actual trace of the electron beam in either a camera or a picturetube. It goes evenly from left to right, then snaps back quickly to the left. The process repeatsitself over and over. To make the beam do this, we apply a scanning voltage to coils of wirearound the neck of the tube that act as electromagnets moving the beam around. Thescanning voltage is called a sawtooth and looks like, well, the teeth of a saw - a smoothgradual ramp in voltage, followed by a sudden return to the start of the ramp again. Theelectron beam moves from left to right across the screen, and then rapidly back, following thewave shape of the scanning voltage.

The vertical scanning in the television system is done similarly, as the beam moves 60 timesa second from the top of the screen to the bottom. We have to control these sawtoothwaveforms in some way.

Sawtooth wave

Basic television scanning process (click on the picture for a bigger view) Sync Signals

We have already created a constantly changing voltage called the "video" signal. In itsprimitive form, it is just the changes in an electrical signal that represent the light and darkareas of a scene. These signals go from an arbitrary "zero percent" or "no light level" to "100percent" or "maximum light level"; our scale is actually from 7.5 to 100 . As this signal isapplied to a CRT electron beam, it reproduces with light the various areas of the picture.

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How we tell monitors how to scan from one line to the next

Horizontal Interval

But when we want to retrace the scan line back to the left again, we don't want to see it- we want it "blanked" out. So, during the "right to left" retrace period, we insert intoour video signal a "horizontal blanking pulse" (at "zero units," so we won't see theretrace).

This is fine, but to ensure that our sawtooth doesn't drift off frequency over a long time (whichwould "skew" our picture in unpredictable ways), we send, within the horizontal blankingperiod a "horizontal sync pulse." This can be used to feed the sawtooth generator circuit,to give it a jolt, to re-synchronize it at the end of every line. We'll place this sync pulse at an

intensity where it can easily be detected by the sawtooth circuit, and will never be seenby the viewer - at "-40" on our relative scale. The pulses will be sent 15,734 times asecond (one for each line of video).



The vertical interval, featuring blanking, vertical sync pulse, and equalizing pulses (click on the picture for a bigger view)

Vertical Interval

A similar process occurs with the vertical sweep. A vertical sync pulse is created. This pulse

triggers the second sawtooth wave generator - the one that controls the "top to bottom, andback to the top" part of scanning. It tells this generator "better make your way back to the topof the screen now." The shape of the vertical sync pulse is actually six small pulses. It's madeup this way to provide synchronization for the horizontal sawtooth generator during thevertical retrace period.

In addition to the vertical sync pulses, another group of pulses is required when usinginterlaced scanning. Interlacing occurs because the second field of scanning starts half aline's distance across the screen, relative to the first field. This means that the vertical

sawtooth voltage (inside the monitor) for one field must occur one half line later than

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for the other one.

Now, since our vertical sweeps are locked into the vertical sync pulses, they must occur onehalf line after the last horizontal sync pulse in one field, and one full line after the lasthorizontal sync pulse in the other field. One group of six "equalizing" pulses precedes thevertical sync pulse to allow this to happen properly; another group follows it. So, the

equalizing pulses make interlacing happen and start the scans at the proper points ineach of the two video fields.

No horizontal hold, due to missing horizontal sync pulses Vertical roll due to lack of luck with vertical sync pulse

The vertical intervals at the end of each field differ a bit... There's one more thing you should realize: there are two vertical intervals, one after thefirst field, and another after the second field. They differ slightly. Note that in Fig. A, the

first field finishes after a half line of video; the first equalizing pulse is now displaced only ahalf line away from the last horizontal sync pulse in the previous field. Likewise, the first lineof video is only a half line. The second field (Fig. B) is completed with a full line of video, and



its corresponding vertical interval begins with a full line of video (which is the start of field one,again.) Otherwise, the vertical blanking intervals for the even and odd line fields are identical.

An interesting point for pay-TV subscribers: often, scrambled video is just video without thesync - what you may be paying for is the horizontal sync signal!

Scrambled pay-TV

How the parts of the composite video signal affect the monitor (click on the picture for a bigger view)

Try This At Home!

Okay, you can try this stuff on any TV set...

