One of the earliest electronicTV images clearly showing scanning lines

A television image is painted on the screen in a series of lines starting at the uppermost left, and continuing in a pattern to the lower right. As an analogy to film, a complete scan of the entire picture (each thirtieth of a second) is called a "frame". For a number of reasons, a repetition rate of 30 frames (scans) per second was chosen.

But thirty frames per second tended to show flicker on the cathode ray tube. Upping the repetition rate of the frames also ups the amount of information that needs to be sent, upping the demands on the equipment, raising the cost, and eating up more space in the radio spectrum….. but there is a way to double the flicker rate for free: INTERLACE. Interlace allows the screen to be refreshed twice as often with no increase in the amount of information sent. All broadcast forms of analog television are interlaced.

An interlaced scan begins just as you would think, but reaches the bottom after only half the number of scan lines has been completed. Halfway across the bottom, the scan is reset to the top, and continues scanning the last half of the horizontal line. The scan then continues to the bottom again, this time ending at the right side. Each half of the scan is called a "field", and it takes two interlaced fields to make one frame. The half-line timing offset causes the second field to fall in between the scan lines of field 1. Because of the half-line trick, all interlaced TV systems have an odd number of lines.... we have 525, Britain has 625, etc.

In the picture above, field one starts at the black arrow. The blue lines are rapid retrace from the left back to the right. The vertical red line is the scan returning to the top, and ending up halfway across the screen, and halfway between the first two lines of field one. The retrace lines for field two have been omitted. Field two ends at the lower right, and retraces to the start of field one.

The Video Signal

 

 The VIDEO SIGNAL is an electrical voltage that instantaneously varies in proportion to the image it is scanning. This varying voltage can be hooked to a monitor (where it controls the instantaneous brightness of the scanning pattern being displayed), recorded on tape, sent via radio waves through the air or cable. At the scanning speeds chosen for our system, to get sufficient detail in the image, video amplifiers need to be responsive to fluctuations up to 4.2 MILLION times a second.

  The other requirement of the varying video voltage, is that the receiving display needs to know where a new scanning line begins, and when to kick the scanning back to the top of the display. This is done by making the voltage dip very low, very suddenly. The lowest voltage positions can be separated. This is SYNC… short for synchronizing signal. The horizontal sync pulses at the end of each line occur quickly and often (every 63.5 millionths of a second... almost 16,000 times a second), while the vertical sync pulses occur relatively slowly and have a longer duration (sixty times each second... two sync pulses per frame because of the two interlaced fields). A circuit similar to an audio bass and treble control can separate the two.

The above vertical sync interval has no additional information. The thin black line in the thicker grey bar is the actual sync pulse. A special note about vertical sync... the allowed retrace time from bottom to top leaves 21 horizontal lines not usable for picture, these lines being blanked out while the scanning beam resets to the top. This 21 line period is called the VERTICAL BLANKING INTERVAL (VBI), and stuff like timecode, closed captioning, and test signals can be hidden in it....

Here, many of the empty lines have signals riding along. The bottom line of white dashes is closed-captioning. 

Adding Colour

Hmm. This isn't what they told you in school. In school, you learned about paint, which is subtractive, where red, blue, and yellow mix to make a dark colour (theoretically black). Television uses additive colour, where red, green, and blue mix to make white.

  

In the early 1950’s, it was decided to add colour information to the television signal, while allowing existing sets to continue receiving a good black & white picture. At first, this seems impossible. All good color reproduction involves at least three colour signals. Television uses a separation of RED, GREEN, and BLUE. The best way to reproduce colour in television is to split the incoming light using re, green, and blue filters to three separate sensors. This results in THREE VIDEO SIGNALS… the red channel is brighter where red is present, etc. A white object will be the highest voltage on all three channels, and so on.

One way to transmit good colour, is to send all three video signals on separate paths. In Television, this would have required three television channels for every station, and three tuners in every TV set. A different approach was needed. Incredibly, it was decided to send the additional information in the same space that the black and white signal already occupied! Worse yet, it was decided that all existing black and white sets should be able to display the new colour broadcasts as good black and white programs. The solution has been ridiculed for 50 years, but was, and is pure genius.

OK, let's squeeze the color in. FIRST, we so some mixing and subtracting.

The Colour Matrix

Where's the G-Y ???

We don't need it, because Y is made from R, G, and B. Since we will be recovering R and B in our decoder, we simply apply G = Y-R-B in the correct proportions to recover G. Now, now.... settle down, it works!  

The resulting R-Y and B-Y are the smallest signals we can pack into the Y (Black & White) signal that will allow us to recover the three colour channels. They are known as the color difference signals, and/ or color components. Some newer systems use these three signals (Y, R-Y, B-Y) on three separate wires. This is called "Component Video", and is found in BetaCam (and Panasonic MII) systems. If you can stand to have three wires, component analog video is the best analog connection available. (Component video calls R-Y and B-Y "U" and "V").

To squeeze the three signals into ONE wire, we need a SUBCARRIER. A subcarrier is any signal hidden in another signal that can carry extra information without affecting the main signal (well, not too much). 

A Very Important Note About the Subcarrier Chroma System

The actual workings of the subcarrier have not been explained at all.... just the result of it. Quadrature subcarrier modulation is pretty interesting, but it is just a means of delivering R-Y and B-Y to the receiver or monitor on one wire. The need for video with a subcarrier is rapidly disappearing from the video world, as you will see, so this tutorial has decided to "gloss over" it.

What's that Frame Rate Again?

Sneaking the subcarrier at 3.58 megahertz into the video signal had one other lasting effect. The actual subcarrier frequency had to be an odd-multiple of one-half of the horizontal scanning frequency (see "I'm not telling you the whole story", above). The needed subcarrier frequency was too high to get through the transmitters. antennas, and tuners of the day, so the scanning was slowed down by a factor of about 1000/1001. Even though we slip-up a lot and refer to thirty frames per second, the real frame rate of video in the NTSC system is 29.97 frames per second. No one noticed until timecode was invented, as you will see.

 Black

  

Sometimes we want "nothing" on the screen. If we used "nothing", when we switched to "something", the monitors and tape machines would have to scramble to synchronize the picture and colour information. This takes a second or so, and is ugly. The official "nothing" of video is black, also known as "colour black" by older techs. Black consists of horizontal and vertical sync, and the colour burst with no actual picture information.

 

This author does not like "setup", also known as "pedestal". Having invented a very nice video system, with a friendly scale of zero to one hundred, it was decided that black wouldn't be at zero. Rather, it would be at 7.5 units, and no parts of the picture would be darker than that. It was feared that some receivers might have the brightness a little high, and the the retracing electron beam might be visable on the screen (simulated at right). The idea never worked, so since 1960 or so, all TV sets and receivers have simply blanked themselves during the retrace of the scanning beam. We are still stuck with the 7.5 unit setup in NTSC, but the idea has been dropped from all of the new standards (including DVCam!). Setup steals 7.5 percent of our contrast, and messes up all of the math, while failing to do what it was supposed to do. Good riddance!

These are "SMPTE" (sem-tee) bars. SMPTE- the Society of Motion Picture and Television Engineers

"PLUGE" is an acronym for Pulse Line-up Generation Equipment. No Kidding. It's British. In this example, the brightness is set too high. Only the slightly lighter than black bar will be visible if the brightness is set correctly. The "reverse" blocks have the same amount of blue as the bars above them. This will come in handy later. The unlabelled purplish boxes are the "I" and "Q" axes. Ignore 'em. These are the most common bars in use, but there are several variations, for instance "Full Field", which is just the coloured stripes for the entire height of the picture.