1. If your set has a "vertical hold" knob, rotate it - go ahead, it won't bite. You'll see the

vertical interval (that's what that "black bar" is when your set goes wonky.) Look at it realclose. Try and find the parts you just read about. Get acquainted with it; make it your friend.

2. Turn on some scrambled pay-TV. Watch the horizontal and vertical intervals jump allaround.

3. Put the set back the way you found it when you're done, or your room mates will be reallyannoyed with you.

Colour Television

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The additive colour wheel

Colour television employs the basic principles of black-and-white television. The essentialdifference is that a colour picture is like three pictures in one.

The screen of a colour monitor, in effect, displays three images superimposed on eachother . These images present, respectively, the red, green, and blue components of thecolours in the scene. Colour television achieves reproduction of the wide range of natural colours by adjusting the relative brightness of these red, green, and blue

images.

If two images are suppressed (for example, red and green), only the remaining colour (blue)is seen. If one image is suppressed (for example, blue), the other two (green and red) cancover the range of colours from green to red, including the intermediate colours orange andyellow, by changing the proportions of the red and green channels. When all three colours arepresent in the proper proportions, white light is produced - in fact, the whole range of graysfrom black to white can be reconstructed. By allowing one or two of the three colours topredominate, the white light can be given the tint of the stronger colours - pastel shades canbe reproduced.

This colour representation process is called the additive colour system.

Colour Picture Tubes

At the rear of the picture tube is the electron gun, which produces three separate beams of electrons. These three beams hit the coloured dots, and the tube is designed so each beamcan hit dots of only one colour; a mask prevents each beam from striking the others' colour dots. Because the coloured dots are so small that they cannot be seen separately by theviewer, the effect is three superimposed images in the primary colours. By adjusting thestrength of the respective beams of electrons, the relative brightness of the image producedby each can be changed.



Colour picture tube cutaway, showing electron gun, shadow mask and arrangement of phosphor dots(courtesy Broadcast Engineering )

"Front end" of a modern CCD camera Colour Cameras

The three electrical signals that control the respective beams in the picture tube are, thesedays, produced in the colour television camera by three CCD (Charge Coupled Device)

integrated circuit chips.

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The camera has a single lens, behind which a set of prisms or special dichroic mirrorsproduce three images of the scene. These are focused on the three CCDs. In front of eachone is a colour filter; the filters pass respectively only the red, green, or blue components of the light in the scene to the chips. The three signals produced by the camera are transmittedto the respective electron guns in the picture tube, where they re-create the scene.

Separation of full colour picture into red, green, and blue images

Three Channels of Colour - One Wire?

One way of connecting the camera to the picture tube is to use three separate cables, one for each of the primary colour signals. In fact, a computer monitor takes the separate channels of red, green and blue sent by the computer's video card and displays them directly on thescreen. To broadcast colour programs by this method, however, would require each station touse three channels. The number of channels available for television is so limited that thismethod is impractical. Also, if a black-and-white TV receiver were to tune in on such a colour broadcast, it could receive only one of the three channels, and the grey-scale valuesreproduced would be unnatural. There has to be a better way.

Making Luminance

Let's start by creating the luminance information from these colour channels. It'sproduced by adding, electronically, the three signals from the colour camera, in theratios 30% red, 59% green, and 11% blue. The luminance signal is what a black-and-whitebroadcast is like, so the black and white receiver, which interprets only this signal, gives acorrect rendition of the broadcast.

We already know how to send a composite signal (incorporating black and white informationand synchronization signals) down one wire. How can we add colour and still keep the wholeprocess compatible with our black and white system?

Right now, with the invention of black and white television, we have a signal which hasa full range of frequencies between about 30 Hz and 4,200,000 Hz (4.2 MHz) - that's howwe transmit details in the television picture. Supposing we were to "borrow" a continuoussine wave, of a very particular high frequency within that range, and somehow use it totransmit colour information? How would we use it?

A Special Carrier Wave

One thing we can't do with this wave is change its frequency - that aspect of a wave is howwe tell the television system about the details in a scene. However, there are two other things we can do with this high frequency sine wave - change its amplitude (level) andchange its phase. If, somehow, we could give this signal attributes that corresponded to

how saturated a colour was, and what particular hue it was , we could superimpose thissignal on the luminance, and send them both down the same wire. And that's what we do -with a device called a colour encoder.