	Here the scale is set to display 1 horizontal line period. We are seeing all the scanning lines superimposed. Note the scale at the left. The video goes from 0 to 100 units, with the blackest part of the picture at 7.5 units in North America. The sync extends to -40 on the scale.
	This is the more common view, with the time period set to display two horizontal lines. This is so the entire horizontal sync pulse and colour burst can be seen un-broken.
	This is H-MAG... the circled interval above is expanded for a better look.
	Colour bars with the colour (subcarrier/chroma) filtered out. This filter is often labeled "IRE"... the same as the scale. This is the common view used for adjusting video levels from studio cameras.... the chroma just gets in the way. PLUGE is circled... a dip below 7.5 black, 7.5 black, and about 10 units (lighter than black).
	he reverse filter is available, too... of less use, but note that the burst correctly spans from -20 to +20 (40 units total.... the same size as the sync pulses. The peak chroma in Bars extends from -40 to +40.
	2V is the view.... two complete fields visible. The vertical sync pulse is circled. This view is used for checking for low-frequency problems, like power-line HUM... as seen below on the waveform and picture monitors. The name hum comes from the fact that this same problem in audio is heard as a humming sound.
	2V MAG... as with "H", a magnified view of the (vertical) sync pulse. The vertical sync is a big blob (squint at it to see) with serrations that serve to keep the horizontal section syncronized.
	The above are beautiful looking waveforms, so how come my video on the waveform monitor looks like GRASS? This is a typical scene seen on a waveform monitor. The colour (chroma) is filtered out, leaving just the greyscale information. It looks like grass because all of the video lines are seen at once, repeated over and over on top of each other.
	Here, one scan line of the picture has been isolated. It is more like what you would expect, with the video signal representing the brightness of the image from left to right. Remember, most Waveform Monitors show two side-by side displays so that the sync pulse can be seen in its entirety. Yeah, yeah, yeah, the sync level is low...

If it happens that you can't get all of the dots in the boxes, favor the red dot for perfection, as it is closest to flesh colour. All varieties of flesh are about the same vector.... just different brightness's!	The Vectorscope plots colour on a circular scale. Highly saturated colours are the furthest from the centre. This is colour bars on a vectorscope. The six outermost dots are landing on targets labelled "R", "M", "B", etc. These are Red, Magenta, and Blue, etc. The short arrow points to the zero degree mark, and the long arrow is pointing to the colour burst (sync) which lies on the zero line. (The un-mentioned dots are the I and Q axies again!)
	Here, the bars have the wrong tint.... known as "Hue" The burst is correct, but the bars are wrong. Since the Magenta target has nothing near it, it can be seen clearly. The trapezoidal large target was the goal for bars in the old days. These days, with our more stable equipment, we can usually adjust until all of the bars are in the small boxes. They should all land on the small cross in the centre of the small box, but that's usually hoping for too much.
	This is a typical "talking head" shot.... just a ball of grass in the red/yellow quadrant. The colour burst is prominent.

	A Picture Monitor is the display portion of a high quality TV set. Home sets are designed to hide some problems with video, but expensive monitors are designed to show every flaw! (Irony) Here, like a home set, the picture is stretched high and wide enough that the edges disappear under the plastic mask around the picture tube.
	This is the same monitor with the "Underscan" button pushed. Sometimes the button is just labeled "Size". Now, all edges of the picture can be seen, and we are sure where the centre is... you never know how much of the picture is missing in "overscan".  Under the RED bar, you can just barely see the "lighter than black" bar in the PLUGE. The monitor brightness is correct.
	This mode is "Pulse Cross", or "H and V Delay". This feature is used for inspecting all of the sync stuff in the video. The white arrow is pointing to vertical sync. The Yellow arrow point is on the Horizontal sync, and the red arrow is on the murky, yellowish-green colour burst. Gee, it's just to the right of the horizontal sync.... just like on the waveform monitor!
	Blue Only. This button shuts off the RED and GREEN channels in the monitor, leaving only blue. Since the little reverse-blocks on bars have the same amount of blue as the bar above, the wedges match the bars... BUT ONLY if the tint (hue) and colour saturation settings are correct.

The red curve at the left shows a cathode ray tube's (CRT) response to electrical input. The "light out" doesn't correspond to the "video volts in" linearly, resulting in darker mid-tones than intended.  The blue curve (which is exactly opposite) is applied to the video signal for correction, and the green curve shows the result.

CRT native response

Gamma Correction Applied

Even Grayscale

HOME | BACK | HOW IT WORKS | TIMING | RECORDING | EDITING & TIMECODE | AUDIO | COMPUTERS

---

DIGITAL INTRO | SDI | DIGITAL AUDIO | WHAT'S IN SDI | DIGITAL MONITORING | DIGITAL TIMING | STREAMS | ATSC | GRAPHICS