We have, in fact, chosen a particular frequency to represent colour information, and it isexactly 3.579545 MHz, but you can remember it as 3.58 MHz. This is high enough afrequency so it won't be seen on black and white television sets (except as occasional small"dots"), but will still be within the bandwidth of what we're allowed to transmit over theairwaves.

Through a process involving manipulation of this high frequency (using the variations presentwithin the colour and luminance signals), we are able to produce what we call "colour subcarrier" that has within it all of the possible hues ("which colours") we would ever want toreproduce, and also information about how saturated ("colourful") our colours are.

A Burst of Colour Information

There is also a separate reference "colour burst" that is added at the beginning of each videoline, just after the horizontal sync signal. This is a short blip of colour subcarrier, and is usedas a reference to give the colour monitor a "starting point" as to which colour is supposed tobe represented, and how saturated that colour is. As various hues are displayed based onwhat phase of subcarrier is being transmitted, the NTSC designers have decided that colour

burst will be sent at 180 phase.

Mix Ingredients Thoroughly...

This colour signal (the colour subcarrier, continuously changing its amplitude and phase, andthe colour burst itself) is mixed with our already available black and white signal, so that theentire composite can be sent down a single wire.

The Colour Encoding Process - In Detail

Want to know how the colour encoding process really works? In all the nitty-gritty detail? For

a description of the colour encoding process in all its glory, please refer to theAppendix. Here's a thought - if you aren't sure about this colour encoding stuff, sit with afriend and work through the section in the Appendix. It does no harm to have a look, andsomething may come clear to you that you hadn't understood before.

One last hitch about this colour encoding stuff and then we're done - honest.

Video Is Not 30 Frames Per Second???

Up until now, we've been saying how television scans 525 lines in 1/30 of a second. Well,that's not exactly true. You see, it was true in the days of black and white television. But, to

keep the visibility of the colour subcarrier in the monitor to a minimum (the "little dots" wereferred to earlier), a couple of the specifications got changed.

In black and white television, 525 lines scanned in 1/30 of a second gave us a line scan rateof 15,750 Hz (525 x 30). With the invention of colour television, that was changed to beprecisely related to the subcarrier frequency - 2/455 of it, in fact - which made it 15,734 Hz(2/455 x 3,579,545 Hz).

Having changed the line scan rate, the frame rate also had to change, from 30 frames asecond, to 29.97 frames a second (15,734 / 525). You might think that this doesn't matter

too much - after all, 29.97...30...whateverrr...close enough, right? But when you start editingvideotape, on the frame, these little discrepancies have a tendency of adding up, making your show too short or too long. This problem comes back to haunt us when we start thinking

about time code (a frame numbering system used in editing.) See the chapter on Editing for more details on this conundrum.

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Transmitting The Image & NTSC Resolution - ALittle History, and A Look To The Future

Back in 1936, most of the new science and technology involved in television had been

worked out by two committees of the Radio Manufacturers Association (RMA), and in theyear before, RCA had demonstrated a fully electronic 343-line television system.

In 1936 (before there was an NTSC), the committees decided: the television transmissionchannel should be 6 MHz wide. This was a huge chunk of radio frequency spectrum to beassigned for each channel - it was 600 times as wide a band as an AM broadcast stationused, but it still allowed lots of channels, with reasonable resolution in each.

Most of the channel is used to transmit the primary video signal, which occupies a sidebandof 4.2 MHz (as decided by the RMA Television Standards Committee in 1938) - the other portions of the channel are required for a vestigial sideband of the video signal and for soundtransmission. It's the standard by which we live today, and as such, puts very real limitsonto the maximum resolution of NTSC television.

So, what are the limits of NTSC resolution? This is a question hotly debated by everybodyfrom broadcast engineers to audio/video salesmen, so we'll tread safely on some knownplanks of information.

Vertical Resolution

Let's start with the easy one: vertical resolution. We speak of NTSC video as having 525scanning lines. This number includes 42 of them (21 per field) that are blanked out and usedup during periods of vertical retrace (the electron beam has to get from the bottom to the topof the frame, and this takes a certain amount of time). Therefore, we really have 483 linesavailable to us for picture material. Now, consider a test chart with a series of fine black

and white lines, running horizontally. Place this in front of a camera. How many lines can yousee on the screen before they begin to blend?

If we're really, really careful, we might just be able to scan exactly 483 black and white lines -one chart line being scanned exactly by one line of video. The odds of this happening arepretty dismal. In fact, if by some chance, we vertically re-position the chart within the camera'sframing just a little bit, so our video scanning lines each straddle a black and a white line, theresult will be a totally grey screen with no lines visible! As we play around with this game of chance, it turns out that we can successfully reproduce 340 lines as often as we like. That's our practical vertical resolution. This fooling around with horizontal stripes and a TVcamera that we have just done can be mathematically estimated, and it has a name. The Kellfactor, as it's called, is equal to .7, so, if we use it, we get 483 x .7 = 340 lines.



Using a test card with a series of horizontal lines, to check vertical resolution of NTSC TV

Using a test card with a series of vertical stripes to test the horizontal resolution of NTSC TV Horizontal Resolution

Now comes the trickier one: horizontal resolution. The arithmetic on this is a little deeper, sobear with me as we go through it.

We think of our NTSC scheme as a 525-line, 29.97 frame per second, television system, thattakes 1/15,734 of a second for each line to scan across the screen.

Some of that scan line time is used to get the electron beam back from the right side of thescreen over to the left again. This is called horizontal retrace, and it leaves us with a practicalvisible scan line that takes about 1/18975 of a second to track across the screen from left to

right.

(1)

If we take our 4.2 MHz (4,200,000 cycles per second) of bandwidth that we're allowed attransmission, and divide it by the 1/18975 of a second that we have to display our video in(4,200,000 / 18975), we get about 220 cycles of signal per line.

Yuck. Television pictures would look pretty chunky if you only were allowed a little more than200 picture elements per line. But these are cycles, not pixels. Consider that a "cycle" is apositive going voltage followed by a negative going voltage - like in audio - and we representvideo by a series of ever-changing voltages corresponding to the light level read by thecamera.

If we were to take our "bunch of black and white lines" chart and tip it so the lines werevertical, how many lines (black lines and white lines) would we see before we blurred? Eachblack line would be a low voltage, followed by each white line - a high voltage. That would be

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one "cycle." So with 220 cycles available to us, we could see 440 lines of video across thescreen. That's more like it.

To re-cap then, the practical resolution of our broadcast transmission environment is340 lines top to bottom, and 440 lines left to right.

(2)

TV Lines Per Picture Height (TVL/PH)

We're now going to express these two resolutions a little differently, since with DTV andHDTV, we'll be dealing with screen sizes that aren't always 4:3. Today, we refer toresolutions expressed in "TV lines per picture height" (TVL/PH). What this means is that

we take a "square" piece of the television picture (an area with equal height and width) andsee how much resolution we have in the horizontal and vertical directions within that shape.This makes a certain amount of sense in that we are comparing the same distance in eachdirection on the television screen and speaking of the relative resolution in each of thosedirections. This makes it possible for us to compare NTSC resolution to DTV digital resolutionto HDTV resolution since we're looking at the same square for all these formats.

Let's try it with NTSC. Since our picture here is 4:3, we'll take a piece of the picture that'sessentially 3:3. This makes the vertical resolution number easy to figure out: it's the same asthe full height of the 4:3 screen, or 340 TVL/PH. For horizontal resolution, let's make our wayacross the screen, left to right, for the same distance, or about 3/4 of the way across theNTSC screen. That will give us 440x.75=330 TVL/PH. You'll notice how the horizontal andvertical resolutions (330 vs. 340 TVL/PH) are almost identical, resulting in what could bethought of as "square pixels" on the screen, with equal resolutions in both directions.

The next time you walk into a mega-hi-fi/video store and the salesperson tries to sell you anexpensive TV set with "over 600 lines of resolution", just remember to ask them why this isnecessary, since no broadcaster sends out that much resolution in the first place...I wonder what their answer would be? Oh, and while you're at it, impress 'em with your new knowledgeof "lines per picture height".

Things To Think About:

The purpose of our television system is to allow us to send

television down a single transmission channel.

Some parts of the process to do this include scanning, interlace,

synchronization signals, and, in the case of colour television,

colour encoding of separate colour channels of picture information.

This system has inherent within it certain limits of resolution. What

are those limits?





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